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CORNELL
UNIVERSITY
LIBRARY
BOUGHT WITH THE INCOME
OF THE SAGE ENDOWMENT
FUND GIVEN IN 1891 By
HENRY WILLIAMS SAGE
—————
“Nicngana
A LABORATORY MANUAL AND TEXT-BOOK
of
EMBRYOLOGY
By
CHARLES WILLIAM PRENTISS, A. M., Pu. D.
LATE PROFESSOR OF MICROSCOPIC ANATOMY, NORTHWESTERN UNIVERSITY MEDICAL SCHOOL, CHICAGO
Revised and Extensively Rewritten by
LESLIE BRAINERD AREY, Pu. D.
ASSOCIATE PROFESSOR OF ANATOMY IN THE NORTHWESTERN UNIVERSITY MEDICAL SCHOOL, CHICAGO
SECOND EDITION, ENLARGED
WITH 388 ILLUSTRATIONS
MANY IN COLOR
PHILADELPHIA AND LONDON
W. B. SAUNDERS COMPANY
1917
Copyright, 1915, by W. B. Saunders Company. Reprinted August, 1915. Revised, entirely
reset, reprinted, and recopyrighted October, 1917
Copyright, ror7, by W. B. Saunders Company
PRINTED IN AMERICA
PRESS OF
W. &. SAUNDERS COMPANY
PHILADELPHIA
PREFACE TO THE SECOND EDITION
THE untimely death of Professor Prentiss has made necessary the transfer
of his ‘Embryology’ into other hands. In this second edition, however, the
general plan and scope of the book remain unchanged although the actual descrip-
tions have been extensively recast, rewritten, and rearranged. A new chapter
on the Morphogenesis of the Skeleton and Muscles covers briefly a subject not
included hitherto. Forty illustrations replace or supplement certain of those
in the former edition.
In preparing the present manuscript a definite attempt has been made to
render the descriptions as clear and consistent as is compatible with brevity and
accuracy. It has likewise been essayed to properly evaluate the embryological
contributions of recent years, and, by incorporating the fundamental advances,
to indicate the trend of modern tendencies. Since no page remains in its entirety
as originally penned by Professor Prentiss, the reviser must assume full respon-
sibility for the subject-matter as it now stands.
It is hoped that those who read this text will co-operate with the writer by
freely offering criticisms and suggestions.
L. B. A.
Curcaco, ILL.,
October, 1917.
lil
PREFACE
Tus book represents an attempt to combine brief descriptions of the verte-
brate embryos which are studied in the laboratory with an account of human
embryology adapted especially to the medical student. Professor Charles Sedg-
wick Minot, in his laboratory textbook of embryology, has called attention to the
value of dissections in studying mammalian embryos and asserts that “dissection
should be more extensively practised than is at present usual in embryological
work. .” The writer has for several years experimented with
methods of dissecting pig embryos, and his results form a part of this book. The
value of pig embryos for laboratory study was first emphasized by Professor Minot,
and the development of my dissecting methods was made possible through the
reconstructions of his former students, Dr. F. T. Lewis and Dr. F. W. Thyng.
The chapters on human organogenesis were partly based on Keibel and
Mall’s Human Embryology. We wish to acknowledge the courtesy of the pub-
lishers of Kollmann’s Handatlas, Marshall’s Embryology, Lewis-Stéhr’s Histology
and McMurrich’s Development of the Human Body, by whom permission was
granted us to use cuts and figures from these texts. We are also indebted to
Professor J. C. Heisler for permission to use cuts from his Embryology, and to
Dr. J. B. De Lee for several figures taken from his “Principles and Practice of
Obstetrics.” The original figures of chick, pig and human embryos are from
preparations in the collection of the anatomical laboratory of the Northwestern
University Medical School. My thanks are due to Dr. H. C. Tracy for the loan
of valuable human material, and also to Mr. K. L. Vehe for several reconstruc-
tions and drawings.
C. W. PRENTISS.
NORTHWESTERN UNIVERSITY MEDICAL SCHOOL.
CONTENTS
PAGE
TINTRODUCTIONS 45 cco-was. spud aaie eagnarcyoraieratitis# ache Gilosacean aca SR aeanend ns Gia gs & Sade sear ean ea est 1
CHAPTER U— Tn, ‘GERM OBELS: sfrdiagn ap iararsat ena docseia he ke paar hen a oa saan eG 7
SEM OV ATA a tee ahah ceva rsinic oeiutcendds peat w ect ae rama ded reese nee uaoed tains ama auasumnate Gea BERRI ite aes 7
Ovulationsand: Menstruation’: 05.005 Wwe ocala bu no Mao han eigen hahaa pyar lanes eo se 10
ENE SPEEMALOZOON ys sacs dor a Mee ns oe cree Peet net eens boner ewes Cees wan eh3 10
Mitosis and: Amiltosiss: ea iy yg ccc bu od £gacadaon ba wes daheoewiaydrar ob cnsete node Biusedaranes ae 12
IME aetna ti Omer: § sarte a ate eececte wthceisuy nd 2 Tena BELG Dea Maw hen ee DS aE Tad he kMestoeuae Saal! 14
OTE IZA C1 OT a shasticga:d ous sg eed mE aspohoanais malas GAL cas Bohs dtades Be MaMa UN aaa ay Sd, oe Seis etd Oaridauanee eetned4 19
Heredity and the Determination of Sex... 0... 0.0.00. nen nbn nen tenes 21
CHAPTER IT.—CLEAVAGE AND FORMATION OF THE GERM LAYERS.............-..22000000 0000s 23
Cleavage in Amphioxus, Amphibia, Birds, and Reptiles............0..0....0000 00220000 23
Cleavage win Miaminiall Sites ie Anant wa kr ila oma egies aud seen Alas aba hohds ouainainewicinanmn nent a 26
Origin of the Metoderm and! Entoderm. 9.x nex dx fede Alnaibeoe say eee eg ogo ead 4 28
Origin of the Mesoderm, Notochord and Neural Tube................0 00000. e cece 29
Whe: Notochond)s 4%. ccte"s eed 2 oe S22 es ri ale gga ane a nee oo eas ae Aes 35
CHAPTER III].—TuHe Stupy or CHIcK EMBRYOS. ........200 0000 c cece eee 36
Chick: Embryoof Twenty Hoursé cc. .cn acu au 64 ak Sane Rodd dd eka cheka Thea ed 36
Chick Embryo of Twenty-five Hours (7 Segments)... 0.2.0... 0c eee eee eee 38
ATATIS VETSE SECEONS ce cecccy alas shy Sey ee dioerreanetire tdl ac elevates A eeR nce aunaned Wlunees Wea Ea ANCES 40
Chick Embryo of Thirty-eight Hours (17 segments)... ... 0.00.00. 0 0000000 cece eee eee 43
Gemerall Amato in iy cts, wera esta Baiciavs inh aeisde exsetelic hte avis cot MAMMA ape iebea td alee ate a eave clalinina ae 43
PraANSVerSesSECHONSH fats angi syiadiisay eas aad oanshieaedthn ad» anes Pen gia clonmeshineitnds 47
Derivatives:oh the Germ Layers’ oc. a5 2c0 5 ges ose deb eases se oe ee eR eae bn eee 54
Chick Embryo of Fifty Hours (27 segments)... 00000 cece ee eene 55
General Anatomy y's aoch lenny vgs howe eevee steam se eo ERE Ee eed ees awe deelahee 55
Tratisverse-Sections sa. ices aioe ga sareka 2 oc RG dd eeoweding aes Cea be Aka BOS Sabana Aa 59
CHAPTER IV.—TuHeE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS...............0..00000- 68
Fetal Membranes of the Pig: Embryo)... gocc3 0 teen wa tiuautaadeh ogous Soave wid bobs Aya Rds ded ened 68
Wim Diliea Conds coe 3.8 6 ects eee ay Sy 5 cc oy op acted decide decent WMO epi ty Reto lala dea pense gh eataNG 70
Early Human Embryos and Their Membranes... ..........0.00 0000 e eee eee 71
Anatomy of a 4.2 mm. Human Embryo... 2.0.0... 79
Age iok Humanck mbtyOSa 1k dai aiid Ames 4 eed aoe dois nom ned oe Moaung nina tian aaa 87
CaAPTER V.—THE STUDY OF PIG EMBRYOS vcd dust e ached GRA MAAGE Ses OY eaaak eed oem benel 89
The Anatomy of a 6 mm. Pig Embryo... 1.0.0... cnet eee eens 89
External Form and Internal Anatomy... ... 0.0... 0... cece ccc e eee nen . 89
PLPANSVCLSE SE CHO MS) 56 pict asec ssccutn stipes ie dh Sem Sie Vem e Pusyale BosiGnacensuuameo nana mtecd olgh & docsters 104
The Anatomy of 10-12 min. Pig EMbEV OS! 4 .0 ie econ esac Koln a doe dae knw eminaueee 112
External Form and Internal Anatomy... .............00 00020 cece eee eee a ae OLED,
-BTAMSV CESE SECTIONS 5 ca vutrd aiwasuwnraidn ie oe hesunbune Bue ors Seneetelacei aes hae ieeielslale, deat ENA 125
CHAPTER VI.—MEeEtTHops oF DissEcTING Pic EmBryos: DEVELOPMENT OF THE FACE, PALATE,
ToncurE, TEETH AND SALIVARY GLANDS... 00... es 137
Directions for Dissecting Pig Embryos... . 0.0.2.2... 000006 e ccc eee eee ee 137
Dissections of 18-35 mm. Embryos... 0.0.0.0... o occ eee e eee e ees 140
Development: Of the Faces 552 sca hacen an Wino sot dada ait edb nd deanordaettapabeentfaiedd ta edaueed & Aameateten teases 144
Development ofthe: Hard: Palate: 3: cana t5.2-kasee eet tape coed meedelaleeedd ubedharan toh. dobsesbchen cop an atime ses 146
Development: of the Tong tei. .s 0 oe echaate! vaste aod once: beta a euenicngudega aitwrn ow geaee wavesnd Dab eheAende 149
Developmentofthe: salivary Glands... cis scciceem data om aeivein Wa baal eau: + coy ieee y 152
Development ofthe: Feethe icc ncsaw aecniet 4 ancnn yey, veel erate oe. Hs gee Gonans Pa ements 153
vili CONTENTS
PAGE
CHAPTER VIT.—ENTODERMAL CANAL AND ITS DERIVATIVES... 0.00. c cece cece cece eee eee etees 159
Pharyngeal Pouches and their Derivatives. ........00 00... c cece cece cece cece cece etenneee 160
Why reid (Gland. sa sxoseuinen atten th Gk & dts Anal cdskoavae to mhebal matacedna) Wa ove bod ovis a beck Mbeducheuatuaree Meee SAE 164
atynx,; Prachea:and: Ian ese, cs. theca dca yot mont a ators in ndeuedaleoayandcaan dela h ake auteereae ala a ue eS 164
Digestive (Cama lane fc uekco cuyecamalt wes leh atv moe cesslekel all lanes leas RA sen NAME os yao eB Soe 169
Ti OTe sess geen ees 1 Ssbaeteabe he wctchchete Satoh. SD Rdele GSA SOONER ON atone SLaTE He dibudit We eIOA tok ace ch Fe .. 175
DAM Grea Sia tec fs toner eaeeeat ove ids irae a cauoie ictal Mad dosed A tonccttals iain edt a Ach tacos decoherence amet _. 178
Body Cavities, Diaphragm and Mesenteries... 1.2.0.0... 0. cece eee e eee nee ence et eeee 179
CHAPTER VIITL—UROGENITAL- SYSTEMS. 2.555 ne eat a ati Sate bin toe See ee RoR AS SEE OMe 195
PLONE EOS sm seg cosce se @ uses tes ARAN ay Aah cha shen pads ve onte we atesne au Sem le Bia: Slate rethaand Grease Sa Reakeeeeeae .. 195
Miesonephross: a sen Aide wacntdeuee aco aarett iar: alba fl eidlaialntlecitiain acon nainrnben inte ancien a a een oneratra teed 197
Meétanephrosinass oes pass uianeas edie sae PP ede dese gue eye bee ed gence) SORE 199
Cloaca, Bladder, Urethra and Urogenital Sinus.......... 00.0000 0. e ee cece eee ae 205
Genital: Glandsand: Ductsi ss givsi- te pha eee HS eS FLEE OE HONE BOER 4G 8 ABER ES LO eee ae 208
External Gemitania cr, 2 a:6.008 Rosteociciniues wae ae outa IES he Liebe’ PAGE SURES TEAR ESR akg 224
The Uterus during Menstruation and Pregnancy..... 2.0.2... 6.0 o eee 230
The Decidual Membranes...................- relists ptddeesl dB eR OS ren NeA piSauncrer ad Smee tenets 231
CTS er PAGO nit as ocnxa-a a naonserstg? aeesainanceend ves 2 be Suse alge Shc MN ea Ss AAS ayaa aus aug ee tea a Teak a eta 237
The Relation-of Fetus to Placemtais icccc 2 c-cseseie aren au tateciivenncti cone tvere once moda Anal A dies dtc aveale i Gee 241
CHAPTER: IX. VASCUEAR SISTEM :.2 na) pre os PEER: Sea ReRS 14 ee Eee eee teen ee 243
The Primitive Blood Vessels and Blood Cells... 1.2.0.2... 0.0: cc cece cece ee eee eee e eee 243
Development..of the Hearts ccoces cg eae oats Seaa be SR kee eee eRe ee ewes FEE RAS 247
Primitive Blood: Vascular System: 5 ca.cuci008 CHa Mok Pease ae tee oe pee eae pease 259
Developmentiof the-ATteMes.. ooo 5c pode a esa Sdcanb hes Kase eed badonte paeheeteas sae 2061
Deévelopment.of the, VeinSs .ocs co agc00 4 nad A edonlitd a dda adidas Shauinet ced Bepeaueune Eales 268
Bhe: BetalCireulatiomts 2. se ss, so ncterese aoe dace as acbgeeaerni A ore Somnb eases Snpaaamab nad b.8 wee dbeyim 207
The Wey phatie Systems, 4. iesceace seeget onus ciseetiaie waqehennie we ghachn Ptvaand ead AV Qdue DN GDS ie b Suaobpeghghane 279
ymph.and.emolymph'Glandsa.2. 5 <.c0 dau sapreten Gre eaeme nies MAI ead rendned QQdeiine 281
Spleemssnrive-sanhisve sz cateunlaypiay afte feces os Joase gt eure te de Aa 4, ai lvoe oar lbepe adele deep a AR 281
CHAPTER Xi.“ FISTOGENESIS acu s aoeuns saa e ete ween Say tie tae na ae Ubud oa eas emaatians 283
The Entodermal Derivatives ic. ncvreeesagareine ss 22406 A Kane Stee eae was Pewenes 283
‘Fhe: Mesodermal Tissites)«.¢., accs.40e4 3 npeeeetun Mant Lag eres Gate beta baa ae Came 284
Tae Ectodermal Derivatives. uic.so 3 ca-ccacudawt noe Seedy dee bd eS ee bee ee aes eee 294
The Ne ous: Disses 22. ee ek acd 2 4 04 6 set deardsivacd y BEA d SE die wre hd MGoweNwii ee EE dees Bees 300
CHAPTER XI.—MORPHOGENESIS OF THE SKELETON AND MUSCLES..... 2.2.0... 00000 e eee eeeeee 309
AhesS Ke letale Sy Stem. (tare ec fontun Sek ta tacad vice MegeN@a nk a hisde ten ewiphel een ae adem Ahe Onelselees 309
GRETA SILC TO Ic cf ae eh cs csr eec ke Meagan we sees dsssb ao RaKANHO RONG Ges Sree Serta A EERSMO HS 309
Appendicular Systema. gcc. csgica wen acedeergavemicn yczcm gaat aoangteineaenvard ode RAAB RSE Ea ave re acta ae 0 315
ARES Miuseula ri Sy St emt) wcrc d deahindecn tecieishitheuss AE aa. otal ac seeds vengha sua eee Bly dears esata SS 316
CHAPTER XII.—MorPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM...........0 000000 ee ee eee 321
he: Spinal: Cord: 4s nue shee sey LEBER ES Ment tee gae han aula k et ecns tune ee S27
THE Brain, 44 G vices enn e eee oat nee needa ha high oe wanuien & cud Grito @ ae BOE wat A et aeee 327
The Differentiation of the Subdivisions of the Brain... ...........0.. ccc cece cece eee: 332
CuarTer XIII.—TueE PErIPHerat NERVOUS SYSTEM... 2... ec cece cece eeeevececes 8 353
ABW SPINAL INC TCS scree casters ee of Aa aded pre awtuia iat tahaeete NNT acdc a dancer aodite Meno eRe eae oe
A WEEG Creb ral AN EVs egies G8 ak soe ch Soschiunstosabsiala a. tnob uae booed ante aed St
The Sympathetic Nervous System... 0.000.000. .0 ccc cece eee nec e cece eueeceenceeeees 366
Chromaffin Bodies: Suprarenal Gland... 00.000. 0 0c cece ccc ccc cece eee eecevecee 368
The Sense Orpamsecscs cease 4.4 aparece va bean wach gia ers eter Gemva dle: sich satirouieban Boles, reciac e m ns tnaeeid a 370
TEXT-BOOK OF EMBRYOLOGY
INTRODUCTION
THE study of human embryology deals with the development of the individual
from the origin of the germ cells to the adult condition. To the medical student
human embryology is of primary importance because it affords a comprehensive
understanding of gross anatomy. It is on this account that only recently a
prominent surgeon has recommended a thorough study of embryology as one of
the foundation stones of surgical training. Embryology not only throws light
on the normal anatomy of the adult, but it also explains the occurrence of many
anomalies, and the origin of certain pathological changes in the tissues. From
the theoretical side, embryology is the key with which we may unlock the secrets
of heredity, of the determination of sex, and, in part, of organic evolution.
There is, unfortunately, a view current among graduates in medicine that the
field of embryology has been fully reaped and gleaned of its harvest. On the
contrary, much productive ground is as yet unworked, and all well-preserved
human embryos are of value to the investigator. An institute of embryology
for the purpose of collecting, preserving, and studying human embryos has re-
cently been established by Professor F. P. Mall of the Johns Hopkins Medical
School. Aborted embryos and those obtained by operation in case of either normal
or ectopic pregnancies should always be saved and preserved at once by immersing them
intact in 10 per cent. formalin or in Zenker’s fluid.
Historical.—The science of modern embryology is a comparatively new one,
originating with the use of the compound microscope and developing with the
improvement of microscopical technique. Aristotle (384-322 B. c.), however,
centuries before had followed the general development of the chick day by day.
The belief that slime and decaying matter was capable of giving rise to living
animals, as asserted by Aristotle, was disproved by Redi (1668).
A few years after Harvey and Malpighi had published their studies on the
chick embryo, Leeuwenhoek reported the discovery of the spermatozoén by
Ham in 1677. At this period it was believed either that fully formed animals
existed in miniature in the egg, needing only the stimulus of the spermatozoén to
I
2 INTRODUCTION
initiate development, or that similarly preformed bodies, male and female, con-
stituted the spermatozoa and that these merely enlarged within the ovum.
According to this doctrine of preformation all future generations were likewise
encased, one inside the sex cells of the other, and serious computations were made
as to the probable number of progeny (200 million) thus present in the ovary of
Mother Eve, at the exhaustion of which the human race would end! Dalenpatius
(1699) believed that he had observed a minute human form in the spermatozoén.
The preformation theory was strongly combated by Wolff (1759) who saw
that the early chick embryo was differentiated gradually from unformed living sub-
stance. This theory, known as epigenesis, was proved correct when, in 1827, von
Baer discovered the mammalian ovum and later demonstrated the germ layers of
the chick embryo.
About twenty years after Schleiden and Schwann (1839) had shown the cell
to be the structural unit of the organism, the ovum and spermatozoon were recog-
nized as true cells. O. Hertwig, in 1875, was the first to observe and appreciate
the events of fertilization. Henceforth all multicellular organisms were believed
to develop each from.a single fertilized ovum, which by continued cell division
eventually gives rise to the adult body, containing, it is estimated, 26 million
million cells. In the case of vertebrates, the segmenting ovum differentiates first
three primary germ layers. The cells of these layers are modified in turn to form
tissues, such as muscle and nerve, of which the various organs are composed,
and the organs together constitute the organism, or adult body.
Primitive Segments—Metamerism.—In studying vertebrate embryos we
shall identify and constantly refer to the primitive segments or metameres. These
segments are homologous to the serial divisions of an adult earth worm’s body,
divisions which, in the earth worm, are identical in structure, each containing a
ganglion of the nerve cord, a muscle segment, or myotome, and pairs of blood ves-
sels and nerves. In vertebrate embryos the primitive segments are known as
mesodermal segments, or somites. Each pair gives rise to a vertebra, to a pair of
myotomes, or muscle segments, and to paired vessels; each pair of mesodermal
segments is supplied by a pair of spinal nerves, consequently the adult verte-
brate body is segmented like that of the earth worm. As a worm grows by
the formation of new segments at its tail-end, so the metameres of the vertebrate
embryo begin to form in the head and are added tailward. There is this dif-
ference between the segments of the worm and the vertebrate embryo. The seg-
mentation of the worm is complete, while that of the vertebrate is incomplete
ventrally.
GROWTH AND DIFFERENTIATION OF THE EMBRYO 3
GROWTH AND DIFFERENTIATION OF THE EMBRYO
A multicellular embryo develops by the division of the fertilized ovum to
form daughter cells. These are at first similar in structure, and, if separated, any
one of them may develop into a complete embryo, as has been proved by the
experiments of Driesch on the ova of the sea urchin. The further development of
the embryo depends: (1) upon the multiplication of its cells by division; (2) upon
the growth in size of the individual cells; (3) upon changes in their form and
structure.
The first changes in the form and arrangement of the cells give rise to three
definite plates, or germ layers, which are termed from their positions the ectoderm
(outer skin), mesoderm (middle skin) and entoderm (inner skin). Since the ecto-
derm covers the body it is primarily protective in function, but it also gives rise to
the nervous system through which sensations are received from the outer world.
The entoderm, on the other hand, lines the digestive canal and is from the first
nutritive in function. The mesoderm, lying between the other two layers,
naturally performs the functions of circulation, of muscular movement, and
of excretion; it also gives rise to the skeletal structures which support the
body. While all three germ layers form definite sheets of cells known as
epithelia, the mesoderm takes also the form of a diffuse network of cells, the
mesenchyma.
The Anlage.—This German word, which lacks an entirely satisfactory
English equivalent, is a term applied to the first discernible cell, or aggregation of
cells, which forms any distinct part or organ of the embryo. In the broad sense
the fertilized ovum is the anlage of the entire adult organism; furthermore, in the
early cleavage stages of certain embryos it is possible to recognize single cells or
cell groups from which definite structures will indubitably arise. The term anlage,
however, is more commonly applied to the primordia that differentiate from the
various germ layers. Thus the thickening of the epithelium over the optic vesicle
is the anlage of the lens. |
Differentiation of the Embryo.—The developing embryo exhibits a progres-
sively complex structure, the various steps in the production of which occur in
orderly sequence. There may be recognized in development a number of com-
ponent mechanical processes which are used repeatedly by the embryo. The
general and fundamental process conditioning differentiation is cell multiplication
and the subsequent growth of the daughter cells. The more important of the
specific developmental processes are the following: (1) cell migration; (2) localized
growth, resulting in enlargements and constrictions; (3) cell aggregation, forming (a)
4 INTRODUCTION
cords, (b) sheets, (c) masses ; (4) delamination, i. e., the splitting of single sheets into
separate layers; (5) folds, including circumscribed folds which produce (a) evagina-
tions, or out-pocketings, e. g., the intestinal villi, (b) cnvaginations, or in-pocket-
ings, e. g., the intestinal glands.
The production of folds, including evaginations and invaginations, due to un-
equal rapidity of growth, is the essential factor in moulding the organs and hence the
general form of the embryo.
Differentiation of the Tissues.—The cells of the germ layers which form
organic anlages may be at first alike in structure. Thus the evagination which
forms the anlage of the arm is composed of a single layer of like ectodermal
cells, surrounding a central mass of diffuse mesenchyma (Fig. 136). Gradually
the ectodermal cells multiply, change their form and structure, and give rise to
the layers of the epidermis. By more profound structural changes the mesen-
chymal cells also are transformed into the elements’ of connective tissue, tendon,
cartilage, bone, and muscle, aggregations of modified cells which are known as
tissues. The development of modified tissue cells from the undifferentiated
cells of the germ layers is known as histogenesis. During histogenesis the struc-
ture and form of each tissue cell are adapted to the performance of some special
function or functions. Cells which have once taken on the structure and func-
tions of a given tissue cannot give rise to cells of any other type. In tissues like
the epidermis, certain cells retain their primitive embryonic characters throughout
life, and, by continued cell division, produce new layers of cells which are later
cornified. In other tissues all of the cells are differentiated into the adult type,
and, during life, no new cells are formed. This takes place in the case of the
nervous elements of the central] nervous system.
Throughout life, tissue cells are undergoing retrogressive changes. In this
way the cells of certain organs like the thymus gland and mesonephros degenerate
and largely disappear. The cells of the hairs and the surface layer of the epider-
mis become cornified and eventually are shed. Thus, normally, tissue cells may
constantly be destroyed and replaced by new cells.
The Continuity of the Germ Plasm.—According to this important conception
of Weismann the body-protoplasm, or soma, and the reproductive-protoplasm
differ fundamentally. The germinal material is a legacy that has existed since
the beginning of life, from which representative portions are passed on intact
from one generation to the next. Around this germ plasm there develops in
each successive generation a short-lived body, or soma, which serves as a vehicle
for insuring the transmission and perpetuation of the former. The reason, there-
METHODS OF STUDY 5
fore, why offspring resembles its parents is because each develops from portions
of the same stuff.
The Law of Biogenesis.—Of great theoretical interest is the fact, con-
stantly observed in studying embryos, that the individual in its development
tends to repeat the evolutionary history of its own species. This law of recapitu-
lation was first stated clearly. by Miiller in 1863 and was termed by Haeckel the
law of biogenesis. According to this law, the fertilized ovum is compared to a
unicellular organism like the Amceba; the blastula is supposed to represent an
adult Volvox; the gastrula, a simple sponge; the segmented embryo a worm-like
stage, and the embryo with gill slits may be regarded as a fish-like stage. The
blood of the human embryo in development passes through stages in which its
corpuscles resemble in structure those of the fish and reptile; the heart is at first
tubular, like that of the fish; the kidney of the embryo is like that of the amphib-
ian, as are also the genital ducts. Many other examples of this law may readily
be observed.
Methods of Study—Human embryos not being available for individual
laboratory work, the embryos of the lower animals which best illustrate certain
points are employed instead. Thus the germ cells of Ascaris, a parasitic round
worm, are used to demonstrate the phenomena of mitosis and maturation; the
larvee of echinoderms, or of worms, are frequently used to demonstrate the cleav-
age of the ovum and the development of the blastula and gastrula larve; the
chick embryo affords convenient material for the study of the early vertebrate em-
bryo, of the formation of the germ layers and of the embryonic membranes, while
the structure of a mammalian embryo, similar to that of the human embryo, is
best observed in the readily procured embryos of the pig. An idea of the anatomy
of embryos is obtained first by examining the exterior of whole embryos and study-
ing dissections and reconstructions of them. Finally, each embryo is studied in
serial sections, the level of each section being determined by comparing it with
figures of the whole embryo.
Along with his study of the embryos in the laboratory, the student should
do a certain amount of supplementary reading. Only the gist of human organo-
genesis is contained in the following chapters. A very complete bibliography
of the subject is given in Keibel and Mall’s “Human Embryology,” to which
the student is referred. Below are given the titles of some of the more important
works on vertebrate and human embryology, to which the student is referred
and in which supplementary reading is recommended.
6 INTRODUCTION
TITLES FOR REFERENCE
Duval, M. Atlas D’Embryologie. Masson, Paris, 1889.
His, W. Anatomie menschlicher Embryonen. Vogel, Leipzig, 1885.
Keibel, F. Normentafel zur Entwicklungsgeschichte der Wirbelthiere. Bd.
I. Fischer, Jena, 1897.
Keibel and Elze. Normentafel zur Entwicklungsgeschichte des Menschen, , , ,
Jena, 1908.
Keibel and Mall. Human Embryology. Lippincott, 1910-1912.
Kellicott, W. E. A Textbook of General Embryology. Henry Holt, 1913.
Kollmann, J. Handatlas der Entwicklungsgeschichte des Menschen. Fischer,
Jena, 1907.
Lee, A. B. The Microtomist’s Vade Mecum. Blakiston, 1913.
Lewis, F.T. Anatomy of a 12 mm. Pig Embryo. Amer. Jour. Anat., vol. 2.
Lillie, F. R. The Development of the Chick. Henry Holt, 1908.
Minot, C.S. A Laboratory Text-book of Embryology. Blakiston, 1910.
Thyng, F.W. The Anatomy of a 7.8mm. Pig Embryo. Anat. Record, vol. 5.
Wilson, E. B. The Cell in Development and Inheritance. Macmillan, 1911.
CHAPTER I
THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION
THE GERM CELLS
THE highly differentiated human organism, like all other vertebrates and
most invertebrates, develops from the union of two germ cells, the ovum and
spermatozoon.
The Ovum.—The female germ cell, or ovum, is a typical animal cell pro-
duced in the ovary. It is nearly spherical in form and possesses a nucleus with
nucleolus, chromatin network, chromatin knots, and nuclear membrane (Figs. 1 and
2). The cytoplasm of the ovum is distinctly granular, containing more or less
numerous yolk granules and rarely a minute centrosome. The nucleus is essential
to the life, growth, and reproduction of the cell. The function of the nucleolus
is unknown; the chromatin probably bears the hereditary qualities of the cell.
The yolk granules, containing a fatty substance termed Jecithin, furnish nutrition
for the early development of the embryo. A relatively small amount of yolk
is found in the ova of the higher mammals, since the embryo develops within,
and is nourished by, the uterine wall of the mother. A much larger amount occurs
in the ova of fishes, amphibia, reptiles, birds, and the primitive mammalia, the
eggs of which are laid and develop outside of the body. The so-called yolk of
the hen’s egg (Fig. 3) is the ovum proper and its yellow color is due to the large
amount of lecithin which it contains.
Ova become surrounded by protective membranes, or envelopes. The
vitelline membrane, secreted by the egg itself, is a primary membrane (Fig. 2).
The follicle cells about the ovum usually furnish other secondary membranes,
e. g., the zona pellucida. Tertiary membranes may be added as the egg passes
through the oviduct and uterus—the albumen, shell membrane, and shell of the
hen’s egg are of this type (Fig. 3).
The human ovum is of small size, measuring from 0.22 to 0.25 mm. in diam-
eter (Fig. 1). The cytoplasm is surrounded by a relatively thick radially striated
membrane, the zona pellucida. The striated appearance of the zona pellucida
is said to be due to fine canals which penetrate it and through which nutriment
is carried to the ovum by smaller follicle cells during its growth within the ovary.
7
8 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION
The origin and growth of the ovum within the ovary (odgenesis) are described on
pp. 214-217. We may state here that each growing ovum is at first surrounded
Fic. 1.—Human ovum examined fresh in the liquor folliculi (Waldeyer). > 415. The zona pellu-
cida is seen as a thick, clear girdle surrounded by the cells of the corona radiata. The nearly mature
egg itself shows a central granular deutoplasmic area and a peripheral clear layer, and encloses the
nucleus in which is seen the nucleolus. At the right is a spermatozoén correspondingly enlarged.
2 Ae
: your"
Fic. 2.—Ovum of monkey. X 430.
by small nutritive cells known as follicle cells. These increase in number during
the growth of the ovum until several layers surround it (Fig. 229). A cavity
appearing between these cells becomes filled with fluid and thus forms a sac,
THE GERM CELLS 9
yy. wy. wy. bl
Ub. x wW.
Fic. 3—Diagrammatic longitudinal section of an un- Fic. 4.—Section of human ovary,
incubated hen’s egg (Allen Thomson in Heisler): b.J, germ- including cortex: a, germinal epithe-
inal disc; w.y, white yolk, which consists of a central flask- lium of free surface; 6, tunica albu-
shaped mass, and a number of concentric layers surrounding ginea; c, peripheral stroma contain-
the yellow yolk (y..); v.t, vitelline membrane; x, a some- ing immature Graafian follicles (d); e,
what fluid albuminous layer which immediately envelops well-advanced follicle from whose wall
the yolk; w, albumen, composed of alternating layers of membrana granulosa has partially
more and less fluid portions; ch.l, chalaze; a.ch, air chamber separated; f, cavity of liquor follic-
at the blunt end of the egg—simply a space between the uli; g, ovum surrounded by cell mass
two layers of the shell membrane; 7.s.m, inner, s.m, outer constituting cumulus odphorus (Pier-
layer of the shell membrane; s, shell. sol).
Fic. 5.—Section of well-developed Graafian follicle Fic. 6.—Uterine tube and ovary with
from human embryo (von Herff); the enclosed ovum mature Graafian follicle about ready to burst
contains two nuclei. (Ribemont-Dessaignes).
the Graafian follicle, within which the ovum is eccentrically located (Figs. 4 and
230). The cells of the Graafian follicle immediately surrounding the ovum form
the corona radiaia (Fig. 1) when the ovum is set free.
Io THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION
Ovulation and Menstruation.— When the ovum is ripe, the Graafian follicle
is large and contains fluid, probably under pressure. The ripe follicles form
bud-like projections at the surface of the ovary (Fig. 6), and at these points
the ovarian wall has become very thin. It is probable that normally the bursting
of the Graafian follicle and the discharge of the ovum are periodic and associated
with the phenomena of menstruation, as maintained by Fraenkel and Villemin.
That ovulation or discharge of the ovum from the ovary may occur independently
of the menstrual periods has been proven by the observations of Leopold and
of Ravano. Also in young girls ovulation may precede the inception of men-
struation and it may occur in women some time after the menopause.
Ovum
Follicle cells
At birth, or shortly after, all of the ova are formed in the ovary of the female
child. Hensen estimates that a normal human female may develop in each ovary
200 ripe ova. Most of the young ova, which may number 50,000, degenerate and
never reach maturity. At ovulation but one ovum is normally ripened and dis-
charged from the ovary. Several ova, however, may be produced in a single
follicle in rare cases. Such multiple follicles have been observed in human ane
and are of frequent occurrence in the ovary of the monkey (Fig. 7).
The Spermatozoén.—The male cell or spermatozoén of man is a minute cell
0.055 mm. long, specialized for active movement. Because of their active move-
THE GERM CELLS II
ments, spermatozoa were, when first discovered, regarded as parasites living in
the seminal fluid. The sperm cell is composed of a flattened head, short neck, and
thread-like tai) (Fig. 8).
The head is about 0.005 mm. in length.
It appears oval in side view, pear-shaped
in profile. When stained, the anterior
two-thirds of the head may be seen to
form a cap, and the sharp border of this
cap is the perforatorium by means of which
the spermatozoén penetrates the ovum.
The head contains the nuclear elements of
the sperm cell. The disc-shaped neck con-
tains the anterior centrosomal body. The
tail begins with the posterior centrosomal
body and is divided into a short connecting
piece, a chief piece or flagellum, which forms
about four-fifths of the length of the sperm
cell, and a short end piece, or terminal fila-
ment. The connecting piece is marked off
The
connecting piece is traversed by the axial
from the chief piece by the annulus.
filament (filum principale), and is sur-
rounded (1) by the sheath common to it
and to the flagellum; (2) by a sheath con-
taining a spiral filament; and (3) by a
mitochondrial sheath. The chief piece is
composed of the axial filament surrounded
by a cytoplasmic sheath, while the end piece
comprises the naked continuation of the
axial filament.
The spermatozoa are motile, being
propelled by the movements of the tail.
They swim always against a current at the rate of about 2.5 mm. a minute.
Perforatorium
Cap
Head
“Ant. centrosomal body
Neck | E
1) Post. centrosomal body
Spiral filament
Sheath of axial
filament
«| Mitochondrial sheath
Connect-
ing piece
of tail
Annulus
Axial filament
Chief piece } H
apie saat
End piece
of tail
Fic. 8.—Diagram of a human spermat-
ozoén, highly magnified, in side view
(Meves, Bonnet).
This
is important, as the outwardly directed currents induced by the ciliary action of
the uterine tubes and uterus direct the spermatozoa by the shortest route to the
infundibulum. Keibel has found spermatozoa alive three days after the execution
of the criminal from whom they were obtained. They have been found motile
12 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION
in the uterine tube three and one-half weeks after coitus. They have been kept
alive eight days outside the body by artificial means. It is not known for how
long a period they may be capable of fertilizing ova, but, according to Keibel, this
period would certainly be more than a week. Lode estimates that 200 million
spermatozoa are liberated at an average ejaculation.
MITOSIS AND AMITOSIS
Before the discharged ovum can be fertilized by the male germ cell, it must
undergo a process of cell division and reduction of chromosomes known as matu-
ration. As the student may
oy a not be familiar with the proc-
Sm ON
i pas %
esses of cell division, a brief
description is appended. (For
details of mitosis see text-
books of histology and E. B.
Wilson’s ‘The Cell.’’)
Amitosis.—Cells may di-
vide directly by the simple
fission of their nuclei and
cytoplasm. This rather in-
frequent process is called
amilosis. Amitosis is said
by many to occur only in
moribund cells. It is the
type of cell division demon-
strable in the epithelium of
the bladder.
Mitosis.—In the repro-
duction of normally active
cells, complicated changes
take place in the nucleus.
These changes give rise to
Ny. Pia ae es, se oe thread-like structures, hence
Fic. 9.—Diagram of the phases of mitosis (Schafer). the Process is termed mitosis
(thread) in distinction to
amitosis (no thread). Mitosis is divided for Convenience into four phases
(Fig. 9).
\
oa (
& i : ;
wt é Se,
MITOSIS AND AMITOSIS 13
Prophase.—1. The centrosome divides and the two minute bodies resulting
from the division move apart, ultimately occupying positions at opposite poles
of the nucleus (I-III).
2. Astral rays appear in the cytoplasm about each centriole. They radiate
from it and the threads of the central or achromatic spindle are formed between
the two asters, thus constituting the amphiaster (II).
3. The nuclear membrane and nucleolus disappear, the nucleoplasm and
cytoplasm becoming continuous.
4. During the above changes the chromatic network of the resting nucleus
resolves itself into a skein or spiveme, which soon shortens and breaks up into
distinct, heavily-staining bodies, the chromosomes (II, III). A definite number
of chromosomes is always found in the cells of a given species. The chromosomes
may be block-shaped, rod-shaped, or bent in the form of a U. °
5. The chromosomes arrange themselves in the equatorial plane of the central
spindle (IV). If U-shaped, the base of each U is directed toward a common center.
The amphiaster and the chromosomes together constitute a mitotic figure and at
the end of the prophase this is called a monaster.
Metaphase.—The longitudinal splitting of the chromosomes into exactly
similar halves constitutes the metaphase (IV, V). The aim of mitosis is thus ac-
complished, an accurate division of the chromatin between the nuclei of the
daughter cells.
Anaphase.—At this stage the two groups of daughter chromosomes separate
and move up along the central spindle fibers, each toward one of the two asters.
Hence this is called the diaster stage (V, VI). At this stage, the centrioles may
each divide in preparation fot the next division of the daughter cells.
Telophase.—1. The daughter chromosomes resolve themselves into a retic-
ulum and daughter nuclei are formed (VII, VIII).
2. The cytoplasm divides in a plane perpendicular to the axis of the mitotic
spindle (VIII). Two complete daughter cells have thus arisen from the mother
cell.
The complicated processes of mitosis, by which cell division is brought about
normally, seem to serve the purpose of accurately dividing the chromatic sub-
stance of the nucleus in such a way that the chromatin of each daughter cell may
be the same qualitatively and quantitatively.
This is important if we assume that the chromatic particles of the chromosomes bear the
hereditary qualities of the cell. The number of chromosomes is constant in the sexual cells
of a given species. The smallest number of chromosomes, two, occurs in Ascaris megalo-
14 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION
cephala univalens, a round worm parasitic in the intestine of the horse. The largest number
known is found in the brine shrimp, Artemia, where 168 have been counted.
The number for the human cell is in doubt. Guyer (1910) and Montgomery (1912)
found 22 in the spermatogonia of negroes, and Guyer (1913) reported considerably larger
numbers (count not given) for white spermatogonia. According to Winiwarter’s recent
work on whites (Arch. de Biol., T. 27, 1912), the number of chromosomes in each immature
ovum or odcyte is 48, in each spermatogone 47. Wieman (1913) found the most frequent
number in various white somatic cells to be 34, but recently (Amer. Jour. Anat., vol. 21, 1917)
he asserts that the number in both negro and white spermatogonia is 24, thereby agreeing
with Duesberg (1906).
We have seen that reproduction in mammals is dependent upon the union of
male and female germ cells. The union of two germinal nuclei (pronuclei)
would necessarily double the number of chromosomes in the fertilized ovum and
also the number of hereditary qualities which their particles are supposed to bear.
This multiplication of hereditary qualities is prevented by the processes of matu-
vation which take place in both the ovum and spermatozoon.
MATURATION
Maturation may be defined as a process of cell division during which the
number of chromosomes in the germ cells is reduced to one-half the number
characteristic for the species.
The spermatozoa take their origin in the germinal epithelium of the testis.
Their development, or spermatogenesis, may be studied in the testis of man or of
the rat; their maturation stages in the tubular testis of Ascaris. Two types of
cells may be recognized in the germinal epithelium of the seminiferous tubules,
the sustentacular cells (of Sertoli), and the male germ cells or spermatogonia (Fig.
10). The spermatogonia divide, one daughter cell forming what is known as a
primary spermatocyte. The other daughter cell persists as a spermatogone, and,
by continued division during the sexual life of the individual, gives rise to other
primary spermatocytes. The primary spermatocytes correspond to the ova
before maturation. Each contains the number of chromosomes typical for the
male of the species. The process of maturation consists in two cell divisions of
the primary spermatocytes, each producing first, two secondary spermatocytes,
and these in turn four cells known as spermatids. During these cell divisions the
number of chromosomes is reduced to half the original number, the spermatids
possessing just half as many chromosomes as the spermatogonia. Each spermatid
now becomes transformed into a mature spermatozoon (Fig. 11). The nucleus
forms the larger part of the head; the centrosome divides, the resulting moieties
passing to the extremities of the neck. The posterior centrosome is prolonged to
MATURATION 15
Sp’c. IT (telophase) .___ No
Sp’c. II (metaphase)___Aylf} fons.
Wi Z. Primary spermatocyte
Accessory chromo-
, , Th Te A . Uf - some (?)
SPevd Spnaplease) ENT] LO ~Sp’c. I (metaphase)
-Sp’c. I (prophase)
wall
Nee
Sp’g. (anaphase)
Fic. 10.—Stages in the spermatogenesis of man arranged in a composite to represent a portion of a
seminiferous tubule sectioned transversely. X 900.
Cc
Fic. 11.—Diagrams of the development of spermatozoa (after Meves in Lewis and Stthr).
a.c., Anterior centrosome; a.f., axial filament; c.p., connecting piece; ch.p., chief piece; g.c., cap; n.,
nucleus; #%., neck; p., protoplasm; ?.c., posterior centrosome.
form the axial filament, and the cytoplasm forms the sheaths of the neck and tail.
The spiral filament of the connecting piece is derived from the cytoplasmic mito-
chondria.
16 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION
The way in which the number of chromosomes is reduced may be seen in the
spermatogenesis of Ascaris (Fig. 12). Four chromosomes are typical for Ascaris
megalocephala bivalens and each spermatogone contains this number. In the
early prophase of the primary spermatocyte there appears a spireme thread con-
*«Q Fi:
Fic. 12.—Reduction of chromosomes in the spermatogenesis of Ascaris megalocephala bivalens
(Brauer, Wilson). XX about 1100. A-G, successive stages in the division of the primary spermatocyte.
The original reticulum undergoes a very early division of the chromatin granules which then form a
doubly split spireme (B, in profile). This becomes shorter (C, in profile) and then breaks in two to
form two tetrads (D, in profile), (EZ, in end). F, G, H, first division to form two secondary spermato-
cytes, each receiving two dyads. J, secondary spermatocyte. J, K, the same dividing. L, two
resulting spermatids, each containing two single chromosomes.
sisting of four parallel rows of granules (B). This thread breaks in two and
forms two quadruple structures known as tetrads (D-F); each is equivalent to two
original chromosomes split lengthwise to make a bundle of four. At the meta-
phase (G) the two fetrads split each into two chromosomes which already show
evidence of longitudinal fission and are termed dyads. One pair of dyads goes to
MATURATION 17
each of the daughter cells, or secondary spermatocytes (G-I). Without the
formation of a nuclear membrane, the second maturation spindle appears at once,
the two dyads split into four monads, and each daughter spermatid receives two
single chromosomes, or one-half the number characteristic for the species. The
tetrad, therefore, represents a precocious division of the chromosomes in prepara-
tion for two rapidly succeeding cell divisions which occur without the intervention
of the customary resting periods. The easily understood tetrads are not formed
in most animals, although the outcome of maturation is identical in either case.
A diagram of maturation is shown in Fig. 13. The first maturation division in
Ascaris is probably reductional, each daughter nucleus receiving two complete
A B
Spermatogonium Odgonium
Proliferation
period
Growth
. iod
Spermatocyte 1 Odcyte x pane
Odcyte 2 (ovum
Spermatocyte 2 and polocyte r)
Maturation
é period
: vum and
Spermatids three polo-
J cytes Transforma-
Spermatozoa tion period of
1 2 3 4 1 2 3 4 Spermatozoa
Fic. 13.—Diagrams of maturation, spermatogenesis and odgenesis (Boveri).
chromosomes of the original four, whereas in the second maturation division, as in
ordinary mitosis, each daughter nucleus receives a half of each of the two chromo-
somes, these being split lengthwise. In the latter case the division is equational,
each daughter nucleus reciving chromosomes bearing similar hereditary qualities.
In some animals the sequence of events is reversed, reduction occurring at
the second maturation division. In many insects and some vertebrates it has
been shown that the number of chromosomes in the oégonia is even, the number
in the spermatogonia odd, and that all the mature ova and half the spermatids
contain an extra or accessory chromosome (see p. 32).
During odgenesis, the ova undergo a similar process of maturation. Two
2
18 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION
cell divisions take place but with this difference, that the cleavage is unequal,
and, instead of four cells of equal size resulting, there are formed one large ripe
ovum or odcyte and three rudimentary or abortive ova known as polar bodies
or polocytes. The number of chromosomes is reduced in the same manner as in
the spermatocyte, so that the ripe ovum and each polar cell contain one-half
the number of chromosomes found in the immature ovum or primary odcyte.
The female germ cells, from which new ova are produced by cell division, are
called odgonia and their daughter cells after a period of growth within the ovary
are the primary odcytes, comparable to the primary spermatocytes of the male
(Fig. 12). During maturation the ovum and first polocyte are termed secondary
odcytes (comparable to secondary spermatocytes), the mature ovum and second
polocyte, with the daughter cells of the first polocyte, are comparable to the |
spermatids. Each spermatid, however, may form a mature spermatozoén, but
only one of the four daughter cells of the primary odcyte becomes a mature ovum.
The ovum develops at the expense of the three polocytes which are abortive and
degenerate eventually, though it has been shown that in the ova of some insects
the polar cell may be fertilized and segment several times like a normal ovum.
In most animals, the actual division of the first polocyte into two daughter cells
is suppressed. The maturation of human ova has not been observed, but such a
process undoubtedly takes place. The reduction of the chromosomes may be
best observed in the germ cells of Ascaris and of insects. The mouse offers a
favorable opportunity for studying the maturation of a mammalian egg as the ova
are easily obtained. Their maturation stages have recently been studied by Long
and Mark (Carnegie Inst. Publ. No. 142).
Maturation of the Mouse Ovum.—The nucleus of the ovum after maturation
is known as the female pronucleus. When the spermatozoon penetrates the mature
ovum it loses its tail, and its head becomes the male pronucleus. The aim and
end of fertilization consists in the union of the chromatic elements contained in the
male and female pronuclei and the initiation of cell division. In the mouse, the
first polocyte is formed while the ovum is still in the Graafian follicle. In the
formation of the maturation spindle no astral rays and no typical centrosomes
have been observed. The chromosomes are V-shaped. The first polar cell is
constricted from the ovum and lies beneath the zona pellucida as a spherical mass
about 25 micra in diameter (Fig. 14). Both ovum and polar cell (secondary
oécytes) contain 20 chromosomes, or half the number normal for the mouse.
The first maturation division is the reductional one and the chromosomes take
the form of tetrads.
FERTILIZATION Ig
After ovulation has taken place, the ovum lies in the ampulla of the uterine
tube. If fertilization occurs, a second polocyte is cut off, the nucleus of the
ovum forming no membrane between the production of the first and second polar
bodies (Fig. 14 A-D). The second maturation spindle and second polar cell are
Fic. 14.—Maturation and fertilization of the ovum of the mouse (after Sobotta). A, C-J, X 500;
BX 750. A-—D, entrance of the spermatozoén and formation of the polar cells. D-—E, development of
the pronuclei. F—J, successive stages in the first division of the fertilized ovum.
smaller than the first. Immediately after the formation of the second polar cell,
the chromosomes resolve themselves into a reticulum and the female pronucleus
is formed (Fig. 14 D).
FERTILIZATION
Fertilization of the Mouse Ovum.—Normally, a single spermatozoén enters
the ovum six to ten hours after coitus. While the second polar cell is forming,
20 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION
the spermatozoén penetrates the ovum and loses its tail. Its head is converted
into the male pronucleus (Fig. 14D). The pronuclei, male and female, approach
each other and resolve themselves first into a spireme stage, then into two groups
of 20 chromosomes. A centrosome, possibly that of the male cell, appears be-
tween them, divides into two, and soon the first segmentation spindle is formed
(F-H). The 20 male and 20 female chromosomes arrange themselves in the
equatorial plane of the spindle, thus making the original number of 40 (J). Fer-
‘tilization is now complete and the ovum divides in the ordinary way, the daughter
cells each receiving equal numbers of maternal and paternal chromosomes. The
fundamental results of the process of fertilization are: (1) the wnion of the male and
female chromosomes to form the cleavage nucleus of the fertilized ovum, (2) the
initiation of cell division or cleavage of the ovum.
These two factors are separate and independent phenomena. It has been shown by
Boveri and others that fragments of sea urchin’s ova containing no part of the nucleus may be
fertilized by spermatozoa, segment, and develop into larve. The female chromosomes are
thus not essential to the process of segmentation. Loeb, on the other hand, has shown that
the ova of invertebrates may be made to develop by chemical and mechanical means without
the codperation of the spermatozoon (artificial parthenogenesis). Even adult frogs have been
reared from mechanically stimulated eggs. It is well known that the ova of certain inver-
tebrates develop normally without fertilization, that is, parthenogenetically. These facts
show that the union of the male and female pronuclei is not the means of initiating the
development of the ova. In all vertebrates it is, nevertheless, the end and aim of fertiliza-
tion.
Lillie (Science, vols. 36 and 38; 1912, 1913) has recently shown that the cortex of sea
urchin’s ova produces a substance which he terms fertilizin. This substance he regards as an
amboceptor essential to fertilization, with one side chain which agglutinates and attracts the
spermatozoa, and another side chain which activates the cytoplasm and initiates the cleavage
of the ovum. According to Loeb, the spermatozoén activates the ovum to develop by in-
creasing its oxidations and by rendering it immune to the toxic effects of oxidation.
Spermatozoa may enter the mammalian ovum at any point. If fertilization
is delayed and too long a period elapses after ovulation, the ovum may be weak-
ened and allow the entrance of several spermatozoa. This is known as polvy-
spermy. In such cases, however, only one spermatozo6n unites with the female
pronucleus.
Fertilization of the Human Ovum.—This has not been observed, but prob-
ably takes place in the uterine tube some hours after coitus. Ova may be fertil-
ized and start developing before they enter the uterine tube. If they attach them-
selves to the peritoneum of the abdominal cavity, they give rise to abdominal
pregnancies. If the ova develop within the uterine tube ‘wbal pregnancies result.
Ovarian pregnancies are known also. Normally, the embryo begins its develop-
FERTILIZATION 21
ment in the uterine tube, thence passes into the uterus and becomes embedded in
the uterine mucosa. The time required for the passage of the ovum from the
uterine tube to the uterus is unknown. It probably varies in different cases and
may occupy a week or more. The ovum may in some cases be fertilized within
the uterus. Fertilization is favored by the fact that the spermatozoa swim always
against a current. As the cilia of the uterus and uterine tube beat downward and
outward the sperms are directed upward and inward. They may reach the ova-
rian ends of the uterine tubes within two hours of a normal coitus.
Twin Development.—Usually but one human ovum is produced and fertilized at
coitus. The development of two or more embryos within the uterus is commonly due to the
ripening, expulsion, and subsequent fertilization of an equal number of ova. In such cases
ordinary or fraternal twins, triplets, and so on, of the same or opposite sex, result. Identical
twins, that is, those always of the same sex and strikingly similar in form and ‘feature, are
regarded as arising from the daughter cells of a fertilized ovum, these having separated and
each having developed like a normal ovum. Separate development of the cleavage cells can
be produced experimentally in many of the lower animals. The offspring of the armadillo
are normally produced in this manner (Patterson).
The Significance of Mitosis, Maturation and Fertilization.—It is assumed by
students of heredity that the chromatic particles of the nucleus bear the hereditary qualities
of the cell. During the course of development these particles are probably distributed to the
various cells in a definite way by the process of mitosis. The process of fertilization would
double the number of hereditary qualities and they would be multiplied indefinitely were it
not for maturation. At maturation not only is the number of chromosomes halved, but it is
assumed also that the number of hereditary qualities is reduced by half. In the case of the
ovum, maturation takes place at the expense of three potential ova, the polocytes, which de-
generate, but to the advantage of the single mature ovum which retains more than its share
of cytoplasm and nutritive yolk.
Mendel’s Law of Heredity.—Experiments show that most hereditary characters fall
into two opposing groups, the contrasted pairs of which are termed allelomorphs. As an
example, we may take the hereditary tendencies for black and blue eyes. It is supposed that
there are paired chromatic particles which are responsible for these hereditary tendencies,
and that paired spermatogonial chromosomes bear one each of these particles. Each chro-
mosome pair in separate germ cells may possess similar particles, both bearing black-eyed
tendencies or both blue-eyed tendencies, or opposing particles, bearing the one black, the
other blue-eyed tendencies. It is assumed that at maturation these paired particles are
separated along with the chromosomes, and that one only of each pair is retained in each germ
cell, in order that new and favorable combinations may be formed at fertilization. In our
example, either a blue-eyed or a black-eyed tendency bearing particle would be retained. At
fertilization the segregated tendency-bearing particles of one sex may enter into new combina-
tions with their allelomorphs from the other sex, combinations which may be favorable to
the offspring.
Three combinations are possible. If the color of the eyes be taken as the hereditary
character, (1) two ‘“‘black” germ celis may unite; (2) two “‘blue” germ cells may unite; (3)
a “black” germ cell may unite with a “blue” germ cell. The offspring in (1) will all have
black eyes, and, if interbred, their progeny will likewise inherit black eyes exclusively.
Similarly, the offspring in (2), and if these are interbred their progeny as well, will include
22 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION
nothing but blue-eyed individuals. The first generation from the cross in (3) will have black
eyes solely, for black in the present example is dominant, as it is termed. Such black-eyed
individuals, nevertheless, possess blue-eyed bearing chromatic particles in their germ cells;
in the progeny resulting from the interbreeding of this class the original condition is repeated
—pure blacks, impure blacks which hold blue recessive, and pure blues will be formed in the
ratio of 1: 3:1 respectively. It is thus seen that blue-eyed children may be born of black-
eyed parents, whereas blue-eyed parents can never have black-eyed offspring. Many such
allelomor phic pairs of unit characters are known.
DETERMINATION OF SEX
The assumption that the chromosomes are the carriers of hereditary ten-
dencies is borne out by the observations of cytologists on the germ cells of inver-
tebrates, especially insects, and of some vertebrates. According to Winiwarter
(Arch. de Biol., T. 27, 1912) the nuclei of human spermatogonia contain 47 chro-
mosomes, while those of the odgonia contain 48. When maturation and reduction
of the chromosomes take place in the male cells, one unpaired chromosome fails
to divide and passes intact to one or the other daughter cells; hence half of the
spermatids contain 24 chromosomes, the other half only 23. All the odcytes and
polocytes, on the contrary, contain 24. There is thus one extra chromosome in
each mature ovum and in each of half the spermatozoa. This chromosome,
because of peculiarities of size or shape, can be identified easily m many animals,
and is termed the accessory chromosome. McClung was the first to assume that
the accessory chromosome is a sex determinant. It has since been shown by
Wilson, Davis, and others that the accessory chromosome carries the female
sexual characters. When, in the case under consideration, a spermatozoan with
24 chromosomes fertilizes an ovum, the resulting embryo is a female, its somatic
nuclei containing 48 chromosomes. An ovum fertilized by a sperm cell containing
only 23 chromosomes (without the accessory chromosome) produces a male with
2
somatic nuclei containing only 47 chromosomes. These ob “ons of Wini-
warter on man have yet to be confirmed by otLer investigators. 1t 1s probable,
however, that sex is transmitted by the human chromosomes essentially in the
manner described, which agrees with the easily observed phenomena in insects.
CHAPTER II
CLEAVAGE OF THE FERTILIZED OVUM AND ORIGIN OF THE
GERM LAYERS
CLEAVAGE
THE processes of cleavage, or segmentation, not having been observed in
human ova, must be studied in other vertebrates. It is probable that the early
development of all vertebrates is, in its essentials, the same. Cleavage may be
modified, however, by the presence in the ovum of large quantities of nutritive
yolk. In many vertebrate ova the yolk collects at one end, termed the vegetal
pole, in contrast to the more purely protoplasmic animal pole. Such ova are said
to be telolecithal. Examples are the ova of fishes, amphibians, reptiles, and birds.
When very little yolk is present, the ovum is said to be isolecithal. Examples
are the ova of Amphioxus, the higher mammals, and man. The typical processes
of cleavage may be studied most easily in the fertilized ova of invertebrates
(Echinoderms, Annelids, and Mollusks). Among Chordates, the early processes
in development are primitive in a fish-like form Amphioxus. The yolk modifies
the development of the amphibian and bird egg, while the early structure of the
mammalian embryo can be explained only by assuming that the ova of the
higher Mammalia at one time contained a considerable amount of yolk, like the
ovum of the bird and of the lower mammals, and the influence of this condition
persists.
Cleavage in, ‘.nphioxus.—The ovum is essentially isolecithal since it contains
but little yolk (Lig. 15). About one hour after fertilization it divides vertically
into two nearly equal daughter cells, or blastomeres. The process is known as
cell cleavage, or segmentation, and takes place by mitosis. Within the next hour
the daughter cells again cleave in the vertical plane, at right angles to the first
division, thus forming four cells. Fifteen minutes later a third division takes
place in a horizontal plane. As the yolk is somewhat more abundant at the vege-
tal pole of the four cells cne mitotic spindles lie nearer the animal pole. Conse-,
quently in the eight-celled stage the upper tier of four cells is smaller than the
lower four. By successive cleavages, first in the vertical, th. 1 in the horizontal
plane a 16- and 32-celled embryo is formed. The upper two tiers are now smaller,
23
24 CLEAVAGE AND THE GERM LAYERS
and a cavity, the blastocele, is enclosed by the cells. The embryo at this stage is
sometimes called a morula (mulberry). In subsequent cleavages, as development
proceeds, the size of the cells is diminished while the cavity enlarges (Fig. 15).
Fic. 15.—Cleavage of the egg of Amphioxus (after Hatschek). 200. 1. The egg before the
commencement of development; only one polar body, P.B., is present, the other having been lost during
ovulation. 2. The ovum in the act of dividing, by a vertical cleft, into two equal blastomeres. 3. Stage
with four equal blastomeres. 4. Stage with eight blastomeres; an upper tier of four slightly smaller ones
and a lower tier of four slightly larger ones.’ 5. Stage with sixteen blastomeres in two tiers, each of eight.
6. Stage with thirty-two blastomeres, in four tiers, each of eight; the embryo is represented bisected to
show the cleavage cavity or blastoccele, B. 7. Later stage; the blastomeres have increased in number by
further division. 8. Blastula stage bisected to show the blastoccele, B.
The embryo is now a blastula, nearly spherical in form and about four hours old.
The cleavage of the Amphioxus ovum is thus holoblastic, i. e., complete, and nearly
equal.
CLEAVAGE 25
Cleavage in Amphibia—These ova contain so much yolk that the nucleus
and most of the cytoplasm lies at the upper or animal pole. The first cleavage
spindle lies in this cytoplasm. ‘The first two cleavage planes are vertical and at
Fic. 16.—Cleavage of the frog’s ovum (Hatschek in Marshall). X 20. B, Blastoceele or cleavage cav-
ity; NV, nucleus.
right angles, and the four resulting cells are nearly equal (Fig. 16,1). The
spindles for the third cleavage are located near the animal pole and the cleavage
takes place in a horizontal plane. As a result, the upper four cells are much
srhaller than the lower four (2 and 3). The large yolk-laden cells divide more
26 CLEAVAGE AND THE GERM LAYERS
slowly than the upper small cells. At the blastula stage, the cavity is small,
and the cells of the vegetal pole are each many times larger than those at the
animal pole (4 and 5). The cleavage of the frog’s ovum is thus complete but
unequal.
Cleavage in Reptiles and Birds.—The ova of these vertebrates contain a
large amount of yolk. There is very little pure cytoplasm except at the animal
pole and here the nucleus is located (Fig. 3). When segmentation begins, the
first cleavage plane is vertical but the inert yolk does not cleave. The segmen-
tation is thus incomplete or meroblastic. In the hen’s ovum the cytoplasm is
divided by successive vertical furrows into a mosaic of cells, which, as it increases
in size, forms a cap-like structure upon the surface of the yolk. These cells are
separated from the yolk beneath by horizontal cleavage furrows, and successive
horizontal cleavages give rise to several layers of cells. The space between
cells and yolk mass may be compared to the blastula cavity of Amphioxus and
the frog (Fig. 18). The cellular disc or cap is termed the germinal disc or blasto-
derm. The yolk mass, which forms the floor of the blastula cavity and the greater
part of the ovum, may be compared to the large yolk-laden cells at the vegetal
pole of the frog’s blastula. The yolk mass never divides but is gradually used up
in supplying nutriment to the embryo which is developed from the cells of the
germinal disc. At the periphery of the germinal disc new cells constantly form
until they enclose the yolk.
Cleavage in Mammals.—The ovum of all the higher mammals, like that
of man, is isolecithal and nearly microscopic in size. Its cleavage has been studied
in several mammals but the rabbit’s ovum will serve as an example. The cleav-
age is complete and nearly equal (Fig. 17), a cluster of approximately uniform
cells being formed within the zona pellucida. This corresponds to the morula
stage of Amphioxus. Next an inner mass of cells is formed which is equivalent
to the germinal disc, or blastoderm, of the chick embryo (Fig. 17). The inner cell
mass is overgrown by an outer layer which is termed the troplecioderm, because,
in mammals, it later supplies nutriment to the embryo from the uterine wall.
Fluid next appears between the outer layer and the inner cell mass, thereby sepa-
rating the two except at the animal pole. As the fluid increases in amount, a hol-
low vesicle results, its walls composed of the single-layered trophectoderm except
where this is in contact with the inner cell mass. This stage is known as that of
the blastodermic vesicle. It is usually spherical or ovoid in form, as in the rabbit,
and probably this is the form of the human ovum at this stage. In the rabbit
the vesicle is 4.5 mm. long before it becomes embedded in the wall of the uterus.
Outer cell. Outer cetts.
Zona
pellucida
Outer cell-
DHASS,
Polar bodies
Inner cell-
MASS.
Inner cell
Outer cells.
Inner cells.
Inner cells.
Outer cells Inner cells.
Tebheckede< n
Cua, G t(,
\nnev
Cell macs
Outer cedls.
Fic. 17—Diagrams showing the cleavage of the mammalian (rabbit’s) ovum and the formation of the
blastodermic vesicle (Allen Thomson, after van Beneden). X 200.
CLEAVAGE a
Among Ungulates (hoofed animals) the vesicle is greatly elongated and attains a
length of several centimeters, as in the pig.
If we compare the mammalian blastodermic vesicle with the blastula stages
of Amphioxus, the frog, and the bird, it will be seen that it is to be homologized
with the bird’s blastula, not with that of Amphioxus (Fig. 18). In each case
there is an inner cell mass of the germinal disc. The trophectoderm of the
mammal represents a precocious development of cells, which, in the bird, later
envelop the yolk. The cavity of the vesicle is to be compared, not with the
Frc. 18.—Diagrams showing the blastule: A, of Amphioxus; B, of frog; C, of chick; D, blastodermic
vesicle of mammal.
blastula cavity of Amphioxus and the frog, but with the yolk mass plus the rudi-
mentary blastocele of the bird’s ovum. The mammalian ovum, although almost
Evol alk, thus develops much like the yolk-laden ova of reptiles and birds. This
similarity has an evolutionary significance. Its cleavage, however, is complete
and the early stages in its development are abbreviated.
In Primates, but one cleavage stage has been observed. This, a four-celled ovum of
Macacus nemestrinus figured by Selenka, shows the cells nearly equal and oval in form.
This ovum was found in the uterine tube of the monkey and shows that, in Primates and prob-
ably in man, cleavage as in other mammals takes place normally in the oviducts.
28 CLEAVAGE AND THE GERM LAYERS
THE FORMATION OF THE ECTODERM AND ENTODERM (GASTRULATION)
The blastula and early blastodermic vesicle show no differentiation into
layers. Such differentiation takes place later in all vertebrate embryos, giving
tise first to the ectoderm and entoderm, and finally to the mesoderm. From these
three primary germ layers all tissues and organs of the body are derived.
The processes of gastrulation, by which ectoderm and entoderm arise, and of
mesoderm formation will be treated separately.
Amphioxus and Amphibia.—In these animals the larger cells at the vegetal
pole of the blastula either fold inward, i. e., invaginate (Amphioxus, Fig. 19), or
are for the most part overgrown by the more rapidly dividing cells of the animal
Fic. 19.—Gastrulation of amphioxus (Hatschek in Heisler). 220. A, Blastula: a, animal cells;
v, vegetative cells; c.c., cleavage cavity. B, Beginning invagination of vegetative pole. C, Gastrula, the
invagination of the vegetative cells being complete: ect., ectoderm; ent., entoderm; arch., archenteron;
bl, blastopore.
pole (amphibia). Eventually the invaginating cells obliterate the blastula cavity
and come in contact with the outer layer of cells (Fig. 19). The new cavity thus
formed is the primitive gut or archenteron and its narrowed mouth is the blastopore.
The outer layer of cells is the ectoderm, the inner, newly formed layer is the ento-
derm. The entodermal cells are henceforth concerned in the nutrition and metab-
olism of the body. The embryo is now termed a Gastrula (little stomach).
Reptiles and Birds.—The germinal disc, or blastoderm, in these animals lies
like a cap on the surface of inert yolk (Fig. 3). Since the enormous amount of
yolk makes gastrulation as in Amphioxus and amphibians impossible, the process
exhibits marked modifications.
ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 29
There appears caudally on the blastoderm of reptiles a pit-like depression.
From this slight invagination a proliferation of cells forms a layer which spreads
beneath the ectoderm (cf. Fig. 21.4). The inner layer originating in this manner
is the entoderm, and the region of the pit where ectoderm and entoderm are con-
tinuous is the blastopore.
In birds the caudal portion of the blastoderm is rolled or tucked under, the
inner layer formed in this way constituting the entoderm. The marginal region
where ectoderm and entoderm meet bounds the dlastopore, while the space be-
tween entoderm and yolk is the archenteron._
~~ Mammals.—As in cleavage, so also in gastrulation the mammalian ovum
exhibits a modified behavior indicative of an ancestral yolk-rich condition. The
entoderm apparently arises by a splitting off, or delamination, of cells from the
under side of the inner cell mass (Figs. 16,74 A and 75). In the blastoderm of the
rabbit, opossum, and mole, however, a minute pore has been observed at which
the ectoderm and entoderm are continuous. This opening is believed by some to
represent a true blastopore where the ingrowth of entodermal cells has occurred.
ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL ‘TUBE
Amphioxus and Amphibia.—The dorsal plate of entoderm, which forms the
roof of the archenteron in Amphioxus, gives rise to paired lateral diverticula or
celomic pouches (Fig. 20). These separate both from the plate of cells in the mid-
Fic. 20.—Origin of the mesoderm in Amphioxus (after Hatschek). XX about 425. x.g., Neural
groove; m.c., neural canal; ch., anlage of notochord; mes. som., mesodermal segment; ect., ectoderm; ent.,
entoderm; al., cavity of gut; cw., coelom or body cavity.
dorsal line (which forms the notochord), and from the entoderm of the gut, and
become the primary mesoderm. ‘The mesodermal pouches grow ventral and their
cavities form the celom'or body cavity. Their outer walls, with the ectoderm,
30 CLEAVAGE AND THE GERM LAYERS
form the body wall or somatopleure ; their inner walls, with the gut entoderm, form
the intestinal wall or splanchnopleure. In the meantime, a dorsal plate of cells, cut:
off from the ectoderm, has formed the neural tube (anlage of the nervous system),
and the notochordal plate has become a cord or cylinder of cells (axial skeleton)
extending the length of the embryo. In this simple fashion the ground plan of
the chordate body is developed.
In Amphibia the mesodermal diverticula grow out from the dorsal entoderm
as solid plates between the ectoderm and entoderm. Later, these plates spht
into two layers and the cavity so formed gives rise to the coelom.
Ectoderm
TB iol PO MOS HOLS
FICCI PRS Hea I
7 GIMVAPA aos
Siitoinraconace
elo ale arstarerath
Mat
i
WN
vir
bP LT Tarn
LR 2
RRR =
Palais rena
— I
SENDS #0 CSRS
Entoderm Notochordal x
plate Remnant of floor
Fic. 21.—Longitudinal sections of the snake’s blastoderm at various stages to show the oxic of the
notochordal plate (adapted after Hertwig).
Reptiles.—The same pocket-like depression in the caudal portion of the
blastoderm, which gave rise to the cells of the entodermal layer, now invaginates
more extensively and forms a pouch which pushes in between ectoderm and ento-
derm (Fig. 21 A and B). The size of the invagination cavity varies in different
species; in some it is elongated and narrow, being confined to the middle line of
the blastoderm. ‘The floor of this pouch soon fuses with the underlying entoderm
and the two thin, rupture, and disappear, thus putting the cavity of the pouch in
ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 31
communication with the space (archenteron) beneath the entoderm (Fig. 21 C).
The cells of the roof persist as the notochordal plate which later gives rise to the
notochord. The neural folds arise before the mouth of the pouch (blastopore)
closes up, and, fusing to form the neural tube, incorporate the blastopore in its
floor. This temporary communication between the neural tube and the primitive
enteric cavity is the neurenteric canal (Fig. Di C); it is found in al the vertebrate
groups (cf. Fig. 78). A transverse section through the invaginated pouch, at the
time of rupture of its floor, and the underlying entoderm will make clear the rela-
tively slight lateral extent of these changes (Fig. 22).
From about the blastopore, and from the walls of the pouch, mesodermal
plates arise and extend like wings between the ectoderm and entoderm (Fig. 22).
As in amphibia they later separate into outer (somatic) and inner (splanchnic)
layers enclosing the coelom (cf. Fig. 29 B). The relation between notochordal
plate, mesoderm, and entoderm shown in Fig. 22 resembles strikingly the condi-
tions in Amphioxus (Fig. 20 A).
Ectoderm Mesoderm
atarCTeele
Notochordal Entoderm Pitrare ;
plate
Fic. 22,—Transverse section of a snake’s blastoderm at a level corresponding to the middle of Fig.
21 C (adapted after Hertwig).
Birds.—Due to the modified gastrulation in reptiles, birds, and mammals
through the influence of yolk, a structure known as the primitive streak ee
important. ‘An account of its formation and significance based on conditions
found in the bird may be introduced conveniently at this place.
Shortly after the formation of entoderm there appears in the median line at
the more caudal portion of the blastoderm an elongated opaque band (Fig. 23).
Along this primitive streak there forms a shallow primitive groove, bounded later-
ally by primitive folds. Cranially the groove ends in a depression, the primitive
pit. In front of this pit the streak ends in a knob, the primitive knot (of Hensen).
The primitive streak becomes highly significant when interpreted in the
light of the theory of concrescence, a theory of general application in vertebrate
development. It will be remembered that the entode i ises_by a
rolling under of the outer ng the caudal margi . As
the blastoderm expands it is believed that a middle point on this margin remains
32 CLEAVAGE AND THE GERM LAYERS
fixed while the edges of the margin on each side are carried caudad and brought
together. Thus a crescentic margin is transformed into a longitudinal slit as in
Fig. 24. Since this marginal lip originally bounded the blastopore (p. 29) the
longitudinal slit must also be an elongated blastopore whose direction has merely
been changed. The lips of the slit fuse,
forming the primitive streak. The primi-
tive groove may be interpreted as a further
j
__. Area opaca
| bese futile attempt at invagination in the region
tive k .
Erimitz ot of the blastopore. The teachings of com-
Primitive fold Patative embryology support these con-
MW Primitive enue clusions, for the neurenteric canal arises
at the cranial end of the primitive streak,
] Area pellucida
ane | the anus at its caudal end, while the
fe ‘ Blood island
primary germ layers fuse in its substance.
All these relations exist at the blastopore
Fic. 23.—Blastoderm of a chick embryo of the lower animals.
at the stage of the primitive streak and Hiei the thickened e@odenn ot the
groove (16 hours). X 20.
primitive streak a proliferation of cells
takes place and there grows out laterally and caudally between the ectoderm and
entoderm a solid plate of mesoderm (Fig. 31 Band C). From the primitive knot
_a mesode mesodermal sheet also extends cephalad forming along the midline-a thicker
“ayer: the er, the so-called head process or notochordal plate, which fuses intimately with
the entoderm (Figs. 25, 30 and 31 A).
eee
Since the primitive streak and groove = é je
represent a modified blastopore, it is no, aw fig SN
evident that this cranial extension, the {|} ie A Wk AW
head process, corresponds to the pouch- — a = SEY
like invagination concerned in the forma- Fic. 24.—Diagram elucidating the forma-
tion of the primitive streak (Duval in Heisler).
The increasing size of the germ disc in the
tiles. In birds the fusion of the head course of the development is indicated by dot-
ted circular lines. The heavy lines represent
the crescentic groove and the primitive streak
of mesodermal sheets to it laterally, the which arises from it by the fusion of the edges
of the crescent.
tion of mesoderm and notochord in rep-
process with the entoderm, the relation
tormation of the notochord from its tis-
sue, and the occasional traces in it of a cavity continuous with the primitive pit
(i. e., neurenteric canal), all recall the conditions described for the less modified
invagination in reptiles.
Mammals.—On the blastoderm of mammals appear a primitive streak and
ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 33
knot essentially as in birds (Figs. 26 A and 28). Similarly from the keel-like
ectodermal thickening of the primitive streak, mesoderm grows out laterally and
caudally, and from the primitive knot it is continued cranially as the head process.
ell epee ne oe Ma eat oD
All three primary germ layers fuse in the primitive streak and knot, this condition
Neural plate Primitive knot
Head fold ; Primitive pit Primitive streak Ectoderm
Ecto-.
derm
Head process Entoderm Mesoderm
Yolk
Fic. 25.—Median longitudinal section of a chick embryo at the stage of the primitive streak and head
process. X 100.
being known in man. The head process of many mammalian embryos contains a
cavity (xotochordal canal), which in some cases is of considerable size, opening at
the primitive pit. As in reptiles, the floor of this cavity fuses with the entoderm
and the two rupture and disappear. A still persistent portion of the floor is shown
in Fig. 27. Thus a neurenteric
B
canal, later enclosed by the
neural folds, puts the dorsal
surface of the blastoderm into
communication with the enteric
cavity beneath the entoderm
(Figs. 77 and 78). The roof of
the head process or notochordal
canal is for a time continuous
with the mesoderm and ento-
derm (compare these relations
in reptiles, Fig. 22), but it event-
ually becomes the notochord.
y Fic. 26.—The primitive streak of pig embryos (Kei-
The extent of mesoderm bel). X 20. A, Embryo with primitive streak and prim-
in rabbit embryos is shown in itive knot; B, a later embryo in which the neural groove is
also present, cephalad in position.
Fig. 28. Cranial to the primi-
tive node the notochord is differentiated in the midline, the mesoderm being
divided into two wings. The mesoderm rapidly grows around the wall of the
blastodermic vesicle until it finally surrounds it and the two wings fuse ven-
trally (Fig. 29). The single sheet of mesoderm soon splits into two layers, the
cavity between being the celom_or body cavity. The outer mesodermal layer
(somatic), with the ectoderm, forms the somatopleure or body wall, the inner
3
34 CLEAVAGE AND THE GERM LAYERS
splanchnic layer, with the entoderm, forms the intestinal wall or Splanchnopleure.
The neural tube having in the meantime arisen from the neural folds of the ecto-
derm, there is present the ground plan of the vertebrate body, the same in man
as in Amphioxus.
No stages of gastrulation or mesoderm formation have yet been observed in
the human embryo, but the primitive streak may be recognized in later stages
Post. opening of notochordal
canal
Primitive streak
Ant. opening of
notochordal canal
Ant. persisting portion of 7 Seah,
notochordal canal Neurenteric canal
Fic. 27.—Median longitudinal section through the blastoderm of a bat (Vespertilio murinus) (after Van
Beneden).
(Fig. 77), and there is evidence also of an opening, the neurenteric canal, leading
from the exterior into the cavity of the primitive gut (archenteron). In Tarsius,
an animal classed by Hubrecht,with the primates, the mesoderm has two sources:
(1) From the splitting of ectoderm at the caudal edge of the blastoderm; this forms
bate a
Fic. 28.—Diagrams showing the extent of the mesoderm in rabbit embryos (Kolliker). In A the
mesoderm is represented by the pear-shaped area about the primitive streak at the caudal end of the
embryonic disc; in B, by the circular area which surrounds the embryonic disc.
the extra-embryonic mesoderm and takes no part in forming the body of the embryo.
(2) The intra-embryonic mesoderm, which gives rise to body tissues, takes its origin
from the primitive streak and knot as in the chick and lower mammals. The
origin of mesoderm in the human embryo is probably much the same as in
Tarsius.
ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 35
The Notochord or Chorda Dorsalis.—Unlike in Amphioxus anu amphibia, the
head (notochordal) process and mesoderm of higher vertebrates are not clearly of
entodermal origin, but are derived from the ectoderm, any union with the entoderm
being secondary. As the primitive streak recedes caudalward during development
the head process is progressively lengthened at the expense of the former. UIti-
mately the primitive streak becomes restricted to the tail region, whereas the entire
remainder of the body is built up around the head process as an axis. In later
Mesodermal segment Neural tube __
Ectoderm. Nephrotome
Somatic Notochord
Splanchnic
mesoderm
Fic. 29.—Diagrams showing the origin of the germ layers of mammals as seen in transverse section
(modified from Bryce).
stages, the rod-like notochord extends in the midline beneath the neural tube from
the tail to a dorsal out-pocketing of the oral entoderm, known as Seessel’s pocket
(p. 81). It becomes enclosed in the centra of the vertebrz and in the base of the
cranium, and eventually degenerates. In Amphioxus it forms the only axial
skeleton and it is persistent in the axial skeleton of fishes and amphibians. In
man, traces of it are found as pulpy masses (nuclei pulpost) in the intervertebral
discs.
CHAPTER III
THE STUDY OF CHICK EMBRYOS
Cuick embryos may be studied whole and most of the structures identified up to the
end of the second day. The eggs should be opened in normal saline solution at 40° C. With
scissors cut around the germinal disc, float the embryo off the yolk, and remove the vitelline
membrane. ‘Then float the embryo dorsal side up on a glass slide, remove enough of the saline
solution to straighten wrinkles, and carefully place over the embryo a circle of tissue paper
with opening large enough to leave the germinal disc exposed. Add a few drops of fixative
(5 per cent. nitric acid gives good fixation) and float embryo into a covered dish. After fix-
ing and hardening, stain in Conklin’s acid hematoxylin or in acid carmine. Extract sur-
plus stain, clear, and mount on slide supporting cover-slip to prevent crushing the embryo.
Acid hematoxylin gives the best results for embryos of the first two days. For a detailed
account of embryological technique see Lee’s ““Microtomist’s Vade Mecum.”’
In the following descriptions we shall use the terms dorsad and ventrad to indicate
“toward the back” or ‘toward the belly”; cephalad and craniad to denote ‘‘headward’’;
caudad to denote “‘tailward’’; /aterad to indicate “toward the side”; and mesad, “toward the
middle line.”
EMBRYOS OF ABOUT TWENTY HOURS’ INCUBATION
The events of cleavage and the formation of the primary germ layers in
birds have been described in an earlier chapter. The appearance on the disc-
like blastoderm (Fig. 3) of the primitive streak and groove (Fig. 23), and of its
cranial extension, the head process (Fig. 25), has likewise received brief treatment
(p. 31).
In a chick embryo of twenty hours’ incubation (Fig. 30) the primitive streak
is formed as a linear opacity near the posterior border of the germinal disc.
Over a somewhat pear-shaped clear area the yolk has been dissolved*away from
the overlying entoderm. This area, from its appearance, is termed the area
pellucida. It is surrounded by the darker and more granular area opaca. Whether
or not the primitive streak represents the fused lips of the blastopore, it is certain
that it represents the point of origin for the middle germ layer, the extent of
which is indicated by the shaded area of Fig. 30. It also indicates the future
longitudinal axis of the embryo. The mesoderm extends at first more rapidly
caudal to the primitive streak, at the cranial end of which appears a shaded
_thickenin the primitive knot or node (of Hensen). From the primitive knot it
grows cranially, forming along the midline a thicker layer of tissue, the noto-
36
EMBRYOS OF ABOUT TWENTY HOURS’ INCUBATION 37
chordal plate or head process, which is temporarily united with the entoderm
(Fig. 30).
Neural
Head groove
process Mesoderm
At A
Primitive }
knot '
Primitive
groove
C Primitive
.C streak
Area opaca
: Blood island ;
Fic. 30.—Dorsal surface view of a twenty-hour chick embryo showing primitive streak and
extent of mesoderm (after Duval). X17. The lines A, B,and C indicate the levels of the corres-
ponding sections shown in Fig. 31.
Ectoderm Neural plate
4 A
aie: 3
1 Pay BE.
gee RRR.
Set ease
Mesoderm Notochordal plate Entoderm
+ ms ‘
Ectoderm Primitive knot Ule NStTNNS Nove)
Be nis ee
Mesoderm , Enloderm
Ectoderm Primitive groove
Mesoderm Entoderm
Fic. 31.—Transverse sections through the embryonic area of a twenty-hour chick. X 165. A, through
the head process; B, through the primitive knot; C, through the primitive streak.
A transverse section through the primitive streak at twenty hours (see guide
line C, Fig. 30) shows the three germ layers distinct laterally (Fig. 31C). In the
38 THE STUDY OF CHICK EMBRYOS
midline, a depression in the ectoderm is the primitive groove. In this region there
is no line of demarcation between ectoderm and mesoderm. A transverse section
through the primitive knot (Fig. 31 B; guide line B, Fig. 30) shows the three germ
layers intimately fused (cf. Fig. 51). There is a marked proliferation of cells,
which are growing cephalad to form the notochordal plate (head process) (cf. Fig.
25).
A transverse section through the notochordal plate, just beginning to form
at this stage (Fig. 31 A; guide line A, Fig. 30), shows the thickening near the
midline which will separate from the lateral mesoderm and form the notochord.
It is fused with the entoderm but not with the ectoderm.
After the notochordal plate becomes
eae e tied
BRA oc |
ae
Ie ree
fos
a é
#
: r
prominent at twenty hours the differ-
entiation of the germinal disc is rapid.
A curved_fold, at involving the
ectoderm and entoderm alone, is formed
Sn ee
——___—_.
cephalad of the notochordal_process.
This is the head fold and is the anlage of
the head of the embryo (Figs. 25 and 32).
The ectoderm _has_thickened_on_ each
sie OF she mud-dorsal Ie, fonning the
4 Primilive
4 segment
b, Primitive
neural folds. The groove between these
The closure of this
groove will form the neural tube, the an-
is the neural groove.
BAS PUP BT Ue
= ai lage of the central nervous system. The
Fic. 32.—Surface view of a twenty-one
hour chick embryo, in which the head fold and
first two pairs of primitive mesodermal seg-
ments are present. The head process is seen
under the neural groove (after Duval). x 13.
notochord is now differentiated from the
mesoderm and may be seen in the mid-
dorsal line through the ectoderm. Inthe
mesoderm lateral to the notochord and
cephalad to the primitive node, transverse furrows have differentiated two pairs
of block-like mesodermal segments, one incomplete cranially. As development
proceeds these increase in number, successive pairs being developed caudally.
They will be described in detail later.
EMBRYO OF SEVEN SEGMENTS (TWENTY-FIVE HOURS’ INCUBATION)
In this embryo (Fig. 33) there is a prominent network of blood vessels and
blood cells in the saudal portion of the area opaca. In its cranial portion isolated
groups of blood and blood vessel-forming cells are seen as blood islands. To-
EMBRYO OF SEVEN SEGMENTS 39
gether, they constitute the angioblast from which arises the extra-embryonic
blood vascular system. The area pellucida has the form of the sole of a shoe with
broad toe directed forward. The head fold has become cylindrical and the head
of the embryo is free for a short distance from the germinal disc. The mesoderm_
extends on each side beyond the head leaving a median. clear space, the proam-
niotic area. The entoderm is carried forward in the head fold as the fore-gut, from
Anterior neuro pore Pore-brain
fj : Free portion of head
Pharynx
>. Fovea cardiaca
~ Right vilelline
vein
——Neural groove
a
y
5
“Segmental
sone
Primitive knot
Notochord
Area opaca Blood island
Primitive streak
Fic. 33.—Dorsal view of a twenty-five-hour chick embryo with seven primitive segments. X 20.
which later arise the pharynx, esophagus, stomach, and a portion of the small
intestine. The opening into the fore-gut faces caudad and is the fovea cardiaca.
The way in which the entoderm is folded up from the germinal disc and forward
into the head is shown well in a longitudinal section of an older embryo (Fig. 42).
The tubular heart lies ventral to the fore-gut and cranial to the fovea cardiaca. In
later stages it is bent to the right. Converging forward to the heart, on each side
of the jones are the vitelline veins, just making their appearance at this stage.
40 THE STUDY OF CHICK EMBRYOS
The lips of the neural folds have met throughout the cranial two- thirds of the
embryo but have not fused. The neura al tube, formed thus by. 7 the closing of the
ectodermal folds, is open at either end at the neuropores. Cephalad, the neural
tube has begun to expand to form the brain vesicles. Of these only the fore-brain
is prominent, and from it the oplic vesicles are budding out laterally. The
paraxial mesoderm is divided | by transverse furrows into seven pairs of block- like
primitive “segments. Caudally, between the segments and the primitive streak,
there is there is undifferentiated mesoderm, but new pairs of segments will develop in this
region. Looking through the open neural tube (rhomboidal sinus), one may see
in the midline the notochord extending from the primitive node cephalad until it
is lost beneath the neural tube in the region of the primitive segments. The
primitive streak is still prominent at the posterior end of the area pellucida, forming
about one-fourth the length of the embryo. Transverse sections through the
primitive streak and open neural groove show approximately the same conditions
as in the twenty-hour embryo (Figs. 30 and 31).
A Transverse Section through the Fifth Primitive Segment (Fig. 34) is characterized
by the differentiation of the mesoderm, the approximation of the neural folds and the presence
of two vessels, the descending aorta, on each side between the mesodermal segments and the
entoderm. The neural folds are thick and the ectoderm is thickened over the embryo. The
Neural groove
Mesodermal segment
Splanchnic mesoderm
AD Recuesguees Cn sea
Descending aorta Entoderm
Fic. 34.—Transverse section through the fifth pair of mesodermal segments of a twenty-five-hour chick
embryo. X 90.
notochord is a sharply defined oval mass of cells. The mesodermal segments are somewhat
triangular in outline and connected by the intermediate cell mass, or nephrotome, with the
lateral mesoderm. This is partially divided by irregular flattened spaces into two layers, the
dorsal of which is the somatic, the ventral the splanchnic layer of mesoderm. Later, the
spaces unite on either side to form the celom or primitive body cavity.
Transverse Section Caudal to the Fovea Cardiaca (Fig. 35).—The section is charac-
terized: (1) by the closing together of the neural folds to form the newral tube; (2) by the
dorsal and lateral folding of the entoderm, which, a few sections nearer the head end, forms
the fore-gut or pharynx; (3) by the presence of the vitelline veins laterally between the ento-
derm and mesothelium, (4) by the wide separation of the somatic and splanchnic mesoderm
and the consequent increase in the size of the coelom. In this region the ccelom later sur-
rounds the heart and forms the pleuro-pericardial cavity.
EMBRYO OF SEVEN SEGMENTS 4I
The neural tube at this level forms the third brain vesicle or hind-brain. The neural
folds have not yet fused and at their dorsal angles are the neural crests, the anlages of the
spinal ganglia. Mesodermal segments do not develop in this region; instead a diffuse net-
work of mesoderm partly fills the space between ectoderm, entoderm, and mesothelium. This
is termed mesenchyme and will be described later.
Transverse Section through the Fovea Cardiaca (Fig. 36).—This section passes
through a vertical fold of entoderm at the point where the latter is reflexed into the head as the
Neural crest
Ectoderm
Descending aorta
Se
? a
ee —___Entoderm of fore-gut
"ihe
i Vitelline vein
a ae He 3
Entoderm a ts
”,
¢
Fic. 35.—Transverse section caudal to the fovea cardiaca of a twenty-five-hour chick embryo. X 90.
fore-gut (cf. Fig. 42). The entoderm forms a continuous mass of tissue between the vitelline
veins thereby closing the fore-gut ventrally. The splanchnic mesoderm is differentiated into
a thick-walled pouch on each side, lateral to the endothelial layer of the veins.
Transverse Section through the Heart (Fig. 37).—Passing cephalad in the series
of sections the vitelline veins open into the heart just in front of the fovea cardiaca. The
entoderm in the head fold now forms the crescentic pharynx or fore-gut, separated by the heart
and splanchnic mesothelium from the entoderm of the germinal disc. The descending
Neural tube
Descending aorta
Notochord
Fore-gut
Splanchnic mesoderm
(myocardium) :
Endothelium of heart tube
Celom—
"ieecesete Entoderm
ne
Splanchnic mesoderm j#
Fic. 36.—Transverse section through the fovea cardiaca of a twenty-five-hour chick embryo. X 90.
aorte are larger, forming conspicuous spaces between the neural tube (hind-brain) and the
pharynx. The heart, as will be seen, is formed by the union of two endothelial tubes, similar
to those constituting the vitelline veins in the preceding sections. The median walls of these
tubes disappear at a slightly later stage to form a single tube, the endocardium. Thickened
layers of splanchnic mesoderm, which, in the preceding section, invested the vitelline veins
laterally, now form the mesothelial wall of the heart. In the median ventral line, the layers
of splanchnic mesoderm of each side have fused and separated from the splanchnic mesothe-
42 THE STUDY OF CHICK EMBRYOS
lium of the germinal disc; thus the two pleuro-pericardial cavities are put in communication.
The mesothelial wall of the heart forms the myocardium and epicardium of the adult. Dor-
sally, the splanchnic mesoderm, as the dorsal mesocardium, suspends the heart, while still
more dorsally it is continuous with the somatic mesoderm.
Origin of Primitive Heart.—From the two sections last described, it is seen that
the heart arises as a pair of endothelial tubes lying in the pockets of the splanchnic mesoderm.
Later, the endothelial tubes fuse to form a single tube. The heart then consists of an endo-
thelial tube within a thick-walled tube of mesoderm. The origin of the endothelial cells of
the heart—whether they arise from entoderm or mesoderm—is not surely known. The vas-
cular system is primitively a paired system, the heart arising as a double tube with two
veins entering and two arteries leaving it.
Origin of the Blood Vessels and Blood.—We have seen that in the area opaca a
network of blood vessels and blood islands is differentiated as the_angioblast. This tissue
gives rise to primitive blood vessels and blood cells and probably is derived from the splanch-
nic mesoderm. The vessels arise first as reticular masses of cells, the so-called blood islands.
These cellular thickenings undergo differentiation into two cell types, the innermost hecom-
ing blood cells, the outermost forming a flattened endothelial layer which encloses the blood
Ecloderm Neural tube
Somatic mesoderm seer
Notochord Descending aorta
Pharynx
Myocardium ;
Endocardium
Fic. 37.—Transverse section through the heart of a twenty-five-hour chick embryo. X 90
cells. All the primitive blood vessels of the embryo are composed of an endothelial layer
only. The endothelial cells continue to divide, forming vascular sprouts and in this way new
vessels are in part produced. The first vessels arising in the vascular area of a chick embryo
unite into a close network, some of the branches of which enlarge to form vascular trunks.
One pair of such trunks, the vile/line veins, is differentiated adjacent to the posterior end of
the heart and later connects with it. Another pair, the vifelline artcrics, are developed in
continuation with the aorte of the embryo. The vessels of the vascular area thus appear
before those of the embryo have developed; they probably arise from the splanchnic meso-
derm, and, both arteries and veins, are composed of a simple endothelial wall. As the coelom
develops in the region of the vascular area of the embryo soon after the differentiation of the
angioblast, the anlages of the blood vessels are formed only in the splanchnic layer. (For
the development of the heart and blood vessels see Chapter IX.)
Transverse Section through the Pharyngeal Membrane (Fig. 38).—This section
passes through the head fold and shows the head free from the underlying germinal disc
(cf. Fig. 42). The ectoderm surrounds the head and near the mid-ventral line it is bent dor-
sad, is somewhat thickened, and comes in contact with the thick entoderm of the pharynx.
The area of contact between ectoderm and pharvngeal entoderm forms the pharyngeal plate
EMBRYO OF SEVENTEEN SEGMENTS 43
or membrane. Later, this membrane breaks through and thus the oral cavity arises. The
expanded neural tube is closed in this region and forms the middle brain vesicle or mid-brain.
The descending aortz appear as small vessels dorsal to the lateral folds of the pharynx. The
blastoderm in the region beneath the head is composed of ectoderm and entoderm
This is the proamntolic area. Laterad may be seen the layers of the mesoderm.
Ectoderm
aes
oi fess ; Neural tube
ORAL LEAS
Ae EER
Mesenchyma Gs hts ae i pM :
AEE hopeet fab) he
Su oe
dh we
Descending aorta
x : SPA wtf ; ==— Fore-gut
Pharyngeal € ; aN
membrane con : SLE ;
er gous
egy. oars ero aaa Beers age
lm tes a *
Entoderm of Ke Tesoro w en nets Ectoderm of proamnion
proamnion ae oo neetaeen noe
NS
Fic. 38.—Transverse section through the pharyngeal membrane of a twenty-five-hour chick embryo.
x 90.
Transverse Section through the Fore-brain and Optic Vesicle (Fig. 39).—The
neural tube is open here and constitutes the first brain vesicle or fore-brain. The opening is
the anterior neuropore. The ectoderm is composed of two or three layers of nuclei and is con-
tinuous with the much thicker wall of the fore-brain. The lateral expansions of the fore-
Neuropore
Ectoderm
Lctoderm—
Optic vesicle
'plic vesicle —_.
poeta |
ean Proamnion
— tig ee
Fic. 39.—Transverse section through the fore-brain and optic vesicles of a twenty-five-hour chick. X 90.
brain are the optic vesicles, which eventually give rise to the retina of the eye. The two ecto-
dermal layers are in contact with each other except in the mid-ventral region, where the
mesenchyma is beginning to penetrate between and separate them. The prearanide consists
merely of a layer of ectoderm and of entoderm.
CHICK EMBRYO OF SEVENTEEN PRIMITIVE SEGMENTS (THIRT Y-EIGHT HOURS)
The long axis of this embryo is nearly straight (Fig. 40), the area pellucida
is dumb-bell shaped and the vascular network is well differentiated throughout
the area opaca. The tubular heart is bent to the embryo’s right, and opposite its _
posterior end the vascular network converges and becomes continuous with the
trunks of the vitelline veins. Connections have also been formed between the
descending aorte and the vascular area, but as yet the vitelline arteries have not
44 THE STUDY OF CHICK EMBRYOS
appeared as distinct trunks. The proamniotic area is reduced to a small region
in front of the head, which latter is now larger and more prominent. In the
posterior third of the vascular area blood islands are still prominent.
Central Nervous System and Sense Organs.—The neural tube is closed save
at the caudal end where the open neural folds form the rhomboidal sinus. In
PRT EY EA
t
UN ee
GP LF Ie
7
4 Proamnion
A
ne)
~ Oplic vesicle
Ve
Fore-brain |
Mid-brain« | Free porlion of head
Hind-brain —
re Heart
it m {
Vitelline vein ——~ =! 4
ee 2. Neural tube
Mesodermal ‘”
ay
segment §
t
YT
He
a
{; Notochord \_ : se
AE SESE
ee Le pe Pio ae oN. es
Si I 2 HR ime Hill
Fic. 40.—View of the dorsal surface of a thirty-eight-hour chick embryo. X 20.
the head the neural tube is differentiated into the three brain vesicles, marked off
from each other by constrictions. The fore-brain (prosencephalon) is charac-
terized by the outgrowing optic vesicles. The mid-brain (mesencephalon) is
merges caudally with the spinal cord. It shows a number of secondary constric-
tions, the neuromeres. The ectoderm is thickened laterally over the optic ves-
EMBRYO OF SEVENTEEN SEGMENTS 45
icles to form the lens placode of eye (Fig. 43). The optic vesicle is flattened
at this point and will soon invaginate to produce the inner, nervous_layer_of the_
invaginated as the auditory placode (Fig. 45). This placode later forms the
Pg »y Fore-brain
Optic vesicle fe \\
Paired ventral aorta | \
4 / Vy Phervnse! pouch 1
WG fr ot é
yf + Descending ao
Ventral aorta +42 le scending aorta
Bulbus cordis
Ventricle
Splanchnic mesoderm —r
5 Left vitelline vein
Fovea cardiaca —
Entoderm
§
Right descendi ta
eS aaa Section medullary tube
Vascular plexus —
Splanchnic mesoderm a
~—— Somatopleure
Notochord A Descending aorta
ff |
Hl 4 |
Mesbdermal t =I} |
esodermal segmeni 4 | A | CD stetullary be
| asm j yoo (|| id |
ji S@\ 4 i | \ ‘
t \ \ —— Splanch ds
Satyd j j Splanchnic mesoderm
Beer ae
, ated est 2-4 _____— Capillary plexus
| 382
y
oe
Segmental zone
Somatopleure
_ ———— Neural groove
Fic. 41.—Ventral reconstruction of a thirty-eight-hour chick embryo. The entoderm has been removed
save about and caudal to the fovea cardiaca. X 38.
otocyst or otic vesicle from which is differentiated the epithelium of the internal
Digestive Tube.—The entoderm is still flattened out over the surface of the
yolk caudal to the fovea cardiaca. In Fig. 41 the greater part of the entoderm
iscutaway. The flattened fore-gut, folded inward at the fovea, shows indications
46 THE STUDY OF CHICK EMBRYOS
of three lateral diverticula, the pharyngeal pouches. Cephalad the pharynx is
closed ventrally by the pharyngeal membrane.
Heart and Blood Vessels.—After receiving the vitelline veins cephalad to
the fovea cardiaca the double-walled tube of the heart dilates and bends ventrad
and to the embryo’s right (Fig. 41). It then is flexed dorsad and to the median
line, and narrows to form the ventral aorta. The aorta lies ventrad to the pharynx
and divides at the boundary line between the mid- and hind-brain into two
ventral aorte. These diverge and course dorsad around the pharynx. Before
reaching the optic vesicles they bend sharply dorsad and caudad, and, as the
paired descending aortw, may be traced to a point opposite the last primitive seg-
ments. In the region of the fovea cardiaca they lie close together and have fused
to form a single vessel, the dorsal aorta. They soon separate and opposite the
last primitive segments they are connected by numerous capillaries with the
Hind-brain Fore-gut Neural tube
Mid-brain
Fore-brain =
Amnion fold 4 Breasts
Pharyngeal membrane Ventral
aorta Dorsal Heart Pericardial cavity
mesocardium
Fic. 42.—A median longitudinal section of the head of a thirty-eight-hour chick embryo. X about 50.
vascular network. In this region at a later stage the trunks of the paired vitelline
arteries will be differentiated. The heart beats at this stage; the blood flows from
the vascular area by way of the vitelline veins to the heart, thence by the aorte
and vitelline arteries back again. This constitutes the vilelline circulation and
through it the embryo receives nutriment from the yolk for its future develop-
ment.
In studying transverse sections of the embryo it is not sufficient merely to:
identify the structures seen. The student should determine also the exact level
of each section with respect to Figs. 40, 41 and 42, and trace the organs from sec-
tion to section in the series. It is important to remember that the transverse
sections figured and described in this manual (except those of the fifty-hour chick)
are all drawn viewed from the cephalic surface; hence the right side of the embryo
is at the reader’s left.
EMBRYO OF SEVENTEEN SEGMENTS 47
TRANSVERSE SECTIONS
Transverse Section through the Fore-brain and Optic Vesicles (Fig. 43).—The
optic stalks connect the optic vesicles laterally with the ventral portion of the fore-brain.
Dorsally the section passes through the mid-brain due to the somewhat ventrally flexed head
Mid-brain
Celom
Somatopleure
Fic. 43.—Transverse section through the fore-brain of a thirty-eight-hour chick embryo. X 75.
(cf. Fig. 42). We have alluded to the thickening of the Jens placode. Note that there is now
a considerable amount of mesenchyme between the ectoderm and the neural tube. Layers
of mesoderm are present in the underlying blastoderm.
Ectoderm Mid-brain
Mesenchyme
Descending aorta—f
Fore-gut
Veniral aorta
Pharyngeal membrane
Fic. 44.—Transverse section through the pharyngeal membrane of a thirty-eight-hour chick embryo.
x 75.
Transverse Section through the Pharyngeal Membrane and Mid-brain (Fig.
44).—In the mid-ventral line the thickened ectoderm bends up into contact with the entoderm
of the rounded pharynx of the fore-gut. At this point the oral opening will break through.
48 THE STUDY OF CHICK EMBRYOS
On either side of the pharynx a pair of large vessels are seen; the ventral pair are the ventral
aorte. Two sections cephalad their cavities open into those of the dorsal pair, the descending
aorte. The section is thus just caudad of the point where the ventral aorta bend dorsad and
caudad to form the descending aorte. The section passes through the caudal end of the
Ectoderm Hind-brain
Ant. cardinal vein
Notochord
Auditory placode
Descending aorta
Pericardial cavity
Somatic mesoderm : Bay Ectoderm
Endothelium
of ventricle
Endothelium of bulbus
Myocardium
Fic. 45.—Transverse section through the hind-brain and auditory placodes of a thirty-eight-hour chick
embryo. X 75.
mesencephalon which is*here thick walled with an oval cavity. Note the large amount of
undifferentiated mesenchyme in the section. The structure of the blastoderm is complicated
by the presence of collapsed blood vessels.
Transverse Section through the Hind-brain and Auditory Placodes (Fig. 45).—
Besides the auditory placodes already described as the anlages of the internal ear, this sec-
Ectoderm Hind-brain
Mesodermal segment Fore-gut
Anterior cardinat vein
Descending aorta
Somatic mesoderm Calom
: Ectoderm
Entoderm
Splanchnic mesoderm
Vitelline vein
Heart Myocardium
Fic. 46.—Transverse section through the caudal end of the heart of a thirty-eight-hour chick embryo.
x 75.
tion is characterized by (1) the large hind-brain, somewhat flattened dorsad; (2) the broad
dorso-ventrally flattened pharynx, above which on each side lie the descending aorte; (3) the
presence of the bulbar and ventricular portions of the heart. The bulbus is suspended dorsally
EMBRYO OF SEVENTEEN SEGMENTS 49
by. the mesoderm, which here forms the dorsal mesocardium. The ventricle lies on the right
side of the embryo; a few sections caudad in the series it is continuous with the ventral aorta
(cf. Fig. 41). Between the somatic and splanchnic mesoderm is the large pericardial cavity.
It surrounds the heart in this section. Dorsal to the aorta are the anterior cardinal veins,
which return blood from the head region.
Transverse Section through the Caudal End of the Heart (Fig. 46).—The
section passes through the hind-brain. The descending aorte are separated only by a thin
septum which is ruptured in this section. The anterior cardinal veins are cut at the level
where they bend ventrad to enter the heart. The mesothelial wall of the heart is continuous
with the splanchnic mesoderm. On the right side of the section there is apparent fusion
between the myocardium of the heart and the somatic mesoderm. A pair of primitive meso-
dermal segments may be seen in this section lateral to the hind-brain. It may be noted here
that the primitive segments were not present in the sections of the head previously studied.
Transverse Section through the Fovea Cardiaca (Fig. 47).—The descending
aorie now form a single vessel, the dorsal aorta, the medium septum having disappeared. The
Ectoderm
Neural tube
Mesodermal segment
Notochord
Dorsal aorta
Calom Ant. card-
inal vein
Extra-embryonic celom
NEenioderm
Left vitelline vein
Right vitelline vein Splanchnic
mesoderm Entoderm Fore-gut
Fic. 47.—Transverse section through the fovea cardiaca of a thirty-eight-hour chick embryo. X 90.
section passes through the entoderm at the point where it is folded dorsad and cephalad into
the head as the fore-gut (cf. Fig. 42). Two sections caudad is found the opening (fovea car-
diacay where the fore-gut communicates with the flattened open gut between the entoderm
and the yolk. On each side of the fore-gut are the large vitelline veins, sectioned obliquely.
As the splanchnic mesoderm overlies these veins dorsad, it is pressed by them on each side
against the somatic mesoderm and the cavity of the ccelom is thus interrupted.
Transverse Section Caudal to the Fovea Cardiaca (Fig. 48).—This section re-
sembles the preceding save that the primitive gut is without a ventral wall. The right
vitelline vein is still large.
Section through the Fourteenth Pair of Primitive Segments (Fig. 49)—The
body of the embryo is now flattened on the surface of the yolk. Here the descending aorte
are still separate and occupy the depressions lateral to the primitive segments. The section
is characterized by the notochord and the differentiated mesoderm which forms the primitive
segments, nephrotomes, somatic and splanchnic mesoderm, structures soon to be described.
Arising from the nephrotomes are sprout-like pronephric tubules. The tips of these hollow
out and unite to form the primary excretory or mesonephric duct. :
4
5° THE STUDY OF CHICK EMBRYOS
Transverse Section through the Rhomboidal Sinus (Fig. 50).—The ‘neural groove
is open, the notochord is ovalin form. The ectoderm is characterized by the columnar form of
its cells. At the point where the ectoderm joins the neural fold a ridge of cells projects ven-
Neural tube Neural cavity
Mesodermal segment
Celom
Splanchnic mesoderm
Splonchnablene Open gut Entoderm
Fic. 48.—Transverse section caudal to the fovea cardiaca of a thirty-eight-hour chick embryo. X 90.
Neural tube
Mesodermal segment Ectoderm
Central cells of segment.
Somatic. mesoderm
Pronephric tubule
Sblanchnic mesoderm
Descending aorta Entoderm
Notochord
Fic. 49.—Transverse section through the fourteenth pair of mesodermal segments of a thirty-eight-hour
chick embryo. X 90.
Neural groove
Ectoderm Neural crest
Segmental sone
Somatic mesoderm
ON
AVS
wee
Splanchnic mesoderm Calom Notochord — Entoderm Blood vessel
Fic. 50.—Transverse section through the rhomboidal sinus of a thirty-eight-hour chick embryo. X 90
trally on either side. These projecting cells form the nenral crests, and from them the spinal
ganglia are formed. The mesodermal plates have split laterally into layers, but the coelomic
cavities are mere slits. Between the splanchnic mesoderm and the entoderm blood vessels
may be seen.
EMBRYO OF SEVENTEEN SEGMENTS 51
Transverse Section through the Primitive (Hensen’s) Knot or Node (Fig. 51).
—The section shows the three germ layers fused inseparably at the “knot” into a mass of
Somatic mesoderm Ectoderm Primitive knot
Celom Entoderm Splanchnic mesoderm
Fic. 51.—Transverse section through the primitive (Hensen’s) knot of a thirty-eight-hour chick embryo.
x 90.
undifferentiated tissue. The mesoderm is split laterally into the somatic and splanchnic
layers.
Transverse Section through the Primitive Streak (Fig. 52).—In the mid-dorsal
line is the primitive groove. The germ layers may be seen taking their origin from the undif-
Somatic mesoderm Primitive groove
Ectoderm
DEUTER CAB Splanchnic mesoderm
Entoderm
Fic. 52.—Transverse section through the primitive streak of a thirty-eight-hour chick embryo. X 90.
ferentiated tissue of the primitive streak beneath the primitive groove. Between the splanchnic
mesoderm and entoderm blood vessels are present laterad as in the preceding sections.
Mesodermal Segments.—We have seen that these are developed by the ap-
pearance of transverse furrows in the mesoderm (Fig. 53). Later a longitudinal
furrow partially separates the paired segments from the lateral unsegmented
mesoderm. The segments are block-like with rounded angles when viewed
dorsally, triangular in transverse sections (Figs. 49 and 53). They are formed
cranio-caudally, the most cephalad being the first to appear. The first four lie
in the head region. The segments contain no definite cavity but a potential
cavity representing a portion of the ccelom is filled with cells, and the other cells
of the segments form a thick mesothelial layer about them (Fig. 49). The ventral
wall and a portion of the median wall of each primitive segment become trans-
formed into mesenchyma which surrounds the neural tube and notochord (Fig.
290). The remaining portion of the segments persist as the dermo-muscular plates.
The cells of the mesial portions of the plates, the myotomes, elongate and give rise
52 THE STUDY OF CHICK EMBRYOS
to the voluntary muscle of the body. The voluntary or skeletal muscles are thus
at first all segmented but later many of the segments fuse. In the trunk muscles
of the adult fish the primitive segmented condition is retained.
The Intermediate Cell Masses or Nephrotomes.—The bridge of cells con-
necting the primitive segments with the lateral mesodermal layers constitutes
the nephrotome (Figs. 49 and 53). In the chick the nephrotomes of the fifth to
sixteenth segments give rise dorsad to pairs of small cellular sprouts, the rudi-
mentary kidney tubules of the pronephroi, segmentally arranged in the fur-
row lateral to the primitive segments. By the union of these cell masses dis-
tally solid cords are formed which run lengthwise in the furrow. These cords
fs
a
Mesonephric duct ——@
Neural tube
Mesodermal segment
Somatic mesoderm
Splanchno pleure
Descending aorla
Notochord Entoderm Ci st
Fic. 53.—Semi-diagrammatic reconstruction of five mesodermal segments of a forty-eight-hour chick
embryo. The ectoderm is removed from the dorsal surface of the embryo.
hollow out, grow caudad, and become the primary excretory (mesonephric) ducts
(Fig. 53). More caudally the intermediate cell masses form the embryonic _kid-
ney or mesonephros, the tubules of which open into the primary excretory duct.
Further details concerning these provisional kidneys are given on pages 195-199,
Since the genital glands develop in connection with the mesonephros, and the
kidney of the adult (metanephros) is partly developed as an outgrowth of the
primary y excretory duct, the intermediate cell mass may be regarded as the anlage
of the urogenital glands and their ducis. These structures are thus of mesodermal
origin. a es
Somatopleure and Splanchnopleure.—In the embryo of seven primitive seg-
ments the mesoderm was seen to split laterally into two layers, the somatic
EMBRYO OF SEVENTEEN SEGMENTS 53
(dorsal) and the splanchnic (ventral) mesoderm (Fig. 34). These layers per=
sist in the adult, the somatic mesoderm giving rise to the > pericardium of the
heart, to the par. rietal plet pleura of the thorax and to the peritoneum | of the
while the splanchnic layer forms the epicardium and myocardium of t
the visceral pleura of the Jungs, and the mesenteries and mesodermal layer of the
gut. The somatic mesoderm and the ectoderm, with the tissue developed be-
tween them, constitute the body wall, which is termed the somatopleure. In
the same way the splanchnic mesoderm and the entoderm, with the mesenchymal
tissue between them, constitute the wall of the gut, or the splanchnopleure.
Coelom.—The cavity between the somatopleure and splanchnopleure is the
celom (body cavity). With the splitting of the mesoderm, isolated cavities are
produced. These unite on
Notochord Neural tube
each side and eventually
- Nephrotome
form one cavity—the cce-
lom. With the extension Archenteron
of the mesoderm, the coe- SPlanchnic
mesoderm
Somatic
mesoderm
Sb leack aafle are.
Yolk
lom surrounds the heart
and gut ventrally (Fig.54). --Euloderm
Later, it is subdivided into Sia atofl uve
the pericardial cavity about
the heart, the pleural cavity
of the thorax, and the
peritoneal cavity of the ab-
Celom
dominal region. In the
Fic. 54.—Diagrammatic transverse section of a vertebrate
stages already studied, the embryo (adapted from Minot).
embryo was flattened on
the surface of the yolk and the somatopleure and splanchnopleure did not
meet ventrally. If this union occurred they would conform to the structural
relations shown in Fig. 54, which is essentially the ground plan of the vertebrate
body.
Mesenchyme.—In the sections through the head of this embryo, and through
that of the preceding stage, but four primitive segments were found. The greater
part of the mesoderm in the head appears in the form of an undifferentiated net-
work of cells which fill in the spaces between the definite layers (epithelia). This
tissue is mesenchyme (Fig. 55). The mesoderm may be largely converted into
mesenchyme, as in the head, or any of the mesodermal layers may contribute
to its formation. Thus it may be derived from the primitive segments and
54 THE STUDY OF CHICK EMBRYOS
from the somatic and splanchnic mesoderm. The cells of the mesenchyme form
a syncytium or network, and are at first packed closely together. Later, they
may form a more open network with cytoplasmic processes extending from cell
to cell (Fig. 55). The mesenchyme is an inportant Asshe Of Le em iye tissue of the embryo, from
it. are differentiated the blood and 1 together ms. tomether with niost most of the
smooth muscle, connective tissue, and skele-
tal tissue of the body.
The body of the embryo is now com-
posed (1) of cells arranged in layers—epi-
The
term “epithelium” may be used in a general
thelia, and (2) of diffuse mesenchyme.
sense, or restricted to layers covering the
surface of the body or lining the digestive
canal and its derivatives. Layers lining the
body cavities are termed mesothelia, while
those lining the blood vessels and heart are
Fic. 55.—Mesenchyme from the head of a
thirty-eight-hour chick embryo.
x 495.
called endothelia.
Derivatives of the Germ Layers.—The tissues of the adult are derived from
the epithelia and mesenchyme of the three germ layers as follows:
Ectoderm
1. Epidermis and its derivatives
special sense.
Mesoderm
A. Mesothelium.
. Serous layer of intestine.
Entoderm
1. Epithelium of digestive
4, Epithelium of pharynx.
(hair, nails, glands). 1. Pericardium. tract.
2. Conjunctiva and lens of eye. 2. Pleura. 2. Liver.
3. Sensory epithelia of organs of 3. Peritoneum. 3. Pancreas.
4
5
4. Epithelium of mouth, enamel
. Epithelium .of most of urogeni-
Eustachian tube.
of teeth, oral glands. Hypo- tal organs. Tonsils.
physis. 6. Striated muscle. Thymus.
5. Epithelium of anus. 1. Skeletal. Thyreoids.
6. Male urethra (distad). 2. Cardiac. Parathyreoids.
7. Epithelium of amnion and B. Mesenchyme. 5. Epithelium of respiratory
chorion. 1. Blood cells. tract.
8. Nervous, neuroglia, and chro- 2. Bone marrow. Larynx.
maffin cells of nervous sys- 3. Endothelium of blood vessels. Trachea.
tem. Retina and optic nerve. 4. Endothelium of lymphatics. Lungs.
9. Notochord (?). 5. Spleen and lymphoid organs. 6. Epithelium of most of blad-
10. Smooth muscle of sweat glands 6. Supporting tissues. (Connect- der, of female urethra,
and of iris.
ing tissue, cartilage, and bone.)
. Smooth muscle.
male prostatic urethra
and prostate.
7. Notochord (?).
For the histological development (histogenesis) of the various tissues from
the primary germ layers see Chapter X.
EMBRYO OF TWENTY-SEVEN SEGMENTS 55
x
CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS (FIFTY HOURS)
This embryo, of nearly fifty hours’ incubation, lies in the center of the vascular
area and is peculiar in that the head is twisted 90° to the right. In a dorsal view,
therefore, one sees the right side of the head but the dorsal side of the body. In
the region of the mid-brain is a very marked bend, the cephalic flexure. Below
the head, and ventral in position, lies the tubular heart, now bent in the form of
Mid-brain
Hind-brain § ; ;
‘ = Fore-brain
Oplic vesicle
| Lens vesicle
Otic vesicle
ire
Branchial cleft 3 Bz
Amnion fold
L. vitelline artery +
t R. vitelline artery
Neural tube ;
Segment 24
Area pellucida
Primitive streak *
and tail bud }
Fic. 56.—Dorsal view of a fifty-hour chick embryo, stained and mounted in balsam. X 14.
a letter S. Dorsal to the heart, in the region of the pharynx, three transverse
grooves or slits may be seen. These are the branchial clefts or gill slits. The
head of the embryo is now covered by a double fold of the somatopleure, the
head fold of the amnion. It envelops the head like a veil. Caudally, a fold and
opacity mark the position of the tail bud from which develops the caudal end of
the body. The curved fold embracing this is the ¢azl fold of the amnion, which will
eventually meet the head fold and completely envelop the embryo.
Central Nervous System and Sense Organs (Fig. 57).—Cephalad, the neural
56 THE STUDY OF CHICK EMBRYOS
tube is divided by constrictions into four vesicles. The fore-brain of the previous
stage is now subdivided into two regions, the telencephalon and Hiencephalon.
The cephalic flexure has been established in the region of the mesencephalon.
The hind-brain, as yet undivided, equals the combined length of the other three
vesicles. The lens of the eye has invaginated, pushing in the wall of the optic
Mid-brain
Optic vesicle FO xe
Aperture of lens vesicle,
. Hind-brain
Ky YN, ‘\, Notochord
’ f SN \ Otocyst
Pharynx i ey Ss {|
Bulb of heart
Fore-brain-+
Aortic arches 1, 2, 3
ey i | Ant. cardinal vein
Ventricle Atrium
R. vitelline vein Common cardinal vein
. vitelline
Post cardinal vein
Fore-gut Descending aorta
Splanchnopleure Liver anlage
io Wi Fovea cardiaca
Splanchnic mesoderm
Entoderm
Dorsal aorta Somatopleure
R. vitelline sna,
Mes. segment -
. vilelline artery
Edge of splanchnic mesoderm
Mes. segment
Vascular plexus
Segmental zone
Neural plate
Entoderm 7
Primitive node “&
ee
Fic. 57.—Semi-diagrammatic reconstruction of a fifty-hour chick embryo, in ventral view. X 18.
The entoderm has been removed save in the region of the fovea cardiaca and of the hind-gut. Owing
to the torsion of the embryo, the cranial third of the embryo is seen from the left side, the caudal two-
thirds in ventral view.
vesicle and thus forming a double-walled structure, the optic cup. The audi-
tory placode has become a sac, the ofocyst, which overlies the hind-brain opposite
the second branchial groove and is still connected with the outer ectoderm, cut
away in Fig. 57. The rhomboidal sinus is still open at the caudal end of the
neural tube.
EMBRYO OF TWENTY-SEVEN SEGMENTS 57
Digestive Canal (Fig. 57).—In a reconstruction from the ventral side, the
digestive canal shows differentiation into three regions. Of these, the fore-gut
has been seen in earlier stages. A greater part of the mid-gut has been cut away
to show the underlying structures; it is without a ventral wall and overlies the yolk.
Caudad, a small fovea leads into the hind-gut which is just beginning to evag-
inate into the tail fold. The pharyngeal membrane now lies in a considerable
cavity, the stomedgum, formed by the invaginated ectoderm. The median ecto-
dermal pouch next the brain wall is known as Rathke’s pocket and is the anlage
of the anterior lobe of the hypophysis. The pharynx shows laterally three out-
pocketings, of which the first is wing-like and is the largest. These pharyngeal
pouches occur opposite the three branchial grooves and here entoderm and ecto-
derm are in contact, forming the closing plates. At about this stage the first
closing plate ruptures, thereby forming a free opening, or branchial cleft, into the
pharynx. Between the pouches are developed the branchial arches, in which
course the paired aortic arches. Towards the fovea cardiaca the fore-gut is flat-
tened laterally and before it opens out into the mid-gut there is budded off ven-
teally a bilobed structure, the anlage of the liver (Figs. 57 and 63). It lies be-
tween the vitelline veins and in its later development the veins are broken up into
the sinusoids or blood spaces of the liver.
Just as the entoderm participates in the head fold to form the fore-gut so in
the tail fold it forms the hind-gut. This at once gives rise to a tubular outgrowth
which becomes the allantois, one of the fetal membranes to be described later
(Fig. 70).
Blood Vascular System.—The tubular heart is flexed in the form of a letter
S when seen from the ventral side. Four regions may be distinguished: (1)
the sinus venosus, into which open the veins; (2) a dilated dorsal chamber, the
atrium; (3) a tubular ventral portion flexed in the form of a U, of which the left
limb is the ventricle, the right limb (4) the bulbus cordis. From the bulbus is
given off the ventral aorta. There are now developed three pairs of aortic arches
which open into the paired descending aorte. The first aortic arch passes
cranial to the first pharyngeal pouch and is the primitive arch seen in the thirty-
six-hour embryo. The second and third arches course on either side of the
second pharyngeal pouch. They are developed by the enlargement of channels
in primitive capillary networks between ventral and descending aorte. Op-
posite the sinus venosus the paired aortic trunks fuse to form the single dorsal
aorta which extends as far back as the fifteenth pair of primitive segments.
At this point the aorte again separate, and, opposite the twentieth segments, each
58 THE STUDY OF CHICK EMBRYOS
connects with the trunk of a vitelline artery which was developed in the vascular
area and conveys the blood to it (Fig. 57). Caudal to the vitelline arteries the
dorsal aorte rapidly decrease in size and soon end.
As in the previous stage, the blood is conveyed from the vascular area to the
heart by the vitelline veins, now two large trunks. In the body of the embryo
there have developed two pairs of veins. In the head have appeared the anterior
cardinal veins, already of large size and lying lateral to the ventral region of the
Chorion
Hind-brain Amnion
Ant. cardinal vein
Notochord
\ Aorlic arch 1
Ectoderm
Lens vesicle}
A—O plic vesicle
ic Prosencephalon
Cavity of fore-brain
Fic. 58.—Transverse section through the fore-brain and eyes of a fifty-hour chick embryo. X SO.
brain vesicles (Fig. 60). Caudal to the atrium of the heart, two small posterior
cardinal veins are developed. They lie in the mesenchyma of the somatopleure
laterad in position (Fig. 63). Opposite the sinus venosus the anterior and pos-
terior cardinal veins of each side unite and form the common cardinal veins (ducts
of Cuvier) which open into the dorsal wall of the sinus venosus (Fig. 57). The
primitive veins are thus paired like the arteries, and like them develop by the en-
largement of channels in a network of capillaries.
The following series of transverse sections from an embryo of this stage shows
EMBRYO OF TWENTY-SEVEN SEGMENTS 59
the more important structures. The approximate plane and level of each section
may be ascertained by referring to Figs. 56 and 57.
TRANSVERSE SECTIONS
Section through the Fore-brain and Eyes (Fig. 58).—The section passes cranial
to the optic stalks, consequently the optic vesicles appear unconnected with the fore-brain.
The thickened ectoderm is invaginated to form the anlages of the lens vesicles. The thicker
wall of the optic vesicles next the lens anlage will give rise to the nervous layer of the retina, the
thinner outer wall becomes the pigment layer of the retina. Ventrad in the section are the
wall and cavity of the fore-brain, dorsad the hind-brain with its thin, dorsal ependymal layer.
Amnion
Hind-brain_ KAA i Chorion
Ant. cardinal vein
. aoe
Descending aorta— Ecloderm
; : Pharynx
Ant. cardinal vein ’
Rathke’s pocket
Ventral aorta
Optic vesicle.
Fic. 59.—Transverse section through the optic stalks and hypophysis of a fifty-hour chick embryo.
x 50.
Between the brain vesicles on either side are sections of the first aortic arches and lateral to the
hind-brain are the smaller paired anterior cardinal veins, which convey the blood from the
head to the heart.
Section through the Optic Stalks and Hypophysis (Fig. 59).—The section passes
just caudal to the lens which does not show. The offic vesicles are connected with the wall of
the fore-brain by the optic stalks which later form the path by which the fibers of the optic
nerve pass from the retina to the brain. Both the ventral and the descending aorte are seen in
section about the cephalad end of the pharynx. Between the ventral wall of the fore-brain
and the pharynx is an invagination of the ectoderm, Rathke’s pocket (anterior lobe of
hypophysis).
Missing Page
Missing Page
62 THE STUDY OF CHICK EMBRYOS
The somatopleuric folds of the amnion envelop the right side of the embryo and the
ectoderm of these folds now forms the outer layer of the chorion and the inner layer of the
amnion. The mesodermal components of these folds have not united.
Section through the Anlage of the Liver (Fig. 63).—In this section the cavity of
the fore-gut is narrow, the gut being flattened from side to side. Ventrad there are evaginated
from the entoderm two elongate diverticula which form the anlages of the /iver. On either side
of the anlages of the liver are sections of the vifelline veins on their way to the sinus venosus
at a higher level in the series. Note the intimate relation between the entodermal epithelium
of the liver and the endothelium of the vitelline veins. In later stages, as the liver anlages
branch, there is, as Minot aptly expresses it, “‘an intercrescence of the entodermal cells consti-
tuting the liver and of the vascular endothelium” of the vitelline veins. Thus are formed
the hepatic sinusoids of the portal system, which surround the cords of hepatic cells.
Fic. 63.—Transverse section through the anlage of the liver of a fifty-hour chick embryo. xX 50.
The septum transversum is still present at this level and lateral to the fore-gut are small
body cavities. Lateral to the body cavities appear branches of the posterior cardinal veins.
Section through the Cranial Portion of the Open Intestine (Fig. 64).—The intestine
is now open ventrad, its splanchnopleure passing directly over to that of the vascular area.
The folds of the amnion do not join, leaving the amniotic cavity open. The dorsal aorta
is divided by a septum into its primitive components, the right and left descending aorte.
Lateral to the aorte are the small posterior cardinal veins. The ccelom is in communication
with the extra-embryonic body cavity.
Section through the Seventeenth Pair of Mesodermal Segments (Fig. 65).—The
body of the embryo is now no longer flexed to the right. On the left side of the figure the
mesodermal segment shows a dorso-lateral muscle plate. The median and ventral portion of
the segment is being converted into mesenchyme. On the right side appears a section of the
primary excretory or mesonephric duct. The embryonic somatopleure is arched and will form
EMBRYO’ OF TWENTY-SEVEN SEGMENTS 63
the future ventro-lateral body wall of the embryo. The fold lateral to the arch of the somato-
pleure gives indication of the later approximation of the ventral body walls, by which the
embryo is separated from the underlying layers of the blastoderm.
Chorion
\ Amnion
Central canal
ays
Ni
2 Spinal cord
Mesodermat segment
R. descending aorta
-—~ Somato pleure
Fic. 64.—Transverse section through the cranial portion of the open intestine of a fifty-hour chick
embryo. X 50.
Section through the Origin of the Vitelline Arteries (Fig. 66).—At this level the
embryo is more flattened and simpler in structure, the section resembling one through the
mid-gut region of a thirty-eight-hour chick (Fig. 49). The amniotic folds have not appeared.
Spinal cord
Mesodermal segment
Descending aorta
Somato pleure
Ectoderm
Notochord
Somatic mesoderm
aS
Splanchnic mesoderm
Celom Entoderm
Fic. 65.—Transverse section through the seventeenth pair of mesodermal segments of a fifty-hour chick
embryo. X SO.
On the left side of the figure the vitelline artery leaves the aorta. On the right side the con-
nection of the vitelline artery with the aorta does not show, as the section is cut somewhat
obliquely. The posterior cardinal vein is present just laterad of the right mesonephric duct.
The other structures were described in connection with Fig. 49.
64 THE STUDY OF CHICK EMBRYOS
Section Caudal to the Mesodermal Segments (Fig. 67).—The mesodermal seg-
ments are replaced by the segmental zone, a somewhat triangular mass of undifferentiated
mesoderm from which later are formed the segments and nephrotomes. The notochord is
Mesodermal segment Spinal cord
Nephrotome Ectoderm
e Mesonéphric duct
Calom Somatic mesoderm
Somato pleure
Entoderm
Aorta and vitelline — Notochord
artery
Fic. 66.—Transverse section of a fifty-hour chick embryo at the level of the origin of the vitelline arteries.
x 50.
larger, the aorte smaller, and a few sections caudad they disappear. Laterally the somato-
pleure and splanchnopleure are straight and separated by the slit-like ccelom.
Section through the Primitive Knot Cranial to the Hind-gut (Fig. 68).—With
the exception of the ectoderm, the structures near the median line are merged into an undiffer-
Spinal cord Ectoderm
Descending aorta
Somatic mesoderm
Segmental sone
Somatic mesoderm
Splanchnic mesoderm
Notochord Entoderm
Fic. 67.—Transverse section of a fifty-hour chick embryo through the segmental zone caudal to the meso-
dermal segments. x 50.
entiated mass of tissue. The cavity of the neural tube and its dorsal outline may still be seen,
but its ventral portion, the notochord, mesoderm, and entoderm, blend in a dense mass of tissue
which is characteristic of the primitive knot. Laterally the segmental zone and the various
layers are differentiated.
Neural tube Ectoaerm
Segmental sone
Splanchnopleure
Entoderm
Notochordal plate
Fic. 68.—Transverse section of a fifty-hour chick embryo through the primitive node cranial to the
hind-gut. X 50.
Section Passing through the Hind-gut (Fig. 69).—In this embryo the caudal evagina-
tion to form the Aind-gut has just begun. The section shows the small cavity of the hind+
EMBRYO OF TWENTY-SEVEN SEGMENTS 65
gut in the midline. Its wall is composed of columnar entodermal cells and it is an out-
growth of the entodermal layer. Dorsal to the hind-gut may be seen undifferentiated cells
Primitive knot
Ectoderm
Hind-gut Entoderm
Fic. 69.—Transverse section passing through the hind-gut of a fifty-hour chick embryo. X 50.
of the primitive streak continuous dorsad with the ectoderm, ventrad with the entoderm of the
hind-gut, and laterally with the mesoderm.
In the chick embryos which we have studied, there are large areas developed
which are extra-embryonic, that is, lie outside the embryo. The splanchnopleure
of the area vasculosa, for instance, forms the wall of the yolk sac, incomplete in
the early stages. The amnion, chorion, and allantois are extra-embryonic mem-
branes which make their appearance at the fifty-hour stage. These structures
are important in mammalian and human embryos and a description of their
further development in the chick, where their structure and mode of develop-
ment is primitive, will lead up to the study of mammalian embryos in which the
amnion and chorion are précociously developed.
Amnion and Chorion.—These two membranes are developed in all amniote
vertebrates (Reptiles, Birds, and Mammals). They are derived from the extra-
embryonic somatopleure. The amnion is purely a protective structure, but the
chorion of mammals has a trophic function, as through it the embryo derives its
nourishment from the uterine wall. Fig. 70 A shows the amnion and chorion
developing. The head fold of the somatopleure forms first and envelops the head,
the tail fold makes its appearance later. The two folds extend laterad, meet
and fuse (Fig. 70 B, C). The inner leaf of the folds forms the amnion, the re-
mainder of the extra-embryonic somatopleure becomes the chorion. The actual
appearance of these structures and their relation to the embryo have been seen in
Figs. 63 and 64. The amnion, with its ectodermal layer inside, completely sur-
rounds the embryo at'the end of the third day, enclosing a cavity filled with am-
niotic fluid (Fig. 71). In this the embryo floats and is thus protected from injury.
The chorion is of little importance to the chick. It is at first incomplete, but
eventually entirely surrounds the embryo and its other appendages.
Yolk Sac and Yolk Stalk.—While the amnion and chorion are developing
during the second and third day, the embryo grows rapidly. The head- and tail
5
66 THE STUDY OF CHICK EMBRYOS
folds elongate and the trunk expands laterally until only a relatively narrow stalk
of the splanchnopleure connects the embryo with the | yolk. This portion of the
splanchnopleure has grown more slowly than the body of the embryo and is termed
the yolk stalk. It is continuous with the splanchnopleure which envelops the
yolk and forms the yolk sac. The process of unequal growth by which the embryo
becomes separated from the yolk has been described as a process of constriction.
This, as Minot points out, is anerror. The splanchnopleure at first forms only an
oval plate on the surface of the yolk, but eventually encloses it. In Fig. 70, C and
D, the relation of the embryo to the yolk sac is seen at the end of the first week of
incubation. The vitelline vessels ramify on the surface of the yolk sac and
Fic. 70.—Diagrams showing the development of the amnion, chorion and allantois in longi-
tudinal section (Gegenbaur in McMurrich). Ectoderm, mesoderm, and entoderm represented by
heavy, light, and dotted lines respectively. Af. amnion folds; Al., allantois; Am., amniotic
cavity; Ch., chorion; Vs., yolk sac.
through them all the food material of the yolk is conveyed to the chick during the
incubation period (about twenty-one days).
Allantois.—-We have seen that in the fifty-hour chick a ventral evagination,
the hind-gut, develops near its caudal end (Fig. 69). From it develops the anlage
of the allantois, which, as an outgrowth of the splanchnopleure, is lined with
entoderm and covered with splanchnic mesoderm (Fig. 70). It develops rapidly
into a vesicle connected to the hind-gut by a narrow stalk, the allantoic stalk.
At the fifth day the allantois is nearly as large as the embryo (Fig. 71). Its wall
flattens out beneath the chorion and finally it lies close to the shell but is attached
only tothe embryo. The functions of respiration and excretion are ascribed to it.
EMBRYO OF TWENTY-SEVEN SEGMENTS 67
In its wall ramify the allantoic vessels, which have been compared to the umbilical
arteries and veins of mammalian embryos.
The chick embryo is thus protected by the amnion which develops from the
inner leaf of the folded somatopleure and is composed of an inner ectodermal and
an outer mesodermal layer. Nutriment for the growth of the embryo is supplied
by the yolk sac and carried to the embryo by the vitelline veins. The allantois,
Allantois Embryo Amnion Chorion
Shell
Y, Shell membrane
Volk sac d,
Margin of area vasculosa
Air chamber
Fic. 71.—Diagram of a chick embryo at the end of the fifth day showing amnion, chorion and allan-
tois (Marshall). X 1.5.
which takes its origin from the splanchnopleure of the hind-gut and is composed
of an inner layer of entoderm and an outer layer of splanchnic mesoderm, func-
tions as an organ of respiration and serves as a reservoir for the excreta of the
embryonic kidneys. As we shall see, the allantois becomes more important, the
yolk sac less important in some mammals, while in human embryos both yolk
sac and allantois are unimportant when compared to the chorion.
CHAPTER IV
THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
THE fetal membranes of mammals include the amnion, chorion, yolk sac, and
. allantois, structures which we have seen are present in chick embryos. Most
important in mammals is the manner in which the embryo becomes attached to
the uterine wall of the mother, and in this regard mammalian embryos fall into
two groups. Among the Ungulates or hoofed mammals (e. g., the pig) the
fetal membranes are of a primitive type, resembling those of the chick. Among
Unguiculates (clawed animals like the bat and rabbit) and Primates (e. g., Man)
the fetal membranes of the embryo show marked changes in development and
structure.
FETAL MEMBRANES OF THE PIG EMBRYO
The amnion and chorion develop very much as in the chick embryo (Fig. 70
A, B). Folds of the somatopleure form very early and envelop the whole embryo.
= Seon cord
Ny olochord
Mesodermal segment
Amniotic cavily_ 4
Upper limb bud 4)
i:
Posterior cardinal vein
Dorsal aorla——
ye
~~ Entoderm of gut
Splanchnic mesoderm
Fic. 72.—Transverse section through the yolk sac and stalk of a 5 mm. pig embryo showing attachment
of amnion.
The amnion (Fig. 72) is closed in embryos with but a few pairs of segments, but
for some time remains attached to the chorion by a strand of tissue (Keibel).
The yolk sac develops early as in all mammals. In the pig it is small and the greater
part of it soon degenerates. It is important only in the early growth of the
68
FETAL MEMBRANES OF THE PIG EMBRYO 69
embryo, its functions then being transferred to the allantois. Branches of the
vitelline vessels ramify in its wall, as in that of chick embryos, but soon degener-
ate. The trunks of the vitelline vessels, however, persist within the body of the
embryo. The allantois, developing as in the chick from the ventral wall of the
hind-gut (Fig. 70 A-D), appears when the embryo is still flattened out on the
germinal disc. In an embryo 3.5 mm. long it is crescent-shaped and as large as
the embryo. It soon becomes larger and its convex outer surface (splanchnic
mesoderm) is applied to the inner surface (somatic mesoderm) of the chorion.
Entoderm of primitive gut Amnion
Ectoderm
oko
Z 3s Somatic and
ya splanchnic mesoderm
ear
9
‘0 ig os Hi 75 >
pA aaa y a/\% O35 Yolk sac
Tunica propria of uterus
Fic. 73.—Diagram of the fetal membranes and allantoic placenta of a pig embryo in median sagittal sec-
tion (based on figures of Heisler and Minot).
These surface layers fuse more or less completely. A pair of allantoic veins and
arteries branch in the splanchnic layer of the allantois. These branches are
brought into contact with the mesodermal layer of the chorion and invade it.
The outer ectodermal layer of the chorion in the meantime has closely applied
itself to the uterine epithelium, the ends of the uterine cells fitting into depressions
in the chorionic cells (Fig. 73). When the allantoic circulation is established,
waste products given off from the blood of the embryo must pass through the
epithelia of both chorion and uterus to be taken up by the blood of the mother.
iy
7O THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
In the same way nutritive substances and oxygen must pass from the maternal
blood through these layers to enter the allantoic vessels. This exchange does take
place, however, and thus in Ungulates the allantois has become important not
only as an organ of respiration and excretion but as an organ of nutrition. Through
its vessels it has taken on a function belonging to the yolk sac in birds, and we
now see why the! yolk sac becomes a rudimentary structure in the higher mam-
mals. Excreta from the embryonic kidneys are passed into the cavity of the
allantois which is relatively large. The name is derived from a Greek word mean-
ing sausage-like, from its form in some animals. The chorion is important only
as it brings the allantois into close relation to the uterine wall, but in man we shall
see that it plays a more important rdle.
UMBILICAL CORD
Pig Embryos.—In their early development the relation of the amnion, allan-
tois, and yolk sac to each other and to the embryo is much the same as in the
chick of five days (Fig. 71). With the increase in size of the embryo, however,
the somatopleure in the region of the attachment of the amnion grows ventrad
(Fig. 70D). Asa result, it is carried downward about the yolk sac and allantois,
forming the umbilical cord (cf. Fig. 241). Thus in a pig embryo 10 to 12 mm.
long the amnion is attached at a circular line about these structures some distance
from the body of the embryo (cf. Fig. 119). The ccelom at first extends ventrad
into the cord, but later the mesodermal layers of amnion, yolk stalk, and allantois
fuse and form a solid cord of tissue. This is the umbilical cord of fetal life and its
point of attachment to the body is the wmbilicus or navel. The cord is covered by
a layer of ectoderm continuous with that of the amnion and of the embryo and
contains, embedded in a mesenchymal (mucous) tissue, (1) the yolk stalk and (in
early stages) its vitelline vessels; (2) the allantoic stalk; (3) the allantoic vessels.
These latter, two arteries and a single large vein, are termed, from their position,
the umbilical vessels. At certain stages (Figs. 122 and 123) the gut normally
extends into the coelom of the cord, forming an umbilical hernia. Later, it re-
turns to the coelom of the embryo and the cavity of the cord disappears. The
umbilical cord of the pig is very short.
Human Umbilical Cord.—This develops like that of the pig and may attain a
length of more than 50 cm. It becomes spirally twisted, just how is not known.
In embryos from 10 to 40 mm. long the gut extends into the ccelom of the
cord (Figs. 179 and 180). At the 42 mm. stage, according to Lewis and Mall,
the gut returns to the coelom of the body. The mucous tissue peculiar to the cord
EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 71
arises from mesenchyme. It contains no capillaries and no nerves, but embed-
ded in it are the large umbilical vein, the two arteries, the allantois, and the yolk
stalk. The umbilical cord may become wound about the neck of the fetus, caus-
ing its death and abortion, or by coiling about the extremities it may lead to their
atrophy or amputation.
EARLY HUMAN EMBRYOS AND THEIR MEMBRANES
Referring to the blastodermic vesicle of the mammal (Figs. 17 and 18), it
is found to consist of an outer layer, which we have called the trophectoderm, and
the inner cell mass (p. 36). The trophectoderm forms the primitive ectodermal
layer of the chorion in the higher mammals and probably in man. From the inner
cell mass are derived the primary ectoderm, entoderm, and mesoderm. In the
earliest known human embryos described by Teacher, Bryce, and Peters, the germ
layers and amnion are present, indicating that they are formed very early. We
can only infer their early origin from what is known of other mammals. The
diagrams (Fig. 74 A and B) show two hypothetical stages seen in median longi-
tudinal section. In the first stage (A) the blastodermic vesicle is surrounded by
the trophectoderm layer. The inner cell mass is differentiated into a dorsal mass
of ectoderm and a ventral mass of entoderm. Mesoderm more or less completely
fills the space between entoderm and trophectoderm. It is assumed that as the
embryo grows (B) a split occurs in the mass of ectoderm cells, giving rise to the
amniotic cavity and dividing these cells into the ectodermal layer of the embryo
and into the extra-embryonic ectoderm of the amnion. At the same time a
cavity may be assumed to form in the entoderm, giving rise to the primitive gut.
About this stage the embryo embeds itself in the uterine mucosa. In the third
stage, based on Peter’s embryo (C), the extra-embryonic mesoderm has extended
between the trophectoderm and the ectoderm of the amnion and the extra-
embryonic ccelom appears. At first strands of mesoderm, known as the magma
reticulare, bridge across the ccelom between the somatic and splanchnic layers of
mesoderm (Fig. 76). The amniotic cavity has increased in size and the embryo
is attached to the trophectoderm by the unsplit layer of mesoderm between the
ectoderm of the amnion and the trophectoderm of the chorion. The latter shows
thickenings which are the anlages of the chorionic villi surrounded by trophoderm
cells. In the fourth stage, based on Graf Spee’s embryo (D), the chorionic villi
are longer and branched. The mesoderm now remains unsplit only at the pos-
terior end of the embryo, where it forms the body stalk peculiar to Unguiculates
and Primates. It connects the mesoderm of the embryo with the mesoderm of the
72 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
chorion. Into it there has grown from the gut of the embryo the entodermal diver-
ticulum of the allantois.
The Chorion.—The human chorion is derived directly from the outer troph-
ectoderm layer of the blastodermic vesicle and from the extra-embryonic somatic
mesoderm. Its early structure resembles that of the pig’s chorion. The troph-
ectoderm of the human embryo early gives rise to a thickened outer layer, the
trophoderm (syncytial and nutrient layer—Figs. 74 C and 239). When the develop-
Trophectoderm
Archenteron:
Entoderm
Mesoderm
Ectoderm of amnion
Ectoderm of embryo
f yo | Ectoderm of embryo
Cavity of amnion
ALesoderm o,
Tro phectoderm amn a
Amniotic cavily
Volk sae Ectoderm of _ Sy
chorion
= Entoderm Cavity of yolk
: . : Sac=BF
a 3 Splanchnic Entoderm of:
mesoderm yolk sac
Extra-embry- Mfesoderm
onic celom of yolk sac
Extra-embry-
onic celom
derm Mesoderm
Trophoderm of chorion
Chorionic villi
Fic. 74.—Four diagrams of early human embryos (based on figures of Robinson and Minot). 4A, Hy-
pothetical stage; B, Bryce-Teacher embryo (modified); C, Peter’s embryo; D, Graf Spee’s embryo.
ing embryo comes into contact with the uterine wall the trophoderm destroys the
maternal tissues. The destruction of the uterine mucosa serves two purposes:
(1) the embedding and attachment of the embryo, it being grafted, so to speak, to
the uterine wall; and (2) it supplies the embryo with a new source of nutrition.
To obtain nutriment to better advantage, there grow out from the chorion into
the uterine mucosa branched processes or villi. The villi are bathed in maternal
blood, and in them blood vessels are developed, the trunks of which pass to and
EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 73
from the embryo as the umbilical vessels. ‘The embryo receives its nutriment and
oxygen, and gets rid of waste products through the walls of the villi. The region
where the attachment of the chorionic villi to the uterine wall persists during fetal
life is known as the placenta. It will be described later with the decidual mem-
branes of the uterus. We saw how the allantois of Ungulates had assumed the
Inner cell mass
Embryonic ectoderm Entoderm
Maternal blood vessels
Lo Trophoderm
oT
6) BES 5 a Cytolrophoblast
, G (Gr
ares
file Ly.
geese!
Of Oe re
See Amniotic cavity
gO, wih fli?
ph
as
Embryonic ectoderm Entoderm
Fic. 75.—Sections showing the formation of the amnion in bat embryos (after Van Beneden).
X about 160.
nutritive functions performed by the yolk sac in birds, with a consequent degene-
ration of the ungulate yolk sac. In man and Unguiculates the functions of the
allantois are transferred to the chorion, and the allantois, in turn, becomes a rudi-
mentary structure.
The Amnion.—This is formed precociously in Unguiculates and in a manner
quite different from its mode of origin in Ungulates and birds. It is assumed that
74 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
its cavity arises as a split in the primitive ectoderm of human embryos, as in bat
embryos (Fig. 75). Later,a somatic layer of mesoderm envelops its ectodermal
layer, its component parts then being the same as in birds and Ungulates—an
inner layer of ectoderm and an outer layer of mesoderm (F ig. 74D). It becomes
a thin, pellucid, non-vascular membrane and about a month before birth is in
contact with the chorion. It then contains about a liter of amniotic fluid, the
origin of which is unknown. During the early months of pregnancy the
embryo, suspended by the umbilical cord, floats in the amniotic fluid. The
embryo is protected from maceration by a whité fatty secretion, the vernix
caseéosa.
At birth the amnion is ruptured either normally or artificially. If not ruptured,
the child may be born enveloped in the amnion, popularly known as a veil or ‘“‘caul.”” The
hte
v
ae
oe
&
e
3
9, amas
a eee
Va S =e.
ee 3 | |
rN So i
= f
z
i a wa 028 © ATH Oh
LS a= as ’ aoe ae ea" oo eh, mes.
70 © aes go aur? Gor ees
MCS. ei ge ae 2 ae . > eo
° *: ah “i ©
ex. Ca.
ent.
Fic. 76.—Section of Peter’s embryo of 0.2 mm. (about fifteen days). ect., Ectoderm of chorion;
mes., mesoderm; am., amniotic cavity; em. pl., embryonic plate; y.s., yolk sac; ent., entoderm; ex. c@.,
portion of extra-embryonic ccelom limited by a strand of the magma reticulare.
amniotic fluid may be present in excessive amount, the condition being known as hydram-
mios. If less than the normal amount of fluid is present, the amnion may adhere to the
embryo and produce malformations. It has been found, too, that fibrous bands or cords
of tissue may extend across the amniotic cavity, and, pressing upon parts of the embryo
during its growth, may cause scars and splitting of eyelids or lips. Such amniotic threads
may even amputate a limb or cause the bifurcation of a digit.
The Allantois.—The allantois appears very early in the human embryo be-
fore the development of the fore-gut or hind-gut. In Peter’s embryo the amnion,
chorion, and yolk sac are present, but not the allantois (Fig. 76). In an embryo
1.54 mm. long, described by Von Spee (Fig. 77), there is no hind-gut, but the
EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 75
allantoic diverticulum of the entoderm has invaded the mesoderm of the body
stalk. This embryo, seen from the dorsal side with the amnion cut away, shows
Neurenteric canal =
Primitive streak
Body stalk _—_—_4.—
Ba
: Villi of chorion
ae
Embryonic disc
LS
Rudiment of heart
Mesoderm
Fic. 77.—Views of a human embryo 1.54 mm. long (Graf Spee). > 23. A, Dorsal surface; B, median
sagittal section.
a marked neural groove and primitive streak. In front of the primitive knot a
pore is figured leading from the neural groove into the primitive intestinal
76 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
cavity, and hence called the neurenteric canal (p. 33). The fore-gut and head
fold have formed at this stage and there are branched chorionic villi. Somewhat
more advanced conditions are found in
> Amnion (cut) an embryo of 1.8 mm. with five to six
pairs of segments (Fig. 78).
A reconstruction by Dandy of
Volk sac Mall’s embryo, about 2 mm. long with
Neurenteric canal S&VeN pairs of segments, shows well
5 Primitive streak the embryonic appendages (Fig. 79).
—— Body stalk
~~~ Chorion
The fore- and hind-gut are well de-
om s veloped, the amniotic cavity is large,
Fic. 78 —Kroémer human embryo of 1.8 mm. in and the yolk sac still communicates
dorsal view (after Keibel and Elze). > 20. with the gut through a wide opening.
The allantois is present as a long curved tube somewhat dilated near its
blind end and embedded in the mesoderm of the body stalk. As the hind-gut
Chorion
Amnion .
on ~
, Body stalk
Pharyngeal oO ee W Allantoic
membrane f
Fore-gut
Heart’ #f
Splanchnic
mesoderm
Fic. 79.—A human embryo of 2 mm. in median sagittal section (adapted from reconstructions of
Mall’s embryo by F. T. Lewis and Dandy). X 23.
develops, the allantois comes to open into its ventral wall. A large umbilical
artery and vein are present in the body stalk.
In an embryo of 23 somites 2.5 mm. long, described by Thompson, the allan-
EARLY HUMAN EMBRYOS AND THEIR MEMPRANES 77
tois has elongated and shows three irregular dilatations (Fig. 80). A large cavity
never appears distally in the human allantois as in Ungulates. When it becomes
included in the umbilical cord its distal portion is tubular and it eventually atro-
phies. That part of the allantois extending from the umbilicus to the cloaca of the
hind-gut possibly takes part in forming the bladder and the wrachus, a rudiment
extending as a solid cord from the fundus of the bladder to the umbilicus.
The human allantois is thus small and rudimentary as compared with that
of birds and Ungulates. As we have seen, the cavity is very large in the pig, and
Haller found an allantoic sac two feet
ee Ss .
ae > long connected with a goat embryo of
fi .
\. two inches. In human embryos it ap-
Pharynx-___..... Tm ‘
Pharyngeal f. MM ‘ Go, /
membrane 7s \ oe
Thyreoid \ }
Pericardium
|
Hepatic diverticulum __ ; |
Septum lransversum i j | Yolk-sac
4 | Cut edge of
re | amnion
Yolk stalk. Primitive
Segments
Allantois
Cloacal membrane
Cloaca _[, Neural folds
wee — Neurenteric canal
ay
Fic. 80.—Median sagittal section of a 2.5 Fic. 81—Human embryo of 2.11 mm. (Eternod).
mm. human embryo showing digestive tract xX 35.
(after Thompson). X 40.
pears very early and is not free, but embedded in the body stalk. Its func-
tions, so important in birds and Ungulates, are in man performed by the chorion.
Yolk Sac and Yolk Stalk.—In the youngest human embryos described, the
entoderm forms a somewhat elongated vesicle (Fig. 76). With the development
of the fore-gut and hind-gut in embryos of 1.54 and 2 mm. (Figs. 77 and 78), the
entodermal vesicle is divided into the dorsal intestine and ventral yolk sac, the two
being connected by a somewhat narrower region. This condition persists in
an embryo 2.5 mm. long (Fig. 80). In the figure most of the yolk sac has been
78 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
cut away. Embryos with 9 and 14 pairs of segments, with three brain vesicles
and with the amnion cut away are seen in Figs. 81 and 324. The relation of the
Amnion
Fic. 82.—Human embryo of about 2.5 mm. (His, after Coste). 15.
fetal appendages to the embryo shows well in the embryo of Coste (Fig. 82).
The dorsal concavity is probably abnormal. A robust body stalk attaches the
Amnion ———_~ &
Mavillary process
Branchial clefts 1-3 Mandibular process
Heart
CTERT IR
Body stalk
Yolk-sac
Fic. 83.—Human embryo 2.6 mm. long showing amnion, yolk stalk and body stalk (His). 25.
embryo to the inner wall of the chorion. With the growth of the head- and tail
folds of the embryo, there is an apparent constriction of the yolk sac where it
THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 79
joins the embryo. This will become more marked in later stages and form
the yolk stalk. His’ embryo, 2.6 mm. long, shows the relative size of yolk sac and
embryo and the yolk stalk (Fig. 83). The relations of the fetal membranes to the
embryo are much the same as in the chick embryo of five days, save that the al-
lantois of the human embryo is embedded in the body stalk. The embryo shows a
regular convex dorsal curvature, there is a marked cephalic bend in the region of
the mid-brain and there are three gill clefts. The head is twisted to the left, the
tail to the right. At the side of the oral sinus are two large processes; the dorsal
of these is the maxillary, the ventral the mandibular process. The heart is large
and flexed in much the same way as the heart of the fifty-hour chick embryo.
In later stages, with the development of the umbilical cord, the yolk stalk
becomes a slender thread extending from the dividing line between the fore- and
hind-guts to the yolk sac or umbilical vesicle (Figs. 84 and 119). It loses its at-
tachment to the gut in 7mm. embryos. A blind pocket may persist at its point of
Fic. 84.—Yolk sac and stalk of a20 mm. humanembryo. X 11.
union with the intestine and is known as Meckel’s diverticulum, a structure of
clinical importance because it may telescope and cause the occlusion of the intes-
tinal lumen. The yolk stalk may remain embedded in the umbilical cord and
extend some distance to the yolk sac which is found between the amnion and
chorion. The yolk sac may be persistent at birth.
THE ANATOMY OF A 42 MM. HUMAN EMBRYO
This embryo, studied and described by His, is regarded by Keibel as not quite
normal. Viewed from the left side (Fig. 85), with the amnion cut away close to its
line of attachment, there may be seen the yolk stalk, and a portion of the yolk sac
and of the body stalk. There is an indication of the primitive segments along the
dorso-lateral line of the trunk. The head is bent ventrad almost at right angles
in the mid-brain region (cephalic flexure). There are also marked cervical and
caudal flexures, the trunk ending in a short blunt tail. The heart is large and
80 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
flexed as in the earlier stage. Three gill clefts separate the four branchial arches.
The first has developed two ventral processes. Of these, the maxillary process
is small and may be seen dorsal to the stomodeum. The mandibular process is
large and has met its fellow of the right side to form the mandible or lower jaw.
Dorsal to the second gill cleft may be seen the position of the oval olocyst, now
a closed sac. Opposite the atrial portion of the heart, and in the region of the
caudal flexure, bud-like outgrowths indicate the anlages of the upper and lower
extremities.
Central Nervous System and Sense Organs.—The neural tube is closed
throughout its extent and is differentiated into brain and spinal cord. The
brain tube, or excephalon, is divided
Mid-brain
2 Horie by constrictions into four regions, or
5 ; ‘ '
‘Povecbraia v4 a Auditory vesic vesicles, as in the fifty-hour chick
d C \ (Fig. 57). Of these, the most ceph-
Stomod: Pe 3 i : :
Ue Ee Ly y a "iia alad is the felencephalon. It is a
Mandibular a fe, A » od he " -
rote hig paired outgrowth from the fore-brain,
Heart seal the persisting portion of which is
the diencephalon. The mid-brain or
mesencephalon, located at the cephalic
flexure, is not subdivided. The hind-
brain, or rhombencephalon, which is
Amnion (cut)
long and continuous with the spinal
cord, later is subdivided into the
metencephalon (region of the cere-
fs
Body stalk
bellum and pons) and myelencephalon
The
cord forms a closed tube extending
Fic. 85.—Left side of a human embryo of 4.2 mm.
co. ee (medulla oblongata).
spinal
from the brain to the tail and containing the neural cavity, flattened from side
to side.
The eye is represented by the optic vesicles and the thickened ectodermal
anlage of the lens. Its stage of development is between that of the thirty-six
and fifty-hour chick embryos.
The olocyst is a closed sac, no longer connected with the outer ectoderm as
in the fifty-hour chick.
Digestive Canal.—In a reconstruction of the viscera viewed from the right
The pharvn-
geal membrane, which we saw developed in the chick between the stomodeum
side (Fig. 86), the entire extent of the digestive canal may be seen.
THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 81
and the pharynx, has broken through so that these cavities are now in communi-
cation. The fore-gut, which extends from the oral cavity to the yolk stalk, is
differentiated into pharynx, thyreoid, trachea and lungs, esophagus and stomach,
small intestine and digestive glands (pancreas and liver). The gut is suspended
from the dorsal body wall by the dorsal mesentery.
Mesencephalon and cephalic flexure
Hy pophysis
Diencephalon
Internal carotid artery
Atrium of heart
_Umbilical vein
Liver anlage
Splanchnic mesoderm
-Mid-gut
Entoderm of yolk stalk
Tail gut
Umbilical artery
Mesone phric duct
Allantois
Fic. 86.—Diagrammatic reconstruction of a 4.2 mm. human embryo, viewed from the right side (adapted
froma model by His). X 25.
The ectodermal limits of the oral cavity are indicated dorsad by the diver-
ticulum of the hypophysis (Rathke’s pocket). The fore-gut proper begins with
a shallow out-pocketing known as Seessel’s pocket. As the pharyngeal’ mem-
brane disappears between these pockets, it would seem that Seessel’s pocket
represents the persistence of the blind anterior end of the fore-gut. No other
significance has been assigned to it.
The pharynx is widened laterally and at this stage shows four pharyngeal
6
82 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
pouches (Fig. 87). Later a fifth pair of pouches is developed (Fig. 168). The
four pairs of pharyngeal pouches are important as they form respectively the
following adult structures: (1) the auditory tubes; (2) the palatine tonsils; (3) the
thymus anlages; (4) the parathyreoids or epithelial bodies. Between the pharyn-
geal pouches are the five branchial arches in which are developed five pairs of
aortic arches. Between the bases of the first and second branchial arches, on
Mouth cavity
Thyreoid anlage
Pharyngeal pouches 1-4
Trachea
‘Lung bud
Stomach
Hepatic diverticulum Dorsal pancreas
Ventral pancreas
Yolk stalk
Mesonebhric tubule with glomerulus
Hind-gul
Ht— Mesonephros
Allgatoss Mesonephric duct
Fic. 87.—Diagrammatic ventral view of pharynx, digestive tube, and mesonephroi of a 4-5 mm.
embryo (based on reconstructions by Grosser and His). > about 30. The liver and yolk sac are cut
away. The tubules of the right mesonephros are shown diagrammatically...
the floor of the pharynx, is developed the tuberculum impar which perhaps forms a
portion of the anterior part of the tongue. Posterior to this unpaired anlage of
the tongue there grows out ventrally the anlage of the thyreoid gland. From
the caudal end of the trachea have appeared ventrally the lung buds. The
trachea is still largely a groove in the ventral wall of the pharynx and esophagus
(Fig. 86). Caudal to the lungs a slight dilation of the digestive tube indicates the
position of the stomach. The liver diverticulum has grown out from the fore-gut
Aortic arches 1-4
Atrium
Vitello-umbilical vein
Liver
| a L. umbilical vein
|
R. vitelline vein |
I\\
i
~ Common cardinal vein
- Vitelline vein
—-—-Umbilical vein
Placental attachment
Fic. 89.—Lateral view of human embryo of 4.2 mm., showing aortic arches and venous trunks (His).
mx, Maxillary process; jv., anterior cardinal vein; c.v., posterior cardinal vein; of, otocyst.
THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 83
into the ventral mesentery cranial to the wall of the yolk stalk. It is much larger
than in the fifty-hour chick, where its paired anlage was seen cranial to the
fovea cardiaca, and is separated from the heart by the septum transversum. The
small intestine between the liver and yolk stalk is short and broad. In later
stages it becomes enormously elongated as compared with the rest of the diges-
tive tube. The yolk stalk is still expansive. The region of its attachment
to the gut corresponds to the open mid-gut of the chick embryo. The hind-gut
and tail fold of this embryo are greatly elongated as compared with the chick
embryo of fifty hours. The hind-gut terminates blindly in the tail. Near its
caudal end it is dilated to form the cloaca. Into the ventral side of the cloaca opens
the stalk of the allantois. Dorso-laterally the primary excretory (Wolffian) ducts )
which we saw developed in the fifty-hour chick have connected with the cloaca
and open into it. Caudal to the cloaca, on thé ventral side, is the cloacal mem-
brane, which later divides and breaks through to form the genital aperture and
anus. That part of the hind-gut between the cloaca and the yolk stalk forms
the rectum, colon, cecum, and appendix, with a portion of the small intestine
(ileum).
Urogenital Organs.—The opening of the primary excretory (Wolffian)
ducts into the cloaca has been noted. These are the ducts of the mid-kidney
or mesonephros. At this stage the nephrotomes, which in the chick embryos
formed the anlages of these ducts, are also forming the kidney tubules of the meso-
nephros which open into the ducts (Fig. 87). The mid-kidneys project into the
peritoneal cavity as ridges on each side. A thickening of the mesothelium along
the median halves of the mesonephroi forms the anlage of the genital glands or
gonads (Fig. 220).
Circulatory System.—The /eart is an S-shaped double tube as in the fifty-
hour chick. The outer myocardium is confined to the heart while the inner
endothelial layer is continuous, at one end with the veins, at the other end with
the arteries. The disposition of the heart tube is well seen in a ventral view of a
younger embryo (Fig. 88). The veins enter the sinus venosus just cranial to
the yolk sac. Next in front is the afrium with the convexity of its flexure di-
rected cephalad. The ventricular portion of the heart is U-shaped and is flexed
to the right of the embryo. The left limb is the ventricle, the right the bulbus.
The arteries begin with the ventral aorta which bends back to the midline and
divides into five branches on each side of the pharynx (Figs. 88 and 89). These
are the aortic arches and they unite dorsally to form two trunks, the descending
aorie. ‘The aortic arches pass around the pharynx between the gill clefts in the
84 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
Frc. 90.—Embryos of the first six weeks (2.1 to 11 mm.).
x 3.
From His’ Normentafel (Keibel and Elze).
THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 85
branchial arches. The arrangement is like that of the adult fish which has gill
slits, branchial arches, and aortic arches to supply the gills. The descending
4
L
By
Eb
TRE,
ek
f
Frc. 91.—Embryos of six to eight weeks (12.5 to 23 mm.). From His’ Normentafel (Keibel and Elze).
X 2.5. Stage w(22) marks the transition from embryo to fetus.
aorte run caudad and opposite the lung buds unite to form a single median dorsal
aorta. This, in the region of the posterior limb buds, divides into the two um-
86 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
bilical arteries, which, curving cephalad and ventrad, enter the body stalk on
each side of the allantois and eventually ramify in the villi of the chorioa. The
vilelline arteries, large and paired in the chick, are represented by a single small
trunk which branches on the surface of the yolk sac (Fig. 271). Compared with
the arterial circulation of the chick of fifty hours the important differences are
(1) the development of the fourth and the fifth pairs of aortic arches, and (2)
the presence of the chorionic circulation by way of the umbilical] arteries in addi-
tion to the vitelline circulation found in the fifty-hour chick.
The veins are all paired and symmetrically arranged (Figs. 88 and 279).
There are three sets of them: (1) The blood from the body of the embryo is
drained, from the head end by the anterior cardinal veins; from the tail end of the
body by the posterior cardinal veins. These veins on each side unite dorsal to
the heart and form a single common cardinal vein which receives the vitelline and
umbilical veins of the same side before joining the heart. (2) Paired vitelline
veins in the early stages of the embryo drain from the yolk sac the blood carried to
it by the vitelline arteries. The trunks of these veins pass back into the body on
each side of the yolk stalk and liver, and with the paired umbilical veins form a
trunk which empties into the sinus venosus of the heart. As the liver develops
it may be seen that the vitelline veins break up into blood spaces called by Minot
sinusoids (Fig. 279). When the liver becomes large and the yolk sac rudimentary
the vitelline veins receive blood chiefly from the liver and intestine. (3) A pair
of large umbilical veins which drain the blood from the villi of the chorion and are
the first veins to appear. These unite in the body stalk, and, again separating,
enter the somatopleure on each side. They run cephalad to the septum trans-
versum where they unite with the vitelline veins to form a common vitello-umbil-
ical trunk which joins the common cardinal and empties into the sinus venosus.
The veins of this embryo are thus like those of the fifty-hour chick save that
the umbilical vessels are now present and take the place of the allantoic veins of later
chick embryos. The veins, like the heart and arteries, are primitively paired and
symmetrically arranged. As development proceeds, their symmetry is largely
lost and the asymmetrical venous system of the adult results.
The later stages of the human embryo cannot be described in detail here.
The student is referred to the texts of Minot, and Keibel and Mall. Two embryos
will be compared with the pig embryos described in Chapter V. Figs. 90 and 91
show the human embryos described by His, the ages of which were estimated by
him at from two weeks to two months. The figures show as well as could any
description the changes which lead to the adult form when the embryo may be
THE AGE OF HUMAN EMBRYOS 87
called a fetus (stage w). The external metamorphosis is due principally: (1) to
changes in the flexures of the embryo; (2) to the development of the face; (3) to
the development of the external structure of the sense organs (nose, eye, and ear);
(4) to the development of the extremities and disappearance of the tail. The
more important of these changes will be dealt with in later chapters.
THE AGE OF HUMAN EMBRYOS
The ages of the human embryos which have been obtained and described can-
not be determined with certainty, because fertilization does not necessarily fol-
low directly after coitus. It has been shown also that ovulation does not always
coincide with menstruation so that the menstrual period cannot be taken as the
starting point of pregnancy. In 1868, Reichert, from studying the corpus luteum
in ovaries obtained during menstruation, concluded that ovulation takes place as
a rule just before menstruation and that if the ovum is fertilized the approaching
menstruation does not occur. Reichert then decided that a human embryo of 5.5
mm., which he had obtained from a woman two weeks after menstruation failed
to occur, must be two weeks, not six weeks, old. His accepted Reichert’s views and
since then the ages of embryos have largely been estimated on this basis. Ac-
cording to this view, Peter’s ovum, obtained thirty days after the last period, is
only three or four days old. This does not agree at all with what is known of the
age of other mammalian embryos.
From the observations of Mall and obstetricians of the present day, we must
conclude that ovulation does not immediately precede menstruation, but that
most pregnancies take place during the first or second week after the menstrual
period. It is therefore more correct to compute the age of the embryo from the end
of the last menstruation, or, according to Grosser, from the tenth to the twelfth day
before the first missed menstrual period. Peter’s embryo then would be about
fifteen days old. To compare an embryo with one of known age, the crown-rump
length (i. e., from vertex to breech) is usually taken. Embryos of the same age
vary greatly in size so that their structure must be taken into account. At the
present time the exact relation of ovulation to menstruation is not known, nor the
exact time required for the fertilized ovum to reach the uterus. The computed
age of the embryo thus can be only approximate.
The period of gestation of the human fetus is usually computed from the
beginning of the last menstrual period. Forty weeks or two hundred and eighty
days is the time usually allowed. As some women menstruate once or more
after becoming pregnant this is not a certain basis for computation.
88 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS
The following are the estimated ages and lengths of human embryos, accord-
ing to Mall, and their weights, according to Fehling:
Crown-HEEL LENGTH CROWN-RUMP LENGTH WEIGHT
AGE. (CH), or STANDING (CR), or SITTING IN
HEIGHT (MM.). HEIGHT (MM.). Grams.
Twenty-one days..............-0.0000 0.5 0.5
Twenty-eight days..................00. 2.5 2.5
Thirty-five days..............-...-00005 5:5 5:5
Porty=twodaySir.o1-on aise weaned ans 11.0 11.0
Forty-nine days iisnysxaase eae a qnteenetels 19.0 17.0
Second lunar month.................... 30.0 25.0 es
Third dunar monthi.. 2.03 eeecn2ee 4 cases 98.0 68.0 20
Fourth lunar month.................... 180.0 121.0 120
Fifth lunar month...................... 250.0 167.0 285
Sixth lunar month. ..................... 315.0 210.0 635
Seventh lunar month................... 370.0 245.0 1220
Fighth lunar month.................... 425.0 284.0 1700
Ninth lunar month................00... 470.0 316.0 2240
Tenth lunar month..................... 500.0 345.0 3250
Wee
CHAPTER V
THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
A. THE ANATOMY OF A SIX MM. PIG EMBRYO
VERY young pig embryos of the primitive streak and neural fold stages have
been seen already (Fig. 26). In its early stages the pig embryo is flattened out
on the surface of the yolk sac like a chick embryo (Fig. 92), but as the head and
tail folds elongate the body becomes B
flexed and twisted spirally, making it
difficult to study. In embryos 5 to
7 mm. long the twist of the body ye }
begins to disappear and its structure Wy
may be seen to better advantage.
External Form of 6 mm. Em-
bryo.—When compared with the
form of the 4 mm. human embryo,
the marked difference in the 6 mm.
pig is the convex dorsal flexure which
brings the head and tail regions
close together (Fig. 93). The ceph-
alic flexure at the mesencephalon
forms an acute angle and there is a
marked neck or cervical flexure. As
a result, the head is somewhat tri-
See ae
Fic. 92.—Pig embryos (A) of seven and (B) of
dorsad in an even convex curve _ eleven primitive segments, in dorsal view with amnion
and the tail is flexed sharply dorsad. ‘Yt *¥4Y (Aerbel, Nemmentatel). 2, 20)
Lateral to the dorsal line may be seen the segments, which become larger and more
angular in form. The body is bent
differentiated from tail to head. At the tip of the head a shallow depression marks
the anlage of the olfactory pit. The lens vesicle of the eye is open to the exterior.
Caudal to the eyes at the sides of the head are four branchial arches separated by
three grooves, the branchial clefts. The fourth arch is partly concealed in a tri-
89
go THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
angular depression, the cervical sinus, formed by the more rapid growth of the first
and second arches (cf. Fig. 97). The first, or mandibular arch, forks ventrally
into two processes, a smaller maxillary and a larger mandibular process, and the
latter with its fellow forms the mandible or lower jaw. The position of the
mouth is indicated by the cleft between these processes. The groove between the
eye and the mouth is the lacrimal groove.
The second or hyoid arch is separated from the mandibular arch by a hyo-
mandibular cleft which persists as the external auditory meatus. About the dorsal
end of the cleft develops the external ear.
Maxillary process Mandibular process
\
Branchial arch 2
Coppa Branchial arch 3
Cervical sinus
Atrium of heart
Olfactory pil —— ; A
c iy \ a, Liver
ey 3
4h,
Yolk suey are ha 4
os Sn fr ia
\ a
LU pper limb bud
a)
Fic. 93.—Pig embryo of 6 mm., viewed from the left side. The amnion has been removed and its cut
edge is shown in the figure. X 12.
The heart is large and through the transparent body wall may be seen the
dorsal atrium and ventral ventricle. Caudal to the heart a convexity indicates
the position of the diver. Dorsal to the liver is the bud of the anterior extremity,
now larger than in the 4mm. human embryo. Extending caudal to the anlage
of the upper extremity a curved convexity indicates the position of the left”
mesonephros. At its caudal end is the bud of the lower limb. The amnion has
been dissected away along the line of its attachment ventral to the mesonephros.
There is as yet no distinct umbilical cord and a portion of the body stalk is
attached to the embryo.
Due to a shorter term of development, a young pig embryo is somewhat pre-
THE ANATOMY OF A SIX MM. PIG EMBRYO gt
cociously developed in comparison with a human embryo ot the same size (Fig.
94). Ina human embryo 7 mm. long the head is larger, the tail shorter. The
Myelence phalon
Ugg AIS HD ya “= Metencephalon
S . ( al
Spinal cord— tN SOx d 1 Y
$
Fire
Cervical segment §
E WOH,
yor
Future milk line
Yolk sac ana
umbilical cord
Lumbar segment 5
Fic. 94.—A human embryo 7 mm. long, viewed from the right side (Mall in Kollmann).
xX 14.
I, II, IIT, Branchial arches 1, 2, and 3; H, Ht, heart; L, liver; L’, otic vesicle; R, olfactory placode; Tr,
semilunar ganglion of trigeminal nerve.
cervical flexure is more marked, the olfactory pits larger and deeper. ‘The liver
is more prominent than in the 6 mm. pig, the mesonephros and segments less so.
DISSECTIONS OF THE VISCERA
To understand the sectional anatomy of an embryo, a study of dissections
and reconstructions is essential. For methods of dissection see p. 137, Chapter
VI. Before studying sections, the student should become as well acquainted
as possible with the anatomy of the embryo and compare each section with the
figures of reconstructions and dissections.
Nervous System.—Fig. 95 shows the central nervous system and viscera
exposed on the right side of a 5.5 mm. embryo. The ventro-lateral wall of the
head has been left intact with the lens cavity, olfactory pit, and portions of the
Qg2 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
maxillary and mandibular processes, second and third branchial arches, and
cervical sinus (cf. Fig. 93). The brain is differentiated into the five regions,
telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.
The spinal cord is cylindrical and gradually tapers off to the tail. The anlages
of the cerebral and spinal ganglia and the main nerve trunks are shown. The
oculomotor nerve begins to appear from the ventral wall of the mesencephalon.
Sup. gang.n.9g Oltocyst
Acoustic ganglion
Jugular gang. n. 10 Geniculate gang. n.7
Gang. nodosum n. 10
N. accessorius Semilunar gang. n. 5
Melencephalon
Gang. cerv. I hi i - aa ) / Qo tresorceptaton
N. hypoglossus Lif
Cervical sinus
Diencephalon
Petrosal gang.
vn. 0
Lens opening
: Olfactory pit
a RS actor pt
Dorsal lobe liver— ee Paget si ee a4 Telencephalon
s , . y : i = NR!
—_ Yolk sac
t } PN
Ventral lobe liver —
Allantots
Veniricle
Small intestine NS :
Allantoic stalk
Hind-gut
Fic. 95.—Dissection of a 5.5 mm. pig embryo, showing the nervous system and viscera from the right
side. X 18.
Ventro-lateral to the metencephalon and myelencephalon occur in order the
semilunar ganglion and three branches of the trigeminal nerve; the geniculate
ganglion and nerve trunk of the n. facialis; the ganglionic anlage of the ”. acusticus
and the ofocyst. It will be observed that the nerve trunks are arranged with
reference to the branchial arches and clefts. Caudal to the otocyst'a continuous.
chain of cells extends lateral to the neural tube into the tail region. Cellular
THE ANATOMY OF A SIX MM. PIG EMBRYO 93
enlargements along this neural crest represent developing cerebral and spinal
ganglia. They are in order the superior or root ganglion of the glossopharyngeal
nerve with its distal petrosal ganglion; the ganglion jugulare and distal ganglion
nodosum of the vagus nerve; the ganglionic crest and the proximal portion of the
spinal accessory nerve; and the anlage of Froriep’s ganglion, an enlargement on the
neural crest just cranial to the first cervical ganglion. Between the vagus
and Froriep’s ganglion may be seen the numerous root fascicles of the hypoglossal
nerve, which take their origin along the ventro-lateral wall of the myelencephalon
and unite to form a single trunk. The posterior roots of the spinal ganglia are
very short; their anterior or ventral roots are not shown.
The position of the heart with its ventricle, atrium, and sinus venosus are
shown. The diver is divided into a small dorsal and a large ventral lobe. The
fore-gut emerges from between the liver lobes and curves ventrad to the yolk
stalk and sac. The hind-gut is partly hidden by the fore-gut; it make a U-
shaped bend from the yolk stalk to the caudal region. The gut is attached to
the dorsal body wall by a double layer of splanchnic mesoderm which forms the
mesentery. ‘The long, slender mesonephros lies ventral to the spinal cord and
curves caudad from a point opposite the eighth cervical ganglion to the tail re-
gion. The cranial third of the mesonephros is widest and its size diminishes
tailwards. Between the yolk sac and the tail the allantois is seen, its stalk
curving around from the ventral side of the tail region.
Digestive Canal.—The arrangement of the viscera may be seen in median
sagittal and ventral dissections (Figs. 96 and 97), also in the reconstruction
shown in Fig. 105. The mouth lies between the mandible, the median fronto-
nasal process of the head, and the maxillary processes at the sides. The divertic-
ulum of the hypophysis (Rathke’s pocket), flattened cephalo-caudad and expanded
laterad, extends along the ventral wall of the fore-brain (Fig. 105). Near its
distal end, the wall of the brain is thickened and later the posterior lobe of the
hypophysis will develop from the brain wall at this point.
The pharynx is flattened dorso-ventrally and is widest near the mouth. Its
lateral dimension narrows caudad, and opposite the third branchial arch it makes
an abrupt bend, a bend which corresponds to the cervical flexure of the embryo’s
body (Figs. 104 and 105). In the roof of the pharynx, just caudal to Rathke’s
pocket, is the somewhat cone-shaped pouch known as Seessel’s pocket, which may
be interpreted as the blind cephalic end of the fore-gut. The lateral and ven-
tral walls of the pharynx and oral cavity are shown in Fig. 98. Of the four arches
the mandibular is the largest and a groove partly separates the processes of the
94 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
two sides. Posterior to this groove and extending in the median line to the
hyoid arch is a triangular rounded elevation, the tuberculum impar, which later
forms a small part of the tongue. At an earlier stage the median thyreoid anlage
grows out from the mid-ventral wall of the pharynx just caudal to the tuberculum
impar. The ventral ends of the second arch fuse in the mid-ventral line and
form a prominence, the copula. This connects the tuberculum impar with a
Pharynx Neuromere 4 Rathke’s pocket
Anlage of
tongue
I. sthmus
AMesencephalon
R. atrium Se
Esophagus —~__
Diencephalon
Interatrial ( ; Ma\ 5,’ ne , — Bulbus cordis
foramen | a aa a
Lung bud | aa ae) Os Be ~~ ——— Telencephalon
Stomach MBNA! \t ;
Ventricle
Hepatic
~~ Septum trans-
diverticulum
versumt
Ventral Liver
ancreas
P Yolk sac
Allantois
Cranial limb 3
intestine j Poa gut
Cloaca
L. genital fold Metanephros
Spinal cord os Caudal limb of intestine
R. mesone phros Mesonephric duct
Fic. 96.—Median sagittal dissection of a pig embryo of 6 mm., to show viscera and neural tube. X 18.
rounded tubercle derived from the third and fourth pairs of arches, the anlage
of the epiglottis. Its cephalic portion forms the root of the tongue (Fig. 156).
Caudal to the epiglottis are the arytenoid ridges, and a slit between them, the
glottis, leads into the trachea.
The branchial arches converge caudad and the pharynx narrows rapidly
before it is differentiated into the trachea and esophagus (Figs. 104 and 105).
THE ANATOMY OF A SIX MM. PIG EMBRYO 95
Laterally and ventrally. between the arches are the four paired outpocketings of
the pharyngeal pouches. The pouches have each a dorsal and ventral divertic-
ulum (Fox, Thyng). The dorsal diverticula are large and wing-like (Fig. 104);
they meet the ectoderm of the gill clefts and fuse with it to form the closing plates.
Between the ventral diverticula of the third pair of pouches lies the median thy-
reoid anlage. The fourth pouch is smaller than the others. Its dorsal diverticu-
a ronto-nasal process
Olfactory pit
Maxillary process
Mandibular process
Mouth
Branchial arch 2
Branchial arch 3 Aortic bulb
Branchial arch 4
Upper limb bud
Hepatic diverticulum
Yolk sac
Body stalk
Allantois
Umbilical arlery ,__———“er aN
VP wy Lower limb bud
Mesonephric duct eas ee
Dorsal aorta and umbilical artery
Rectum
Fic. 97.—Ventral dissection of a 6 mm. pig embryo. X 14. The head has been bent dorsally.
lum just meets the ectoderm; its ventral portion is small, tubular in form, and is
directed parallel to the esophagus (Fig. 104).
The groove on the floor of the pharynx caudal to the epiglottis is continu-
ous with the tracheal groove. More caudally, opposite the atrium of the heart,
the trachea has separated from the esophagus (Fig. 96). The trachea at once
bifurcates to form the primary bronchi and the anlages of the lungs (Fig. 97).
The lungs consist merely of the dilated ends of the bronchi surrounded by a layer
of splanchnic mesoderm. They bud out laterally on each side of the esophagus
96 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
near the cardiac end of the stomach, and project into the pleural cwlom. The
esophagus is short and widens dorso-ventrally to form the stomach. The long
axis of the stomach is nearly straight, but its entodermal walls are flattened
together and it has revolved on its long axis so that its dorsal border lies to the
left, its ventral border to the right, as seen in transverse section (Fig. 111).
Caudal to the pyloric end of the stomach, and to its right, is given off from
the duodenum the hepatic diverticulum. Its opening into the gut is seen in the
ventral dissection (Fig. 97). The hepatic diverticulum is a sac of elongated oval
form from which the liver and part of the pancreas take origin, and which later
gives rise to the gall bladder, cystic duct, and common bile « duct. ti is connected
by several cords of cells with the trabeculz of the liver.
The liver is divided incompletely into four lobes, a small dorsal and a large
ventral lobe on each side (Figs. 95 and 112). The lobation does not show in a
Branchial arch I
‘
f
Lateral lingual anlage \ ye Y
f
Tuberculum impar —~———_ > Branchial arch II
Branchial arch III
Branchial arch IV
Glottis
Epiglottis
Arylenoid ridge
Fic. 98.—Dissection of the tongue and branchial arches of a 7 mm. pig embryo, seen in dorsal view. X 15.
median sagittal section. The pancreas is represented by two outgrowths. The
ventral pancreas originates from the hepatic diverticulum near its attachment
to the duodenum (Fig. 96). It_grows to the right of the duodenum and-ventral
to the portal vein. The dorsal pancreas takes origin from the dorsal side of the
duodenum caudal tothe hepatic diverticulum and grows dorsally into the sub-
stance of the gastric mesentery (Figs. 105 and 113). It is larger than the v entral
pancreas, and its posterior lobules grow to the right and dorsal to the portal vein
and in later stages anastomose with the lobules of the ventral pancreas.
The intestine of both fore-gut and hind-gut has elongated and curves ven-
trally into the short umbilical cord where the yolk stalk has narrowed at its point
of attachment to the gut (Fig. 96). As the intestinal tube grows ventrad, the
layers of splanchnic mesoderm which attach it to the dorsal body wall grow at an
equal rate and persist as the mesentery.
THE ANATOMY OF A SIX MM. PIG EMBRYO 97
The cloaca, a dorso-ventrally expanded portion of the hind-gut, gives off
cephalad and ventrad the allantoic stalk. This is at first a narrow tube, but soon
expands into a vesicle of-large size, a portion of which is seen in Fig. 95. Dorso-
laterad the cloaca receives the primary excretory (mesonephric) ducts. The hind-
gut is continued into the tail as the tail gut (postanal gut) which dilates at its ex-
tremity as in the 7.8 mm. pig described by Thyng. The mid-ventral wall of the
cloaca is fused to the adjacent ectoderm to form the cloacal membrane. In this
region later the anus arises (Fig. 105). The postanal gut soon disappears.
Urogenital System.—This consists of the mesonephroi, the mesonephric (Wolf-
fian) ducts, the anlages of the metanephrot, the cloaca, and the allantois. The form
of the mesonephroi is seen in Figs. 95 and97. Each consists of large vascular glom-
eruli associated with coiled tubules lined with cuboidal epithelium and opening
into the mesonephric duct (Figs. 114 and 208). The Wolffian ducts, beginning
at the anterior end of the mesonephros,
curve at first along its ventral, then along Bulbus cordis
its lateral surface. At its caudal end AGL. atrium
each duct bends ventrad and to the mid-
line where it opens into a lateral expansion
of the cloaca (Fig. 96). Before this junc-
tion takes place, an evagination into the
'L. ventricle
mesenchyme from the dorsal wall of each Fic. 99.—Ventral and cranial surface of the
mesonephric duct gives rise to the anlages heart from a 6 mm. pig embryo. xX 14.
of the metanephroi, or permanent kid-
neys. A slight thickening of the mesothelium along the median and ventral sur-
face of each mesonephros forms a light-colored area, the genital fold (Fig. 96).
This area is pointed at either end and confined to the middle third of the kidney.
It is the anlage of the genital gland from which either testis or ovary is developed.
Blood Vascular System.—The /eart lies in the pericardial cavity, as seen in
Fig. 96. The atrial region (Fig. 99), as in the 4.2 mm. human embryo, has given
rise to two lateral sacs, the right and left atria. The bulbo-ventricular loop has
become differentiated into right and left ventricles much thicker walled than
the atria. The right ventricle is the smaller and from it the bulbus passes be-
tween the atria and is continued as the ventral aorta. Viewed from the caudal
and dorsal aspect (Fig. 100) the szus venosus is seen dorsal to the atria. It opens
into the right atrrum and receives from the right side the right common cardinal
vein, from the left side the left common cardinal. These veins drain the blood
from the body of the embryo. Caudally the sinus’ venosus receives the two
7
98 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
vitelline veins. Of these, the left is small in the liver and later disappears.
The right vitelline vein, now the common hepatic, carries most of the blood to
the heart from the umbilical veins, and from the liver sinusoids, gut, and
yolk sac.
Transverse sections of the embryo through the four chambers of the heart
show the atria in communication with the ventricles through the atrie-ventricular
ee ( See 7 — Bulbus cordis
See
A. comin Se es y, XS R. atrium
cardinal vein / ¥ OG fe \
Soe. %
L. common i = in .
cardinal vein “S28 y
L. vitelline vein —+—- 3 —R. vilelline vein
}
Left ventricle —— ——R, ventricle
Fic. 100—Dorsal and caudal view of the heart from a 6 mm. pig embryo. X 21.
foramina, and the sinus venosus opening into the right atrium (Fig. 109). This
opening is guarded by the right and left valves of the sinus venosus. Septa in-
completely separate the two atria and the two ventricles. In Fig. 109 the atrial
septum (sepium primum) appears complete due to the plane of the section.
In Fig. 101, from a slightly smaller embryo, it is seen that the sepium primum
grows from the dorsal atrial wall
Bulbus cordis eee of the heart and does not yet
s<—loramen ovale 7
Wall of 1. atrium meet the endocardial cushions be-
“1 nteratrial oramen : :
Z oe cushions tween the atrio-ventricular canals.
R. ventricle-fy
Interventricular¢
foramen This opening between the atria is
Wall of 1. ventricle . ;
known as the iuteratrial foramen.
Before it closes, another opening
Fic. 101.—Dissection of a 5.5 mm. pig’s heart appears in the septum, dorsal in
from the left side, showing the septum primum and dae ae at
the interatrial and oval foramina. X 14. position. This is the foramen ovale
and persists during fetal life. In
Fig. 101 these two openings may be seen, as may also the dorsal and ventral
endocardial cushions which bound the atrio-ventricular foramina. The outer
mesothelial layer of the ventricles has become much thicker than that of the
atria. It forms the epicardium and the myocardium, the sponge-like meshes of
which are now being developed.
THE ANATOMY OF A SIX MM. PIG EMBRYO 99
The Arteries.—These begin with the ventral aorta, which takes origin from the
bulbus cordis. F ‘rom the ventral aorta are given off five pairs of aortic arches.
These run dorsad in the five branchial arches (Figs. 104 and 105) and join the
paired dorsal or descending aorte. The first and second pairs of aortic arches are
very small and take origin from the small common trunks formed by the bifurca-
tion of the ventral aorta just caudal to the median thyreoid gland. The fourth
aortic arch is the largest. From the fifth arch small pulmonary arteries are devel-
oping. There is evidence that this pulmonary arch is really the sixth in the
series, the fifth having been suppressed in development (cf. Fig. 272 B). Cranial
to the first pair of aortic arches, the descending aorte are continued forward
into the maxillary processes as the internal carotids. Caudal to the aortic arches
the descending aorta « converge, unite opposite the cardiac end of the stomach,
and form the median dorsal aorta. From this vessel and from the descending
aorte paired dorsal intersegmental arteries arise. From the seventh pair of these
arteries (the first pair to arise from the medial dorsal aorta) there are developed
a pair of lateral branches to the upper limb buds. These vessels are the sub-
clavian arteries. From the dorsal aorta there are also given off ventro-lateral
arteries to the glomeruli of the mesonephros, and median ventral arteries. Of the
latter the celiac artery arises opposite the origin of the hepatic diverticulum. The
vitelline artery takes origin by two or three trunks caudal to the dorsal pancreas.
Of Of these trunks the posterior is the larger and persists as the superior mesenteric
artery. Thyng (Anat. Record, vol. 5, 1911) has figured three trunks of origin in
the 7.8 mm. pig. These unite and the single vitelline artery branches in the wall
of the yolk sac.
Opposite the lower limb buds the dorsal aorta is divided for a short distance.
From each division there arises laterad ‘three short trunks which unite to form the
single umbilical artery on each side. The middle vessel is the largest and appa-
rently becomes the common iliac artery. A pair of short caudal arteries, much
smaller in size, continue the descending aorte into the tail region.
The Veins.—The vitelline veins, originally paired throughout, are now repre-
sented distally by a single vessel, which, arising in the wall of the yolk sac, enters
the embryo and courses cephalad of the intestinal loop (Fig. 102). Crossing to
the left side of the intestine and ventral to it, it is joined by the superior mesenteric
vein which has developed in the mesentery of the intestinal loop. The trunk
formed by the union of these two vessels becomes the portal vein. It passes along
the left side of the gut in the mesentery. Opposite the origin of the dorsal pan-
creas it gives off a small branch, a rudimentary continuation of the left vitelline
100 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
vein, which courses cephalad and in earlier stages connects with the sinusoids
of the liver. The portal vein then bends sharply to the right, dorsal to the duo-
denum, and, in the course of the right vitelline vein, passing between the dorsal
and ventral pancreas to the right of the duodenum, it soon enters the liver and
connects with the liver sinusoids. The portal trunk is thus formed by persisting
portions of both vitelline veins, and receives a new vessel, the superior mesenteric
vein. The middle portions of the primitive vitelline veins are connected with the
Spinal cord Nolochord
Ant. cardinal
vein
Pharynx
Ant. cardinal vein
Cervical sinus
a < . ovr carvAly,
Pericardial cavily- Pericardial
Y cavily
Left common
cardinal vein
Left horn of sinus venosus
Atrial junction sinus venosus
Sinus venosus
Right vitelline vein
Liver
Large venous sinusoid of liver
Left vilelline vein
Duclus venosus
Ant. limb bud
Hepatic diverticulum (cut) Inf. vena cava
Volk stalk
Dorsal pancreas
Portal vein
Cephalic limb
intestinal loop
Right umbilical vein
Left vitelline vein
Common vitelline vein
Left umbilical vein
Vitelline artery
: Sup. mesenteric vein
Caudal limb
intestinal loop
: ea Le ft bili ti
Right umbilical artery ane
Post. limb bud
Dorsal aorta Spinal cord
Fic. 102.—Reconstruction in ventral view of a 6 mm. pig embryo to show the vitelline and umbilical
veins, the latter opened (original drawing by Dr. K. L. Vehe). > 22. In the small orientation figure
(cf. Fig. 105) the various planes are indicated by broken lines—*------- --*, e
network of liver sinusoids. Their proximal vitelline trunks drain the blood from
the liver and open into the sinus venosus of the heart. The right member of this
pair is much the larger (Fig. 100) and persists as the proximal portion of the infe-
rior vena cava. For the development of the portal vein see Chapter IX.
The umbilical veins, taking their origin in the walls of the chorion and allan-
toic vesicle, lie caudal and lateral to the allantoic stalk and anastomose (Figs.
102 and 105). Before the allantoic stalk enters the body, the umbilical veins sepa-
THE ANATOMY OF A SIX MM. PIG EMBRYO IOl
rate and run lateral to the umbilical arteries. The left vein is much the larger.
Both, after receiving. branches from the posterior limb buds and from the body
wall, pass cephalad in the somatopleure at each side (Fig. 72). Their course is
first cephalad, then dorsad, until they enter the liver. The left vein enters a wide
channel, the ductus venosus, which carries its blood through the liver, thence to
the heart by way of the right vitelline trunk. The right vein joins a large sinu-
soidal continuation of the portal vein in the liver. This common trunk drains
into the ductus venosus.
Spinal cord
Anterior cardinal vein
Notochord
Cervical sinus
Pericardial cavity Evachea
R. common cardinal vein L. common
cardinal vein
Lung
Post. cardinal vein
Eso phagus
be live e
Large venous sinusoid liver
Stomach (cut edge)
rior limb bud
Anterior limb bu Omental bursa
Inf. vena cava :
. ; Mesogastrium
Post. cardinal vein
Mesonephros (cut surface) Mesonephros (cut surface)
Capillary anastomosis between
subcardinal veins
Vitelline artery in dorsal
mesentery
Capillary anastomosis between
subcardinal veins
R. subcardinal vein
Venous sinusoid on dorsum of
mesonephros
Venous sinusoid on dorsum of
mesonephros
Spinal cord
Fic. 103.—Reconstruction of the cardinal and subcardinal veins of a 6 mm. pig embryo showing the
early development of the inferior vena cava (Kk. L. Vehe). X 22. In the small orientation figure (cf.
Fig. 105) the various planes are indicated by broken lines—*---------*.
The anterior cardinal veins (Figs. 103 and 104) are formed to drain the plexus
of veins on each side of the head. These vessels extend caudad and lie lateral to
the ventral portion of the myelencephalon. Each anterior cardinal vein receives
branches from the sides of the myelencephalon, then curves ventrad, is joined by
the linguo-facial vein from the branchial arches and at once unites with the pos-
terior cardinal of the same side to form the common cardinal vein. This, as we
have seen, opens into the sinus venosus.
102 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
The posterior cardinal veins develop on each side in the mesonephric ridge,
dorso-lateral to the mesonephros (Figs. 103, 104 and 112). Running cephalad,
they join the anterior cardinal veins. When the mesonephroi become prominent,
as at this stage, the middle third of each posterior cardinal is broken up into sinu-
soids (Minot). Sinusoids extend from the posterior cardinal vein ventrally
Int. car- Ant. car-
Metencephalon otid artery dinal vein
Thyreoid Ph. Pe
ss M yelencephalon
PUP Eg
Mesence phalon
Notochord
Ventral aorla ‘f Descending aorta
Ore | i : a — Se PhP
Pulmonary artery
Linguo-facial
vein
R. com.
Nasal pil cardinal vein
R. alrium
R. ventricle
Sinus venosus
Com. hepatic
vein
Post. car-
dinal vein
R. subclavian
vein
R. subcar-
dinal vein
Pulmonary
vein
Intersegmental
vein
Spinal cord
JLesone phros
Post. cardinal vein Mesonephric arteries
Fic. 104.—Reconstruction of 7.8 mm. pig embryo showing veins and aortic arches from the left side
(after Thyng). X15. Ph. P. 1, 2, 3, 4, Pharyngeal pouches.
around both the lateral and medial surfaces of the mesonephros. The median
sinusoids anastomose longitudinally and form the subcardinal veins, right and left.
The subcardinals lie along the median surfaces of the mesonephroi, more ventrad
than the posterior cardinals with which they are connected at either end. There
is a transverse capillary anastomosis between them, cranial and caudal to the
THE ANATOMY OF A SIX MM. PIG EMBRYO 103
permanent trunk of the vitelline artery (Fig. 103). The right subcardinal is
connected with the liver sinusoids through a small vein which develops in the
mesenchyme of the plica ven cavee (caval mesentery) located to the right of the
mesentery (Fig. 112). This vein now carries blood direct to the heart from the
Aortic arch 1 Seessel’s pocket
Aortic arch 2 ja sth mus
Pharynx j ;
Thyreoid
Aortic arch 3 <<
w
oo
Int. carotid artery
106
Hypophysis (pharyngeal »
Jobe)
— AO RG FECES
107
~ Telencephalon
, Ventral aorta
+— 1085
we Bulbus cordis
Y— 109 :
I nterventricular foramen
Aortic arch 4 —4
I07
Aortic arch 5 and_{
pulmonary artery 7]
I08
Esophagus
LOY Linh
100
rach Bi ;
Trachea He sy —-— L. horn of sinus venosus
110,_} | “fp— TIO L. umbilical vein
R. ling ate hr ae ee Tail gut
III—} I ——_ ae
| Bb Volk sac (cut) 2s. _Cloaca
| ose ru ie
- PK ES 112
. Ii2 YN Spinal cord
Stomach + ee)
113 4 17
¥ 4 Hee oes
4 —~ Metanephric anlage
Jee
7 Pi L. umbilical artery
f~ -
Anastomosis between
dorsal aorle
Allantoic stalk
Ventral pancreas —®
Dorsal Cee
Gall bladder
L. umbilical vein L. dorsal aorta
Vitelline artery
Mesonephric duct
Cephalic limb of intestinal loop
Artery to mesonephros
Dorsal aorta
Mesentery Caudal limb of intestinal loop
Fic. 105.—Reconstruction of a 6 mm. pig embryo in the median sagittal plane, viewed from the
The numbered heavy lines indicate the levels of the transverse sections shown in Figs.
right side.
106-117. The broken lines indicate the outline of the left mesonephros and the course of the left um-
The latter may be traced from the umbilical cord to the liver where it is sec-
bilical artery and vein.
tioned longitudinally. (Original drawing and reconstruction by Dr. K. L. Vehe). x 16.5.
right posterior cardinal and right subcardinal, by way of the liver sinusoids and the
right vitelline trunk (common hepatic vein). Eventually the unpaired inferior
(For the development of the
vena cava forms in the course of these four vessels.
inferior vena cava see Chapter IX.)
104 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO
Aaving acquainted himself with the anatomy of the embryo from the study
or dissections and reconstructions, the student should examine serial sections cut
in the plane indicated by guide lines on Fig. 105. Refer back to the external
structure of the embryo (Fig. 93), to the lateral dissection of the organs (Fig.
95), and having determined the exact plane of each section, interpret the struc-
_ tures seen by comparing with Fig. 105. The various structures may be recog-
nized by referring to the figures of sections in the text, and they should be traced
section by section through the series as carefully as time will allow. Remember
that the sections of pig embryos figured here are drawn from the cephalic surface,
so that the right side of the section is the left side of the embryo.
Transverse Section through the Myelencephalon and Otocysts of a6 mm. Embryo
(Fig. 106).—As the head is bent nearly at right angles to the body, this section passes length-
Fourth ventricle Myelencephalon
Neur. 6
Gang. superior n. 9
Ant. cardinal vein
#-Gang. acust.n. 8
Neur. 3 ie Gang. geniculat, n. 7
Neur. 2- fg ;
Neur. 1-* "
Gang. semilunar. n. 5
Ant. cardinal vein
Int. carotid artery
‘Ant. cardinal vein
Dience phalon
Fic. 106.—Transverse section through the myelencephalon and otocysts of a 6 mm. pig embryo.
X 26.5. Gang. acust. n. 8, acoustic ganglion of acoustic nerve, etc.; Newr. 1-6, neuromeres 1-6.
wise through the myelencephalon. The diencephalon is cut transversely. The cellular
walls of the myelencephalon show a series of six pairs of constrictions, the neuromeres. Lateral
to the fourth pair of neuromeres are the otocysts, which show a median outpocketing at the
point of entrance of the endolymph duct. The ganglia of the nn. trigeminus, facialis, acus-
ticus, and the superior ganglion of the glossopharyngeal nerve occur in order on each side.
Sections of the anterior cardinal vein and its branches show on the left side. Ventral to the
diencephalon are sections of the internal carotid arteries.
TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO I05
Passing along down the series into the pharynx region, observe the first, second, and
third pharyngeal pouches. Their dorsal diverticula come into contact with the ectoderm of the
branchial clefts and form the closing plates.
Transverse Section through the Branchial Arches and the Eyes (Fig. 107).—
The section passes lengthwise through the four branchial arches, the fourth sunken in the
cervical sinus. Dorsad is the spinal cord with the first pair of cervical ganglia. The pharynx
is cut across between the third and fourth branchial pouches. In its floor is a prominence,
the anlage of the epiglottis. Ventral to the pharynx the ventral aorta gives off two pairs of
vessels. The larger pair are the fourth aortic arches which curve dorsad around the pharynx
to enter the descending aorte. The smaller third aortic arches enter the third branchial arches
on each side. A few sections higher up in the series the ventral aorta bifurcates and the right
Neural tube
Myotome
Notochord
Ant. cardinal vein ~ Descending aorta
Pharynx “Branchial arch 4
Pharyngeal pouch 3. Branchial arch 3
Aortic arch
ros Branchial arch 2
Pharyngeal pouch 2
Lens of eye
Optic vesicle
Diencephalon
Fic. 107.—Transverse section through the branchial arches and eyes of a 6 mm. pig embryo.
X 26.5. X, aortic arch 4.
and left trunks thus formed give off the first and second pair of aortic arches. Cranially in
the angle between their common trunks lies the median thyreoid anlage. The anterior cardinal
veins are located lateral and dorsal to the descending aorte. The end of the head is cut
through the diencephalon and the optic vesicles. On the left side of the figure the lens vesicle
may be seen still connected with the ectoderm. The optic vesicle now shows a thick inner, and
a thin outer layer; these form the nervous and pigment layers of the retina respectively.
Transverse Section through the Tracheal Groove, Bulbus Cordis and Olfactory
Pits (Fig. 108).—The ventral portion of the figure shows a section through the tip of the
head. The telencephalon is not prominent. The ectoderm is thickened and slightly invagi-
nated ventro-laterad to form the anlages of the olfactory pits. These deepen in later stages
and become the nasal cavities. In the dorsal portion of the section may be seen the cervical
106 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
Spinal cord
Notochord
Ant. cardinal vein
Somato pleure
Olfactory pil
Fic. 108.—Transverse section through the bulbus cordis and olfactory pits of a 6 mm. pig embryo.
X 26.5.
Myotome
L. common cardinal vein
Descending aorla—f 4
pret
Trachea
Sinus venosus
Valve of sinus venosus L. atrium
R. atrium -Seplum I
' , LL. ventricle
Atrio-ventricular foramen 2
FI nterventricular foramen
Interventricular sc plum
Pericardial cavity
R. ventricle
Somatopleure
Fic. 109.—Transverse section through the four chambers of the heart of a6 mm. pig embryo. X 26.5.
TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO 107
portion of the spinal cord, the notochord just ventral to it, the descending aorta, and ventro-
lateral to them the anterior cardinal veins. The nasopharynx now is small with a vertical
groove inits floor. This is the tracheal groove and more caudad it will become the cavity of
the trachea. The bulbus cordis lies in the large pericardial cavity. On either side the section
cuts through the cephalic portions of the atria. These will become larger as we go caudad in
the series.
Transverse Section through the Heart (Fig. 109).—Lateral to the descending aorte
are the common cardinal veins. The right common cardinal opens into the sinus venosus
which in turn empties into the right atrium, its opening being guarded by the two valves of
the sinus venosus. The entrance of the left common cardinal into the sinus venosus is some-
what more caudad in the series. The ¢rachea has now separated from the esophagus and lies
ventraltoit. Both trachea and esophagus are surrounded by a condensation of mesenchyme.
‘The myocardium of the ventricles has formed a spongy layer much thicker than that of the
Spinal cord
Upper limb bud
Descending aorta Post. cardinal vein
Pleuro-peritoneal cavity
R. lung bud
Esophagus
Dorsal lobe of liver
R. vitelline vein
Lesser sac
eptum transversum ei a A
Septum transv L. vitelline vein
Pericardial cavity
L. ventricle
Fic. 110.—Transverse section through the right lung bud and septum transversum of a 6 mm. pig
embryo. X 26.5.
atrial wall. An incomplete interventricular septum leaves the ventricles in communication
dorsad. The septum primum is complete in this section, but higher up in the series there is an
interatrial foramen (cf. Fig. 101). The foramen ovale is hardly formed.
Transverse Section through the Lung Buds and Septum Transversum (Fig.
110).—The section passes through the bases of the upper limb buds. The tips of the ventricles
lying in the pericardial cavity still show in this section. Dorsally the pericardial cavity has
given place to the pleuro-peritoneal cavity. Projecting ventrad into this cavity are the meso-
nephric (Wolffian) folds in which the posterior cardinal veins partly lie. Into the floor of the
pleuro-peritoneal cavities bulge the dorsal lobes of the liver, embedded in mesenchyma.
This mesenchyma is continuous with that of the somatopleure, and forms a complete trans-
verse septum ventrally between the liver and heart. This is the septum transversum which
takes part in forming the ligaments of the liver and is the anlage of a portion of the diaphragm.
The two proximal trunks of the vitelline veins pass through the septum. Projecting laterally
~
108 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
into the pleuro-peritoneal cavities are ridges of mesencyhma. covered by splanchnic mesoderm
in which the lungs develop as lateral buds from the caudal end of the trachea. The right
lung bud is shown in the figure. Between ‘the esophagus and the lung is a crescent-shaped
cavity, the cranial end of the lesser peritoneal sac.
Transverse Section through the Stomach (Fig. 111).—The section passes through
the upper limb buds and just caudal to the point at which the descending aorte unite to form
the median dorsal aorta. As the liver develops in early stages, it comes into relation with the
plica vene cave along the dorsal body wall at the right side of the dorsal mesogastrium. The
space between the liver and plica to the right, and the stomach and its omenta to the left, is
a caudal continuation of the lesser peritoneal sac. The dorsal wall of the stomach is rotated
to the left, its ventral wall to the right. The liver shows a pair of dorsal lobes and contains
large blood spaces and networks of sinusoids lined with endothelium. Ventral to the liver,
the tips of the ventricles are seen.
: Spinal cord
Spinal ganglion
— Spinal nerve
Notochord
Dorsal aorta Post. cardinal vein
Upper limb bud
Peritoneal cavily
Lesser sac - Stomach
Common hepatic vein
(R. vilelline)
L. ventral lobe of liver
R, ventral lobe liver
R. ventricle L. ventricle
Fic. 111.—Transverse section through the stomach of a 6 mm. pig embryo. X 26.5.
Transverse Section through the Hepatic Diverticulum (Fig. 112).—The upper
limb buds are prominent in this section. The mesonephric folds show the tubules and glom-
eruli of the mesonephroi and the posterior cardinal veins are connected with the mesonephric
sinusoids. From the dorsal attachment of the liver there is continued down into this section
a ridge on the dorsal body wall just to the right (left in figure) of the mesentery. In this ridge
lies a small vein which connects cranially with the liver sinusoids, caudally with the right
subcardinal vein. As it later forms a portion of the inferior vena cava, the ridge in which it
lies is termed the plica vene cave or caval mesentery. The right dorsal lobe of the liver contains
a large blood space into which the portal vein opens. The duodenum is ventral to the position
occupied by the stomach in the previous section. There is given off from it ventrad and to
the right the /epatic diverticulum. In the sections higher up small ducts from the liver tra-
becule may be traced into connection with it. In the left ventral lobe of the liver, a large
blood space indicates the position of the eft umbilical vein on its way to the ductus venosus.
Transverse Section through the Dorsal Pancreas (Fig. 113).—At this level the
upper limb buds still show; the mesonephroi are larger and marked by their large glomeruli.
TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO 10g
The right posterior cardinal vein is broken up into mesonephric sinusoids. The vein in the
plica venz cave will, a few sections lower, connect with the right subcardinal vein. The an-
lage of the dorsal pancreas is seen extending from the duodenum dorsad into the mesenchyme
Spinal cord
Notochord
Post. cardinal vein Post. cardinal vein
f
Dorsal aorta i ng
Es Upper limb bud
Inf. vena cava Dorsal mesogastrium
Portabaein Dorsal lobe liver
R. umbilical vein L. vitelline vein
Hepatic diverticulum
L. umbilical vein
Peritoneal cavily
Fic. 112.—Transverse section through the hepatic diverticulum of a 6 mm. pig embryo. X 26.5.
ofthe mesentery. It soon bifurcates into a dorsal and right lobe, of which the latter is slightly
lobulated. Ventro-lateral to the duodenum the anlage of the ventral pancreas is seen cut
across. It may be traced cephalad in the series to its origin from the hepatic diverticulum.
Myotome:
Dorsal aorta L. post. cardinal vein
R. post. cardinal vein Mesonephros
Glomerulus of mesone phros Eonar oan ee
Mesenter
Inf. vena cava é y
Dorsal pancreas
Portal vein L. vitelline vein
R. umbilical vein Duodenum
Distal end of hepatic diverticulum L. umbilical vein
Ventral pancreas
Fic. 113.—Transverse section through the dorsal pancreas of a6 mm. pig embryo. X 26.5
To the right of the ventral pancreas lies the portal vein (at this level a portion of the right
vitelline). To the left of the dorsal pancreas is seen the remains of the left vitelline vein.
The ventral lobes of the liver are just disappearing at this level. In the mesenchyme which
IIo THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
connects the liver with the ventral body wall lie on each side the umbilical veins, the left being
the larger. Between the veins is the extremity of the hepatic diverticulum. The body wall is
continued ventrad to form a short umbilical cord.
Transverse Section at the Level of Origin of the Vitelline Artery and Umbilical
Arteries (Fig. 114).—As the posterior half of the embryo is curved in the form of a half
circle, sections caudal to the liver, like this one, pass through the lower end of the body
at the level of the posterior limb buds. Two sections of the embryo are thus seen in one,
Spinal cord
Spinal nerve
Notochord
R. post. cardinal vein Post. cardinal vein
Dorsal aorta y \esonephros
+1. subcardinal vein
R. subcardinal vein
£-L. vitelline vein
Mesentery
Cephalic limb of intestine ;
L. umbilical vein
R. umbilical vein
Caudal limb of intestine
R. umbilical vein
Tail
L. umbilical vein
Lower limb bud Rectum
Mesonephric duct
Ce AMesone phric tubule
Dorsal aorta
AMesodermal segment
Spinal cord
Fic. 114.—Transverse section of a 6 mm. pig embryo at the level of the origin of the vitelline
artery. The lower end of the section passes through the posterior limb buds. X 26.5.
their ventral aspects facing each other and connected by the lateral body wall. In the dorsal
part of the section the mesonephroi are prominent with large posterior cardinal veins lying dor-
salto them. The trunk of the vifelline artery takes origin ventrally from the aorta. It may
be traced into the mesentery, and through it into the wall of the yolk sac. On either side of
the vitelline artery are the subcardinal veins, the right being the larger. In the mesentery
may be seen two sections of the intestinal loop (the small intestine being cut lengthwise, the
large intestine transversely), and also sections of the vitelline artery and veins. In the lateral
TRANSVERSE SECTIONS OF A SIX MM. PIG EMBRYO III
body walls ventral to the mesonephros occur the umbilical veins. The left vein is large and
cut lengthwise. The right vein is cut obliquely twice.
In the ventral portion of the section, the lower limb buds are prominent laterally. A
large pair of arteries, the common iliacs, are given off from the aorta and may be traced into
connection with the umbilical arteries. The large intestine supported by a short mesentery lies
in the coelom near the midline. On each side are the mesonephric folds, here small and each
showing a section of the mesonephric duct and a single vesicular anlage of the mesonephric
tubules. The mesonephric ducts are sectioned as they curve around from their position in the
dorsal portion of the section.
Section through the Primitive Segments and Spinal Cord (Fig. 115).—This sec-
tion is near the end of the series and as the body is here curved it is really a frontal section.
At the left side of the spinal cord the oval cellular masses are the spinal ganglia cut across.
The ectoderm, arching over the segments, indicates their
position. Each segment shows an outer dense layer, the cutis
plate, lying just beneath the ectoderm. This plate curves
lateral to the spindle-shaped muscle plate which gives rise to
the voluntary muscle. Next comes a diffuse mass of mesen-
chyma, the sclerotome, which eventually, with its fellow of the
Spinal ganglionstpits
Intersegmental X:
artery { = : dee PICO oa q
Muscle plateyg R. umbilical vein : NA ( J L. umbilical
artery
R. umbilical artery
Tail
. t ki es
Cutis plate L. umbilical vein
Sclerolome Allantois and cloaca
L. umbilical artery
Ectoderm i
Mesonephric duct
Spinal cord
Notochord
Fic. 115.—Transverse sec- Fic. 116.—Transverse section through the umbilical vessels, allan-
tion through the primitive seg- tois and cloaca of a6mm. pigembryo. x 45.
ments and spinal cord of a 6 mm.
pigembryo. X 45.
opposite side, surrounds the spinal cord’and forms the anlage of a vertebra. A pair of spinal
nerves and spinal ganglia are developed opposite each somite, and pairs of small vessels are
seen between the segments. These are dorsal intersegmental arteries.
Section through the Umbilical Vessels, Allantois and Cloaca (Fig. 116).—Hav-
ing now studied sections at various levels to near the end of the series we shall next examine
sections through the caudal region and. study the anlages of the urogenital organs. Owing
to the curvature of the embryo, it is necessary to go cephalad in our series. The first section
passes through the bases of the limb buds at the level where the allantoic stalk, curving in-
ward from the umbilical cord, opens into the cloaca. At either side of the allantoic stalk
may be seen oblique sections of the umbilical arteries and lateral to these the large left and
small right umbilical vein. The mesonephric ducts occupy the mesonephric ridges which
project into small caudal prolongations of the celom. Midway between the ducts lies the
hind-gut, dorsal to the cloaca. The tip of the tail is seen in section at the left of the figure.
112 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
Section through the Anlages of the Metanephroi, Cloaca and Hind-gut (Fig.
117).—The metanephroi are seen as dorsal evaginations from the mesonephric (Wolffian) ducts
just before their entrance into the cloaca. Each consists of an epithelial layer surrounded by
a condensation of mesenchyme. Traced a few sections cephalad the mesonephric ducts open
Gag £-Veniral body wall
@Al 4 — L. umbilical artery
-L. umbilical vein
Allantoic stalk
R. umbilical vein
R. umbilical artery
Mesonephric duct f :
Metanephric anlage \ +
Spinal cord
Fic. 117.—Transverse section through the anlages of the metanephroi of a 6 mm. pig embryo. X 45.
into the lateral diverticula of the cloaca, which, irregular in outline because it is sectioned
obliquely, lies ventral to them and receives dorsad the hind-gut. Caudal to the cloaca in
this embryo the tail bends abruptly cephalad and to the right. The blind prolongation of the
hind-gut may be traced out into this portion of the tail until it ends in a sac-like dilatation.
B. THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS
The study of embryos at this stage is important as they possess the anlages
of most of the organs. The anatomy of a 12 mm. pig embryo has been carefully
studied and described by Lewis (Amer. Jour. Anat., vol. 2, 1903).
External Form (Fig. 118) —The head is now relatively large on account of
the increased size of the brain. The third branchial arch is still visible in the
embryo, but the fourth arch has sunken in the cervical sinus; usually both have
disappeared at a slightly later stage. The olfactory pits form elongated grooves
on the under surface of the head and the Jens of the eye lies beneath the ectoderm
surrounded by the optic cup. The maxillary and mandibular processes of the
first branchial arch are larger and the former shows signs of fusing with the median
nasal process to form the upper jaw. Small tubercles, the anlages of the external
ear, have developed about the first branchial cleft which itself becomes the
external auditory meatus.
At the cervical bend the head is flexed at right angles with the body, bringing
the ventral surface of the head close to that of the trunk, and it is probably owing
to this flexure that the third and fourth branchial arches buckle inward to form
the cervical sinus. Dorsad, the trunk forms a long curve more marked opposite
THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 113
the posterior extremities. The reduction in the trunk flexures is due to the in-
creased size of the heart, liver, and mesonephroi. These organs may be seen
M yelencephalon
Branchial cleft 1
Hyoid Le
Cervical flexure oS ee flexure
Branchial arch 3 ~f:
Cervical sinus ,
Maxillary process
Mandibular process
Upper limb bud Olfactory pit
Milk line A
Ve" iD
Mesodermal segment
ay
va
So
e Lower limb bud
Fic. 118.—Exterior of a 10 mm. pig,embryo viewed from the right side. X 7.
through the translucent body wall and the position of the septum transversum
may be noted between the heart and the diaphragm, as in Fig. 120. The limb
Cervical flexure
External ear
Mandibular process
Upper limb bud
Mesodermal segment Yolk stalk
_ Lower limb bud Yolk sac
Fic. 119.—Exterior of a human embryo of 12 mm., viewed from the right side, showing attachment
: of amnion (cut away) and yolk stalk and sac. X 4.
buds are larger and the umbilical cord is prominent ventrad. Dorsally the meso-
dermal segments may be seen, and extending in a curve between the bases of the
8
,
II4 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
limb buds is the milk line, a thickened ridge of ectoderm which forms the anlages
of the mammary glands. The tail is long and tapering. Between its base and the
umbilical cord is the genital eminence (Fig. 120).
Human embryos of this stage or slightly older vary considerably in size
(Fig. 119). They differ from pig embryos in the greater size of the head, the
Metencephalon N. trochlearis
Mesencephalon
Gang. nm. 5 .
N. oculomotorius
Gang. nn. 7 and 8
N. facialis
Gang. superior n. 9 \
Gang. jugulare n. 10 | A
Gang. pelrosal n. 9
Dicence phalon
Gang. Froriep Ophthalmic r. n. 5
Gang. nodos. 1. 10 :
. oplicus
N. accessorius
Maxillary ron. 5
N. hypoglossus
Telencephalon
Atrium
Lung Mandibular r. n. 5
Gang. cerv. 8
Septum transversum Chorda tymp. n. 7
Livers i
“entricle
Mesonephros
Gang. thorac. 10
Umbilical cord
Genital eminence
Fic. 120.—Lateral dissection of a 10 mm. pig embryo, showing the viscera and nervous system from
the right side. The eye has been removed and the otic vesicle is represented by a broken line. The
ventral roots of the spinal nerves are not indicated. % 10.5. m., Nerve; r., ramus.
shorter tail, the much smaller mesonephric region, the longer umbilical cord, and
the less prominent segments. The yolk sac is pear-shaped and the yolk stalk is
long and slender.
Central Nervous System and Viscera.—Dissections show well the form and
relations of the organs (Figs. 120, 121 and 122). Directions for preparing dis-
sections are given in Chapter VI.
THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS II5
Brain.—Five distinct regions may be distinguished (Figs. 120 and 122):
(1) The telencephalon with its rounded lateral outgrowths, the cerebral hemispheres.
Their cavities, the Jateral ventricles, communicate by the interventricular foramina
with the third ventricle. (2) The diencephalon shows a laterally flattened cavity,
the third ventricle. Ventro-laterally from the diencephalon pass off the optic
stalks and an evagination of the mid-ventral wall is the anlage of the posterior
hypophyseal lobe. (3) The mesencephalon is undivided, but its cavity becomes the
cerebral aqueduct leading caudally into the fourth ventricle. (4) The metencephalon
is separated from the mesencephalon-by a constriction, the isthmus. Dorso-later-
Accessory gang. I
Accessory gang. 2
Acc. gang. 3 Myelencephalon
Acc. gang. 4
Gang. sup.
Gang. jugulare
Gang. Froriep —>
N.ar
Cerv. gang. I Gang. petrosum
N.9g
Cerv. gang. 2 N.1r
Gang. nodosum
N. 12
Fic. 121.—Duissection of the head of a 15 mm. pig embryo from the right side to show the accessory
vagus ganglia with peripheral roots passing to the hypoglossal nerve. X 25.
ally it becomes the cerebellum, ventrally the pons. (5) The elongated myelen-
cephalon is roofed over by a thin non-nervous ependymal layer. Its ventro-lateral
wall is thickened and still gives internal indication of the neuromeres. The cavity
of the metencephalon and myelencephalon is the fourth ventricle.
Cerebral Nerves.—Of the twelve cerebral nerves all but the first (olfactory)
and sixth (abducens) are represented in Fig. 120. For a detailed description
of these nerves see Chapter XIII. (2) The optic nerve is represented by the optic
stalk cut through in Fig. 120. (3) The oculomotor, a motor nerve to four of the eye
muscles, takes origin from the ventro-lateral wall of the mesencephalon. (4)
116 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
The trochlear nerve fibers, motor, to the superior oblique muscle of the eye, arise
from the ventral wall of the mesencephalon, turn dorsad and cross at the isthmus,
thys emerging on the opposite side. From the myelencephalon arise in order
(5) the trigeminal nerve, mixed, with its semilunar ganglion and three branches,
the ophthalmic, maxillary, and mandibular; (6) the . abducens, motor, from the
ventral wall to the external rectus muscle of the eye; (7) the 2. facialis, mixed,
with its geniculate ganglion and its chorda tympant, facial, and superficial petrosal
branches in the order named; (8) the 2. acusticus, sensory, arising cranial to the
otocyst, with its acoustic ganglion and sensory fibers to the internal ear; (9)
caudal to the otocyst the n. glossopharyngeus, mixed, with its superior and petrosal
ganglia; (10) the vagus, sensory, with its jugular and nodose ganglia; (11) accom-
panying the vagus the motor fibers of the spinal accessory which take origin
between the jugular and sixth cervical ganglia from the lateral wall of the spinal
cord and myelencephalon; the internal branch of the n. accessorius accompanies
the vagus; the external branch leaves it between the jugular and nodose ganglia
and supplies the sternocleidomastoid and trapezius muscles; (12) the ~. hypoglos-
sus, motor, arising by five or six fascicles from the ventral wall of the myelen-
cephalon; its trunk passes lateral to the nodose ganglion and supplies the muscles
of the tongue.
A nodular chain of ganglion cells extends caudad from the jugular ganglion of the vagus.
These have been interpreted as accessory vagus ganglia. They may, however, be continuous
with Froriep’s ganglion which sends sensory fibers to the n. hypoglossus. In pig embryos of
15 to 16 mm. this chain is frequently divided into four or five ganglionic masses, of which
occasionally two or three (including Froriep’s ganglion) may send fibers to the root fascicles
of the hypoglossal nerve. Such a condition is shown in Fig. 121.
Spinal Nerves.—These have each their spinal ganglion, from which the dorsal
root fibers are developed (Figs. 120 and 136). The motor fibers take origin from
the ventral cells of the neural tube and form the ventral roots which join the
dorsal roots in the nerve trunk.
In Fig. 120 the heart with its right atriwm and ventricle, the dorsal and ven-
tral lobes of the liver, and the large mesonephros are prominent. Dorsal and
somewhat caudal to the atrium is the anlage of the right lung. The sepium
transversum extends between the heart and the liver.
Pharynx and its Derivatives.—Dorsally the anterior lobe of the hypophysis
is long and forks at its end (Figs. 122 and 123). In the floor of the pharynx are
the anlages of the tongue and epiglottis (Fig. 156 A). From each mandibular arch
arises an elongated thickening which extends caudal to the second arch. Be-
THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 117
‘tween, and fused to these thickenings, is the triangular tuberculum impar. The
opening of the thyreoglossal duct between the tuberculum impar and the second
arch is early obliterated. A median ridge, or copula, between the second arches
connects the tuberculum impar with the epiglottis, which seems to develop from
the bases of the third and fourth branchial arches. On either side of the slit-like
Metencephalon., Mesencephalon
Tela choroidea
Neuromeres of myelence phalon
\
a Diencephalon
Notochord ;
Tongue xe
LG
KG
fi Post. lobe hypophysis
Spinal cord \_f/
Pf Oplic recess
Esophagus ™ j
Trachea ff
lf
Telencephalon
Ant. lobe hypophysis
Atrium . Bulbus cordis
mse. F
Lung Se iy f Ventricle
4 i
@ Yolk sac
Stomach} x ee
ye eee) > Seplum transversum
Dorsal Pancreas Y ; Yolk stalk
s\ Liver
5 a 1 a £9
Hepatic diverticulum YT” SL
<< Y Cacum
ee ae
Duodenum aaa ‘Small intestine
L. genital fold a ‘Allantois
. , J C Nr . :
L. mesonephros’ “ aS — Urogenital sinus
Dorsal aorta x baemecer oe \ Ureter
Colon ali Mesonephric duct
Umbilical artery (cut away)
Metanephros Rectum
Fic. 122.—Median sagittal dissection of a 10 mm. pig embryo, showing the brain, spinal cord and
viscera from the right side. X 10.5.
glottis are the arytenoid folds of the larynx. (For the development of the tongue,
see p. 149.) The pharyngeal pouches are now larger than in the 6 mm. pig (Fig.
123). The first pouch persists as the auditory tube and middle ear cavity,
the closing plate between it and the first branchial cleft forming the tympanic
membrane. The second pouch later largely disappears; about it, develops the
118 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
palatine tonsil. ‘The third pouch is tubular, directed at right angles to the phar-
ynx, and meets the ectoderm to form a “‘closing plate.” Median to the plate, the
ventral diverticulum of the third pouch is the anlage of the thymus gland. Its
dorsal diverticulum forms an epithelial body, or parathyreoid. The fourth
pouch is smaller and its dorsal diverticulum gives rise to a second parathyreoid
Gang. nn.7 and 8 Gang. n. 5
Otocyst
Pharyngeal pouch 2
Gang. jugulare n. 10
Post. lobe hypophysis
SHEE
\ Ant. lobe hy pophysis
\
Caudal root n.
hy poglossus Pharyngeal pouch r
Maxillary process
Thyreoid gland
Aorlic arch 4
Gang. cerv. I
Pharyngeal pouch 4 Pulmonary artery
ey
cn
Aorta
NI
Aortic arch 5
AN
ni
a7
SEU
SS
I
R. descending aorta
AWAY
oS JUS
Eso phag _ Septum
transversum
Trachea
sss
ANG
Ss
Vertebral artery
Subclavian artery Hepatic
diverticulum
Cloaca
~ Allantois
Rectum
10.
body. Its ventral diverticulum is a rudimentary thymus anlage. A tubular
outgrowth, caudal to the fourth pouch, is regarded as a fifth pharyngeal pouch
in human embryos and forms the ultimobranchial body on each side (see p. 164).
The thyreoid gland, composed of branched cellular cords, is located in the mid-
line between the second and third branchial arches (Fig. 123).
THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS I1IQ
Trachea and Lungs.—Caudal to the fourth pharyngeal pouches the esoph-
agus and trachea separate and form entodermal tubes (Figs. 122 and 123).
Cephalad of the point where the trachea bifurcates to form the primary bronchi
there appears on its right side the tracheal bud of the upper lobe of the right lung
(Fig. 124). This bronchial bud is developed only on the right side and appears in
embryos of 8 to9 mm. Two secondary bronchial buds arise from the primary
bronchus of each lung, and form the anlages of the symmetrical lobes of each lung.
Olfactory pit
Lateral nasal process ——~___
Lacrymal groove ~~ SS ive
~— Median nasal process
Maxillary process
— Branchial arch 2
_____ Branchial arch 3
— Branchial arch 4
Mandibular process
Cervical sinus
Trachea
Tracheal lung bud , oy) L 7— Esophagus
Upper limb bud i Ua
Septum transversum
Mesonephric duct
Hepatic diverticulum Ventral pancreas
Yolk sac § iy !) —~ Mesonephros
— Cephalic limb of intestine
——~ Caudal limb of intestine
Allantois =
R. umbilical artery —
~~ Rectum
a
oe SG yf - ~~ Metanephros
Lower limb bud ~ ue
Mesonephric duct Spinal cord
Rectum
Fic. 124.—Ventral dissection of a 9 mm. pig embryo. The head is bent dorsad. X 9.
Esophagus and Stomach.—The esophagus extends as a narrow tube caudal
to the lungs, where it dilates into the stomach. The stomach is wide from its
greater to its lesser curvature and shows a cardiac diverticulum (Lewis). The
pyloric end of the stomach has rotated more to the right, where it opens into the
duodenum, from which division of the intestine the liver and pancreas develop.
The liver, with its four lobes, fills in the space between the heart, stomach,
and duodenum (Fig. 122). Extending from the right side of the duodenum along
the dorsal and caudal surface of the liver is the hepatic diverticulum. It lies to
120 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
the right of the midline and its extremity is saccular. This saccular portion
becomes the gall bladder. Several ducts connect the diverticulum with the liver
cords. One of these persists as the hepatic duct which joins the cystic duct of the
gall bladder. The portion of the diverticulum proximal to this union becomes the
common bile duct, or ductus choledochus. The ventral pancreas arises from the
common bile duct near its point of origin (Fig. 123). It is directed dorsad and
caudad to the right of the duodenum. The dorsal pancreas arises more caudally
from the dorsal wall of the duodenum and its larger, lobulated body grows dor-
sally and cranially (Figs. 123, 127 and 140). Between the pancreatic anlages
courses the portal vein. In the pig, the duct of the dorsal pancreas persists as
the functional duct.
Intestine.—Caudal to the duodenum, the intestinal loop extends well into
the umbilical cord (Figs. 122 and 123). At the bend of the intestinal loop is the
slender yolk stalk. The cephalic limb of the intestine lies to the right, owing to
the rotation of the loop. The small intestine extends as far as a slight enlarge-
ment of the caudal limb of the loop, the anlage of the cecum, or blind gut. This
anlage marks the beginning of the large intestine (colon and rectum). The
intestinal loop is supported by the mesentery which is cut away in Fig. 122. The
cloaca is now nearly separated into the rectum and urogenital sints. The cavity
of the rectum is almost occluded by epithelial cells (Lewis).
Urogenital System.—The mesonephros is much larger and more highly dif-
ferentiated than in the 6 mm. embryo (Figs. 120 and 124). Along the middle
of its ventro-median surface the genital fold is now more prominent (Fig. 122).
In a ventral dissection (Fig. 124) the course of the mesonephric ducts may be
traced. They open into the urogenital sinus, which also receives the allantoic
stalk (Fig. 122).
The metanephros, or permanent kidney anlage, lies just mesial to the um-
bilical arteries where they leave the aorta (Fig. 123). Its epithelial portion,
derived from the mesonephric duct, is differentiated into a proximal, slender duct,
the ureter, and into a distal, dilated pelvis. From this grow out later the calyces
and collecting tubules of the kidney. Surrounding the pelvis is a layer of con-
densed mesenchyma, or nephrogenic tissue, which is the anlage of the remainder
of the kidney.
Blood Vascular System.—The Heart.—In Fig. 125 the cardiac chambers of
the right side are opened. The septum primum between the atria is perforated
dorsad and cephalad by the foramen ovale. The inferior vena cava is seen opening
oS
into the sinus venosus, which in turn communicates with the right atrium through
THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 121
a sagittal slit guarded by the right and left valves of the sinus venosus. The right
valve is the higher and its dorsal half is cut away. The valves were united
cephalad as the septum spurium. Between the left valve and the septum primum
the sickle-like fold of the septum secundum is forming; the fusion of these three
components gives rise later to the adult atrial septum. The aortic bulb is divided
distally into the aorta and the pulmonary artery, the latter connecting with the
fifth pair of aortic arches. Proximally the bulb is undivided. The inlerven-
tricular septum is complete except for the interventricular foramen which leads
from the left ventricle into the aortic side of the bulb. Of the bulbar swellings
which divide the bulb into aorta and pulmonary trunk, the Jeft joins the inter-
ventricular septum, while the right extends to the endocardial cushion. These
folds eventually fuse and the partition of the ventricular portion of the heart is
Sept. IT R. atrium
Sept. I
Foramen ovale
Left valve of sinus venosus
Right valve of sinus venosus — Aorta
Inferior vena cava
“Pulmonary artery
s—Interventricular foramen
R. ventricle
Fic. 125.—Heart of 12 mm. embryo dissected from the right side.
completed. The endocardium at the atrio-ventricular foramina is already under-
mined to form the anlages of the tricuspid and bicuspid valves. From the caudal
wall of the left atrium there is given off a single pulmonary vein.
The Arteries —As seen in Fig. 123, the first two aortic arches have dis-
appeared. Cranial to the third arch, the ventral aorte become the external
carotids. The third aortic arches and the cephalic portions of the descending
aorte constitute the internal carotid arteries. The ventral aorte between the
third and fourth aortic arches persist as the common carotid arteries. The de-
scending aorte in the same region are slender and eventually atrophy. The
fourth aortic arch is largest and on the left side will form the aortic arch of the
adult. From the right fourth arch caudad, the right descending aorta is smaller
than the left. Opposite the eighth segment, the two aorte unite and continue
122 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
caudally as the median dorsal aorta. The fifth (sixth ?) aortic arches (cf. p. 99)
are connected with the pulmonary trunk, and from them arise small pulmonary
arteries to the lungs. Dorsal intersegmental arteries arise, six pairs from the de-
scending aorte, others from the dorsal aorta. From the seventh pair, which arise
just where the descending aorte fuse, the subclavian arteries pass off to the
upper limb buds and the vertebral arteries to the head. The latter are formed
Fic. 126 A.—Reconstruction of a 12 mm. pig embryo to show the veins and heart from the left side.
For names of parts see Fig. 126 B on opposite page (F. T. Lewis). X 9.
by a longitudinal anastomosis between the first seven pairs of intersegmental
arteries on each side, after which the stems of the first six pairs atrophy.
Ventro-lateral arteries from the dorsal aorta supply the mesonephros and
genital ridge (Fig. 123). Ventral arteries form the celiac artery to the stomach
region, the vitelline or superior mesenteric artery to the small intestine, and the
inferior mesenteric artery to the large intestine.
THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 123
The umbilical arteries now arise laterally from secondary trunks which
persist as the common iliac arteries.
The Veins.—The cardinal veins have been reconstructed by Lewis in a 12
mm. pig (Fig. 126). The veins of the head drain into the anterior cardinal vein,
WA eer Z
\ y L WAL
. “y
Ul
eS Oo
Mog
i
q:
kh
Fic. 126 B.—Reconstruction of a 12 mm. pig embryo to show the veins from the left side (Lewis).
X 9. A., Umbilical artery; Ao., aorta; Au., right auricle (atrium); card.’, card.”, superior and in-
ferior sections‘of posterior cardinal vein; d, left common cardinal vein; D.C., right common cardinal
vein; D.V., ductus venosus; Jug.’, Jug.”, jugular or ant. cardinal vein; L., liver; L.o., anlage of lateral
sinus; mx, transverse vein; P., pulmonary artery; Sc., subcardinal vein; Sc/., subclavian vein; sls.,
anlage of sup. longitudinal sinus; Um.d., right umbilical vein; Ven., right ventricle, V.H.C., common
hepatic vein; V.op., ophthalmic vein; V.P., portal vein; X, anastomosis between the right and left
subcardinal veins.
which becomes the internal jugular vein of the adult. After receiving the ex-
ternal jugular veins and the subclavian veins from the upper limb buds the anterior
cardinals open into the common cardinal veins (duct of Cuvier).
The posterior cardinal veins arise in the caudal region, course dorsal to the
mesonephroi, and drain the mesonephric sinusoids. The subcardinal veins
124 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
anastomose just caudal to the origin of the superior mesenteric artery and the
posterior cardinals are interrupted at this level. The proximal portions of the
posterior cardinals open into the common cardinal veins as in the 6 mm. embryo.
Of the two subcardinal veins, the right has become very large through its con-
nection with the right posterior cardinal vein and the common hepatic vein, and
now forms the middle portion of the inferior vena cava. For the development
of this vein, see Chapter IX.
Notochord Spinal cord
Ant. cardinal vein
Esophagus
Pharynx
R. ant. cardinal vein
Trachea
Pericardial cavily Uber lamb
Common car-
dinal vein
Sinus venosus
Inf. vena cava-$ Duclus venosus
Liver
Portal vein —* a J ee Pyloric stomach
Ventral pancreas Ilepalic diverticulum
Caecum
Dorsal pancreas
L. vitelline vein
Small intestine
Duodenum
L. umbilical vein
Sup. mesenteric vein
eas i Allantois
R. umbilical vein
R. umbilical artery
Fic. 127.—Reconstruction of a 10 mm. pig embryo to show the umbilical and vitelline veins from
the ventral side. x indicates sinusoidal connection between left umbilical vein and portal vein. X 15.
In the small orientation figure (cf. Fig. 123) the various planes are indicated by broken lines—
The umbilical veins (Figs. 126 and 127) anastomose in the umbilical cord,
separate on entering the embryo, and course cephalad in the ventro-lateral body
wall of each side to the ventral lobe of the liver. The Jeff vein is much the
larger, and, after entering the liver, its course is to the right and dorsad. After
connecting with the portal vein, it continues as the ductus venosus and joins the
proximal end of the inferior vena cava. The smaller right umbilical vein after
entering the liver breaks up into sinusoids. It soon atrophies, while the left vein
persists until after birth.
The Vitelline Veins.—Of these, a distal portion of the left and a proximal
TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO |. 125
portion of the right are persistent. The left vitelline vein, fused with the right,
courses from the yolk sac cephalad of the intestinal loop. Near a dorsal anas-
tomosis between the right and left vitelline veins, just caudal to the duct of the
dorsal pancreas, the left receives the superior mesenteric vein, a new vessel arising
in the mesentery of the intestinalloop. Cranial to its junction with the superior
mesenteric vein, the left vitelline, with the dorsal anastomosis and the proximal
portion of the right vitelline vein, form the portal vein, which gives off branches to
the hepatic sinusoids and connects with the left umbilical vein. For the develop-
ment of the portal vein, see Chapter IX.
TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO
Figures are shown of sections passing through the more important regions
and should be used for the identification of the organs. The level and plane of
each section is indicated by guide lines on Fig. 128. The student should compare
Metencephalon
Myelence phalon
Aesencephalon
Gang. sup. n. 9
Gang. and n. access. Diencephalon
129
Lelence phalon
130
I3L
, Olfactory pit
Pulmonary artery os
134
136
137
138
139
140
T4I
142
T43
“) or
ee
ay
‘by Lip 2 Z
: fi) baad Ged oad
Fic. 128.—Reconstruction of a 10 mm. pig embryo, showing the chief organs of the left side. The
numbered lines indicate the levels of transverse sections shown in the corresponding figures (129-143).
For the names of the various structures not lettered see Fig. 123. X 8.
this with Figs. 118 and 123, and orient each section with reference to the embryo
asa whole. To avoid repetition most of the levels illustrated in the transverse
sections of the 6 mm. pig are not represented in the 10 mm. series. For this reason
126 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
the former series will be found very instructive in supplementing the following
descriptions.
Transverse Section through the Eyes and Otocysts (Fig. 129).—The brain is
sectioned twice, lengthwise through the myelencephalon, transversely through the fore-brain.
The brain wall shows differentiation into three layers: (1) an inner ependymal layer densely
cellular; (2) a middle manile layer of nerve cells and fibers; (3) an outer marginal layer chiefly
fibrous. These same three layers are developed in the spinal cord. A thin vascular layer
differentiated from the mesenchyma surrounds the brain wall and is the anlage of the pia
mater. The myelencephalon shows three neuromeres in this section. The telencephalon is
Fourth ventricle Wall of myelencephalon
N. accessorius
Gang. jugulure n. 10
Sle N. glossopharyngeus
Olocyst
Gang. acust. n. 8 Gang. geniculat. n. 7
N. abducens
}
Mandibular ramus n. 5 Basilar artery
. S nt =
Maxillary ramus n. 5 LOUMSSGGDET IOS US
Int. carotid artery
Ant. lobe hypo physis
O plic vesicle
Lens vesicle
Third ventricle of diencephalon ROREINEN: inlerveniriculare
Lat. ventricle of telencephalon
Fic. 129.—Transverse section passing through the eyes and otocysts of a10 mm. pig embryo. X 22.5.
represented by the paired cerebral hemispheres, their cavities, the Jateral ventricles, connecting
through the interventricular foramina with the third ventricle of the diencephalon. Close to the
ventral wall of the diencephalon is a section of the anterior lobe of the hypophysis (Rathke’s
pocket) near which are the ‘nternal carotid and basilar arteries. Lateral to the diencephalon
“is the optic cup and lens vesicle of the eye, which are sectioned caudal to the optic stalk. The
outer layer of the optic cup forms the thin pigment layer; the inner thicker layer, is the mervous
layer of the retina. The lens is now a closed vesicle distinct from the overlying corneal
ectoderm.
The large vascular spaces are the cavernous sinuses, which drain by way of the vv. capitis
laterales into the internal jugular veins. Transverse sections may be seen of the maxillary
TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 127
and mandibular branches of the n. trigeminus; the n. abducens is sectioned longitudinally.
The small nn. oculomotorius and trochlearis should be identified in sections more cephalad in
the series. Ventral to the otocyst are seen the geniculate and acoustic ganglia of the nn. facialis
and acusticus. The wall of the ofocyst forms a sharply defined epithelial layer. More cephalad
in the series the endolymph duct lies median to the otocyst and connects with it. Dorsal to
the otocyst the x. glossopharyngeus and the jugular ganglion of the vagus are cut transversely
while the trunk of the x. accessorius is cut lengthwise.
Section through the First and Second Pharyngeal Pouches (Fig. 130).—The
end of the head, with sections of the felencephalon and of the ends of the olfactory pits, is now
N. accessorius 5 5
Neural cavity
Gang. Froriep
Myelence phalon
Roots of n. hypoglossus
Basilar artery
Int. jugular vein
Notochord Nn. vagus et accessorius
Descending aorta
Gang. petrosal n. 9 Me gaciales
Pharyngeal pouch 2 i Branchial arch 2
Ti
Pharyngeal pouch 1 ss
L Mandible
Maxillary process
Olfactory pit
Telencephalon
Fic. 130.—Transverse section passing through the first and second pharyngeal pouches of a 10 mm. pig
embryo. X 22.5.
distinct from the rest of the section. The pharynx shows portions of the first and second
pharyngeal pouches. Opposite the first pouch externally is the first branchial cleft. A section
of the tuberculum impar of the tongue shows near the midline in the pharyngeal cavity. The
neural tube is sectioned dorsally at the level of Froriep’s ganglion. Between the neural tube
and the pharynx may be seen on each side the several root fascicles of the 2. hypoglossus, the
fibers of the nn. vagus and accessorius, and the petrosal ganglion of the n. glossopharyngeus.
Mesial to the ganglia are the descending aorte and lateral to the vagus is the internal jugular
vein.
128 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
Section through the Third Pharyngeal Pouches (Fig. 131).—The tip of the head
is now small and shows on either side the deep olfactory pits lined with thickened olfactory epi-
thelium. The first, second, and third branchial arches show on either side of the section, the
third being slightly sunken in the cervical sinus. The dorsal diverticula of the third pharyn-
geal pouches extend toward the ectoderm of the third branchial cleft. The ventral diverticula
or thymic anlages may be traced caudad in the series. The floor of the pharynx is sectioned
through the epigloitis. Ventral to the pharynx are sections of the third aortic arches and the
solid cords of the thyreoid gland. Dorsally the section passes through the spinal cord and
first pair of cervical ganglia. Between the cord and pharynx, named in order, are the internal
jugular veins, the hypoglossal nerve, and the nodose ganglion of the vagus. Lateral to the
Spinal ganglion
Notochord Int. jugular vein
N. hypoglossus
Ext. branch n. accessorius
Gang. nodos. n. 10
Epigloltis
Branchial arch 3
Branchial arch 24 Thyreoid anlage
Mandible
Olfactory pit Olfactory epithelium
Fic. 131.—Transverse section through the third pharyngeal pouches of a 10 mm. pig embryo. X 22.5.
ganglion is the external branch of the n. accessorius, and mesial to the ganglia are.the small
descending aorta.
Section through the Fourth Pharyngeal Pouches (Fig. 132).—This region is marked
by the disappearance of the head and the appearance of the heart in the pericardial cavity.
The tips of the atria are sectioned as they project on either side of the bulbus cordis. The
bulbus is divided into the aorta and pulmonary artery, the latter connected with the right
ventricle, which has spongy muscular walls. The pharynx is crescentic and continued laterally
as the small fourth pharyngeal pouches. Into the mid-ventral wall of the pharynx opens the
vertical slit of the trachea. A section of the vagus complex is located between the descending
aorta and the internal jugular vein. At this level the jugular vein receives the linguo-facial
TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 129
Spinal cord
Spinal ganglion
R. descending aorta L, descending aorla
Int. jugular vein
Pharynx
aan ti Pharyngeal pouch 4
Tracheal groove
R. atrium 4
Aorta
Pulmonary artery
R. ventricle
Fic. 132.—Transverse section through the fourth pharyngeal pouches of a 10 mm. pigembryo. X 22.5.
Spinal ganglion Spinal cord
Notochord
R. descending aorta
Esophagus
Trachea
L. atrium
Aorta Pulmonary artery
R. atrium
Cavity of bulbus
‘ R. ventricle
Fic. 133.—Transverse section through the fifth pair of aortic arches and bulbus cordis of a 10 mm. pig
embryo. X 22.5.
130 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
vein. The left descending aorta is larger than the right. The ventral aorta may be traced
cranially in the series to the fourth aortic arches. The pulmonary artery, if followed caudad,
connects with the fifth aortic arches as in Fig. 133.
Section through the Fifth Aortic Arches (Fig. 133).—The fifth aortic arch is com-
plete on the left side. From these pulmonary arches small pulmonary arteries may be traced
caudad in the series to the lung anlages. The esophagus, now separate from the trachea,
forms a curved horizontal slit. All four chambers of the heart are represented, but the
aorta and pulmonary artery are incompletely separated by the right and left bulbar swellings
or folds.
Section through the Sinus Venosus and the Heart (Fig. 134).—The section is
marked by the symmetrically placed atria and ventricles of the heart and by the presence of
Spinal cord
Spinal ganglios
Notochord
Upper limb bud
R. descending aorla Esophagus
g aorta’ dg
L. common cardinal vein
Sinus venosus :
Trac hea
R. valve of sinus venosus L. atrium
Pericardial cavity
R. ventricles Uy Te She & et Body wall
Interventricular se plum—-*
Fic. 134.—Transverse section through the sinus venosus of the heart ina 10 mm. pig embryo. X 22.5.
the upper limb buds. Dorsal to the atria are the common cardinal veins, the right vein forming
part of the sinus venosus. The sinus venosus drains into the right atrium through a slit-like
opening in the dorsal and caudal atrial wall. The opening is guarded by the right and left
valves of the sinus venosus, which project into the atrium. The septum primum completely
divides the right and left atria at this level, which is caudal to the foramen ovale and the
atrio-ventricular openings. The septum joins the fused endocardial cushions. Note that
the esophagus and trachea are now tubular and that the left descending aorta is much larger
than the right. Around the epithelium of both trachea and esophagus are condensations of
mesenchyma, from which their outer layers are differentiated.
Section through the Foramen Ovale of the Heart (Fig. 135).—The level of this
section is cranial to that of the previous figure and shows the septum primum interrupted dor-
sally to form the foramen ovale. Each atrium communicates with the ventricle of the same
TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 131
side through the atrio-ventricular foramen. Between these openings is the endocardial cushion,
which in part forms the anlages of the tricuspid and bicuspid valves. The atria are marked off
externally from the ventricles by the coronary sulcus. Between the two venticles is the inter-
ventricular septum. The ventricular walls are thick and spongy, forming a network of muscu-
lar cords or trabecule surrounded by blood spaces or sinusoids. The trabeculae are composed
of muscle cells, which later become striated and constitute the myocardium. They are sur-
rounded by an endothelial layer, the endocardium. The mammalian heart receives all its
nourishment from the blood circulating in the sinusoids until later, when the coronary vessels
of the heart wall are developed. The heart is surrounded by a layer of mesothelium, the
epicardium, which is continuous with the pericardial mesothelium lining the body wall.
Section through the Liver and Upper Limb Buds (Fig. 136).—The section is
marked by the presence of the upper limb buds, the liver, and the bifurcation of the trachea to
form the primary bronchi of the lungs. The limb buds are composed of dense undifferentiated
mesenchyme surrounded by ectoderm which is thickened at their tips. The seventh pair
of cervical ganglia and nerves are cut lengthwise showing the spindle-shaped ganglia with the
dorsal root fibers taking origin from their cells. The ventral root fibers arise from the ventral
Foramen ovale : : L. atrium
R. atrium
. . Endocardial cushion
R. atrio-ventricular foramen
L. atrio-ventricular foramen
R. ventricle-8 E
L. ventricle
Interventricular septum
Fic. 135.—Transverse section through the foramen ovale of the heart ina 10 mm. pig embryo.
X 22.5.
cells of the mantle layer and join the dorsal root to form the nerve trunk. On the right side
a short dorsal ramus supplies the anlage of the dorsal muscle mass. The much larger ventral
ramus unites with those of other nerves to form the brachial plexus.
The descending aorte have now fused and the seventh pair of dorsal intersegmental arter-
ies arise from the dorsal aorta. From these intersegmental arteries the subclavian arteries are
given off two sections caudad in the series. Lateral to the aorta are the posterior cardinal
veins. The esophagus, ventral to the aorta, shows a very small lumen, while that of the
trachea is large and continued into the bronchi on either side. Adjacent to the esophagus are
the cut vagus nerves. The lung anlages project laterally into the crescentic pleural cavities, of
which the left is separated from the peritoneal cavity by the septum transversum. The liver,
with its fine network of trabecule and sinusoids, is large and nearly fills the peritoneal or
abdominal cavity. The liver cords are composed of liver cells surrounded by the endothelium
of the sinusoids. Red blood cells are developed in the liver at this stage. The large vein
penetrating the septum transversum from the liver to the heart is the proximal portion of the
inferior vena cava, originally the right vitelline vein. Ventral to the bronchi may be seen sec-
tions of the pulmonary veins.
132 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
Spinal ganglion
Spinal nerve Notochord
Post. cardinal vein
Mesonephios
Dorsal aorta
Esophagus
Pleural cavily ¢ Bifurcation of trachea
Upper limb bud Inferior vena cava
Fic. 136.—Transverse section through the liver and upper limb buds of a 10 mm. pig embryo at the level
of the bifurcation of the trachea. X 22.5.
Spinal cord
Notochord
Descending aorta . ° .
& Sympathetic ganglion
Mesone phros : ;
Post. cardinal vein
R. lung bud, ,
AMesonephric tubule
Esophagus
/ Peritoneal cavity
Lesser peritoneal sac raansal tobe of ite
Inferior vena cava Vy §
‘e Tee ly
Simusotds of liver
Duclus venosus
Fic. 137.—Dorsal half of a transverse section through the lung buds cranial to the stomach in a 10 mm.
pig embryo. X 22.5. :
Section through Lung Buds Cranial to Stomach (Fig. 137).—The lungs are sec-
tioned through their caudal ends and the esophagus is just beginning to dilate into the
TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 133
stomach. On either side of the circular dorsal aorta are the mesonephroi, while dorso-laterally
are sympathetic ganglia. The pleural cavities now communicate freely on both sides with the
peritoneal cavity. A section of the lesser peritoneal sac appears as a crescent-shaped slit at
the right of the esophagus. In the right dorsal lobe of the liver is located the inferior vena
cava. Near the median line ventral to the lesser sac is the large ductus venosus.
Section through the Stomach and Liver (Fig. 138).—Prominent in the body cav-
ity are the mesonephroi and liver lobes. The mesonephroi show sections of coiled tubules
lined with cuboidal epithelium. The glomeruli, or renal corpuscles, are median in position
and develop as knots of small arteries which grow into the ends of the tubules. The thickened
epithelium along the median and ventral surface of the mesonephros is the anlage of the genital
gland. The body wall is thin and lined with mesothelium continuous with that which covers
Spinal cord
Spinal ganglion
Notochord
Dorsal aorla
Glomerulus of
mesonephros
Plica vene cave
Greater omentum
Inferior vena cava
e
a Stomach
Lesser omentum
Fic. 138.—Transverse section through the stomach and liver of a 10 mm. pig embryo. X 22.5.
the mesenteries and organs. The mesothelial layer becomes the epithelium of the adult peri-
toneum, mesenteries, and serous layer of the viscera. The stomach lies on the left side and is
attached dorsally by the greater omentum, ventrally to the liver by the Jesser omentum. The
right dorsal lobe of the liver is attached dorsally to the right of the great omentum. Inthe
liver, ventral to this attachment, courses the inferior vena cava and the attachment forms the
plica vene cave. Between the attachments of the stomach and liver, and to the right of the
stomach, is the lesser peritoneal sac. In the liver to the left of the midline is the ductus
venosus, sectioned just at the point where it receives the left umbilical vein and a branch from
the portal vein. The ventral attachment of the liver later becomes the falciform ligament.
Section through the Hepatic Diverticulum (Fig. 139).—The section passes through
the pyloric end of the stomach and duodenum near the attachment of the hepatic divertic-
134 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
ulum. The great omentum of the stomach is larger than in the previous section and to its
right, in the plica vene cave, lies the inferior vena cava. Ventral to the inferior vena cava
is a section of the portal vein. The ventral and dorsal lobes of the liver are now separate and
in the right ventral lobe is embedded the saccular end of the hepatic diverticulum which forms
the gall bladder. To the right of the stomach, the diverticulum is sectioned again just as it
enters the duodenum. Ventrally the left umbilical vein is entering the left ventral lobe of the
liver. It is much larger than the right vein, which still courses in the body wall. On the
left side of the embryo the spinal nerve shows in addition to its dorsal and ventral rami a sym-
pathetic ramus, the fibers of which pass to a cluster of ganglion cells located dorso-lateral to the
aorta. These cells form one of a pair of sympathetic ganglia and are derived from a spinal
ganglion.
Spinal cord
Notlochord —-+}$ ;
Sym pathetic ramus
Dorsal aorta
Plica vene cave Dorsal mesogastrium
Inferior vena cava
Lesser peritoneal sav
Portal vein
Hepatic diverticulun
L. umbilical vein
R. umbilical vein
Fic. 139.—Transverse section through the hepatic diverticulum of a 10 mm. pig embryo. X 22.5.
Section through the Pancreatic Anlages (Fig. 140).—The lesser peritoneal sac just
above the level of this section has opened into the peritoneal cavity through the cpiploic
foramen (of Winslow). The mesonephric ducts are now prominent ventrally in the meso-
nephroi. The duct of the dorsal pancreas is sectioned tangentially at the point where it takes
origin from the duodenum. From the duct the lobulated gland may be traced dorsad in the
mesentery. To the right of the dorsal pancreatic duct is a section of the ventral pancreas,
which may be traced cephalad in the series to its origin from the hepatic diverticulum. Dorsal
to the ventral pancreas is a section of the portal vein. The inferior vena cava appears as a
vertical slit in the dorsal mesentery.
Section through the Urogenital Sinus and the Lower Limb Buds (Fig. 141).—
.The figure shows only the caudal end of a section, in the dorsal portion of which the meso-
.
TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 135
nephroi were sectioned at the level of the subcardinal anastomosis. A portion of the mesentery
is shown with a section of the colon. In the body wall are veins which drain into the umbilical
veins, and on each side are the wmbilical arteries, just entering the body from the umbilical
cord. Between them, in sections cranial to this, the allantoic stalk is located, Here it has
opened into the crescentic urogenital sinus. Dorsal to the urogenital sinus (dorsal now being
Post. cardinal vein
R. vitelline or portal vein—¥
Mesonephric duct
Ventral pancreas
Fic. 140.—Portion of a transverse section through the pancreatic anlages of a 10 mm. pig embryo.
x 22.5.
at the bottom of the figure owing to the curvature of the caudal region) is a section of the
rectum, separated from the sinus by a curved prolongation of the coelom. From the ends of
the urogenital sinus, as we trace cephalad in the embryo (downward in the series), are given
off the mesonephric ducts.
Vein in body wall
L. umbilical artery
Allantois and
urogenital sinus
Mesonephric duct
Fic. 141.—Transverse section through the urogenital sinus and rectum of a 10 mm. pigembryo. X 22.5.
Section through the Mesonephric Ducts at the Opening of the Ureter (Fig. 142).
—The section cuts through both lower limb buds near their middle. Mesial to their besas
are the umbilical arteries, which lie lateral to the mesonephric ducts. From the dorsal wall of
the left mesonephric duct is given off the ureter or duct of the metanephros. Tracing the sec-
tions down in the series, both ureters appear as minute tubes in transverse section. They
136 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS
soon dilate to form the pelvis of the kidney at the level of Fig. 143. Note the undifferentiated
mesenchyme of the lower limb buds and their thickened ectodermal tips.
Caudal limb of intestine
Lower limb bud
R. umbilical ariery
Mesonephric duct
Ureler
Notochord
Spinal cord
Frc. 142,—Transverse section of a 10 mm. embryo passing through the lower limb buds at the level of
the openings of the ureters into the mesonephric ducts. X 22.5.
Section through the Metanephroi and Umbilical Arteries (Fig. 143).—The sec-
tion passes caudal to the mesonephric ducts which curve along the ventral surfaces of the
Calom
Anlage of metanephros
Vein
Nolochord
Spinal cord
Fic. 143.—Transverse section through the anlages of the metanephroiina 10mm. pigembryo. X 22.5.
mesonephroi (Fig. 124). The umbilical arteries course lateral to the metanephroi which con-
sist merely of the thickened epithelium of the pelvis surrounded by a layer of condensed
mesenchyma, the nephrogenic tissue.
CHAPTER VI
THE DISSECTION OF PIG EMBRYOS: DEVELOPMENT OF FACE,
PALATE, TONGUE, SALIVARY GLANDS AND TEETH
THE DISSECTION OF PIG EMBRYOS
As the average student will not have time to study series of embryos sectioned in differ-
ent planes, dissections may be used for showing the form and relations of the organs. Cleared
embryos mounted whole are instructive, but show the structures superimposed and are apt to
confuse the student.. Pig embryos 10 mm. or more in length may be easily dissected, mounted
as opaque objects, and used for several years. Success in dissecting such small embryos de-
pends: (1) on the fixation and hardenirig of the material employed; (2) on starting the dissec
tion with a clean cut in the right plane; (3) on a knowledge of the anatomy of the parts to be
dissected.
Fixation and Hardening of Material—Embryos fixed in Zenker’s fluid have given
the best results. They should then be so hardened in 95 per cent. alcohol that the more
diffuse mesenchyma will readily separate from the surfaces of the various organs, yet the
organs must not be so brittle that they will crumble and break. Embryos well hardened
and then kept for two weeks in 80 per cent. alcohol usually dissect well. Old material is
usually too brittle; that just fixed and hardened may prove too soft. As a test, determine
whether the mesenchyma separates readily from the cervical ganglia and their roots.
Dissecting instruments include a binocular dissecting microscope, a sharp safety
razor blade, large curved blunt-pointed dissecting needles, pairs of small sharp-pointed for-
ceps, and straight dissecting needles small and large.
Methods of Dissection.—In general, it is best to begin the dissection with a clean,
smooth cut ‘made by a single stroke with the safety razor blade, which should be flooded
with 80 per cent. alcohol. The section is made free hand, holding the embryo, protected by a
fold of absorbent cotton, between the thumb and index finger. Having made preliminary
cuts in this way, the embryo may be affixed with thin celloidin to a cover glass and im-
mersed in a watch glass containing alcohol. We prefer not to affix the embryo, as the celloidin
used for this purpose may interfere with the dissection. Instead, a cut is made parallel to
the plane of the dissection so that the embryo, resting in the watch glass upon this flat surface,
will be in a fairly stable position. It may thus be held in any convenient position by resting
the convex surface of a curved blunt dissecting needle upon some part not easily injured.
The dissection is then carried on under the binocular microscope, using the fine pointed for-
ceps, dissecting needles, and a’small pipette to wash away fragments of tissue.
Whole Embryos.—For the study of the exterior, whole embryos may be affixed with
celloidin to the bottoms of watch glasses which may be stacked in wide-mouthed jars of 80
per cent. alcohol. The specimens may thus be used several years at a saving of both time and
material. Preliminary treatment consists in immersion in 95 per cent. alcohol one hour, in
ether and absolute alcohol at least thirty minutes, in thin celloidin one hour or more. Pour
enough thin celloidin into a Syracuse watch glass to cover its bottom, and immerse in this a
circle of black mat paper, first wet with ether and absolute alcohol. Pour off any surplus cel-
loidin, mount embryo in desired position and immerse watch glass in 80 per cent. alcohol,
137
138 THE DISSECTION OF PIG EMBRYOS
in which the specimen may be kept indefinitely. Embryos may also be mounted in gelatin-
formalin solution in small sealed glass jars.
Lateral Dissections of the Viscera.—Dissections like those shown in Figs. 144 and
145 may easily be prepared in less than an hour, and make valuable demonstration and
laboratory specimens. Skill is required to demonstrate most of the cerebral nerves, but the
Mesencephalon
N. oculomotorius
Cerebellum
N. trochlearis
Gang. geniculatum n. 7
Gang. acust. n. 8
Gang. sup. n. 9
Gang. accessor. \ gS
bs 4
Gang. jugulare n. 10
Gang. petrosum n. 9
N. hypoglossus
Cerebrum
Maxillary ramus n. 5
N. accessorius
~Chord.tymp. n. 7
Gang. cerv.
NV. facialis
| Gang. nodos. n. 10
Brachial plexus—
I
Lungs
R. atrium
R. ventricle
Diaphragm
q Q lf ( Nie
ey y
Ventral lobe of liver
Dorsal lobe of liver
4. Umbilical cord
Es
“
\
((
Mesonephros
e
“SS
\Z
Scialic nerve
Fic. 144.—Lateral dissection of an 18 mm. pig embryo, showing the nervous system and viscera from
the right side. X 8.
central nervous system, cerebral and spinal ganglia, and viscera may easily be exposed.
Starting dorsally, make a sagittal section of the embryo slightly to one side of the median line
and avoiding the umbilical cord ventrally. With the embryo resting on the flat sectioned
surface, begin at the cervical flexure and with fine forceps grasp the ectoderm and dural anlage
at its cut edge, separate it from the neural tube and pia mater, and strip it off ventralwards
LATERAL DISSECTIONS OF THE VISCERA 139
exposing the myelencephalon and cervical portion of the cord. As the mesenchyma is pulled
away, the ganglia and roots of the cerebral nerves will be exposed. The mesenchyma be-
tween the ganglia and along the nerves may be removed with the end of a small blunt needle.
Care must be exercised in working over the mesencephalon and telencephalon of the brain not
Semilunar ganglion n. 5 Ophthalmic ramus n. 5
Geniculale gang. n. 7 Cerebrum
gh Hoppin
4 . oplicus
AMesencephalon
Cerebellum
Lobus olfactorius
Gang. n. 8
Maxillary ramus n.
Gang. sup. n. 9 2
Gang. jugulare n. ro Mand. ramus n. 5
Chorda. tymp. n. 7
Auricular r. n. 10
PN. facialis (7)
Gang. n. cerv. I
Gang. petros. n. 9
Ne ert Gin. nodosum n. IO
N. hypoglossus
Gang. cerv. 5-5
Gang. thor. 1
R. atrium
R. ventricle
Ventral lobe of liver
= Umobil. cord
Dorsal lobe of liver
Mesonephros \/ o, ~Lower limb
|
Sciatic nerve
Fre. 145.—Lateral dissection of a 35 mm pig embryo to show the nervous system and viscera from the
right side. Xx 4.
to injure the brain wall, which may be brittle. By starting with a clean dissection dorsally
and gradually working ventrad, the more important organs may be laid bare without injury.
The beginner should compare his specimen with the dissections figured and also previously
study the reconstructions of Thyng (1911) and Lewis (1903).
140 THE DISSECTION OF PIG EMBRYOS
Lateral dissections of embryos 18 mm. and 35 mm. long show infinitely better
than sections the form and relations of the organs, their relative growth, and
their change of position (Figs. 144 and 145). Compare the organs of 6, 10, 18,
and 35 mm. embryos and note the rapid growth of the viscera (see Figs. 95 and
120). Hand-in-hand with the increased size of the viscera goes the diminution
of the dorsal and cervical flecures. In the brain, note the increased size of the
cerebral hemispheres of the telencephalon and the presence of the olfactory lobe of
the rhinencephalon. The cerebellum also becomes prominent and a ventral
flexure in the region of the pons, the pontine flexure, is more marked. The
brain grows relatively faster than the spinal cord, and, by the elongation of
their dorsal roots, the spinal ganglia are carried ventral to the cord. The body
of the embryo also grows faster than the spinal cord, so that the spinal nerves,
at first directed at right angles to the cord, course obliquely caudad in the
lumbo-sacral region.
Median Sagittal Dissections (Figs. 146 and 147).—Preliminary to the dissection, a
cut is made dorsally as near as possible to the median sagittal plane. Beginning caudally at
the mid-dorsal line, an incision is started which extends in depth through the neural tube and-
the anlages of the vertebre. This incision is carried to the cervical flexure, cranial to which
point the head and brain are halved as accurately as possible. The blade is then carried
ventrally and caudally, cutting through the heart and liver fo the right of the midline and of the
umbilical cord until the starting point is reached. A parasagittal section is next made well
to the left of the median sagittal plane and the sectioned portion is removed, leaving on the
left side of the embryo a plane surface. With the embryo resting upon this flat surface, the
dissection is begun by removing with forceps the right half of the head. In pulling this away
caudalwards, half of the dorsal body wall, the whole of the lateral body wall, and the parts of
the heart and liver lying to the right of the midline will be removed, leaving the other struc-
tures intact. Ifthe plane of section was accurate, the brain and spinal cord will be halved in
the median sagittal plane. Wash out the cavities of the brain with a pipette and its internal
structure may beseen. Dissect away the mesenchyma between the esophagus and trachea
and expose the Jung. Remove the right mesonephros, leaving the proximal part of its duct
attached to the urogenital sinus. The right dorsal lobe of the liver will overlie the stomach
and pancreas. Pick it away with forceps and expose these organs. Dissect away the caudal
portion of the liver until the hepatic diverticulum is laid bare. It is whitish in color and may
thus be distinguished from the brownish liver. Beginning at the base of the umbilical cord,
carefully pull away its right wall with forceps, thus exposing the intestinal loop and its attach-
ment to the yolk stalk. If in the caudal portion of the umbilical cord the umbilical artery is
removed, the allantoic stalk may be dissected out. To sce the anlage of the genital gland,
break through and remove a part of the mesentery, exposing the mesial surface of the left
mesonephros and the genital fold. The dissection of the metanephros and ureter is difficult
in small embryos. In 10 to 12 mm. embryos, the umbilical artery, just after it leaves the
aorta, passes lateral to the metanephros and thus locates it. By working carefully with fine
needles the surface of the metanephros may be laid bare and the delicate ureter may be traced
to the base of the mesonephric duct. The extent of the dorsal aorta may also be seen by
removing the surrounding mesenchyma. With a few trials, such dissections may be made in
MEDIAN SAGITTAL DISSECTIONS I41I
a short time, and are invaluable in giving one an idea of the form, positions, and relations of
the different organs. By comparing the early (Figs. 96 and 122) with the later stages (Figs.
146 and 147) a number of interesting points may be noted.
Isthmus Mesencephalon
Melencephaton
Third ventricle
Chorioid plexus of fourth ventricle
Hy pothysis
Diencephalon
Tela chorioidea fourth ventricle a
MM yelencephalon Cor pus striatum
Epiglottis ~~ Cerebrum
a
ms a Ny
Wy / Tongue
Notochord
Trachea — Aorta
Trunk pulm. arlery—
SAX Semilunar valves
all) f
V7] R. ventricle
4 |/
| ]/ Volk sac
y ~~ Diaphragm
~ Volk stalk
~ Wall of atrium -
Foramen ovale
Esophagus
Lung
Dorsal aorta Liver
) Caecum
Stomach
Intersegmental Ue
Se \ ‘ :
arteries ZN Small intestine
Pancreas " O4/ —= ———— Allantois
Common bile duct ‘ ‘ ts , ~ . re ait Bladder
Duodenum / ci Phallus
en
Genital fold < Urogenital sinus
AMetanephros aS
x Rectum
Mesonephric duct Ureter
Frc. 146.—Median sagittal dissection of an 18 mm. pig embryo, showing central nervous system in
section and the viscera in position. X 8.
In the brain, the corpus striatum develops in the floor of the cerebral hemi-
spheres. The interventricular foramen is narrowed to a slit. In the roof of the
diencephalon appears the anlage of the epiphysis, or pineal gland, and the chorioid
plexus of the third ventricle. This extends into the lateral ventricles as the
142 THE DISSECTION OF PIG EMBRYOS
lateral chorioid plecus. The dorso-lateral wall of the diencephalon thickens to
form the thalamus and the third ventricle is narrowed to a vertical slit. The
increased size of the cerebellum has been noted. Into the thin dorsal wall of the
myelencephalon grows the network of vessels which form the chorioid plexus of
Epiphysis Thalamus
Mesencephaion
Pedunculus cerebri Tela chorioidea
Cerebellum Lat. chorioid plexus
Chorioidal plexus ventricle 4
‘pus striatum
Tela of ventricle 4
Myelencephalon Hypo physis
Lobus olfactorius
Epiglottis
PIs Turbinate anlage
Esophagus Palate
Spinal cord
Tongue
Trachea
Aorta Pulmonary artery
R. atri “
ne Ventricle
R. bronchus
Dorsal aorta Diaphragm
\
Inf. vena cava ee
Stomach
Pancreas ‘ecum
Gall bladder
Small intestine
Suprarenal gland
Genital gland
Duodenum
Metanephros Urethra
Colon
L. mesonephric d
Bladder
Ureter me
Urogenital sinus with mesonephric duct Rectum
Fic. 147.—Median sagittal dissection of a 35 mm. embryo. X 4
the fourth ventricle, which is now spread out laterally and flattened dorso-ven-
trally. About the notochord mesenchymal anlages which form the centra of the
vertebre are prominent.
Turning to the alimentary tract, observe that the primitive mouth cavity
VENTRAL DISSECTIONS 143
is now divided by the palatine folds into the upper nasal passages and lower
oral cavity. In the lateral walls of the nasal passages develop the anlages of the
turbinate bones. On the floor of the mouth end pharynx, the tongue and epiglottis
become more prominent. The trachea and esophagus elongate and the lungs lie
more and more caudad. The dorsal portion of the septum transversum, the
anlage of a portion of the diaphragm, is thus carried caudad, and although origi-
nally, when traced from the dorsal body wall, it was directed caudad and ventrad,
now it curves cephalad and ventrad, bulging cephalad into the thorax. The
proximal limb of the intestinal loop elongates rapidly, and, beginning with the
duodenum, becomes flexed and coiled in a characteristic manner. The distal
limb of the intestinal loop is not coiled, but its diverticulum, the cecum, is more
marked. Caudally, the rectum, or straight gut, has completely separated from
the urogenital sinus and opens to the exterior through the anus.
Of the urogenital organs, the genital folds have become the prominent genital
glands attached to the median surfaces of the mesonephroi. The metanephror
have increased rapidly in size and have shifted cephalad. Proximal to the allan-
toic stalk the adjacent portion of the urogenital sinus has dilated to form the
bladder. As the urogenital sinus grows it takes up into its wall the proximal ends
of the mesonephric ducts, so that these and the ureters have separate openings
into the sinus. Owing to the unequal growth of the sinus wall, the ureters open
near the base of the bladder, the mesonephric ducts more caudally into the
urethra. The phallus now forms the penis of the male or the clitoris of the female.
Cranial to the metanephros a new organ, the suprarenal gland, has developed.
This is a ductless gland and is much larger in human embryos.
The heart, as may be seen by comparing Figs. 96 and 147, although at first
pressed against the tip of the head, shifts caudally until, in the 35 mm. embryo, it
lies in the thorax opposite the first five thoracic nerves. Later it shifts even
further caudad. The same is true of the other internal organs, the metanephros
excepted. As the chief blood vessels are connected with the heart and viscera,
profound changes in the positions of the vessels are thus brought about, for the
vessels must shift their positions with the organs which they supply.
Ventral Dissections.—Ventral dissections of the viscera are very easily made. With
the safety razor blade, start a cut in a coronal plane through the caudal end of the embryo
and the lower limb buds (Fig. 148). Extend this cut laterad and cephalad through the
body wall and the upper limb bud. The head may be cut away in the same plane of section,
and the cut continued through the body wall and upper limb bud of the opposite side back
caudally to the starting point. Section the embryo in a coronal plane, parallel with the first
section and near the back, so that the embryo will rest upon the flattened surface. With
144 THE DISSECTION OF PIG EMBRYOS
forceps now remove the ventral body wall. By tearing open the wall of the umbilical cord
along one side it may be removed, leaving the intestinal loop intact. Pull away the heart,
noting its external structure. The liver may also be removed, leaving the stomach and in-
testine uninjured. A portion of the septum transversum covering the lungs may be care-
fully stripped away and the lungs thus laid bare.
Dissections made in this way show the trachea and lungs, the esophagus,
stomach and dorsal attachment of the septum transversum, the course of the
intestinal canal, and also the mesonephroi and their ducts. Favorable sections
through the caudal end of the body may show the urogenital sinus, rectum, and
ro SN | »
Trachea
fe Seplum transversum
Midlerian duct anlage ——
V — Stomach
Caecum Mesoncphric duct
“2 Mesonephros
Large ie ~~ i :
Umbilical artery
Rectum
Allantois —
z
Calom ~~
Fic. 148.—Ventral dissection of a 15 mm. pig embryo, showing lungs, digestive canal and mesonephroi.
The ventral body wall, heart and liver have been removed and the limb buds cut across. X 6.
sections of the umbilical arteries and allantois (Figs. 97, 124 and 148). In late
stages, by removing the digestive organs, the urogenital ducts and glands are beau-
tifully demonstrated (Figs. 223 and 224).
DEVELOPMENT OF THE FACE
The heads of pig embryos have long been used for the study of the development of the
face. The heads should be removed by passing the razor blade between the heart and adja-
cent surface of the head, thus severing the neck. Next cut away the dorsal part of the head
by a section parallel to the ventral surface, the razor blade passing dorsal to the branchial
clefts and eyes. Mount, ventral side up, three stages from embryos 6, 12, and 14 mm. long,
as shown in Figs. 97 and 149.
In the early stages (Figs. 97 and 124) the four branchial arches and clefts
are seen. The third and fourth arches soon sink into the cervical sinus, while
the mandibular processes of the first arch are fused early to form the iower jaw.
Laterally the frontal process of the head is early divided into lateral and median
DEVELOPMENT OF THE FACE 145
nasal processes by the development of the olfactory pits. The processes are dis-
tinct and most prominent at 12 mm. (Fig. 149 A). Soon, in 13 to 14mm. embryos,
the median nasal processes fuse with the maxillary processes of the first arch and
constitute the upper jaw (Fig. 149 B). The lateral nasal processes fuse with the
maxillary processes and form the cheeks, the lateral part of the lips, and the ale
of the nose. Later, the median nasal processes unite and become the median
part of the upper lip. Meanwhile the mesial remainder of the original frontal
process (Fig. 149 A) is compressed and becomes the septum and dorsum of the
nose. The development of the olfactory organ will be traced on p. 371.
Lateral nasal process
Olfactory pit —}
Medial nasal process” 3
Mandible—<
Eye
Lacrimal groove
—Maxillary process
Branchial cleft I
Branchial arch II 4 Nip f
Ventral aorta | ipo m
A
- Branchial cleft II
Lateral nasal process
Maxillary process Medial nasal process
Mandible Oral cavity
Branchial cleft I 4 External ear
Fic. 149.—Two stages showing the development of the face in pig embryos. X 7. A, Ventral view of
face of a 12 mm. embryo; B, of a 14 mm. embryo.
The early development of the face is practically the same in human embryos
(Figs. 150 and 370). In embryos of 8 mm. the lateral and median nasal processes
have formed. The maxillary processes next fuse with the nasal processes, after
which the median nasal processes unite. Coincident with these changes the
mandibular processes fuse and from them a median projection is developed which
forms the anlage of the chin.
Epithelial ingrowths begin to form the lips at the fifth week (Fig. 159). As the median
nasal processes and the maxillary processes take part in their development, the failure of these ©
Io
146 THE DISSECTION OF PIG EMBRYOS
parts to fuse may produce hare lip. The line of fusion of the median nasal processes is evi-
dent in the adult as the philtrum. The lips of the newborn child are peculiar in that their
proximal surfaces are covered with numerous villi, finger-like processes which may be a
millimeter or more in length.
\!
Fic. 150.—Development of the face of the human embryo (His). A, Embryo of 8 mm. (X 7.5);
the median frontal process differentiating into median nasal processes or processus globulares, toward
which the maxillary processes of the first visceral arch are extending. B, Embryo of 13.7 mm. (X 5);
the globular, lateral nasal and maxillary processes are in apposition; the primitive naris is now better
defined. C, Embryo of 17 mm. (X 5); immediate boundaries of mouth are more definite and the nasal
orifices are partly formed, external ear appearing. D, Embryo of nearly eight weeks (X 5).
The external ear is developed around the first branchial cleft by the appearance of small
tubercles which form the auricle. The cleft itself becomes the external auditory meatus and
the concha of the ear. (For the development of the external ear see Chapter XIII.)
DEVELOPMENT OF THE PALATE
This may be studied advantageously in pig embryos of two stages: (a) 20 to 25 mm.
long; (b) 28 to 35 mm. long. Dissections may be made by carrying a shallow incision from
the anlage of the mouth back to the external ear on each side (Fig. 152). The incisions are
then continued through the neck in a plane parallel to the hard palate. Before mounting the
preparation, remove the top of the head by a section cutting through the eyes and nostrils.
DEVELOPMENT OF THE PALATE 147
parallel to the first plane of section. Transverse sections through the snout may also be pre-
pared to show the positions of tongue and palatine folds before and after the fusion of the
latter (Fig. 151).
In pig embryos of 20 to 25 mm. the jaws are close together and the mandible
usually rests against the breast. Shelf-like folds of the maxille, the lateral pala-
tine processes, are separated by the tongue and are directed ventrad (Figs. 151 A
and 152 A). The median nasal processes also give rise to a single heart-shaped
2
Nasal septum ; Z
j
Tongue | i
i)
Lateral palatine process —
Nasal septum
“ Turbinate anlage
as)
Lateral palatine process
(
Mandible —+ i
)
\
he Poe.
Fic. 151.—Sections through the jaws of pig embryos to show development of the hard palate. X 8.
A, 22 mm.; B, 34 mm.
structure, the median palatine process (Fig. 152). In embryos of 26 to 28 mm.
the mandible drops, owing to growth changes, and the tongue is withdrawn from
between the palatine processes (Fig. 151 B). With the withdrawal of the tongue
the palatine folds bend upward to the horizontal plane, approach each other and
fuse, thus cutting off the nasal passages from the primitive oral cavity (Fig. 152 B)-
The primitive choane (cf. Fig. 153), formed by rupture of the membrane separating
the olfactory pits from the oral cavity, now lead into the nasal passages, which in
turn communicate with the pharynx by secondary permanent choane. At the
148 THE DISSECTION OF PIG EMBRYOS
point in the median line where the lateral and median palatine processes meet,
fusion is not complete, leaving the incisive fossa, and laterad between the two
Median palatine
process
Raphé of lateral
palatine process
Median palatine
process
Lateral palatine
process
Internal choane
¥
Oral cavity Nasal passage
Anlage of uvula —
Fic. 152.—Dissections to show the development of the hard palate in pig embryos. X 5. A,
Ventral view of palatine processes of a 22 mm. pig embryo, the mandible having been removed; B, Same
of 35 mm. embryo showing fusion of palatine processes.
processes openings persist for some time, which are known as the incisive canals
(of Stenson).
In human embryos these changes are essentially identical (Fig. 153). The
lateral palatine processes begin to fuse cranio-caudally at about the end of the
Frc. 153.—The roof of the mouth of a human embryo about two and a half months old, showing
the development of the palate (after His). > 9. p.g., Processus globularis; p.g.’, palatine process
of processus globularis; mx, maxillary process; mx’, palatine fold of maxillary process. Close to the
angle between this and the palatine process of the processus globularis on each side are the prim-
itive choane.
second month. At the same time palatine bones first appear in the lateral pala-
tine folds and thus form the hard palate. Caudally the bones do not develop
@
DEVELOPMENT OF THE TONGUE 149
and this portion of the folds forms the soft palate and the wvila (Fig. 152). The
unfused backward prolongations of the palatine folds give rise to the pharyngo-
palatine arches, which are taken as the boundary line between the oral cavity
proper and the pharynx in adult anatomy.
After the withdrawal of the tongue, the lateral palatine processes take up a horizontal
position and their edges are approximated because the cells on the ventral sides of the folds
proliferate more rapidly than those of the dorsal side (Schorr, Anat. Hefte, Bd. 36, 1908).
That the change in position of the palatine folds is not mechanical, but due to unequal growth,
may be seen in Fig. 154, a section through the palatine folds of a pig embryo, which shows the
Nasal sepium
Lateral palatine process+ ‘Proliferating cells
Lateral palatine process
Tongue
Fic. 154.—Section through the jaws of a 25 mm. pig embryo to show the change in the position of the
palatine processes with reference to the tongue.
right palatine fold in a horizontal position, although the left fold projects ventral to the
dorsum of the tongue. A region of cellular proliferation may be seen on the under side of
each process.
Anomalies.—The lateral palatine processes occasionally fail to unite in the middle line,
producing a defect known as cleft palaie. The extent of the defect varies considerably, in
some cases involving only the soft palate, while in other cases both soft and hard palates are
cleft. It may also be associated with hare lip.
DEVELOPMENT OF THE TONGUE
The development of the tongue may be studied from dissections of pig embryos 6, 9,
and 13 mm. long. As the pharynx is bent nearly at right angles, it is necessary to cut away
its roof by two pairs of sections passing in different planes. The first plane of section cuts
through the eye and first two branchial arches just above the cervical sinus (Fig. 155, I).
From the surface, the razor blade should be directed obliquely dorsad in cutting toward the
median line. Cuts in this plane should be made from either side. In the same way make
sections on each side in a plane forming an obtuse angle with the first section and passing
dorsal to the cervical sinus (II). Now sever the remaining portion of the head from the
body by a transverse section in a plane parallel to the first (III). Place the ventral portion
of the head in a watch glass of alcohol, and, under the dissecting microscope, remove that
part of the preparation cranial to the mandibular arches. Looking down upon the floor
of the pharynx, remove any portions of the lateral pharyngeal wall which may still interfere
with a clear view of the pharyngeal arches as seen in Figs. 98 and 156. Permanent mounts
of the three stages mentioned above may be made and used for study by the student.
150 THE DISSECTION OF PIG EMBRYOS
The tongue develops as two distinct portions, the body and the root, separated
from each other by a V-shaped groove, the sulcus terminalis. In both human
and pig embryos the body of the tongue is
developed from three anlages which are
formed in front of the second branchial
arches. These are the median, somewhat
triangular tuberculum impar, and the paired
FF “ lateral swellings of the first, or mandibular,
arches, both of which are present in human
Fic. 155.—Lateral view of the head embryos of 5 mm. (Figs. 98 and 157 A).
of « 7 mm. pig embryo. The three A+ this stage, a median ventral elevation
lines indicate the planes of sections to be
made in dissecting the tongue as de- formed by the union of the second branchial
eee arches (and, according to some workers, the
third as well) forms the copula. This, with the portions of the second arches
lateral to it, forms later the base or root of the tongue. Between it and the
(Be is
Branchial arch My }
Tuberculum impar < oR
Branchial arch 2 -
Branchial arch 3 ‘ ;
Branchial arch 4 a E ‘a eee 3
Arytenoid ridge aa pels
fe.
a
Sat
j —_> lingual anlage
—Copula
Epiglottis
Glottis
Branchial arch r
, .
; ‘il
/ oh ky = = Tuberculum impar
Lateral lingual anlage —\— aie —— ay
a Branchial arch 2
Branchial arch 3- ee ; on ‘ f
Branchial arch 4 e a V : ‘ E piglottis
Arytenoid ridge Glottis
Fic. 156.—Dissections showing the development of the tongue in pig embryos. X 12. A, 9 mm. em-
bryo; B, 13 mm. embryo.
tuberculum impar is the point of evagination of the thyreoid gland. The copula
also connects the tuberculum impar with a rounded prominence which is developed
DEVELOPMENT OF THE TONGUE I5t
in the mid-ventral line from the bases of the third and fourth branchial arches.
This is the anlage of the epiglottis, In later stages (Fig. 156 A and B) the lateral
mandibular anlages, bounded laterally by the alveolo-lingual grooves, increase
rapidly in size and fuse with the tuberculum impar, which lags behind in develop-
ment and is said to form the median septum of the tongue. According to Ham-
mar, it atrophies completely. The epiglottis becomes larger and concave on its
ventral surface. Caudal toit, and in early stages continuous with it, are two thick
rounded folds, the arytenoid ridges. Between these is the slit-like glottis leading
into the larynx (see p. 165).
Lateral tongue swellings Thyreoid diverticulum Lateral tongue swellings
Entrance to
y_ Entrance to larynx
larynx Arylenoid
swellings
A B
Fic. 157.—The development of the tongue in human embryos. A, 5 mm.; B, 7 mm. (modified from
Peters).
The foregoing account applies to the early origin of the mucous membrane alone. The
musculature of the tongue is supplied chiefly by the Aypoglossal nerve, and both nerve and
muscles develop caudal to the branchial region in which the tongue develops. The muscu-
lature migrates cephalad and gradually invades the branchial region beneath the mucous
membrane. At the same time, the tongue may be said to extend caudad until its root is cov-
ered by the epithelium of the third and fourth branchial arches. This is shown by the fact
that the sensory portions of the nn. érigeminus and facialis, the nerves of the first and second
arches, supply the body of the tongue, while the nn. glossopharyngeus and vagus, the nerves of
the third and fourth arches, supply the root and the caudal portion of the body of the tongue.
In fetuses of 50 to 60 mm. (C R) the fungiform and filiform papille may be dis-
tinguished as elevations of the epithelium. Taste buds appear in the fungiform
papille of 100 mm. (C R) fetuses and are much more numerous in the fetus than
in the adult. The vallate papille (Fig. 158 A) appear as a V-shaped epithelial
ridge, the apex of the V corresponding to the site of the thyreoid vagination
(foramen cecum). At intervals along the epithelial ridges circular epithelial
downgrowths develop (85 mm. C R) which take the form of inverted and hollow
truncated cones (Fig. 158). During the fourth month circular clefts appear in
the epithelial downgrowths, thus separating the walls of the vallate papilla from
152 THE DISSECTION OF PIG EMBRYOS
the surrounding epithelium and forming the trench from which this type of
papilla derives its name. At the same time, lateral outgrowths arise from the
bases of the epithelial cones, hollow out and form the ducts and glands of Ebner.
: ya
ea cc yeas erabumueaaeee”
Cc
Fic. 158.—Diagrams showing the development of the vallate papille of the tongue (Graberg in McMur-
rich). u, valley; b, von Ebner’s gland.
The taste buds of the vallate papille are also formed early, appearing in embryos
of three months. Foliate papille probably appear at about six months.
DEVELOPMENT OF THE SALIVARY GLANDS
The glands of the mouth are all regarded as derivatives of the ectodermal.
epithelium. Their development has been studied recently by Hammar (Anat.
Anz., Bd. 19, 1901) and by Schulte (Studies in Cancer etc., N. Y., vol. 4, 1913).
Of the salivary glands, the parotid is the first to appear. Its anlage has been
observed in 8 mm. embryos, near the angle of the mouth, as a keel-like flange
in the floor of the alveolo-buccal (i. e. jaw-cheek) groove (Hammar). The flange
elongates, and, in embryos of 17 mm., separates from the epithelial layer, form-
ing a tubular structure which opens into the mouth cavity near the cephalic end
of the original furrow. The tube grows back into the region of the external ear,
branches, and forms the gland in this region, while the unbranched portion of
the tube becomes the parotid duct.
The submazillary gland arises at 11 mm. as an epithelial ridge in the alveolo-
lingual (i. e. jaw-tongue) groove, its cephalic end located caudal to the frenulum
of the tongue. The caudal end of the ridge soon begins to separate from the epi-
thelium and extends caudad and ventrad into the submaxillary region, where it
enlarges and branches to form the gland proper; its cephalic unbranched portion,
persisting as the duct. soon hollows out.
The sublingual and alveolo-lingual glands develop in 24 mm. embryos as several solid
evaginations of epithelium from the alveolo-lingual groove (Fig. 163). Each group, usually
regarded as a sublingual gland, really consists of the sublingual proper, with its ductus major,
and of about ten equivalent alveolo-lingual glands.
The solid branched anlages of the salivary glands last become hollow at their tips.
The glands continue to differentiate until after birth. Mucin cells may be distinguished
by the sixteenth week and acinus cells in the parotid glands at five months (McMurrich).
DEVELOPMENT OF THE TEETH 153
DEVELOPMENT OF THE TEETH
The enamel organs, which give rise to the enamel of the teeth and are the
moulds, so to speak, of the future teeth, are of ectodermal origin. ‘There first ap-
pears in embryos of about 11 mm. an ectodermal downgrowth, the dental ridge or
lamina, on the future alveolar portions of the upper and lower jaws (Fig. 159).
These laminz are parallel and mesial to the labial grooves. At intervals, on
L Lower lip
Mandible
Dental lamina
~/_Labial groove
Dental papilla
Dental lamina
A B
Fic. 159.—Early stages in the development of the teeth (Rise). A,at 17mm. (X 90); B,at 41 mm.
(X 45).
each curved dental ridge or lamina a series of thickenings develop, the anlages
of the enamel organs (Fig. 160). Soon the ventral side of each enamel organ be-
comes concave (fetuses of 40 mm. C H) forming an inverted cup and the con-
cavity is. occupied by dense mesenchymal tissue, the dental papilla (Tigs. 159 B
and 162). An enamel organ with dental papilla forms the anlage of each decid-
Dental groove
Enamel organs
Oral epithelium
Free edge of
the dental
lamina
tal lami en" \/
Dene denne Papille ce organs Necks of enamel organs
A B Cc D
Fic. 160.—Diagrams showing the early development of three teeth. One of the teeth is shown in vertical
section (Lewis and Stohr).
ual or milk tooth. Ten such anlages are present in the upper jaw and ten in
the lower jaw of a 40 mm. fetus. The connection of the dental anlages with the
dental ridge is eventually lost. The position of the tooth anlage between the
tongue and lip is shown in Fig. 163.
154 THE DISSECTION OF PIG EMBRYOS
The anlages of those permanent teeth which correspond to the decidual, or
milk teeth, are developed in another series along the free edge of the dental
Labial
groove
Dental
lamina
Milk
molar I
Aboral
prolonga-
tion of
dental
lamina
Fic. 161.—Dental lamina and anlages of the milk teeth of the upper jaw from a fetus of 115 mm
(Rése in Kollmann).
Dental lamina
~—Epidermis
Outer enamel layer
Enamel pulp
Inner enamel layer
Dental papilla
me al
Fic. 162.—Section through the upper first decidual incisor tooth froma 65 mm. human fetus. X 70.
lamina (Fig. 160 D) and come to lie mesad of the decidual teeth. In addition,
the anlages of three permanent molars are developed on each side, both above
and below, from a backward or aboral extension of the dental lamina, entirely
DEVELOPMENT OF THE TEETH 155
free from the oral epithelium (Fig. 161). The anlages of the first permanent
molars appear at seventeen weeks (180 mm. C H), those of the second molars at
Tip of tongue
Fp Epidermis of lip
Sulouavitiorp dua ee gs = j Te Enamel organ of tooth
ae = Sf
Sublingual duct— ——+
Dental papilla
Meckel’s cartilage
Bone of mandible
Fic. 163.—Parasagittal section through the mandible and tongue of a 65 mm. human fetus showing
the position of the anlage of the first incisor tooth. X 14.
six weeks after birth, while the anlages of the third permanent molars or wisdom
teeth are not found until the fifth year. The permanent dentition of thirty-two
teeth is then complete.
Inner enamel layer
(ameloblasts)~ \\\
Dentine and___ \
dentinal fibers
Lape Sh
Dental pulpe---- rhe (sc JB
WOE
Fic. 164.—Section through a portion of the crown of a developing tooth showing the various layers
(after Tourneux in Heisler).
The internal cells of the enamel organs are at first compact, but later by the
development of an intercellular matrix the cells separate, forming a reticulum
resembling mesenchyme, termed the enamel pulp (Fig. 162). The outer enamel
156 THE DISSECTION OF PIG EMBRYOS
cells, at first cuboidal, flatten out and later form a fibrous layer. The inner
enamel cells bound the cup-shaped concavity of the enamel organ. Over the
crown of the tooth these cells, the ameloblasts, become slender and columnar in
Dental sac
——-_
Outer layer — Inner layer
\ a Outer enamel cells
Enamel pulp
\ Inner enamel cells
\
ug (ameloblasts)
Enamel
Dentine Epithelial sheath
ie eee
Odontoblasts
Dental papilla (fulure pulp)
wz
Blood vesse!”
Bony trabecula of jaw—
Fic. 165.—Longitudinal section of a deciduous tooth of a newborn dog. X 42. The white
spaces between the inner enamel cells and the enamel are artificial and due to shrinkage (Lewis
and Stéhr).
form, producing the enamel layer of the tooth along their basal ends (Fig. 164).
The enamel is laid down first an an uncalcified fibrillar layer which later becomes
calcified in the form of enamel prisms one for each ameloblast. The enamel is
formed first at the apex of the crown of the tooth and extends downward toward
DEVELOPMENT OF THE TEETH 157
the root. The enamel cells about the future root of the tooth remain cuboidal
or low columnar in form, come into contact with the outer enamel cells, and the
two layers constitute the epithelial sheath of the root which does not produce
enamel prisms (Fig. 165).
The Dental Papilla.—The outermost cells of the dental papilla at the end
of the fourth month arrange themselves as a definite layer of columnar epithelium.
Since they produce the dentine, or dental bone, these cells are known as odonto-
blasts (Fig. 165). When the dentine layer is developed, the odontoblast cells
remain internal to it, but branched processes from them (the dentinal fibers of
Tomes) extend into the dentine and form the dental canaliculi. Internal to the
odontoblast layer, the mesenchymal cells differentiate into the dental pulp, pop-
ularly known as the “nerve” of the tooth. This is composed of a framework of
reticular tissue in which are found blood vessels, lymphatics, and nerve fibers.
The odontoblast layer persists throughout life and continues to secrete dentine,
so that eventually the root canal may be obliterated.
Dental Sac.—The mesenchymal tissue surrounding the anlage of the tooth
gives rise to a dense outer layer and a more open inner layer of fibrous connective
tissue. These layers form the dental sac (Fig. 165). Over the root of the tooth
a layer of osteoblasts or bone forming cells develops, and, the epithelial sheath
formed by the enamel layers having disintegrated, these osteoblasts deposit about
the dentine a layer of bone which is known as the substantia ossea or cement.
The cement layer contains typical bone cells but no Haversian canals. As the
teeth grow and fill the alveoli, the dental sac becomes a thin vascular layer, con-
tinuous externally with the alveolar periosteum, internally with the periosteum of
the cement layer of the tooth.
When the crown of the tooth is fully developed the enamel organ disinte-
grates, and, as the roots of the teeth continue to grow, their crowns approach the
surface and break through the gums. The periods of eruption of the various
milk or decidual teeth vary with race, climate, and nutritive conditions. Usually
the teeth are cut in the following sequence:
DECIDUAL OR MILK TEETH
Median Incisorssic.s0228 es qavtewnaen reed ares sixth to eighth month.
Lateral Incisors:..as.e..ssan cgowmewenne vas ee ee danse eighth to twelfth month.
Hirst; Molarsy ao conti ce ace ucwencnawaeee toe tas twelfth to sixteenth month.
Canines isc: sop is3.5 ciawalae bund aAguuaauigeeGl in Mosel seventeenth to twentieth month.
Second MOlaTS i csctacaed cued tara UNNe EMEA ara ttleey twentieth to thirty-sixth month.
The permanent teeth are all present at the fifth year. They are located
mesial to the decidual teeth (Fig. 166), and, before the permanent teeth begin
I 58 THE DISSECTION OF PIG EMBRYOS
to erupt, the roots of the milk teeth undergo partial resolution, their dental pulp
dies, and they are eventually shed. Toward the sixth year, before the shedding
Permanent second molar ~—¥t\:
Permanent premolars
Mental foramen
Permanent canine smell
Permanent incisors
Fic. 166.—The skull of a five-year-old child showing positions of the decidual and permanent teeth
(Sobotta~-McMurrich).
of the deciduous teeth begins, each jaw may contain twenty-six teeth. The
permanent teeth are “cut” as follows (McMurrich in Keibel and Mall, vol. 2)
Pirst MGlates. 9. o0'sieran ssiey see ey eeenecomin sad eaieahnd seventh year.
Median: In¢isors. sider eye age cee eta os geal eighth year.
Lateral AnCisOrsis if sccgaseee ee ia eae a 4 mE ninth year.
First Premolarss.3.2 Qeecang ae se ede a gectneede Ga eRS tenth year.
Second Premolars - 0.0... 0c cco cn ba eee ha eee es eleventh year.
Ss aes ne £8 GAHAN d Ray Ea ieee Sa Se US thirteenth to fourteenth year.
Third Molars (Wisdom Teeth)...................-. seventeenth to fortieth year.
The teeth of vertebrates are homologues of the placoid scales of elasmobranch fishes
(sharks and skates). The teeth of the shark resemble enlarged scales, and many generations
of teeth are produced in the adult fish. In some mammalian embryos three or even four
dentitions are present. The primitive teeth of mammals are of the canine type, and from
this conical tooth the incisors and molars have been differentiated.
Anomalies.—Dental anomalies are frequent and may consist in the congenital absence
of some or all of the teeth, or in the production of more than the normal number. Defective
teeth are frequently associated with hare lip. Cases have been noted in which, owing to
defect of the enamel organ, the enamel was entirely wanting. Many cases in which a third
dentition occurred have been recorded and occasionally fourth molars may be developed
behind the wisdom teeth.
CHAPTER VII
THE ENTODERMAL CANAL AND ITS DERIVATIVES: THE BODY
CAVITIES
WHEN the head- and tail folds of the embryo develop, there are formed both
cranial and caudal to the spherical vitelline sac blind entodermal tubes, the Sore-
gut ana hind-gut respectively (Figs. 79 and 167 A). The region between these
intestinal tubes, open ventrally into the yolk sac, is sometimes termed the mid-gut.
Pharynx
Pharyngeal
Pharynx membrane
Pharyngeal Thyreoid-_” fg
membrane gland y
Pericardial
Pericardial
cavily cavily
Fore gut ete
Hepatic
diverticulum _ Hepatic
diverticulum
Yolk stalk
Volk stalk
Hind-gut
Cloacal Allantois
membraie
Allantois
a Cloacal
Cloaca membrane
Cloaca
Hind-gut
Fic. 167.—Diagrams showing in median sagittal section the human alimentary canal, pharyngeal
and cloacal membranes. X 35. A, 2 mm. embryo (modified after His); B, 2.5 mm. embryo (after
Thompson).
As the embryo and the yolk sac at first grow more rapidly than the connecting
region between them, this region is apparently constricted and becomes the
yolk stalk or vitelline duct. At either end the entoderm comes into contact ven-
trally with the ectoderm. Thus there are formed the pharyngeal membrane of the
159
160 THE ENTODERMAL CANAL AND THE BODY CAVITIES
fore-gut, the cloacal membrane of the hind-gut. In 2 mm. embryos the pharyngeal |
membrane separates the ventral ectodermal cavity, or stomodeum, from the
pharyngeal cavity of the fore-gut. Cranial to the membrane is the ectodermal
diverticulum, Rathke’s pocket. In 2.5 to 3 mm. embryos (Fig. 167 B) the
pharyngeal membrane ruptures and the stomodeum and pharynx become
continuous. The blind termination of the fore-gut apparently forms Seessel’s
pocket.
The fore-gut later forms part of the oral cavity and is further differentiated
into the pharynx and its derivatives, and into the esophagus, respiratory organs,
stomach, duodenum, jejunum, and a portion of the ileum. From the duodenum
arise the liver and pancreas. The hind-gut, beginning at the attachment of the
yolk stalk extends caudally to the cloaca, into which the allantois opens in 2 mm.
embryos. The hind-gut is differentiated into the ileum, cecum, colon, and rec-
tum. The cloaca is subdivided into the rectum and urogenital sinus (for its de-
velopment see Chapter VIII). At the same time the cloacal membrane is
separated into a urogenital membrane and into an anal membrane. The latter
eventually ruptures, forming the awus. The yolk stalk usually loses its connec-
tion with the entodermal tube in embryos of about 7 mm. (Fig. 179).
We have seen how the palatine processes divide the primitive oral cavity
into the nasal passages and mouth cavity of the adult, and have described the
development of the tongue, teeth, and salivary glands—organs derived wholly or
in part from the ectoderm. It remains to trace the development of the pharynx
and the intestinal tract and their derivatives.
PHARYNGEAL POUCHES
There are developed early from the lateral wall of the pharynx paired out-
growths which are formed in succession cephalo-caudad. In 4 to 5 mm. embryos,
five pairs of such pharyngeal pouches are present, the fifth pair being rudimentary
(Figs. 86 and 87). Meantime, the pharynx has been flattened dorso-ventrally
and broadened laterally and cephalad, so that it is triangular in ventral view
(Figs. 87 and 168).
From each pharyngeal pouch develop small dorsal and large ventral diver-
ticula. All five pouches come into contact with the ectoderm of the branchial
clefts, fuse with it, and form the closing plates. Only occasionally do the closing
plates become perforate in human embryos. The first and second pharyngeal
pouches soon connect with the pharyngeal cavity through wide openings. The
third and fourth pouches grow laterad and their diverticula communicate with
PHARYNGEAL POUCHES 1601
the pharynx through narrow ducts in 10 to 12 mm. embryos (Fig. 168). When
the cervical sinus (p. 97) is formed, the ectoderm of the second, third, and fourth
branchial clefts is drawn out to produce the transient branchial and cervical ducts
and the cervical vesicle. These are fused at the closing plates with the entoderm
of the second, third, and fourth pharyngeal pouches.
The first and second pouches soon differ from the others in form, and give rise
to an entirely different type of permanent structures. With the broadening of
Branchial duct 2 Epithelial body of 34 pouch
Pharyngeal pouch 1
Branchial cleft 1
Pharyngeal pouch 2
Gorvital sins Pharyngeal pouch 3
Cervical vesicle
Thymus anlage
Epithelial body of 4th pouch
Pharyngeal pouch 4
Pharyngeal pouch 5
Esophagus
Trachea hag
5 A pical bud of right lung
Gall bladder
Duodenum
Fic. 168.—A reconstruction of the pharynx and fore-gut of an 11.7 mm. human embryo seen in dorsal
view (after Hammar). The ectodermal structures are stippled.
the pharynx the first two pouches acquire a common opening into it. The
first pouch later differentiates into the tympanic cavity of the middle ear and into
the auditory (Eustachian) tube. By the growth and lateral expansion of the
pharynx, the second pouch is absorbed into the pharyngeal wall, its dorsal angle
alone persisting, to be later transformed into the tonsillar and supratonsillar fosse.
The third, fourth, and fifth pouches give rise to a series of ductless glands,-the
thymus, parathyreoids, and the ultimobranchial bodies.
Ir
162 THE ENTODERMAL CANAL AND THE BODY CAVITIES
A mound of lymphoid tissue presses against the epithelium of the tonsillar fossa in
140 mm. (C R ?) fetuses and forms the palatine tonsil. The lymphocytes are probably of
mesodermal origin (Hammar, Maximow).
Imperfect closure of the branchial clefts (usually the second) leads to the formation of
cysts, diverticula, or even of fistula. According to Hammar (Arch. f. mikr. Anat., Bd. 61,
1903), the lateral pharyngeal recess (of Rosenmuller) is not a persistent portion of the second
pouch as His asserted.
A subepithelial infiltration of lymphocytes during the sixth month gives rise to the
median pharyngeal tonsil, which like the lingual tonsil is not of pharyngeal pouch origin.
Immediately caudad is a recess, the pharyngeal bursa, formed by a persistent connection of
the epithelium with the notochord (Huber).
THE THYMUS
The thymus anlage appears in 10 mm. embryos as a ventral and medial
prolongation of the third pair of pouches (Figs. 168 and 169). The ducts con-
necting the diverticula with the pharynx soon disappear so that the thymus an-
lages are set free. At first hollow tubes, they soon lose their cavities and their
Foramen cecum
Thyreoid anlage
Thymus anlages
Post-branchial body
Fic. 169.—Diagram in ventral view of the pharynx and pharyngeal pouches, showing the origin
of the thymus and thyreoid glands and of the epithelial bodies (modified after Groschuff and Kohn).
J-V, first to fifth pharyngeal pouches.
lower ends enlarge and migrate caudally into the thorax, passing usually ven-
tral to the left vena anonyma. Their upper ends become attentuate and atrophy,
but may persist as an accessory thymus lobe (Kohn). The enlarged lower ends of
the anlages form the body of the gland, which is thus a paired structure (Fig. 170).
At 50 mm. (C R) the thymus still contains solid cords and small closed vesicles of
entodermal cells. From this stage on, in development, the gland becomes more
and more lymphoid in character. Its final position is in the thorax, dorsal to the
cranial end of the sternum. It grows under normal conditions until puberty,
THE EPITHELIAL BODIES OR PARATHYREOIDS 163
after which its degeneration begins. This process proceeds slowly in healthy
individuals, rapidly in case of disease. The thymus may function normally
until after the fortieth year.
The ventral diverticulum of the fourth pouch is a rudimentary thymic anlage.
It usually atrophies.
It is now generally believed that the entodermal. epithelium of the thymus is converted
into reticular tissue and thymic corpuscles. The “lymphoid” cells are regarded by Hammar,
Maximow, and recently by Badertscher (Amer. Jour. Anat., vol. 17, 1915) as immigrant
Jugular Carotid Carotid
vein artery artery
Thyreoid
Parathyreoid IV.
Parathyreoid III.
Thyreoid
Parathyreoid IV.
Parathyreoid III.
Fic. 170.—Reconstruction of the thymus and thyreoid glands in a 26 mm. human embryo (after
Tourneaux and Verdun). X 15.
lymphocytes derived from the mesoderm. According to Stéhr, they are not true lympho-
cytes, but are derived from the thymic epithelium. Weill (Arch. f. mikr. Anat., Bd. 83, 1913)
has observed the development of granular leucocytes in the human thymus gland.
THE EPITHELIAL BODIES OR PARATHYREOIDS
The dorsal diverticula of the third and fourth pharyngeal pouches each give
rise to a small mass of epithelial cells termed an epithelial body (Fig. 169). Two
pairs of these bodies are thus formed, and, with the atrophy of the ducts of the
pharyngeal pouches, they are set free and migrate caudalward. They eventually
lodge in the dorsal surface of the thyreoid gland, the pair from the third pouch
lying one on each side at the caudal border of the thyreoid in line with the thymus
anlages (Fig. 170). The pair of epithelial bodies derived from the fourth pouches
are located on each side near the cranial border of the thyreoid. From their
ultimate relation to the thyreoid tissue the epithelial bodies are often termed
parathyreoid glands. The solid body is broken up into masses and cords of poly-
gonal entodermal cells intermingled with blood vessels. In postfetal life, lumina
may appear in the cell masses and fill with a colloid-like secretion.
164 THE ENTODERMAL CANAL AND THE BODY CAVITIES
THE ULTIMOBRANCHIAL OR POSTBRANCHIAL BODIES
The ultimobranchial body is the derivative of the fifth pharyngeal pouch
(Fig. 169). With the atrophy of the duct of the fourth pouch it is set free and
migrates caudad with the parathyreoids. It forms a hollow vesicle which has
been erroneously termed the lateral thyreoid. According to Grosser (Keibel and
Mall, vol. 2) and Verdun, it takes no part in forming thyreoid tissue, but atrophies.
Kingsbury (Anat. Anz., Bd. 47, 1915) denies the origin of the ultimobranchial
body from any specific pouch, and asserts it is “merely formed by a continued
growth activity in the branchial entoderm.”
THE THYREOID GLAND
In embryos with five to six primitive segments (1.4 mm.) there appears in
the mid-ventral wall of the pharynx, between the first and second branchial arches,
a small out-pocketing, the thyreoid anlage. In 2.5 mm. embryos it has become a
stalked vesicle (Figs. 167 B and 87) Its stalk, the t#yreoglossal duct, opens at the
aboral border of the tuberculum impar of the tongue (Fig. 157 A); this spot is
represented permanently by the foramen caecum (Fig. 180). The duct soon
atrophies and the bilobed gland anlage (Fig. 169) loses its lumen and breaks up
into irregular solid anastomosing plates of tissue as it migrates caudad. It takes
up a transverse position with a lobe on each side of the trachea and larynx (Fig.
170). In embryos of 24 mm. discontinuous lumina begin to appear in swollen
portions of the plates; these represent the primitive thyreoid follicles (Norris,
Amer. Jour. Anat., vol. 20, 1916).
LARYNX, TRACHEA AND LUNGS
In embryos of 23 segments, the anlage of the respiratory organs appears as a
groove in the floor of the entodermal tube just caudal to the pharyngeal pouches.
This groove produces an external ridge on the ventral wall of the tube, a ridge
which becomes larger and rounded at its caudal end (Fig. 171). The laryngo-
tracheal groove and the ridge are the anlages of the larynx and trachea. The
rounded end of the ridge is the unpaired anlage of the lungs.
Externally two lateral longitudinal grooves mark off the dorsal esophagus
from the ventral respiratory anlages. The lung anlage rapidly increases in size
and becomes bilobed in embryos of 4 to 5 mm. A fusion of the lateral furrows
progressing cephalad, constricts first the lung anlages and then the trachea from
the esophagus. At the same time the laryngeal portion of the groove and ridge
advances cranially until it lies between the fourth branchial arches (Fig. 87). At
LARYNX, TRACHEA AND LUNGS 165
5 mm. the respiratory apparatus consists of the laryngeal groove and ridge, the
tubular trachea, and the two lung buds (Fig. 165 D).
The Larynx.—In embryos of 5 to 6 mm. the oral end of the laryngeal groove
is bounded on either side by two rounded prominences, the arytenoid swellings,
which, continuous orally with a transverse ridge, form the furcula of His (Fig.
157 B). The transverse ridge becomes the epiglottis, and, as we saw in connec-
tion with the development of the tongue, it is derived from the third and fourth
branchial arches. In embryos of 15 mm. the arytenoid swellings are bent near
the middle. Their caudal portions become parallel, while their cephalic portions
aA B Cc
Trachea
Respiratory anlage
——Lung bud —
Esophagus
Esophagus
D E
Trachea
Trachea
Apical bud
Primary bronchus
Ventral bud €
Fic. 171.—Diagrams of stages in the early development of the trachea and lungs of human embryos
(based on reconstructions by Bremer, Broman, Grosser, and Narath). X about 50. A, 2.5mm; B,
4mm; C, stage B in side view; D, 5 mm.; E, 7 mm.
diverge nearly at right angles (Fig. 172). The opening into the larynx thus be-
comes T-shaped and ends blindly, as the laryngeal epithelium has fused. In 40
mm. fetuses (C R) this fusion is dissolved, the arytenoid swellings are withdrawn
from contact with the epiglottis, and the entrance to the larynx becomes oval in
form (Fig. 173). At 27 mm. the ventricles of the larynx appear and at 37 mm.
(C R) their margins indicate the position of the-vocal cords. The epithelium of
the vocal cords is without cilia. The elastic and muscle fibers of the cords are
developed by the fifth month.
At the end of the sixth week the cartilaginous skeleton of the larynx is indicated by sur-
rounding condensations of mesenchyme. The cartilage of the epiglottis appears relatively
166 THE ENTODERMAL CANAL AND THE BODY CAVITIES
late. The thyreoid cartilage is formed as two lateral plates, each of which has:two centers of
chondrification. These plates grow ventrad and fuse in the median line.
The anlages of the cricoid and arytenoid cartilages are at first continuous. Later, sepa-
rate cartilage centers develop for the arytenoids. The cricoid is at first incomplete dorsad,
but eventually forms a complete ring.
The cricoid may therefore be regarded
as a modified tracheal ring. The cornic-
ulate cartilages represent separated por- _ pl.ph.e.----
tions of the arytenoids. The cuneiform
cartilages are derived from the cartilage ot es
of the epiglottis.
cun.
4 ------ corn,
ft. ee
Fic. 172.—Entrance to larynx in a 15 to 16 Fic. 173.—The larynx of 160 to 230 mm.
mm. human embryo (from Kallius). x 15. 4, human fetuses (Soulié and Bardier). 6. From
Tuberculum impar; ~, pharyngo-epiglottic fold; a dissection. 06., Base of tongue; ¢., epiglottis;
e, epiglottic fold; /.e, lateral part of epiglottis; f.z.a., interarytenoid fissure; 0./., orifice of larynx;
cun., cuneiform tubercle; corn., corniculate tu- pl.a.e., plica ary-epiglottica; pl.ph.e., plica phar-
bercle. yngo-epiglottica; cun., cuneiform tubercle; corn.,
corniculate tubercle.
The Trachea.—This gradually elongates during development and its colum-
nar epithelium becomes ciliated. Muscle fibers and the anlages of the cartilag-
Fic. 174.—Ventral and dorsal views of the lungs from a human embryo of about 9 mm. (after Merkel).
Ap., Apical bronchus; Dr, D2, etc., dorsal, Vz, V2, etc., ventral bronchi; Jc., infracardial bronchus.
inous rings appear at 17mm. The glands develop as ingrowths of the epithelium
during the last five months of pregnancy.
LARYNX, TRACHEA AND LUNGS 107
The Lungs.—Soon after the lung anlages or stem buds are formed (in 5 mm.
embryos), the right bronchial bud becomes larger and is directed more caudally
(Fig. 171). At 7 mm. the stem bronchi give rise to two bronchial buds on the
right side, to one on the left. The smaller bronchial bud on the right side is the
apical bud. The right and left chief buds, known as ventral bronchi, soon bifur-
cate. There are thus formed three bronchial rami on the right side, two on the
left, and these correspond to the primitive lobes of the lungs (Fig. 174).
On the left side, an apical bud is interpreted as being derived from the first ventral bron-
chus (Fig. 174). It develops later and remains small so that a lobe corresponding to the
upper lobe of the right lung is not developed (Narath). The upper lobe of the left lung thus
would correspond to the upper and middle lobes of the right lung.
Mediastinum
+ Parietal pleura
Pleuro-peritoneal membrane Pleural cavity
Pleuro-peritoneal membrane
Visceral pleura )- Esophagus
;
Coronary ligament \¢
Inferior vena cava
Sinusoids of liver
Ductus venosus
Falciform ligame
EE Wall of umbilical cord
Fic. 175.—Transverse section through the lungs and pleural cavities ofa 10mm. humanembryo. X 23.
The bronchial anlages continue to branch in such a way that the stem bud
is retained as the main bronchial stem (Fig. 174). That is, the branching is mono-
podial, not dichotomous, lateral buds being given off from the stem bud proximal
to its growing tip. Only in the later stages of development has dichotomous
branching of the bronchi and the formation of two equal buds been described.
Such buds, formed dichotomously, do not remain of equal size (Flint, Amer.
Jour. Anat., vol. 6, 1906).
The entodermal anlages of the lungs and trachea are developed in a median
mass of mesenchyme dorsal and cranial to the peritoneal cavity. This tissue
forms a broad mesentery termed the mediastinum (Fig. 175). The right and left
168 THE ENTODERMAL CANAL AND THE BODY CAVITIES
stem buds of the lungs grow out laterad, carrying with them folds of the mesoderm.
The branching of the bronchial buds takes place within this tissue which is cov-
ered by the mesothelial lining of the body cavity. The terminal branches of
the bronchi are lined with entodermal cells which flatten out and form the res-
piratory epithelium of the adult lungs. The surrounding mesenchyme differen-
tiates into the muscle, connective tissue, and cartilage plates of the lung, tracheal,
and bronchial walls. Into it grow blood vessels and nerve fibers. When the
pleural cavities are separated from the pericardial and peritoneal cavities, the
mesothelium covering the lungs, with the connective tissue underlying it, becomes
the visceral pleura. The corresponding layers lining the thoracic wall form the
parietal pleura. These layers are derived respectively from the visceral (splanch-
nic) and parietal (somatic) mesoderm of the embryo.
; } -+--Pulmonary artery
/ #4 »® ’&\
/
‘t
Pulmonary vein
\ 4 a
be ‘ E> —.,
ae FE
/ \. es \
s /
Fic. 176.—Ventral view of the lungs of a 10.5 mm. embryo showing the pulmonary arteries and veins
(His in McMurrich). X 27. Ep., Apical bronchus; J, IZ, primary bronchi.
In 11 mm. embryos the two pulmonary arteries, from the sixth pair of aortic
arches, course lateral then dorsal to the stem bronchi (Fig. 176). The right
pulmonary artery passes ventral to the apical bronchus of the right lung. The
single pulmonary vein receives two branches from each lung, two larger veins
from each lower lobe, two smaller veins from each upper lobe and the middle lobe
of the right side. These four pulmonary branches course ventrad and drain into
the pulmonary trunk. When this common stem is taken up into the wall of the
left atrium, the four pulmonary veins open directly into the latter.
According to Killiker, the air cells or alveoli of the lungs begin to form in the sixth
month and their development is completed during pregnancy. Elastic tissue appears during
the fourth month in the largest bronchi. The abundant connective tissue found between the
bronchial branches in early fetal life becomes reduced in its relative amount as the alveoli
of the lungs are developed.
Before birth the lungs are relatively small, compact, and possess sharp margins. They
lie in the dorsal portion of the pleural cavities. After birth they normally fill with air, ex-
ESOPHAGUS, STOMACH AND INTESTINE 169
panding and completely filling the pleural cavities. Their margins become rounded and the
compact fetal lung tissue, which resembles that of a gland in structure, becomes light and
spongy, owing to the enormous increase in the size of the alveoli and blood vessels. Because
of the greater amount of blood admitted to the lungs after birth, their weight is suddenly
increased.
In the most common anomaly involving the esophagus and trachea the former is divided
transversely, the trachea opening into the lower portion of the esophagus, while the upper
portion of the esophagus ends blindly.
ESOPHAGUS, STOMACH AND INTESTINE
Esophagus.—The esophagus in 4 to 5 mm. embryos is a short tube, flattened
laterally, extending from the pharynx to the stomach. It grows rapidly in length
and in 7.5 mm. embryos its diameter decreases both relatively and absolutely
(Forssner). At this stage the esophageal epithelium is composed of two layers
of columnar cells.
In 20 mm. embryos, vacuoles appear in the epithelium and increase the size of the
lumen which remains open throughout. In later stages the wall of the esophagus is folded,
and ciliated epithelial cells appear at 44mm. (C R). The number of cell layers in the epi-
thelium increases, until, at birth, they number nine or ten. Glands are developed as epi-
thelial ingrowths. The circular muscle layer is indicated at 10 mm. but the longitudinal
muscle fibers do not form a definite layer until 55 mm. (C R). (F. T. Lewis in Keibel
and Mall, vol. 2.) These layers appear in similar time-sequence throughout the entire
digestive tract.
Stomach.—The stomach appears in embryos of 4 to 5 mm. as a laterally
flattened, fusiform enlargement of the fore-gut caudal to the lung anlages (Figs.
177 and 178). Its epithelium is early thicker than that of the esophagus and is sur-
rounded by a thick layer of splanchnic mesoderm. It is attached dorsally to the
body wall by its mesentery, the greater omentum, and ventrally to the liver by
the lesser omentum (Fig. 190 B). The dorsal border of the stomach both enlarges
locally to form the fundus, and also grows more rapidly than the ventral wall
throughout its extent, thus producing the convex greater curvature. The whole
stomach becomes curved and its cranial end is displaced to the left by the en-
larging liver (Fig. 168). This forms a ventral concavity, the lesser curvature, and
produces the first flexure of the duodenum.
The rapid growth of the gastric wall along its greater curvature also causes
the stomach to rotate upon its long axis until its greater curvature, or primitive
dorsal wall, lies to the left, its ventral wall, the lesser curvature, to the right
(Fig. 201). The original right side is now dorsal, the left side ventral in position,
and the caudal or pyloric end of the stomach is ventral and to the right of its
cardiac or cephalic end. The whole organ extends obliquely across the peri-.
170 THE ENTODERMAL CANAL AND THE BODY CAVITIES
toneal cavity from left to right (cf. Fig. 138). The change in position progresses
rapidly and is already completed early in the second month (12 to 15 mm.).
The rotation of the stomach explains the asymmetrical position of the vagus nerves
of the adult organ, the left nerve supplying the ventral wall of the stomach, orig-
inally the left wall, while the right vagus supplies the dorsal wall, originally the
right.
Gastric pits are indicated in 16 mm. embryos, and, at 100 mm. (C R), gland cells of the
gastric glands are differentiated. These undoubtedly arise from the gastric epithelium
(Lewis). The cardiac glands are developed early (91 mm. (C R) fetuses), and, according to
Lewis, there is no ‘evidence in favor of Bensley’s conclusion that the cardiac glands are
decadent . fundus glands.”
Pharynx ons
Root of tongue ao
Thyreoid ~ SS XN
Tip of tongue = SS \
—— Stomach
x
aes
LS
S MW
Sy — Liver
Yolk stalk
Allantoic stalk
Dorsal
pancreas
Cloaca
Metane phros
Mesonephric duct
Fic. 178.—Reconstruction of a 5 mm. human embryo showing the entodermal canal and its derivatives
(His in Kollmann). X 25.
The attachment of the yolk stalk may persist in later stages (12 to 14 mm. embryos,
according to Keibel, Elze, and Thyng). Also in 2 per cent. of adult intestines a pouch 3 to 9
cm. long is found about 80 cm. above the colic valve, where the yolk stalk was formerly
attached. This pouch, the diverticulum of the ileum or Meckel’s diverticulum, is of clinical
importance as it may cause intestinal strangulation in infants.
At the stage shown in Fig. 179, the dorsal pancreatic anlage has been de-
veloped from the duodenum, and, in the caudal limb of the intestinal loop, there
172 THE ENTODERMAL CANAL AND THE BODY CAVITIES
is formed an enlargement, due to a ventral bulging of the gut wall, which marks
the anlage of the cecum and the boundary line between the Jarge and small in-
testine. The cecal anlage gives rise later both to the adult cecum and to a more
distal appendage, the vermiform process, which lags in development and remains
small.
Succeeding changes in the intestine consist (1) in its torsion and coiling due
to its rapid elongation, and (2) in the differentiation of its several regions. As
Rathke’s pocket Hypophysis
Thyreoid
Notochord
Pericardium
Trachea
Hepatic duct
Esophagus
Gall bladder
Stomach
Yolk stalk
Liver
Allantois Dorsal pancreas
Ventral pancreas
Cloacal membrane
Caecum
: : ms
Urogenital sinws °
mae
Peritoneal cavity:
Tail gut Mesonephric duct
Rectum
Fic. 179.—Diagram, in median sagittal section, showing the digestive canal of a 9 mm. human embryo
dapted from Mall). xX 9.
the gut elongates in 9 to 10 mm. embryos, the intestinal loop rotates. Asa resuit,
its caudal limb lies at the left and cranial to its cephalic limb (Fig. 179). At
this stage the intestinal loop enters the ccelom of the umbilical cord, thus causing
a temporary umbilical hernia.
The small intestine soon lengthens rapidly and at 17 mm. (Fig. 180) forms
loops within the umbilical cord. Six primary loops occur and these may be
recognized in the arrangement of the adult intestine (Mall, Bull. Johns Hopkins
Hosp., vol. 9, 1898). In embryos of 42 mm. the intestine has returned from the
ESOPHAGUS, STOMACH AND INTESTINE 173
umbilical cord into the abdominal cavity through a rather small aperture; the
coelom of the cord is soon after obliterated.
In embryos between 10 and 30 mm., vacuoles appear in the wall of the duodenum and
epithelial septa completely block the lumen. The remainder of the small intestine remains
open, although vacuoles form in its epithelium. Vulli appear as rounded elevations of the
epithelium at 23 mm. (Johnson). They begin to form at the cephalic end of the jejunum, and
at 130 mm. (C R) they are found throughout the small intestine (Berry). Intestinal glands
appear as ingrowths of the epithelium about the bases of the villi. They develop first in the
duodenum at 91mm.(C R). The duodenal glands (of Brunner) are said to appear during the
= Brain
Hypophysis
ZN \ Foramen cecum
|_Root of tongue
\=& ,
Tip of tongue = Whe
iy
Esophagus
Thyreoid gland \ Se Trachea
Pericardium Notochord
Gall bladder D | Spinal cord
Small intestine y Ail ives
Caecum ~~ Stomach
Dorsal pancreas
a
Ze s > Ventral pancreas
Allantois NE Lp Colon
Urogenital sinus i
ER ecu
een shonte ee
Anal membrane
Fic. 180.—Diagrammatic median sagittal section of a 17 mm. human embryo, showing the digestive canal
(modified after Mall). Xx 5.
fourth month (Brand). In embryos of 10 to 12.5 mm. the circular muscle layer of the intes-
tine first forms. The longitudinal muscle layer is not distinct until 75 mm. (C R).
The impervious duodenum of the embryo may persist as a congenital anomaly, and
the persistence of the yolk stalk, as Meckel’s diverticulum, has already been mentioned (p. 171).
The large intestine, as seen in 9 mm. embryos (Fig. 179), forms a tube extend-
ing from the cecum to the cloaca. It does not lengthen so rapidly as the small
intestine, and, when the intestine is withdrawn from the umbilical cord (at
42 mm. C R), its cranial or cacal end lies on the right side and dorsal to the small
intestine (Fig. 181). It extends transversely to the left side as the transverse
174 THE ENTODERMAL CANAL AND THE BODY CAVITIES
colon, then bending abruptly caudad as the descending colon, returns by its iliac
flecure to the median plane and forms the rectum.
Fic. 181.—Three successive stages showing the development of the digestive tube and the mesen-
teries in the human fetus (Tourneux in Heisler): r, Stomach; 2, duodenum; 3, small intestine; 4, colon;
5, yolk stalk; 6, caecum; 7, great omentum; 8, mesoduodenum; 9, mesentery; ro, mesocolon. The arrow
points to the orifice of the omental bursa. The ventral mesentery is not shown.
The cecum (Fig. 182) may be distinguished from the vermiform process at
65 mm. (C R) (Tarenetzky). The caecum and vermiform process make a U-
Ascending _{ 2M ;
mesocolon 1,
Ascending __\
colon
Cecum—p
a tiagcse
Processus _- 48
¢ t Processus
vermiformis =
vermiformis
Fic. 182.—The cecum of a human fetus of 50 mm. (Kollmann): A, from the ventral side; B, from the
dorsal side.
shaped bend with the colon at 42 mm. (C R), and this flexure gives rise to the colic
valve (Toldt). In stages between 100 and 200 mm. (C R) the lengthening of the
THE LIVER 175
colon causes the caecum and cephalic end of the colon to descend toward the pelvis
(Fig. 181). The ascending colon is thus formed and the vermiform appendix
takes the position which it occupies in the adult.
The circular muscle layer of the large intestine appears first at 23 mm., the longitudinal
layer at 75 mm. (CR). In 55 mm. (C R) fetuses villi are present. The development
of the entire digestive tract has been described by Johnson (Amer. Jour. Anat., vols. 10,
1910; 14, 1913; 16, 1914).
Glandular secretions and desquamated entodermal cells, together with swallowed
amniotic fluid, containing lanugo hairs and vernix caseosa, collect in the fetal intestine.
This mass, yellow to brown in color, is known as meconium. At birth the intestine and its
contents are perfectly sterile.
.THE LIVER
In embryos of 2.5 mm. the liver anlage is present as a median ventral out-
_growth from the entoderm of the fore-gut just cranial to the yolk stalk (Fig.
167 B). Its thick walls enclose a cavity which is continuous with that of the gut.
This hepatic diverticulum becomes
embedded at once in a mass of
splanchnic mesoderm, the septum
transversum. Cranially, the septum
will contribute later to the forma-
tion of the diaphragm; caudally, in
the region of the liver anlage, it be-
comes the ventral mesentery (Fig.
189). Thus, from the first the liver
is in close relation to the septum
transversum and later when the
septum becomes a part of the dia-
phragm the liver remains attached
to it.
In embryos 4 to 5 mm. long,
solid cords of cells proliferate from
Fic. 183—Model of the liver anlage of a4 mm.
the ventral and cranial portion of human embryo (Bremer). X 160. Jn., Intestine;
Pa., pancreas; V., veins in contact with liver tra-
becule.
the hepatic diverticulum (Fig. 86).
These cords anastomose and form a
crescentic mass with wings extending dorsad lateral to the gut (Fig. 177). This
mass, a network of solid trabecule, is the glandular portion of the liver. The
primitive, hollow diverticulum later differentiates into the gall bladder and the
large biliary ducts.
176 THE ENTODERMAL CANAL AND THE BODY CAVITIES
Referring to Fig. 88, it will be seen that the liver anlage lies between the
vitelline veins and is in close proximity to them laterally. The veins send anas-
tomosing branches into the ventral mesentery. The trabecule of the expanding
liver grow between and about these venous plexuses, and the plexuses in turn
Fic. 184.—The trabecule and sinusoids of the liver in section (after Minot). X 300. Tr., Trabecule
of liver cells; S7., sinusoids.
make their way between and around the liver cords (Fig. 183). The vitelline
veins on their way to the heart are thus surrounded by the liver and largely sub-
divided into a network of vessels termed sinusoids. The endothelium of the sinu-
soids is closely applied to the cords of liver cells, which, in the early stages, contain
A B
Stomach
Hepatic duct
Hepatic duct
Ductus choledochus
a Ventral pancreas
Gall bladder
Cystic duct
Ductus choledochus Duct of dorsal’ ky Ae pancreas
pancreas
f Head of dorsal pancreas
Ventral pancreas’ £
Fic. 185.—Reconstructions showing the development of the hepatic diverticulum and pancreatic anlages.
A, 7.5 mm. human embryo (after Thyng), X 50; B, 10 mm. human embryo, X 33.
no bile capillaries (Fig. 184). The transformation of the vitelline veins into the
portal vein and the relations of the umbilical veins to the liver will be treated in
Chapter IX.
The glandular portion of the liver grows rapidly, and, in embryos of 7 to 8
mm., is connected with the primitive hepatic diverticulum only by a single cord
THE LIVER £77
of cells, the hepatic duct (Fig. 185 A). That portion of the hepatic diverticulum
distal to the hepatic duct is now differentiated into the terminal, solid gall bladder
and its cystic duct. Its proximal portion forms the ductus choledochus. In embryos
of 10 mm. (Fig. 185 B) the gall bladder and ducts have become longer and more
slender. The hepatic duct receives a right and left branch from the corresponding
lobes of the liver. The gall bladder is without a lumen up to the 15 mm. stage.
Later its cavity appears, surrounded by a wall of high columnar epithelium.
The glandular portion of the liver develops fast and is largest relative to the
size of the body at 31 mm. (Jackson). In certain regions the liver tissue under-
goes degeneration, and especially is this true in the peripheral portion of the left
d d
a
Fic. 186.—Diagrams of three successive stages of the portal and hepatic veins in a growing liver:
a, Hepatic side; d, portal side; b and c, successive stages of the hepatic vein; e and f, successive stages of
the portal vein (Mall).
lobe. In general, the external lobes of the liver are moulded under the influence
of the fetal vitelline and umbilical trunks.
The development of the ligaments of the liver is described on p. 192.
During the development of the liver the endothelial cells of the sinusoids become stellate
in outline, and thus form an incomplete layer. From the second month of fetal life to some
time after birth, blood cells are actively developed between the hepatic cells and the endo-
thelium of the sinusoids. Lumina bounded by five or six cells may be observed in some of
the liver trabecule of 10 mm. embryos (Lewis). At 22 mm. hollow periportal ducts develop,
spreading inward from the hepatic duct along the larger branches of the portal vein. In
44 mm. (C R) fetuses, bile capillaries with cuticular borders are present, most numerous near
the periportal ducts with which some of them connect. At birth, or shortly after, the number
of liver cells surrounding a bile capillary is reduced to two, three, or four. Secretion of the
bile commences at about the end of the third fetal month.
I2
178 THE ENTODERMAL CANAL AND THE BODY CAVITIES
The lobules, or vascular units of the liver, are formed, according to Mall, by the peculiar
and regular manner in which the veins of the liver branch. The primary branches of the
portal vein extend along the periphery of each primitive lobule, parallel to similar branches
of the hepatic veins which drain the blood from the center of each lobule (Fig. 186). -As
development proceeds, each primary branch becomes a stem, giving off on either side second-
ary branches which bear the same relation to each other and to new lobules as did the primary
branches to the first lobule. This process is repeated until thousands of liver lobules are
developed. ¥
Until the 20 mm. stage the portal vein alone supplies the liver. The hepatic artery,
from the cceliac axis, comes into relation first with the hepatic duct and gall bladder. Later,
it grows into the connective tissue about the larger bile ducts and branches of the portal vein,
and also supplies the capsule of the liver.
Anomalies.—A common anomaly of the liver consists in its subdivision into multi-
ple lobes. Absence or duplication of the gall bladder and of the ducts may occur. In some
animals (horse, elephant) the gall bladder is normally absent.
THE PANCREAS
Two pancreatic anlages are developed almost simultaneously in embryos of
3 to 4mm. The dorsal pancreas arises as a hollow outpocketing of the dorsal
duodenal wall just cranial to the hepatic diverticulum (Fig. 177). At 7.5 mm. it is
B
Ventral pancreas
Pancreatic duct
Bile duct Bile duct Pancreatic duct
Ventral pancreas
Fic. 187.—Two stages showing the development of the human pancreas: A, Embryo of 8 mm.; B,
embryo of about 20 mm. (after Kollman).
separated from the duodenum by a slight constriction and extends into the dorsal
mesentery (Fig. 185 A). The ventral pancreas develops in the inferior angle be-
tween the hepatic diverticulum and the gut (Lewis) and its wall is at first continu-
ous with both. With the elongation of the ductus choledochus its origin is trans-
ferred to this portion of the diverticulum.
Of the two pancreatic anlages, the dorsal grows more rapidly and in 10 mm.
embryos forms an elongated structure with a central duct and irregular nodules
upon its surface (Fig. 185 B). The ventral pancreas is smaller and develops
a short slender duct which opens into the ductus choledochus. When the stomach
and duodenum rotate the pancreatic ducts shift their positions as well. At the
BODY CAVITIES, DIAPHRAGM AND MESENTERIES 179
same time, growth and bending of the bile duct to the right bring the ventral
pancreas into close proximity with the dorsal pancreas (Figs. 185 and 187).
In embryos of 20 mm. the tubules of the dorsal and ventral pancreatic an-
lages interlock (Fig. 187 B). Eventually, anastomosis takes place between the
two ducts, and the duct of the ventral pancreas plus the distal segment of the
dorsal duct persist as the functional pancreatic duct of the adult. The proximal
portion of the dorsal pancreatic duct forms the accessory duct, which remains
pervious, but becomes a tributary of the adult pancreatic duct. The ventral
pancreas forms part of the head and uncinate process of the adult gland. The
dorsal pancreas takes part in forming the head and uncinate process and com-
prises the whole of the body and tail.
‘The ventral pancreas may arise directly from the intestinal wall (Bremer; Keibel and
Elze), and paired ventral anlages also occur (Debeyre; Helly; Kollmann). Accessory pan-
creases are not uncommon. Both the dorsal and ventral ducts persist in the horse and dog;
in the sheep and man the ventral duct becomes of chief importance; in the pig and ox the
dorsal duct.
In 10 mm. embryos the portal vein separates the two pancreatic anlages and later they
partially surround the vein. The alveoli of the gland are developed from the ducts as darkly
staining cellular buds in fetuses of 40 to 55 mm.(C R). The islands characteristic of the
pancreas also bud from the ducts (and alveoli, Mironescu, 1910) and appear first in the tail
at 55 mm. (C R).
Owing to the shift in the position of the stomach and duodenum during development,
the pancreas takes up a transverse position, its tail extending to the left. To its ventral sur-
face is attached the transverse mesocolon.
BODY CAVITIES, DIAPHRAGM AND MESENTERIES
The Primitive Ceelom and Mesenteries.—In the Peters embryo the primary
mesoderm has already split to form the extra-embryonic ccelom (Fig. 74 C).
When the intra-embryonic mesoderm differentiates, numerous clefts appear on
either side between the somatic and splanchnic layers of mesoderm. ‘These clefts
coalesce in the cardiac region and form two elongated pericardial cavities lateral
to the paired tubular heart. Similarly, right and left pleuro-peritoneal cavities
are formed between the mesoderm layers caudal to the heart. The paired peri-
cardial cavities extend toward the midline cranial to the heart and communicate
with each other (Fig. 188). Laterally they are not continuous with the extra-
embryonic coelom, for the head of the embryo separates early from the underlying
blastoderm. The pericardial cavities also are prolonged caudally until they open
into the pleuro-peritoneal cavities. These in turn communicate laterally with
the extra-embryonic cclom. In an embryo of 2 mm. the ccelom thus consists of
a U-shaped pericardial cavity, the right and left limbs of which are continued
180 THE ENTODERMAL CANAL AND THE BODY CAVITIES
caudally into the paired pleuro-peritoneal cavities; these extend out into the extra-
embryonic ccelom.
When the head fold and fore-gut of the embryo are developed, the layers of
splanchnic mesoderm containing the heart tubes are folded together ventral te
the fore-gut and form the ventral mesentery between the gut and the ventral body
wall (Fig. 190). Owing to the position of the yolk sac, the caudal extent of the
ventral mesentery is limited. At the level of each side, where the vitello-umbilical
trunk (Fig. 88) courses to the heart, the splanchnic mesoderm and the somatic
mesoderm are united (cf. Fig. 110). Thus is formed the septum transversum,
which incompletely partitions the ccelom
[pn ee a ° .
A Pericardial cavity into a cranial and caudal portion (Fig.
189). Cranial to the septum, the heart
is suspended in the ventral mesentery
Surface of fore-gut
which forms the dorsal and ventral meso-
cardia (Fig. 190 A). Into the ventral
Pleuro-peritoneal canal
oe mesentery caudal to the septum grows
Entoderm of gut 3 . 7
the liver. This portion of the ventral
Peritoneal cavity mesentery gives rise dorsally to the lesser
XM Extra-embryonic celom
Vit—Wall of yolk sac
Fic. 188.—Diagrammatic model of the ;
fore-gut and ccelom in an early humanem- versum, It forms the ligaments of the
bryo, viewed from above and behind (modi-
fied after Robinson).
omentum of the stomach, and, where it
fails to separate from the septum trans-
liver. Ventrally it persists as the falciform
ligament (Fig. 190 B).
Dorsal to the gut, the splanchnic mesoderm of each side is folded together in
the median sagittal plane to constitute the dorsal mesentery which extends to
the caudal end of the digestive canal (Figs. 189 and 190 C). This suspends the
stomach and intestine from the dorsal body wall and is divided into the dorsal
mesogastrium or greater omentum of the stomach, the mesoduodenum, the mesen-
tery proper of the small intestine, the mesocolon, and the mesorectum.
The covering layers of the viscera, of the mesenteries, and of the body wall
are continuous with each other and consist of a mesothelium overlying connective
tissue. The parietal lining is derived from the somatic layer of mesoderm and
the visceral covering from the splanchnic layer.
The primitive celom lies in the horizontal plane, as in Fig. 188. Coincident
with the caudal regression of the septum transversum, the pericardial cavity is
bent ventrad and enlarged (Fig. 191). The ventral mesocardium attaching the
heart to the ventral body wall disappears and the right and left limbs of the
BODY CAVITIES, DIAPHRAGM AND MESENTERIES 181
U-shaped cavity become confluent ventral to the heart. The result is a single,
large pericardial chamber, the long axis of which now lies in a dorso-ventral plane
Esophagus Spinal cord
Dorsal mesocar-
Pericardial cavity ‘
dium
Ventricle of heart
phe
Ventral mesocardium Septum bransversum
Liver __
Ventral mesentery
(falciform ligament)
coe
Stomach
B
Ventral mesentery
(lesser omentum)
Dorsal mesogastrium
Dorsal pancreas
>
Mesorectum
Fic. 189.—Diagram showing the primitive mesenteries of an early human embryo in median sagittal sec-
tion. The broken lines (A, B, and C) indicate the level of sections A, B, and C in Fig. 190.
Neural tube
Notochord
Dorsal
mesentery
Fore-gut
Aorta Notochord:
Postcar-
dinal vein
Dorsal mesentery.
Pericardial Peritoneal
celom hie
Ventral Fake
mesocardium a. ciform
ligament
A B Cc
Fic. 190.—Diagrammatic transverse sections. A, Through the heart and pericardial cavities of an early
human embryo; B, through the fore-gut and liver; C, through the intestine and peritoneal cavity.
nearly at right angles to the plane of the pleuro-peritoneal cavities, and con-
nected with them dorsally by the right and left pleuro-peritoneal canals.
The division of the primitive ccelom into separate cavities is accomplished by
182 THE ENTODERMAL CANAL AND THE BODY CAVITIES
,
the development of three membranes which join in a <-shaped fashion (Figs.
194 and 195): (1) the septum transversum, which separates incompletely the peri-
cardial and pleural cavities from the peritoneal cavities; (2) the paired pleuro-
pericardial membranes, which complete the division between pericardial and
pleural cavities; (3) the paired pleuro-peritoneal membranes, which complete the
partition between each pleural cavity, containing the lung, and the peritoneal
cavity, which contains the abdominal viscera.
The Septum Transversum.—The vitelline veins, on their way to the heart,
course in the splanchnic mesoderm lateral to the fore-gut. In embryos of 2 to 3
-Bulbus cordis
. . . Dorsal mesocardium
Pericardial cavity
Sinus venosus
Somatopleure
Lateral mesocardium
ari pil -
Septum transversum Common cardinal vein
Umbilical vein
Vitelline vein overlying
stomach
Pleuro-peritoneal canal
Volk stalk
1 Vb
Peritoneal cavity
Fic. 191.—Reconstruction cut at the left of the median sagittal plane of a 3 mm. human embryo, showing
the body cavities and septum transversum (Kollmann).
mm. these large vessels bulge into the ccelom until they meet and fuse with the
somatic mesoderm (Figs. 88 and 110). Thus there is formed caudal to the heart
a transverse partition filling the space between the sinus venosus of the heart,
the gut, and the ventral body wall, and separating the pericardial and peri-
toneal cavities from each other ventral to the gut. This mesodermal partition
was termed by His the septum transversum. In Fig. 191 it comprises both a
cranial portion (designated ‘septum transversum”), which is the anlage of a large
part of the diaphragm, and a caudal portion, the ventral mesentery, into which
the liver grows.
At first the septum transversum does not extend dorsal to the gut, but leaves
BODY CAVITIES, DIAPHRAGM AND MESENTERIES 183
on either side a plewro-peritoneal canal through which the pericardial and pleuro-
peritoneal cavities communicate (Fig. 191). In embryos of 4 to 5 mm. the lungs
develop in the median walls of these canals and bulge laterally into them. Thus
the canals become the pleural cavities and will be so termed hereafter.
On account of the more rapid
growth of the embryo, there is an ap-
parent constriction at the yolk stalk,
and, with the development of the um-
bilical cord, the peritoneal cavity is
finally separated from the extra-em-
bryonic ccelom. Dorsally, the pleural
and peritoneal cavities are permanently
partitioned lengthwise by the dorsal
mesentery.
The septum transversum in 2 mm.
embryos occupies a transverse position
in the middle cervical region (Fig. 192, 2).
According to Mall, it migrates caudally,
its ventral position at first moving
more rapidly so that its position be-
comes obligue. In 5 mm. embryos (Fig.
192, 5) it is opposite the fifth cervical
segment, at which level it receives the
Fic. 192—Diagram showing the change
in position of the septum transversum in stages
phrenic nerve. In stages later than 7 from 2 to 24 mm. (modified after Mall). The
septum is indicated at different stages by the
numerals to the left, the numbers correspond-
at 24 mm. it is opposite the first lum- ing to the length of the embryo at each stage.
bare gment. During this second period The letters and numbers at the right represent
the segments of the occipital, cervical, thoracic
of migration its dorsal attachment travels and Jumbar regions.
mm. the septum migrates caudad, until
faster than its ventral portion. ‘There-
fore, it rotates to a position nearly at right angels to its plane in 7 mm. embryos
and its original dorsal surface becomes its ventral surface.
The Pleuro-pericardial and Pleuro-peritoneal Membranes.—The common
cardinal veins (ducts of Cuvier) on their way to the heart curve around the pleural
cavities laterally in the somatic body wall (Figs. 191 and 193). In embryos of
7 mm. each vein, with the overlying mesoderm, forms a ridge which projects
from the body wall mesially into the pleural canals. This ridge, the pulmonary
ridge of Mall, is the anlage of both the pleuro-pericardial and pleuro-peritoneal
¢
184 THE ENTODERMAL CANAL AND THE BODY CAVITIES
membranes. Later it broadens and thickens cranio-caudally (Fig. 193), forming
a triangular structure whose apex is continuous with the septum transversum
(Fig. 194). Its cranial side forms the pleuro-pericardial membrane and in 9 to 10
mm. embryos reduces the opening between the pleural and pericardial cavities
to a mere slit. Its caudal side becomes the pleuro-peritoneal membrane, which
eventually separates dorsally the pleural from the peritoneal cavity (Fig. 195).
Pericardial cavity Common cardinal vein
Pleuro-pericardial membrane
Pleuro-
peritoneal
Lung bud | membrane
Liver
Vein to limb
bud
Stomach
Mesonephros
Fic. 193.—Reconstruction of a 7 mm. human embryo showing from the left side the pleuro-pericardial
membrane, the pleuro-peritoneal membrane and the septum transversum (after Mall). X 20. The
phrenic nerve courses in the pleuro-pericardial membrane. Arrow passes from pericardial to peritoneal
cavity through the pleuro-pericardial canal.
The two sets of membranes at first lie nearly in the sagittal plane and a portion
of the lung is caudal to the pleuro-peritoneal membranes (Fig. 193). Between
the stages of 7 and 11 mm. the dorsal attachment of the septum transversum is
carried caudally more rapidly than its ventral portion and its primary ventral
surface becomes its dorsal side (Figs. 192 to 194). The pleuro-peritoneal mem-
brane is carried caudad with the septum transversum until the lung lies in the
angle between the pleuro-peritoneal and pleuro-pericardial membranes and is
BODY CAVITIES, DIAPHRAGM AND MESENTERIES 185
included within the spherical triangle which has been described above (Fig. 194).
During this rotation the dorsal end of the pleuro-pericardial membrane lags behind
and so takes up a position in a coronal plane nearly at right angles to the septum
transversum (Figs. 194 and 195). In 11 mm. embryos the pleuro -pericardia]
membranes have fused completely on each side with the median walls of the
pleural canals and thus separate the pericardium from the paired pleural ca vities.
Pleuro-pericardial membrane Phrenic nerve
Pericardial cavity
Septum transversum
peritoneal
membrane
Mesonephros
Stomach
Frc. 194.—Reconstruction of an 11 mm. human embryo to show the same structures as in Fig. 193 (after
Mall). X 14.
By way of the pleuro-pericardial membranes the phrenic nerves course to the
septum transversum (Fig. 194).
The pleuro-peritoneal membranes are continuous dorsally and caudally with
the mesonephric folds; ventrally and caudally they fuse later with the dorsal
pillars of the diaphragm or coronary appendages of the liver (Lewis) (Fig. 196).
Between the free margins of the membranes and the mesentery a temporary open-
ing is left on each side, through which the pleural and peritoneal cavities com-
municate (Figs. 194 and 200).
186 THE ENTODERMAL CANAL AND THE BODY CAVITIES
Esophagus
Mesoderm of left lung bud
Pleural cavily Pericardial cavity
Bae aie Be Lary Ne: Pleuro-perttoneal
Pleuro-pericardial ier be ‘ membrane
membrane — Phrenic nerve
Wall of heart Septum transversum
Liver
Falciform ligament
Fic. 195.—Transverse section through a 10 mm. human embryo showing the pleuro-pericardial mem-
brane separating the pericardium from the pleural cavities. X 33.
Esophagus _
: Pleural cavity
Right lung a:
Pleuro-peritoneat
membrane
Coronary appendage of
liver Phrenic nerve in septum
Vena cava inferior lransversum
Fic. 196.—Transverse section through a 10 mm. human embryo showing the pleuro-peritoneal mem-
branes. X 16.
Owing to the caudal migration of the septum transversum and the growth of
the lungs and liver, the pleuro-peritoneal membrane, at first lying in a nearly
BODY CAVITIES, DIAPHRAGM AND MESENTERIES 187
sagittal plane (Fig. 193), is shifted to a horizontal position (Fig. 194), and gradu-
ally its free margin unites with the dorsal pillars of the diaphragm and with the
dorsal mesentery. The opening between the pleural and peritoneal cavities is
thus narrowed and finally closed in embryos of 19 to 20 mm.
A B
Esophagus
Common cardinal vein Pleuro-pericardial canal Pericardial cavity
Septum transversum Pleuro-peritoneal membrane Heart — Pericardial membrane
Fic. 197.—Diagrams showing the development of the lungs and the formation of the pericardial mem-
brane (modified after Robinson). A, Coronal section; B, transverse section.
The Diaphragm and Pericardial Membrane.—The lungs grow and expand
not only cranially and caudally but also laterally and ventrally (Fig. 197).
Room is made for them by the obliteration of the very loose, spongy mesenchyme
of the adjacent body wall (Fig. 196). As the lungs burrow laterally and ventrally
Fic. 198.—Diagram showing the origin of the diaphragm (after Broman): z, Septum transver-
sum; 2, 3, derivatives of mesentery; 4, 4, derivatives of pleuro-peritoneal membrane; 5, 5, parts derived
from the body walls; A, aorta; Oe, esophagus; VC, inferior vena cava.
into the body wall around the pericardial cavity, the pleuro-pericardial mem-
branes enlarge at the expense of this tissue and more and more the heart comes to
lie in a mesial position between the lungs (Fig. 197 B). The pleural cavities
thus increase rapidly in size.
188 THE ENTODERMAL CANAL AND THE BODY CAVITIES
At the same time the liver grows enormously, and on either side a portion of
the body wall is taken up into the septum transversum and pleuro-peritoneal
membranes. The diaphragm, according to Broman, is thus derived from four
sources (Fig. 198): (1) its ventral pericardial portion from the septum trans-
versum; its lateral portions from (2) the pleuro-peritoneal membranes plus (3)
derivatives from the body wall; lastly, a median dorsal portion is formed from
(4) the dorsal mesentery. In addition to these, the striated muscle of the dia-
phragm, according to Bardeen, takes its origin from a pair of premuscle masses
which in 9 mm. embryos lie one on each side opposite the fifth cervical segment.
Right umbilical vein Left umbilical vein
Ventral mesentery
Ectoderm of body wall
Left lobe of liver
Right lobe of liver Ventral mesentery
. Duodenum
Lesser peritoneal sac
Plica ven@ cave
Dorsal aorla
Neural tube
Fic. 199.—Diagrammatic model of an embryo of 7 to 9 mm. showing the position of the lesser peri-
toneal sac. The embryo is represented as sectioned transversely, caudal to the liver, so that one looks
at the caudal surface of the section and of the liver, and cranially into the body cavities.
This is the level at which the phrenic nerve enters the septum transversum.
The exact origin of these muscle masses is in doubt, but they probably repre-
sent portions of the cervical myotomes of this region. The muscle masses
migrate caudally with the septum transversum and develop chiefly in the
dorsal portion of the diaphragm (Bardeen, Johns Hopkins Hospital Report,
vol. 9, 1900).
Keith (Jour. Anat. and Physiol., vol. 39, 1905) derives the muscle of the diaphragm also
from the rectus and transversalis muscles of the abdominal wall.
The cavities of the mesodermic segments are regarded as portions of the ccelom, but in
man they disappear early. The development of the vaginal sacs which grow out from the
inguinal region of the peritoneal cavity into the scrotum will be described in Chapter VIII.
BODY CAVITIES, DIAPHRAGM AND MESENTERIES 189
The Omental Bursa or Lesser Peritoneal Sac.—According to Broman, the
omental bursa is represented in 3 mm. embryos by a peritoneal pocket which
extends cranially into the dorsal mesentery to the right of the esophagus. A
similar pocket present on the left side has disappeared in 4 mm. embryos. Lateral
to the opening of the primitive lesser peritoneal sac, a lip-like fold of the mesentery
ay
Body wall Falciform ligament
Coronary attachment of
liver to diaphragm
Inferior vena cava
Sup. recess of lesser
peritoneal sac
Pleuro-peritoneal
foramen
Pleuro-peritoneal
Pleuro-peritoneal membrane
membrane
Lesser omentum
Inferior vena cava
Plica vene cave
Mesonephric fold
Genital fold Lesser peritoneal sac
Aorta
Fic. 200.—A diagrammatic ventral view of the middle third of a human embryo 12 to 15 mm. long.
The figure shows the caudal surface of a section through the stomach and spleen, a ventral view of the
stomach, the liver having been cut away to leave the sectioned edges of the lesser omentum and plica
ven cave, and the caudal surface of the septum transversum and pleuro-peritoneal membrane. Upon
the surface of the septum is indicated diagrammatically the attachment of the liver. (Based on figures
of Mall and F. T. Lewis and model by H. C. Tracy.)
is continued caudally along the dorsal body wall into the mesonephric fold as the
plica vene cave, in which the inferior vena cava later develops (Fig. 199). The
liver, it will be remembered, grows out into the ventral mesentery from the fore-
gut, and, expanding laterally and ventrally, takes the form of a crescent. Its
right lobe comes into relation with the plica vene cave, and, growing rapidly
caudad, forms with the plica a partition between the lesser sac and the peritoneal
Igo THE ENTODERMAL CANAL AND THE BODY CAVITIES
cavity. Thus the cavity of the lesser peritoneal sac is extended caudally from
a point opposite the bifurcation of the lungs to the level of the pyloric end of the
stomach. In 5 to 10 mm. embryos it is crescent-shaped in cross-section (cf. Fig.
111) and is bounded mesially by the greater omentum (dorsal mesentery) and the
right wall of the stomach, laterally by the liver and plica vene cave, and ven-
trally by the lesser omentum (ventral mesentery). It communicates to the
right with the peritoneal cavity through an opening between the liver ventrally
and the plica venz cave dorsally (Fig. 201). This opening is the epiploic fora-
Suprarenal gland OD
Mesone phros
F Greater omentum
Liver
Lesser peritoneal sac Stomach
Duodenum
Vitelline vei -
Intestinal loop—
Left umbilical vein
Fic. 201.—An obliquely transverse section through a 10 mm. human embryo at the level of the epiploic
foramen (of Winslow). X 33.
men (of Winslow). When the dorsal wall of the stomach rotates to the left the
greater omentum is carried with it to the left of its dorsal attachment. Its tissue
grows actively to the left and caudally and gives the omentum an appearance of
being folded on itself between the stomach and the dorsal body wall (Fig. 200).
The cavity of the lesser peritoneal sac is carried out between the folds of the
greater omentum as the inferior recess of the omental bursa.
From the cranial end of the sac there is constricted off a small closed cavity which is
frequently persistent in the adult. This is the bursa infracardiaca and may be regarded as a
third pleural cavity. It lies at the right of the esophagus in the mediastinum.
BODY CAVITIES, DIAPHRAGM AND MESENTERIES Ig!
When the stomach changes its position and form so that its mid-ventral
line becomes the lesser curvature and lies to the right, the position of the lesser
omentum is also shifted. From its primitive location in a median sagittal plane
with its free edge directed caudally, it is rotated through 90° until it lies in a cor-
onal plane with its free margin facing to the right (Fig. 194). The epiploic fora-
men now forms a slit-like opening leading from the peritoneal cavity into the ves-
tibule of the omental bursa. The foramen is bounded ventrally by the edge of
the lesser omentum, dorsally by the inferior vena cava, cranially by the caudate
process of the liver, and caudally by the wall of the duodenum.
During fetal life the greater omentum grows rapidly to the left and caudad
in the form of a sac, flattened dorso-ventrally. It overlies the intestines ven-
Fic. 202.—Diagrams showing the development of the mesenteries (Hertwig). A illustrates the
beginning of the great omentum and its independence of the transverse mesocolon; in B the two come
into contact; in C they have fused; A, stomach; B, transverse colon; C, small intestine; D, duodenum;
E, pancreas; F, greater omentum; G, greater sac; H, omental bursa.
trally and contains the inferior recess of the omental bursa (Fig. 202). The dor-
sal wall of the sac during the fourth month usually fuses with the transverse colon
where it overlies the latter (Fig. 202 B). Caudal to this attachment the walls
of the greater omentum may be fused and its cavity is then obliterated. The
inferior recess of the omental bursa thus may be limited in the adult chiefly to a
space between the stomach and the dorsal fold of the greater omentum, which
latter is largely fused to the peritoneum of the dorsal body wall. The spleen
develops in the cranial portion of the greater omentum and that portion of the
omentum which extends between the stomach and spleen is known as the gastro-
lienic ligament (Fig. 200). The dorsal wall of the omentum between the spleen
and kidney is the /ieno-renal ligament.
Further Differentiation of the Mesenteries.— Ligaments of the Liver —We
192 THE ENTODERMAL CANAL AND THE BODY CAVITIES
have seen (p. 179) that the cranial portion of the ventral mesentery forms the
mesocardium of the heart. In the ventral mesentery caudal to the septum trans-
versum the liver develops. From the first, it is enveloped in folds of the splanch-
nic mesoderm which give rise to its capsule and ligaments as the liver increases
in size (Fig. 190 B). Wherever the liver is unattached, the mesodermal layers
of the ventral mesentery form its capsule (of Glisson), a fibrous layer covered
by mesothelium, continuous with that of the peritoneum (Fig. 190 B). Along
its mid-dorsal and mid-ventral line the liver remains attached to the ventral
mesentery. The dorsal attachment between the liver, stomach, and duodenum
is the lesser omentum. This in the adult is differentiated into the duodeno-hepatic
and gastro-hepatic ligaments. The attachment of the liver to the ventral body
wall extends caudally to the umbilicus and forms the falciform ligament.
In its early development the liver abuts upon the septum transversum, and
in 4 to 5 mm. embryos is attached to it along its cephalic and ventral surfaces.
Soon dorsal prolongations of the lateral liver lobes, the coronary appendages,
come into relation with the septum dorsally and laterally. The attachment of
the liver to the septum transversum now has the form of a crescent, the dorsal
horns of which are the coronary appendages (Fig. 200). This attachment be-
comes the coronary ligament of the adult liver. The dorso-ventral extent of the
coronary ligament is reduced during development and its lateral extensions upon
the diaphragm give rise to the triangular ligaments of each side.
The right lobe of the liver, as we have seen, comes into relation along its
dorsal surface with the plica vene cave in 9 mm. embryos (Figs. 199 and 200).
This attachment extends the coronary ligament caudally on the right side and
makes possible the connection between the veins of the liver and mesonephros
which contributes to the formation of the inferior vena cava. The portion of the
liver included between the plica venz cave and the lesser omentum is the caudate
lobe (of Spigelius).
In a fetus of five months the triangular ligaments mark the position of the
former lateral coronary appendages. The umbilical vein courses in a deep groove
along the ventral surface of the liver, and, with the portal vein and gall bladder,
bounds the guadrate lobe.
Changes in the Dorsal Mesentery.—That part of the digestive canal which
lies within the peritoneal cavity is suspended by the dorsal mesentery, which at
first forms a simple attachment extending in the median sagittal plane between
body wall and primitive gut. That portion of it connected with the stomach
forms the greater omentum, the differentiation of which has been described (p. 191).
BODY CAVITIES, DIAPHRAGM AND MESENTERIES 193
The mesentery of the intestine is carried out into the umbilical cord between the
limbs of the intestinal loop. When the intestine elongates and its loop rotates,
the caecal end of the large intestine comes to lie cranially and to the left, the small
intestine caudally and to the right, the future duodenum and colon crossing in
close proximity to each other (Fig. 179). On the return of the intestinal loop
into the abdomen from the umbilical cord, the cecal end of the colon lies to the
right and the transverse colon crosses the duodenum ventrally and cranially
(Fig. 203 A). The primary loops of the small intestine lie caudal and to the left
of the ascending colon (Fig. 203 B). There has thus been a torsion of the mesen-
tery about the origin of the superior mesenteric artery as an axis. From this focal
Lesser omentum
Dorsal mesogastrium
omentum Transverse
mesocolon
Iliac mesocolon
Fic. 203.—Diagrams showing the development of the mesenteries in ventral view (modified after
Tourneux). * Cut edge of greater omentum; a, area of ascending mesocolon fused to dorsal body wall;
b, area of descending mesocolon fused to dorsal body wall. Arrow in omental bursa.
point the mesentery of the small intestine and colon spreads out fan-like. The
mesoduodenum is pressed against the dorsal body wall, fuses with its peritoneal
layer, and is obliterated (Fig. 202). Since the transverse colon lies ventral to the
duodenum it cannot come into apposition with the body wall; where its mesentery
crosses the duodenum it fuses at its base with the surface of the latter and of the
pancreas. Its fixed position now being transverse instead of sagittal, the mesen-
tery is known as the transverse mesocolon. The mesentery of the ascending colon
is flattened against the dorsal body wall on the right and fuses with the peritoneum
(Fig. 203). Similarly, the descending mesocolon is applied to the body wall of
the left side. There are thus left free: (1) the transverse mesocolon; (2) the mesen-
13
194 THE ENTODERMAIL CANAL AND THE BODY CAVITIES
tery proper of the jejunum and ileum, with numerous folds corresponding to the
loops of the intestine; (3) the iliac mesocolon; (4) the mesorectum, which retains
its primitive relations.
Anomalies of the diaphragm and mesenteries are not uncommon. The persistence of
a dorsal opening in the diaphragm, more commonly on the left side, finds its explanation in
the imperfect development of the pleuro-peritoneal membrane. Such a defect may lead to
diaphragmatic hernia, the abdominal viscera projecting to a greater or less extent into the
pleural cavity.
The mesenteries also may show malformations, due to the persistence of the simpler
embryonic conditions, usually correlated with the defective development of the intestinal
canal. In about 30 per cent. of cases the ascending and descending mesocolon is more or less
free, having failed to fuse with the dorsal peritoneum. The primary sheets of the greater
omentum may also fail to unite, so that the inferior recess extends to the caudal end of the
greater omentum.
A striking anomaly is situs viscerum inversus, in which the various visceral organs are
transposed right for left and left for right as in a mirror image. An independent transposi-
tion of the thoracic or abdominal viscera alone may occur. The larger left great venous
trunks are thought to be chiefly responsible for the usual positions of the viscera.
CHAPTER VIII
THE DEVELOPMENT OF THE UROGENITAL SYSTEM
THE excretory and reproductive systems are intimately associated in develop-
ment. Both arise from the mesoderm of the intermediate cell mass (nephrotome),
which unites the primitive segments with the lateral somatic and splanchnic
mesoderm (p. 52; Fig. 205).
Vertebrates possess excretory organs of three distinct types. The pronephros
is the functional kidney of amphioxus and certain lampreys, but appears only in
immature fishes and amphibians, being replaced by the mesonephros. The em-
bryos of amniotes (reptiles, birds, and mammals) possess first a pronephros, and
then a mesonephros, whereas the permanent kidney is a new organ, the meta-
nephros. Whether these glands represent modifications of an originally continu-
ous organ, or whether they are three distinct structures, is undecided, but how-
ever this may be, the pro-, meso-, and metanephroi of amniotes develop suc-
cessively in the order named, both as regards time and place.
THE PRONEPHROS
The pronephros, when functional, consists of paired, segmentally arranged
tubules, one end of each tubule opening into the ccelom, the other into a longitu-
dinal pronephric duct which drains into the cloaca (Fig. 204 A). Near the
nephrostome (the opening into the ccelom) knots of arteries project into the ccelom,
forming glomeruli. Fluid from the ccelom and glomeruli and excreta from the
cells of the tubules are carried by ciliary movement into the pronephric ducts.
The human pronephros is vestigial. It consists of about seven pairs of rudi-
mentary pronephric tubules, formed as dorsal sprouts from the nephrotomes (Fig.
205) in each segment, from the seventh to the fourteenth, and perhaps from more
cranial segments as well. The nodules hollow out and open into the cclom.
Dorsally and laterally, the tubules of each side bend backward and unite to form
a longitudinal collecting duct (Fig. 204 B, A). The tubules first formed in the
seventh segment begin to degenerate before those of the fourteenth segment have
developed. Caudal to the fourteenth segment no pronephric tubules are devel-
oped, but the free end of the collecting duct, by a process of terminal growth, ex-
tends caudad beneath the ectoderm and lateral to the nephrogenic cord, until it
195
196 THE UROGENITAL SYSTEM
A Pronephric duct B
Mesodermal _]
segments
Neural tube
Anlages of pronephric
duct
Nephrotome
Somatic mesoderm
Splanchnic mesoderm
Pronephric tubule Celom Notochord Entoderm
Fic. 204.—Diagrams showing the development of the pronephric duct and pronephric tubules (modi-
fied from Felix). A shows a later stage than B.
Neural tube.
= ‘Mesodermal segment
coe Cavity of segment
Cavity of gut.
Splanchnic mesoderm.
Mesoderm of yolk sac.
Fic. 205.—Transverse section of a 2.4 mm. human embryo showing the intermediate cell mass or
nephrotome (Kollmann).
reaches, and perforates, the lateral wall of the cloaca. Thus are formed the
paired primary excretory (pronephric) ducts. The pronephric tubules begin to
ra
THE MESONEPHROS (197
appear in embryos of 1.7 mm., with nine or ten primitive segments (Felix, in Keibel
and Mall, vol. 2); in 2.5 mm. embryos (23 segments) all the tubules have devel-
oped and the primary excretory duct is nearly complete. In 4.25 mm. embryos
the duct has reached the wall of the cloaca and soon after fuses with it. The pro-
nephric tubules soon degenerate, but the primary excretory ducts persist and be-
come the ducts of the mesonephroi, or mid-kidneys.
THE MESONEPHROS
The mesonephros, like the pronephros, consists essentially of a series of
tubules, each of which at one end is related to a knot of blood vessels and at the
other end opens into the primary excretory duct. Besides possessing an inter-
nal glomerulus alone they differ from the pronephric tubules in that the nephro-
tomes are transitory, never opening into the mesonephric chamber. The meso-
nephric tubules arise just caudal to the pronephros and from the same general
source, that is, the nephrotomes. Only a few of the more cranial tubules, how-
ever, are formed from distinct intermediate cell masses, for caudal to the tenth
pair of segments this mesoderm constitutes unsegmented, paired nephrogenic
cords. These may extend caudally as far as the twenty-eighth segment. The
primary excretory ducts lie lateral to the nephrogenic cords.
When the developing mesonephric tubules begin to expand there is not room
for them in the dorsal body wall and as a result this bulges ventrally into the
coelom. ‘Thus there is produced on either side of the dorsal mesentery a longi-
tudinal urogenital fold, which may extend from the sixth cervical to the third
lumbar segment (Fig. 220). Later, this ridge is divided into a lateral mesonephric
fold and into a median genital fold, the anlage of the genital gland.
Differentiation of the Tubules.—The nephrogenic cord in 2.5 mm. embryos
first divides into spherical masses of cells, the anlages of the mesonephric tubules.
Four of these may be formed in a single segment. Appearing first in the 13th,
14th and 15th segments, the anlages of the tubules differentiate both cranially
and caudally. In 5.3 mm. embryos the cephalic limit is reached in the sixth
cervical segment, and thereafter degeneration begins at the cephalic end. Hence
the more cranial tubules overlap those of the pronephros. In 7 mm. embryos the
caudal limit is reached in the third lumbar segment.
The spherical anlages of the tubules differentiate in a cranio-caudal direction
(Fig. 206). First, vesicles with lumina are formed (4.25 mm.). Next, the vesicles
elongate laterally, unite with the primary excretory ducts, and become S-shaped
(4.9 mm.). The free, vesicular end of the tubule enlarges, becomes thin walled,
198 THE UROGENITAL SYSTEM
and into this wall grows a knot of arteries to form the glomerulus (embryos
of 5 to 7 mm.). The tubule, at first solid, hollows out and is lined with a low
columnar epithelium. The outer wall of the vesicle about the glomerulus is
Bowman’s capsule, the two constituting a renal corpuscle of the mesonephros (Fig.
206 D). ‘In the human embryo the tubules do not branch or coil as in pig embryos,
consequently the mesonephros is relatively smaller. At 10 mm. about 35
Mesonephric duct Degenerating
mesonephric cor puscle
Degenerating mesonephri
corpuscle and tubule
Anlage of
mesonephric tubule!
a
Glomerulus and Bowman’s,
capsule
Developing mesonephric
cor puscle
Urogenital sinus
f esonephric
duct
AMetanephros
Bowman’s capsule
Fic. 206.—Diagrams showing the differentia-
tion of the mesonephric tubules (modified after
Felix). L., lateral; 1., median.
Fic. 207.—Diagram showing the anlages
of the urinary organs in about 10 mm. human
embryos as seen from the left side (based on
reconstructions by Keibel and Felix).
tubules are present in each mesonephros and the glomeruli are conspicuous
(Fig. 207). Each tubule shows a distal secretory portion and a proximal collect-
ing part which connects with the duct (Fig. 208).
The glomeruli form a single median column, the tubules are dorsal and the
duct is lateral in position. Ventro-lateral branches from the aorta supply the
glomeruli, while the posterior cardinal veins, dorsal in position, break up into a
network of sinusoids about the tubules (see Chapter IX).
THE METANEPHROS 199
The primary excretory duct, or mesonephric duct, is solid in 4.25 mm. em-
bryos. A lumen is formed at 7 mm., wider opposite the openings of the tubules.
The duct is important, as the ureteric anlage of the permanent kidney grows out
from its caudal end, while the duct itself is transformed into the chief genital duct
of the male, and its derivatives. The mesonephros is probably a functional ex-
cretory organ in human embryos even though its tubules degenerate before the
metanephros becomes functional (Bremer, Amer. Jour. Anat., vol. 19, 1916).
Degeneration proceeds rapidly in embryos between 10 and 20 mm. long, begin-
ning cranially. New tubules are formed at the same time caudally. In all, 83
pairs of tubules arise, of which only 26 pairs persist at 21 mm., and these are usu-
ally broken at the angle between the collecting and secretory regions. They are
Suprarenal gland
Post. cardinal vein
Collecting tubule
Glomerulus Secretory tubule
?
Beinn pepsi —Mesonephric duct
v3 Miillerian duct
Anlage of genital gland
Fic. 208.—Reconstruction of the contents of the urogenital fold from transverse sections of a 12 mm.
human embryo. X 95.
divided into an upper group and a lower group. The collecting portions of the
upper group, numbering 5 to 12, unite with the rete tubules of the testis or ovary.
In the male they form the efferent ductules of the epididymis. In the female they
constitute the epodphoron. Of the lower group a few tubules persist in the male,
as the paradidymis with its canaliculus aberrans. In the female they form the
paroophoron.
t
THE METANEPHROS ~ gh
The essential parts of the permanent kidney are the renal corpuscles (glom-
erulus with Bowman’s capsule), secretory tubules, and collecting tubules. The
collecting tubules open into expansions of the duct, the pelvis and calyces. The
duct itself is the ureter, which opens into the bladder. Like the mesonephros, the
200 THE UROGENITAL SYSTEM
metanephros is of double origin. The ureter, pelvis, calyces, and collecting
tubules are outgrowths of the mesonephric duct. The secretory tubules and the
—Ouler zone
Inner zone
Bladder
Fic. 209.—Reconstruction of the anlages of the metanephros in a human embryo of about 9 mm.
(after Schreiner). The layers lettered Inner and Outer sone constitute the nephrogenic tissue of the
metanephros.
capsules of the renal corpuscles are differentiated from the isolated caudal end of
the nephrogenic cord and thus have a similar origin as the mesonephric tubules.
D
Cranial pole tubule
B Cranial
Cranial pole tubule
pole tubule
Ventral central
) tubule
Caudal pole
Central ual
Caudal tubule
pole tubule
Pelvis
Secondary
collecting
ae tubules
pole tubule
Ureler
Ureter Tertiary collecting
tubule
Fic. 210.—Diagrams showing the development of the primitive pelvis, calyces and collecting tubules
of the metanephros (based on reconstructions by Schreiner and Felix).
In embryos of about 5 mm. the mesonephric duct makes a sharp bend just
before it joins the cloaca and it is at the angle of this bend that the ureteric evagi-
THE METANEPHROS 201
nation appears, dorsal and somewhat median in position (Fig. 216 B, C). The
bud grows at first dorsally, then cranially. Its distal end expands and forms the
primitive pelvis. Its proximal elongated portion is the ureter. The anlage grows
into the lower end of the nephrogenic cord (Fig. 209), which, in 4.6 mm. embryos, is
separated from the cranial end of the cord at the twenty-seventh segment. The
nephrogenic tissue forms a cap about the primitive pelvis, and, as the pelvis
grows cranially, is carried along with it. In embryos of 9 to 13 mm. the pelvis,
having advanced cephalad through three segments, attains a position in the
retroperitoneal tissue dorsal to the mesonephros and opposite the second lumbar
segment. Thereafter, the kidney enlarges both cranially and
caudally without shifting its position. The ureter elongates
as the embryo grows in length. The cranial growth of the
kidney takes place dorsal to the suprarenal gland (Fig. 232).
Primary collecting tubules grow out from the primitive
pelvis in 10 mm. embryos. Of the first two, one is cranial,
the other caudal in position, and between these there are usu-
ally two others (Fig. 210 B, C). From an enlargement, the
ampulla, at the end of each primary tubule grow out two,
three, or four secondary tubules. These in turn give rise to
tertiary tubules (Fig. 210 D) and the process is repeated until
the fifth month of fetal life, when it is estimated that twelve
generations of tubules have been developed. ‘The pelvis and
Fic. 211.—Re-
construction of the
both primary and secondary tubules enlarge during develop- ureter, pelvis, caly-
ces and their branch-
es from the meta-
calyces, and the secondary tubules opening into them form the _ nephros of a 16 mm.
minor calyces (Fig. 211). The tubules of the third and fourth aera oe
orders are taken up into the walls of the enlarged secondary
tubules so that the tubules of the fifth order, 20 to 30 in number, open into the
minor calyces as papillary ducts. The remaining orders of tubules constitute the
ment. The first two primary tubules become the major
collecting tubules which form the greater part of the medulla of the adult kidney.
When the four to six primary tubules develop, the nephrogenic cap about the
primitive pelvis is subdivided and its four to six parts cover the end of each pri-
mary tubule. As new orders of tubules arise, each mass of nephrogenic tissue
increases in amount and is again subdivided until finally it forms a peripheral
layer about the ends of the branches tributary to a primary tubule. The con-
verging branches of such a tubular “‘tree” constitute a primary renal unit, or
pyramid, with its base at the periphery of the kidney and its apex projecting into
202 THE UROGENITAL SYSTEM
the pelvis. The apices of the pyramids are termed renal papille and through
them the larger collecting ducts open. The nephrogenic tissue forms the cortex
of the kidney, and each subdivision of it, covering the tubules of a pyramid
peripherally, is marked off on the surface of the organ by grooves or depressions.
The human fetal kidney is thus distinctly lobated, the lobations persisting until
after birth, a condition which is permanent in reptiles, birds, and some mammals
(whale, bear, ox). The primary pyramids are subdivided into several secondary
and tertiary pyramids. Between the pyramids the cortex of nephrogenic tissue dips
down to the pelvis, forming the renal columns (of Bertin). The collecting tubules,
on the other hand, extend out into the cortex as the cortical rays-or pars radiata
of the cortex. In these rays, and in the medulla of the kidney, the collecting
tubules run parallel and converge to the papille.
4
Fic. 212 —Semidiagrammatic figures of the anlage and differentiation of renal vesicles and early
developmental stages of uriniferous tubules of mammals. 1 and 2, Anlage and successive stages in the
differentiation of renal vesicles, as seen in sagittal sections; 3, section and outer form of tubular anlage
before union with collecting tubule at the beginning of S-shaped stage; 4 and 5, successive stages in the
development of the tubules, Bowman’s capsule, and glomerulus beginning with a tubular anlage showing
a well-developed S shape (Huber).
DIFFERENTIATION OF THE NEPHROGENIC TISSUE Os
In stages from 13 to 19 mm., the nephrogenic tissue about the ends of the
collecting tubules condenses into spherical masses which lie in the angles between
DIFFERENTIATION OF THE NEPHROGENIC TISSUE 203,
the buds of new collecting tubules and their parent stems (Fig. 212). One such
metanephric sphere is formed for each new tubule. The spheres are converted
into vesicles with eccentrically placed lumina. The vesicle elongates, its thicker
outer wall forming an S-shaped tubule which unites with a collecting tubule,
its thin inner wall becoming the capsule (Bowman’s) of a renal corpuscle. The
uriniferous tubules of the adult kidney have a definite and peculiar structure and
arrangement (Fig. 213 A). Beginning with a renal corpuscle, each tubule forms
a proximal convoluted portion, a U-shaped loop (of Henle) with descending and
Arch of collecting tubule
A Arch of collecting tubule
Distal convoluted tubule
Stoerck’s loop
Proximal convoluted Proximal convoluted tubule
tubule
Distal convoluted &,
tubule fx"
Renal cor puscle—
Connecting piece
Glomerulus
Bowman’s capsule
Connecting piece
Ascending limb of.
Henle’s loop Arch of collecting tubule
Proximal convoluted tubule
Distal convoluted tubule
Descending limb of__| Connecting piece
Henle’s loop Glomerulus
Large collecting __|
tubule |
Fic. 213.—Diagrams showing the differentiation of the various parts of the uriniferous tubules of
the metanephros (based on the reconstructions of Huber and Stoerck): A, From an adult human kid-
ney; B, C, from human embryos.
“— Bowman’s capsule
Stoerck’s loop
ascending limbs, a connecting piece, which lies close to the renal corpuscle, and a
distal convoluted portion continuous with the collecting tubule. These parts are
derived from the S-shaped anlage, which is composed of a lower, middle, and
upper limb. The middle limb, somewhat U-shaped, bulges into the concavity of
Bowman’s capsule (Fig. 213 B). By differentiation the lower portion of the
lower limb becomes Bowman’s capsule, ingrowing arteries forming the glomerulus
(Fig. 213 B, C). The upper part of the same limb by enlargement, elongation,
and coiling becomes the proximal convoluted tubule. The neighboring portion
of the middle limb forms the primitive loop (of Stoerck); the base of the middle
”
204 THE UROGENITAL SYSTEM
Fic. 214.—Diagram showing the relation of Bowman’s capsule and the uriniferous tubules to
the collecting tubules of the metanephros (Huber). c, Collecting tubules; e, end branches of collecting
tubules; 7, renal corpuscles; ”, neck; fc, proximal convuluted tubule; di, descending limb of Henle’s
loop, J; al, ascending limb of Henle’s loop; dc, distal convoluted tubule; j, junctional tubule.
Fic. 215.—Several stages in the development of the uriniferous tubules and glomeruli of the human
metanephros of the seventh month (reconstructions by Huber). x 160.
limb gives rise to the connecting piece, and the rest of it, with the upper limb of
the S, forms the distal convoluted tubule (intermediate piece of Felix). The prim-
CLOACA, BLADDER, URETHRA AND UROGENITAL SINUS 205
itive loop of Stoerck includes both the ascending and descending limbs of Henle’s
loop and a portion of the proximal convoluted tubule. Henle’s loop is differ-
entiated during the fourth fetal month (Toldt) and extends from the pars radiata
of the cortex into the medulla (Fig. 214). The concavity of Bowman’s capsule,
into which grow the arterial loops of the glomerulus, is at first,shallow. Eventu-
ally the walls of the capsule grow about and enclose the vascular knot, except at
the point where the arteries enter and emerge (Fig. 212, 4 and 5). Renal cor-
puscles are first fully formed in 28 to 30 mm. embryos. The new corpuscles are
formed peripherally from persisting nephrogenic tissue until the tenth day after
birth, hence in the adult the oldest corpuscles are those next the medulla. Recon-
structions of the various stages in the development of the uriniferous tubules are
shown in Fig. 215.
Renal Arteries—One or more of the mesonephric arteries is transformed into the
renal artery of the metanephros (Broman, 1906). As any one of the mesonephric arteries
-may thus form the renal artery, and as they anastomose, the variation of the renal vessels
both as to position and number is accounted for. Bremer (Amer. Jour. Anat., vol. 18, 1915)
derives the renal arteries not from the mesonephric vessels but from a periaortic plexus of
multiple aortic origin. \
Anomalies.—If the uriniferous tubules fail to unite with the collecting tubules, cystic
degeneration may take place. The cystic kidneys of pathology may thus be produced. The
nephrogenic tissue of the paired kidney anlagés may fuse, resulting in the union of their cortex
(“horse-shoe kidney’’). Double or triple ureters and cleft ureters are sometimes present.
DIFFERENTIATION OF CLOACA, BLADDER, URETHRA AND UROGENITAL
SINUS
In embryos of 1.4 mm. the cloaca, a caudal expansion of the hind-gut, is
in contact ventrally with the ectoderm, and ectoderm and entoderm together
form the cloacal membrane (Fig. 216 A). Ventro-cranially the cloaca gives off
the allantoic stalk. At a somewhat later stage, the cloaca receives laterally the
mesonephric ducts and caudally is prolonged as the tail-gut (Fig. 216 B).
In embryos of 5 mm. the ureteric anlages of the metanephroi are present as
buds of the mesonephric ducts (Fig. 216 C, D). Next, the saddle-like partition
between the intestine and allantois grows caudally, dividing the cloaca into a
dorsal rectum and ventral, primitive urogenital sinus. The division is complete
in embryos of 11 to 15 mm., and at the same time the partition, fusing with the
cloacal membrane, divides it into the anal membrane of the gut and the urogenital
membrane. At 11 mm., according to Felix, the primitive urogenital sinus by
elongation and constriction is differentiated into two regions: (1) a dorsal
vesico-urethral anlage which receives the allantois and mesonephric duct, and
206 THE UROGENITAL SYSTEM
A Hind-gut
Hind-gut
» Mesonephric
duct
ind-gut M. heric 5
Hind-gu ene Hindevik
Allantois
“» Cloacal membrane
Tail-gut
Fic. 216.—Four stages showing the differentiation of the cloaca into the rectum, urethra and
bladder (after reconstructions by Pohlman). X about 50. A, from a human embryo of 3.5 mm.;
B, at about 4 mm.; C, at 5mm.; D, at 7 mm.
Mesonephric duct Intestine
Fic. 217.—Reconstructions from a 12 mm. human embryo showing the partial subdivision of the cloaca
into rectum and urogenital sinus (after Pohlman). X 65.
CLOACA, BLADDER, URETHRA AND UROGENITAL SINUS 207
is connected by the constricted portion with (2) the phallic portion of the uro-
genital sinus (Figs. 217 and 218). The latter extends into the phallus of both
Celom Rectum
Allantois Mesonephric ducts
Vesico-urethral anlage
Phallic portion of urogenital sinus
| }
HA
an i ic
Fic. 218.—Reconstruction of the caudal portion of an 11.5 mm. human embryo showing the differen-
tiation of the rectum, bladder and urethra (after Keibel’s model). X 25.
sexes and forms a greater part of the urethra (Fig. 219). The vesico-urethral
anlage enlarges and forms the bladder and a portion of the urethra. In 7 mm.
Genital gland
Mesonephric fold
Anlage of bladder, Vag
Mesonephros
Ureter
Utero-vaginal anlage
Mesonephric duct
Rectum
Anal membrane —“4rrrn 2 Rectum
Fic. 219.—Reconstruction of the caudal end of a 29 mm. human embryo showing the complete
separation of therectum and urogenital sinus and the relations of the urogenital ducts (after Keibel’s
model). X 15.
embryos the proximal ends of the mesonephric ducts are funnel shaped, and
at 10 mm., with the enlargement of the bladder, these ends are taken up into
208 THE UROGENITAL SYSTEM
its wall until the ureters and mesonephric ducts acquire separate openings. The
ureters, having previously shifted their openings into the mesonephric ducts from
a dorsal to lateral position, now open into the vesico-urethral anlage lateral to
the mesonephric ducts. The lateral walls of the bladder anlage grow more rap-
idly than its dorso-median urethral wall, hence the ureters are carried cranially
and laterally upon the wall of the bladder, while the mesonephric ducts, now the
male ducts, open close together on a hillock, Miiller’s tubercle, into the dorsal
wall of the urethra (Fig. 219). The fate of the phallic portion of the urogenital
- sinus is described on p. 226 in connection with the external genitalia.
The apex of the bladder, continuous with the allantoic stalk at the umbilicus, is known
as the urachus. Usually the epithelium of the urachus degenerates, but portions may persist
and produce cysts. In some cases it forms after birth a patent tube opening at the umbilicus.
Its connective tissue layers always persist-as the fibrous lig. wmbilicale medium.
The transitional epithelium of the bladder appears at 60 mm. (C H). The outer
longitudinal layer of smooth muscle develops in 22 mm. embryos, and, in 26 mm. embryos,
the circular muscle appears. The inner Jongitudinal muscle layer is found at 55 mm. (C H)
and the sphincter vesice in fetuses of 90 mm. (C H).
Anomaly.—A conspicuous malformation is that of a persistent cloaca, due to the failure
of the rectum and urogenital sinus to separate.
THE GENITAL GLANDS AND DUCTS—INDIFFERENT STAGE
In origin and early development, the ovary and testis are identical. The
urogenital fold (p. 197) is the anlage of both the mesonephros and the genital
gland (Figs. 122 and 220). At first two-layered, its epithelium in embryos of
5 mm. thickens over the ventro-median surface of the fold, becomes many-
layered, and bulges into the ccelom ventrally, producing the longitudinal genztal
fold. The genital fold thus lies mesial and parallel to the mesonephric fold.
Large primitive sex cells are found in 2.5 mm. embryos in the entoderm of the
future intestinal tract (Fuss). At 3.5 mm. they migrate into the dorsal mesen-
teric epithelium and thence into the epithelium of the genital fold. At 10 to
12 mm. the genital epithelium shows no sexual differentiation (Fig. 221). There
is a superficial epithelial layer and an inner epithelial mass of somewhat open
structure.
Owing to the great development of the suprarenal glands and metanephroi,
the cranial portions of the urogenital folds, at first parallel and close together, are
displaced laterally. This produces a double bend in each fold which, in 20 mm.
embryos, shows a cranial longitudinal portion, a transverse middle portion between
the bends, and a longitudinal caudal portion. In the last named segment, the
mesonephric ducts course to the cloaca and here the right and left folds fuse,
THE GENITAL GLANDS AND DUCTS—INDIFFERENT STAGE 209
Mesencephalon
Prosencephalon
Rhombencephalon
Heart
Right lung
Esophagus
Genital fold
Mesonephric fold
Genital eminence
Lower extremity
Tail
Lateral body wall
Post. cardinal vein
Suprarenal gland
Glomerulus
Mesonephric duct
Mesentery—*
‘
Fic. 221.—Transverse section through the mesonephros, genital gland and suprarenal gland of the right
side; from a12mm.humanembryo. X 165.
14
210 ; THE UROGENITAL SYSTEM
producing the genital cord (Fig. 232). As the genital glands increase in size they
become constricted from the mesonephric fold by lateral and mesial grooves until
the originally broad base of the genital fold is converted into a stalk (Figs. 225 to
227). This stalk-like attachment extends lengthwise and forms in the male the
mesorchium, in the female the mesovarium. ‘The urogenital fold is, at the same
time, constricted from the dorsal body wall until it is attached only by a narrow
mesentery which eventually forms either the ligamentum testis or lig. ovarii.
The Indifferent Stage of the Genital Ducts.—The mesonephric ducts, with
the degeneration of the mesonephroi, become the male genital ducts. In both
Mesonephric tubule
Genital gland
Fic. 222.—Transverse sections through the anlage of the right Miillerian duct from a 10 mm.
human embryo. X 250. A, showing the groove in the urogenital epithelium; B, three sections caudad
showing the tubular anlage of the duct.
sexes there also develop a pair of female ducts (of Miiller). In embryos of 10
mm. the Miillerian ducts develop as ventro-lateral thickenings of the urogenital
epithelium at the level of the third thoracic segment and near the cranial ends
of the mesonephroi. Next, a ventro-lateral groove appears in the epithelium
of the mesonephric fold (Fig. 222 A). Caudally, the dorsal and ventral lips of
the groove close and form a tube which separates from, and lies beneath, the
epithelium (Fig. 222 B). Cranially, the tube remains open as the funnel-shaped
ostium abdominale of the Miillerian duct. The solid end of the tube grows
caudalward beneath the epithelium, lateral to the mesonephric or male ducts
(Figs. 223 to 225). Eventually, by way of the genital cord, the Miillerian
THE GENITAL GLANDS AND DUCTS—INDIFFERENT STAGE 211
—— Pulmonary trunk
Pulmonary artery
Diaphragm
Ostium abdominale
Miillerian duct
Esophagus
Inferior vena cava Mesonephric duct
Genital gland Mesonephros
Colon
Allantois Umbilical artery
Trachea ere an oat
/ SS eee Cun
Mesonephros
Miutllerian duct
Colon
Umbilical artery
Fic. 224.—Ventral dissection of a 24 mm. pig embryo showing the anlages of the Miillerian ducts at a
later stage of development than in Fig. 223. X 6.
212 THE UROGENITAL SYSTEM
ducts reach the median dorsal wall of the urogenital sinus and open into it (Figs.
219 and 238 A). Their further development into uterine tubes, uterus, and vagina
is described on page 219. Embryos not longer than 12 mm. are thus character-
ized by the possession of indifferent genital glands and of both male and female
genital ducts. There is as yet no sexual differentiation. The development and
position of the Miillerian ducts is well shown in ventral dissections of pig em-
bryos (Figs. 223 and 224); the mesonephroi of the pig are of enormous size.
In the lowest vertebrates the Miillerian duct arises by a longitudinal splitting of
the mesonephric duct.
Mesentery
Mesone phric tubule
Mesonephric duct
Mesorchium
Anlage of rete testis
Intermediate cord
Testis cord
Epithelium
Tunica albuginea
Fic. 225.—Transverse section through the left testis and mesonephros of a 20 mm. human embryo.
x 250.
Differentiation of the Testis.—In the male embryos of 13 mm. the genital
glands show two characters which mark them as testes: (1) the occurrence of
branched, anastomosing cords of cells, the ¢estis cords: (2) the occurrence between
epithelium and testis cords of a layer of tissue, the anlage of the tunica albuginea
(Fig. 225). According to Felix, the testis cords are developed suddenly from the
loose, inner epithelial mass by a condensation of its cells. The cords converge
and grow smaller towards the mesorchiurh, where they form the dense, epi-
thelial anlage of the rete testis. Two or three layers of loosely arranged cells
between the testis cords and the epithelium constitute the anlage of the tunica
albuginea. According to Allen (Amer. Jour. Anat., vol. 3, 1904), the testis cords
THE GENITAL GLANDS AND DUCTS—TESTIS 213
of the rabbit and pig are formed as active ingrowths of cellular cords from the
epithelium.
The testis cords soon become rounded and are marked off by connective
tissue sheaths from the intermediate cords, columns of undifferentiated tissue
which lie between them (Fig. 226). Toward the rete testis the sheaths of the
testis cords unite to form the anlage of the mediastinum testis. The testis cords
are composed chiefly of indifferent cells with a few larger genital cells. The cells
gradually arrange themselves radially about the inside of the connective tissue
sheath as a many-layered epithelium, in which, during the seventh month, a
lumen appears. The lumina appear in the peripheral ends of the testis cords,
and, extending toward the rete testis, meet lumina which have formed there.
Thus the solid cords of both are converted into tubules. The distal portions of
Ductus deferens
5 Epithelium
Mesorchium
Intermediate cords Tunica albuginea
{_ Testis cord
Genital cell
Fic. 226.—Section through the testis of a 100 mm. human fetus. X 44.
the testis tubules anastomose and form the éubulz contorti. Their proximal por-
tions remain straight as the tubuli recti. The rete testis becomes a network of
small tubules which finally unite with the collecting tubules of the mesonephros
(see p. 218).
The primitive genital cells of the testis cords form the spermatogonia of the
spermatic tubules and from these at puberty are developed the later generations
of spermatogonia (p. 14). The indifferent cells of the tubules become the sus-
tentacular cells (of Sertoli) of the adult testis. Primitive genital cells of the inter-
mediate cords are transformed into large pale cells, which, after puberty, are
numerous in the interstitial connective tissue and hence are called inéerstitial
cells. The intermediate cords themselves are resorbed, but the connective tissue
sheaths of the tubules unite to form septula which extend from the mediastinum
214 THE UROGENITAL SYSTEM
testis to the tunica albuginea. The latter becomes a relatively thick layer in the
adult testis and is so called because of its whitish appearance.
Anomalies.-—The testis may be congenitally absent; the glands may be fused; or
they may fail to descend into the scrotum (cryptorchism). Duplication of the testis is
rare.
The Differentiation of the Ovary.—The primitive ovary, like the testis,
consists of an inner epithelial mass bounded by the parent peritoneal epithelium.
The ovarian characters appear much more slowly than in the testis. In fetuses
of 50 to 80 mm. (C H) the inner epithelial mass, composed of indifferent cells
and primitive genital cells, becomes less dense centrally and bulges into the
mesovarium (Fig. 227). There may be distinguished a dense, outer cortex beneath
Tubules of mesonephros
(Paroiphoron)
Uterine (Miil-
lerian) tube
© Rete ovarii
Genital cells
Epithelium Medulla
Cortex
Fic. 227.—Section of an ovary from a 65 mm. human fetus. X 44,
the epithelium, a clearer medullary zone containing large genital cells, and a
dense, cellular anlage in the mesovarium, the primitive rete ovarii, which is the
homologue of the rete testis. No epithelial cords and no tunica albuginea are
developed at this stage, as in the testis. Later, three important changes take place:
(1) There is an ingrowth of connective tissue and blood vessels from the hilus,
resulting in the formation of a mediastinum and of septula. (2) Most of the
cells derived from the inner epithelial mass are transformed into young cva,
the process extending from the rete ovarii peripherally (Fig. 227). (3) In fetuses
of from 80 to 180 mm. (C R) length the ovary grows rapidly, owing to the
formation of a new peripheral zone of cells, perhaps derived in part from the
peritoneal epithelium. At the end of this period the septule line the epithelium
with a fibrous sheath, the anlage of the tunica albuginea. Hereafter, although
THE GENITAL GLANDS AND DUCTS—OVARY 215
folds of the epithelium are formed, these do not penetrate beyond the tunica
albuginea, and all cells derived from this source subsequently degenerate. This
new peripheral zone, according to Felix, is always a single cellular mass in man,
cords or “Pfliiger’s tubes” never growing in from the epithelium. Generally it
has been believed that the primary follicles are derived from the subdivision of
such cords.
Coincident with the origin of a new zone of cells at the periphery of the ovary
goes the degeneration of young ova in the medulla. By the ingrowth into this
Primordial egg
oe Germinal epithelium
p Tunica albuginea
Primordial ovum
a, Phitiger’s egg tubes
Blood vessel
Primordial ova
~
Fic. 228.—Ovary of five-months’ fetus, showing primordial follicles (De Lee).
region of connective tissue septa, the ova are separated into clusters or cords, the
genital cells of which all degenerate, leaving in the medulla only a stroma of con-
nective tissue. Late in fetal life indifferent cells, by surrounding the young ova
of the cortex, produce primordial follicles (Fig. 229 A). During the first year
after birth the primitive follicles are transformed into the vesicular (Graafian)
follicles. By cell division the follicle cells form a zone many layers deep about
the young ovum (Fig..229 B). Next a cavity appears in the sphere of follicle
cells, enlarges, and produces a vesicle filled with fluid (Figs. 3 and 230). The
216 THE UROGENITAL SYSTEM
ES FSAOt oN
a
~“ \
SS
We ‘ Ws
Wen 5,
t a Theca folliculi Z > Ect. Fig. 256) and shows three regions:
a om : (1) the atrium, which receives the
blood from the primitive veins; (2)
the ventricle; (3) the bulb, from
which is given off the ventral aorta.
Ms. spl. End.
B
Fic. 255.—Diagrams to illustrate the origin of Fic. 256.—The heart of a 2 mm. human
the mammalian heart. ct., Ectoderm; End., endo- embryoin ventral view (Mall). X65. The
thelial tubes; Eni., entoderm; Fg.,-fore-gut; Msc.d., open tube is the fore-gut.
dorsal mesocardium; Ms.spl., splanchnic mesoderm
(epi- and myocardium).
As the cardiac tube grows faster than the pericardial cavity in which it lies
it bends to the right, the bulbus and ventricle forming a U-shaped loop (Fig.
257). Four regions may now be distinguished: (1) the sinus venosus; (2) the
atrium, also thin walled and lying cranial to the sinus; (3) the thick-walled ventricu-
lay limb, ventrad and caudad in position; (4) the bulbar limb, cranial to the ventric-
ular limb and separated from it by the bulbo-ventricular cleft. Next, in embryos
250 THE DEVELOPMENT OF THE VASCULAR SYSTEM
of 3 to4 mm., the bulbo-ventricular loop shifts its position until its base is directed
caudad and ventrad (Fig. 257 B). At the same time the sinus venosus is brought
dorsal to the atrium, which in turn is cranial with relation to the bulbo-ventricular
A B
Fic. 257.—A, Heart of human embryo of 2.15 mm.: a, Bulbus cordis; b, primitive ventricle; c,
atrial portion. B, Heart of human embryo of about 3 mm.: a, Bulbus cordis; 6, atrial portion (behind);
c, primitive ventricle (in front). Ventral views (His).
loop, and the bulbar limb is pressed against the ventral surface of the atrium and
constricts it (Fig. 258 A).
In embryos of 4 to 5 mm. the right portion of the sinus venosus grows more
rapidly than the left, this being due to the fact that the blood flow of the left
A
Fic. 258.—A, Heart of human embryo of about 4.3 mm.: «, Atrium; 6, portion of atrium corre-
sponding to auricular appendage; c, bulbus cordis; d, atrial canal; c, primitive ventricle. B, Heart of
human embryo of about 10 mm.: a, Left atrium; , right atrium; c, bulbus cordis; d, interventricular
groove; ¢, right ventricle; f, left ventricle. Ventral views (His).
umbilical vein is shifted to the right side through the liver. As a result, the en-
larged right horn of the sinus opens into the right dorsal wall of the atrium through
a longitudinally oval foramen, which is guarded on the right by a vertical fold.
EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 251
This fold, which projects into the atrium, is the right valve of the sinus venosus.
Later, a smaller fold forms the left valve of the sinus venosus (Fig. 260 B). The
atrium is constricted dorsally by the gut, ventrad by the bulbus. It therefore
must enlarge laterally and in so doing forms the right and left atria (Fig. 258 A,
B) with the distal portion of the bulb between them. The deep external groove
between the atria and the bulbo-ventricular part of the heart is the coronary
sulcus. As the bulbo-ventricular region increases in size, the duplication of the
wall between the two limbs lags behind in development and finally disappears
(Fig. 259), leaving the proximal portion of the bulb and the ventricular limb
to form a single chamber, the primitive ventricle. In an embryo of 5 mm. the heart
is thus composed of three undivided chambers: (1) the sinus venosus, opening
dorsad into the right dilation of the atrium; (2) the bilaterally dilated atrium,
Pulmonary artery Aorta
}
a Atrium
Bulbus---%
(/
-- L. ventricle
R. ventricle .. .
R. ventricle.-
Fic. 259.—Diagrams to show the reduction of the bulbo-ventricular fold (represented by diagonal lines)
due to its retarded development. (Modified after Keith.)
opening by the single transverse atrial canal into (3) the primitive undivided ven-
tricle. The three-chambered heart is persistent in adult fishes, but in birds and
mammals a four-chambered heart is developed in which venous blood circulates
on the right, and arterial blood on the left. In amphibians and reptiles transi-
tional types occur.
The important changes leading to the formation of the four-chambered heart
are: (1) the complete division of the atrium and ventricle, each into right and left
chambers; (2) the division of the bulb and its distal continuation, the truncus
arteriosus, into the aorta and pulmonary artery; (3) the incorporation of the sinus
venosus into the wall of the right atrium; (4) the development of the semilunar
and atrio-ventricular valves. The first of these changes is completed only after
birth.
Endocardial Cushions and Atrial Septa.—In embryos of 5 to 7 mm. there
252 THE DEVELOPMENT OF THE VASCULAR SYSTEM
develops a thin sickle-like membrane from the mid-dorsal wall of the atrium (Figs.
260 and 261). This is called the atrial septum primum (I). Simultaneously,
endothelial thickenings appear in the dorsal and ventral walls of the atrial canal
(Figs. 261 A, B). There are the endocardial cushions, which later fuse, thus
; Valves of sinus venosus
Valves of sinus venosus
Septum I x oramen ovale
L Atrium
Sept I
Atrio-ventricular canal
Atrio-ventric-
ular canal
Interventricular
sepium
Sinus venosus
R. valve of sinus venosus
Fic. 260.—Horizontal sections through the chambers of the human heart: A, 6 mm.; B,9mm.; C, 12
mm. (A and B are based on figures of Tandler.) about 50.
dividing the single atrial canal into right and left atrio-ventricular canals (Fig.
266). The atrium is now partly divided into right and left atria, which, how-
ever, communicate ventrad through the interatrial foramen. Next, in embryos
of 9mm., the septum I thins out dorsad and cephalad and asecond opening appears,
EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 253
the foramen ovale (Figs. 260 and 261 B). The atria are now connected by two
openings, the interatrial foramen and the foramen ovale. Soon (embryos of 10 to
12 mm.) the ventral and caudal edge of septum I fuses with the endocardial
cushions, which have in turn united with each other (Figs. 260 and 261 C).
The interatrial foramen is thus obliterated, but the foramen ovale persists
until after birth. In embryos of 9 mm. the septum secundum (IT) is developed
from the dorsal and cephalic wall of the atrium, just to the right of the septum
primum (Fig. 260 C). It is important, as it later fuses with the left valve of the
sinus venosus and with it forms a great part of the atrial septum of the late fetal
and adult heart.
L.atr.
Fic. 261.—Lateral dissections of the human heart viewed from the left side: A,6mm.; B, 9 mm.;
C,12mm. (B is based on a reconstruction by Tandler.) X about 38. Cor. sin., Coronary sinus;
D. end. c., dorsal endocardial cushion; For. ov., foramen ovale; Int. for., interatrial foramen; I. v. ¢.,
inferior vena cava; L. aér., left atrium; L. va.s.v., left valve of sinus venosus; L. vent., left ventricle;
Pul.a., pulmonary artery; Pul.v., pulmonary vein; Sept. I, Sept. II, septum primum, septum secundum;
Sup. v. v., superior vena cava; V. end. c., ventral endocardial cushion.
Sinus Venosus and its Valves.—The opening of the sinus venosus into the
dorsal wall of the right atrium is guarded by two valves (Fig. 260). Along the
dorsal and cephalic wall of the atrium these unite to form the septum spurium.
Caudally the valves flatten out on the floor of the atrium, but, as stated pre-
viously, the left valve later fuses with the atrial septum II. In embryos of 10
to 20 mm. the atria increase rapidly in size and the lagging right horn of the
sinus venosus is taken up into the wall of the right atrium. By this absorption
the superior vena cava now opens directly into the cephalic wall of the atrium,
the inferior vena cava into its caudal wall (Fig. 261 C). The transverse portion
of the sinus venosus, persisting as the coronary sinus in part, opens into the pos-
terior wall of the atrium.
254 THE DEVELOPMENT OF THE VASCULAR SYSTEM
The right valve of the sinus venosus is very high in 10 to 65 mm. embryos (first
to third month) and nearly divides the atrium into two chambers (Fig. 262).
It becomes relatively lower during the third and fourth months. Its cephalic
portion becomes the rudimentary crista terminalis (Fig. 263); the remainder is
divided by a ridge into two parts, of which the larger cephalic division persists as
the valve of the inferior vena cava (Eustachian valve) located at the right of the
opening of the vein, and the smaller caudal portion becomes the valve of the coro-
nary sinus (Thebesian valve).
Foramen ovale ZL
<“——~€& Sup. vena cava
a Seplum II
R. valve of sinus venosus
Inf. vena cava-
Semilunar valve of
pulmonary artery
Septum I
R. ventricle
Fic. 262.—Lateral dissection of the heart of a 65 mm. human fetus viewed from the right side. xX 12.
The Jeft valve of the sinus venosus becomes continuous with the septum se-
cundum, and, in embryos of 20 to 22 mm. or larger, the two bound an oval
opening (Figs. 263 to 265). The bounding wall of the oval aperture is the
limbus ovalis.
Closure of the Foramen Ovale.—The free edge of septum I is, in embryos of
10 to 15 mm., directed dorsad and cephalad (Fig. 261 C). Gradually, in later
stages (Figs. 264 and 265), its caudal and dorsal prolongation grows cephalad and
ventrad until its free edge is so directed. Coincident with this change the septum
II, with its free edge directed at first ventrad and caudad, shifts until its free
edge is directed dorsad and cephalad, and overlaps the septum I (Figs. 261 C,
EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 255
264 and 265). The opening between these septa persists until after birth as the
foramen ovale.
During fetal life the left atrium receives little blood from the lungs, so that
the pressure is much greater in the right atrium. As a result, the septum I is
pushed to the left and the blood flows from the right into the left atrium through
the foramen ovale. After birth,-the left atrium receives from the expanding lungs
as much blood as the right atrium, the septum I is pressed against the limbus of
Crista terminalis
Sept. II+-L. valve of
SINUS VENOSUS
Septum I
Inf. vena cava
Valve of inf. vena cava.
Semilunar valves
Valve of coronary sinus of pulmonary artery
, ; R. ventricle
Tricuspid valve
Katherine Hills
Fic. 263.—Lateral dissection of the heart of a 105 mm. human fetus viewed from the right side. X 7.
septum II, and soon fuses with it. The depression formed by the thinner walled
septum I is the fossa ovalis.
The foramen ovale may fail to close soon after birth and the mixed blood produces a
purplish hue in the child which is known popularly as a “blue baby.” This condition may
be persistent in adult life. Incomplete closure occurs in about one in four cases, but actual
mingling of the blood is rare, due to an approximation of the overlapping septal folds during
atrial systole. ;
Pulmonary Veins.—In embryos of 6 to 7 mm. a single vein (arising in the
cat from a peripulmonary plexus, Brown, Anat. Rec., vol. 7, 1913) drains into
THE DEVELOPMENT OF THE VASCULAR SYSTEM
Septum II Foramen ovale
Sup. vena cava
Seplum I
Aorta
| (ode
Pulmonary trunk >
G4
on Coronary sinus
(A Bicus pid valve
oO
| est entricle
mao
L. atrium
Fic. 264.—Lateral dissection of the heart of a 65 mm. human fetus viewed f om the left ide, showing
the septa and the foramen ovale. X 8.
Sup. v. c.
For. ov.
For. ov. Sup. v. C.
Aorta
L. atr.
vent. C.
L. vent.A
Fic. 265.—Lateral dissections of the human heart viewed from the left side: 4, from a 22 mm.
embryo; B, froma 105 mm. fetus. Bic. va., Bicuspid valve; Cor. sin., coronary sinus; For. ov., foramen
ovale; I.v.c., inferior vena cava; L. atr. vent. «., left atrio-ventricular canal; L. ven., left ventricle; Pu.
a., pulmonary artery; Sept. I, Sept. 17, septum primum and septum secundum.
EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 257
the caudal wall of the left atrium at the left of the septum I (Fig. 261.C). This
vein bifurcates into right and left pulmonary veins which divide again before
Pulmonary arlery —— Aorla
Incomplete bulbar septum Dorsal endocardial cushion
Atrio-ventricular
Ventral endocardial foramen
cushion :
WA Interventricular
(|) septum
mt Interventricular sulcus
é
C=
Aorta oa
iY > \\ yp Pulmonary artery
Arrow in pulmonary artery B faba
ase of aorta
Proximal bulbar septum
Arrow in aorta
Interventricular
foramen '
NL. atrio-ventricular
R. atrio-ventricular foramen
foramen
! | Interventricular
Y]
R. ventricle \—
septum
Fic. 266.—Ventral views of stages in the development of the heart to show the differentiation of the
bulbus cordis into the aorta and pulmonary trunk (Kollman): A, Heart of a5 mm. human embryo; B,
of a 7.5 mm. human embryo.
entering the lungs. As the atrium grows, the proximal portion of the pulmonary
vein is taken up into the atrial wall. As a result, at first two, then four pul-
monary veins open into the left atrium.
17
258 THE DEVELOPMENT OF THE VASCULAR SYSTEM
Origin of the Right and Left Ventricles.—In embryos of 5 to 6 mm. there
appears at the base of the primitive ventricular cavity a sagittally placed eleva-
tion, the interventricular septum (Fig. 260 B). It later grows cephalad and
dorsad toward the endocardial cushions, and forms an incomplete partition be-
tween the right and left ventricles, which still communicate through the persisting
interventricular foramen (Fig. 266 B). Corresponding to the internal attach-
ment of the septum there is formed externally the interventricular sulcus (Fig.
266 A) which marks the external line of separation between the large left ven-
tricle and the smaller right ventricle.
Origin of Aorta and Pulmonary Artery from Bulbus.—Coincident with the
formation of the interventricular septum there arise in the aortic bulb (including
its distal truncus arteriosus) longitudinal thickenings, four in the distal half, two
in the proximal half. Of the four distal thickenings (Fig. 267), two, which may be
OD:
a
Pulmonary artery
Fic. 267.—Scheme showing division of bulbus cordis and its thickenings into aorta and pulmonary
artery with their valves. (Explanation in text.)
designated a and c, are larger than the other thickenings, b and d. Thickenings
a and ¢, which distally occupy left and right positions in the bulb, meet, fuse, and
divide the bulb into a dorsally placed aorta and ventrally placed pulmonary
trunk (Fig. 266). Traced proximally they pursue a clockwise, spiral course, a
shifting from left to ventral, and c from right to dorsal, both becoming con-
tinuous with the proximal swellings. Thickenings b and d are also prominent at
one point proximally; when the bulb in this region is divided by ingrowing con-
nective tissue into the aorta and pulmonary artery, the aorta contains the whole
of the thickenings } and half of a and c, while the pulmonary trunk contains the
whole of d and half of a and ¢ (Fig. 267). Distally the three thickenings now pres-
ent in each vessel disappear, but proximally they enlarge, hollow out on their
distal surfaces, and eventually form the thin-walled semilunar valves (Fig. 267).
The anlages of these valves are prominent in embryos of 10 to 15 mm. as plump
swellings projecting into the lumina of the aorta and pulmonary artery.
THE PRIMITIVE BLOOD VASCULAR SYSTEM 259
The two proximal bulbar swellings fuse and continue the spiral division of the
bulb toward the interventricular septum in such a way that the base of the pul-
monary trunk, now ventrad and to the right, opens into the right ventricle, while
the base of the aorta, now lying to the left and dorsad, opens into the left ven-
tricle close to the interventricular foramen, through which the two ventricles
still communicate (Fig. 266 B).
Closure of the Interventricular Foramen.—The interventricular foramen in
embryos of 15 to 16 mm. is bounded: (1) by the interventricular septum; (2) by
the proximal bulbar septum; and (3) by the dorsal portion of the fused endo-
cardial cushions (Fig. 266). Soon these structures are approximated and fuse,
thereby forming the septum membranaceum, which closes the interventricular
foramen. The atrio-ventricular valves arise as thickenings of the endocardium
and endocardial cushions of the atrio-ventricular foramina (Figs. 260 and 261).
Three such thickenings are formed on the right, two on the left. The anlages of
the valves are at first thick and project into the ventricles. Later, as the ven-
tricular wall differentiates, the valvular anlages are undermined, leaving their
edges attached to the ventricular walls by muscular trabecule, or cords. The
muscle tissue of both the valves and trabecule soon degenerates and is replaced
by connective tissue, forming the chorde tendinee of the adult valves. ‘Thus there
are developed the three cusps of the tricuspid valve between the right chambers of
the heart, and the two flaps of the bicuspid, or mitral valve, between the left
atrium and left ventricle.
Differentiation of the Myocardium.—The myocardium, at first uniformly spongy,
becomes compact at the periphery. The inner bundles remain trestle-like, forming the
trabecule carne and the papillary and moderator muscles around all of which the originally
simple endocardial sac is wrapped. The myocardial layers, at first continuous over the sur-
face of the heart, become divided by connective tissue at the atrio-ventricular canal, leaving
a small bridge alone. This connecting strand, located behind the posterior endocardial
cushion, forms the atrio-ventricular bundle.
Descent of the Heart.—At first the heart lies far cephalad in the cervical region, but
it gradually recedes during development until it assumes its permanent position in the thorax.
PRIMITIVE BLOOD VASCULAR SYSTEM
The first paired vessels of human embryos are formed as longitudinal anas-
tomoses of capillary networks which, however, originate first in the angioblast of
the yolk sac and chorion. In the Eternod embryo of 1.3 mm., in which the somites
are still undeveloped the paired vessels are already formed (cf. Fig. 268). They
are the umbilical veins which emerge from the chorion, fuse in the body stalk, then,
separating, course in the somatopleure to. the paired tubular heart. From the
260 THE DEVELOPMENT OF THE VASCULAR SYSTEM
heart tubes paired vessels, the ventral aorte, extend cephalad, then bend dorsad as
the first aortic arches and extend caudad as the dorsal or descending aorte. These,
as the umbilical arteries, bend sharply ventrad into the belly stalk aad branch
Dorsal intersegmental arteries Descending aorte
Umbilical veins
Primitive aortic arch
Primitive heart
Fiody site Vitello-wmbilical trunk
Umbilical vein
as . Yolk sac
Vitelline arteries
Frc. 268.—Diagram, in lateral view, of the primitive blood vessels in human embryos of 1.5 to 2 mm.
in the wall of the chorion. The chorionic circulation is thus the first to be es-
tablished.
In embryos 2 to 2.5 mm. long (5 to 8 somites) the heart has become a single
tube (Fig. 269). From the yolk sac numerous veins converge cephalad and form
a pair of vitelline veins. These join the umbilical veins, and, as the vitello-umbili-
Dorsal intersegmental arteries Ant. cardinal veins
Descending aorte
Umbilical arteries
Aortic arch 1
Heart
Vitello-umbilical trunk
oe . Vitelline veins
Vitelline arteries
Yolk sac
Fic. 269.—Diagram, in lateral view, of the primitive blood vessels in human embryos of 2 to 2.5 mm.
cal trunk, traverse the septum transversum and open into the sinus venosus.
The descending aortz give off, dorsally and cranially, several pairs of dorsal
intersegmental arteries, and, ventrad and caudad, a series of non-segmental vitelline
arteries to the yolk sac. The umbilical arteries now take their origin from a
DEVELOPMENT OF THE ARTERIES 261
plexus of ventral vessels in series with the vitelline arteries. At this stage the
vitelline circulation of the yolk sac is established.
In embryos of 15 to 23 somites (Fig. 270) the vezms of the embryo proper de-
velop as longitudinal anastomoses of branches from the segmental arteries. The
paired anterior cardinal veins of the head are developed first, and, coursing back
on either side of the brain, they join the vitello-umbilical trunk. In embryos of
23 somites the posterior cardinals are present. They lie dorsal to the nephrotomes,
and, running cranially, join the anterior cardinal veins to form the common cardinal
veins. Owing to the later enlargement of the sinus venosus, the proximal portions
of the common venous trunks are taken up into its wall and thus three veins open
into each horn of the sinus venosus: (1) the wmbilical veins from the chorion; (2)
the vitelline veins from the yolk sac; (3) the common cardinal veins from the body of
the embryo.
Posterior cardinal veins Ant. cardinal veins
Vitelline artery
Descending aorta
Umbilical arteries,
Aortic arches 1 and 2
Sinus venosus
Vitelline veins
Fic. 270.—Diagram of the blood vessels of human embryos of 2.6 mm.
The descending aorte have now fused caudal to the seventh intersegmental
arteries and form the single dorsal aorta as far caudad as the origins of the um-
bilical arteries.
Of the numerous vitelline arteries one pair is prominent; these fuse into a
single vessel which courses in the mesentery and later becomes the superior
mesenteric artery. By the enlargement of capillaries connecting the ventral and
dorsal aortz a second pair of aortic arches is formed at this stage (Fig. 270):
DEVELOPMENT OF THE ARTERIES
Transformation of the Aortic Arches.—In embryos 4 to 5 mm. in length five
pairs of aortic arches are successively developed, the first, second, third, fourth,
and sixth (Fig. 271). An additional pair of transitory vessels which extend from
262 THE DEVELOPMENT OF THE VASCULAR SYSTEM
the ventral aorta to the sixth arch appear later in embryos of 7 mm., but soon
degenerate (Fig. 272 B). They are interpreted as being the fifth pair in the series.
From each dorsal or descending aorta there develop cranially the internal carotid
arteries. These extend toward the optic stalks where they bend dorsad and
caudad, connecting finally with the first intersegmental arteries of each side
(Fig. 271). The descending aortz are now fused to their extreme caudal ends and
the umbilical arteries take their origin ventrally. Twenty-seven pairs of dorsal
3 ‘ Vertebral artery
First cervical artery
Otocyst
4 ( )
IN,
a)
Pulmonary artery
Ant. cardinal vein
Vena capitis medialis
aw
ers
Post. cardinal vein
- Bulbus cordis
Subclavian artery | Ophthalmic artery
Celiac artery Ant. cerebral artery
Common cardinal vein
Vitelline artery
(Superior mesenteric)
Caudal artery
Umbilical artery
Inf. mesenteric artery
Fic. 271.—Arteries and cardinal veins of the right side in a 4.9 mm. human embryo (modified after
Ingalls). XX 20. H, Heart; I-VI, aortic arches.
intersegmental arteries are present. From the seventh cervical pair of these
the subclavian arteries of the upper limbs arise. Of the ventral vitelline vessels
three are now prominent, the celiac artery in the stomach-pancreas region, the
vitelline or superior mesenteric in the small intestine region, and the inferior mesen-
teric of the large intestine region.
Of the aortic arches the third pair is largest at 5 mm. (Fig. 272, A). From
the sixth pair are given off the small pulmonary arteries to the lungs. At 7 mm.
the first and second aortic arches are obliterated (Figs. 272 B and 273), but the
DEVELOPMENT OF THE ARTERIES 263
dorsal and ventral aorte cranial to the third arch persist as parts of the internal
and external carotid arteries respectively. The third arches form the stems of
the internal carotids, while the ventral aorte between the third and fourth arches
become the common carotids. In embryos of 15 mm. the bulbus cordis has been
divided into the aortic and pulmonary trunks, so that the aorta opens into the
left ventricle and the pulmonary trunk into the right ventricle. The dorsal
* aorte between the third and fourth arches disappear, but the fourth arch on the
Dorsal aorta
Aortic arch 4
Aortic arch 6
Esophagus
Trachea
Pulmonary artery
Ventral aorta Bulbus cordis
B
Aortic arch 3
Int. carotid artery
Aortic arch 2
External carotid
Aortic arch 6
Pulmonary artery
Bulbus cordis
SS
\
Fic. 272.—Aortic arches of human embryos: A, of 5mm.; B, of 7 mm. (after Tandler). I-JV, pharyn-
geal pouches.
left side persists as the aortic arch of the adult. On the right side, the fourth aortic
arch persists with the descending aorta as far as the seventh intersegmental artery
and forms part of the right subclavian artery, which is thus longer than the left.
The segment of the fourth arch proximal to the right common carotid becomes the
innominate artery. On the right side, the sixth arch between the origin of the
right pulmonary artery and descending aorta is early lost; on the left side, it
persists as the ductus arteriosus and its lumen is only obliterated after birth. The
.
264. THE DEVELOPMENT OF THE VASCULAR SYSTEM
proximal portion of the right sixth arch forms the stem of the right pulmonary
artery, but the proximal portion of the Jeft arch is incorporated in the pulmonary
trunk.
The aortic arches of the embryo are of especial importance comparatively. Five
arches are formed in connection with the gills of adult fishes. In adult tailed amphibia,
three or four arches, and in some reptiles two arches, are represented on either side. In
birds the right, in mammals the left fourth arch persists as the arch of the aorta.
The different courses of the recurrent laryngeal nerves are easily explained. The vagus
early gives off paired branches which reach the larynx by passing caudal to the primitive
fourth aortic arches. When the latter, through growth changes, descend into the chest,
loops of both nerves are carried with them. Hence, after the transformation of the fourth
arches, the left recurrent nerve remains looped around the arch of the aorta, the right around
the right subclavian artery (cf. Fig. 273).
External carotid
Internal carotid
Innominate Common carotid
artery
Aortic arch
Right sub-
clavian artery
Right pul- Sag
monary artery f
Ductus arteriosus
Vertebral artery
Subclavian artery
Trunk of pul-
Left pul
jane bees oft pulmonary artery
Ventral aorta
Fic. 273.—Diagram showing the aortic arches and their derivatives in human embryos.
Branches of the Dorsal Aorta—From the primitive aorte arise: (1) dorsal,
(2) lateral, and (3) ventral branches (Fig. 274).
1. The dorsal branches are intersegmental and develop small dorsal and large
ventral rami. From the dorsal rami are given off neural branches which bifurcate
and form dorsal and ventral spinal arteries.
Origin of the Vertebral Arteries and Basilar Artery.—As we have seen (Fig.
271), the internal carotids are recurved cranially in the 5 mm. embryo and anas-
tomose with the first two pairs of dorsal intersegmental arteries. By longitudinal
postcostal anastomoses (Fig. 274) of the dorsal rami of the first seven pairs of dor-
sal intersegmental arteries the vertebral arteries arise (Fig. 275). The original
trunks of the first six pairs are lost so that the vertebrals take their origin with the
. DEVELOPMENT OF THE ARTERIES 205
Postcostal anastomosis
Precostal anastomosis
Dorsal ramus
Venibal eaanis Dorsal (intersegmental) artery
Aorta Lateral (visceral) artery
Ventral (splanchnic) artery
Ventral anastomosis
Fic. 274.—A diagram of the trunk, in transverse section, showing the arrangement of the aortic branches.
ADCD.
a
Fic. 275.—The development of the vertebral and subclavian arteries and the costo-cervical trunk
in a young rabbit embryo (modified after Hochstetter). III AB.—IV AB., Aorticarches; Ap, pulmonary
artery; A.v.c.b. and A.v.c.v., cephalic and cervical portions of vertebral artery; A.s., subclavian artery;
C.c., costo-cervical trunk; C.d. and C.v., internal and external carotid arteries; [.S.G., spinal ganglion.
subclavians from the seventh pair of intersegmental arteries (Fig. 276). In embryos
of 9 mm. the vertebrals in the region of the metencephalon fuse to form a single
266 THE DEVELOPMENT OF THE VASCULAR SYSTEM
median ventral vessel, the basilar artery, which thus is connected cranially (by
way of the circulus arteriosus) with the internal carotids, caudad with the verte-
bral arteries.
Fic. 276.—Arterial system of a human embryo of 10 mm. (His). > 18. Jc, Internal carotid artery;
P, pulmonary artery; Ve, vertebral artery; 17 J-VI, persistent aortic arches.
The internal carotids (Fig. 271), after giving off the ophthalmic arteries, give rise cra-
nially to the anterior cerebral artery, from which arise later the middle cerebral artery and the
anterior chorioidal artery, all of which supply the
brain. Caudalward many small branches to the
brain wall are given off and quite late in develop-
ment (48 mm. C R fetuses) these form a true pos-
terior cerebral artery (Mall).
The ventral rami of the dorsal interseg-
mental arteries become prominent in the
thoracic and lumbar regions and persist as
the intercostal and lumbar arteries, segmentally
arranged in the adult. Longitudinal precos-
tal anastomoses (Fig. 274) constitute the
costo-cervical and thyreo-cervical trunks (Fig.
275). The subclavian and a portion of the
internal mammary artery are derived from the
ventral ramus of the seventh cervical seg-
mental artery. The remainder of the internal
Fic. 277.—The development of the : ; 2 2
internal mammary and deep epigastric ™ammary and the superior and inferior epi-
arteries in a human embryo of 13mm. gastric arteries are formed by longitudinal
(Mall in McMurrich).
ventral anastomoses (Fig. 274) between the
extremities of the ventral rami from the thoracic and lumbar intersegmental
arteries, beginning with the second or third thoracic (Fig. 277).
DEVELOPMENT OF THE ARTERIES 267
2. The lateral (visceral) branches of the descending aorte are not segmentally
arranged. They supply structures arising from the nephrotome region (meso-
nephros, sexual glands, metanephros, and suprarenal glands). From them later
arise the renal, suprarenal, inferior phrenic, and internal spermatic or ovarian
arteries.
3. The ventral (splanchnic) branches are at first rather definitely intersegmen-
tal. Primitively they form the paired vitelline arteries to the yolk sac (Figs.
268 to 270). Coincident with the degeneration of the yolk sac the prolongations
of the ventral vessels to its walls disappear, and the paired persisting arteries,
passing in the mesentery to the gut, fuse to form unpaired vessels from which
Seventh segmental
artery
Celiac arter
> Tenth dorsal segmental artery
V. pancreas Dorsal aorta
Celiac axis
Yolk stalk
Vitelline artery
Mesonephric arteries t Vitelline artery
Dorsal aorta
R. umbilical artery (¢ ae
Cloaca pe Inf. mesenteric
ad yg arlery
Common iliac artery
A B
Fic. 278.—Reconstructions showing the development of the umbilical and iliac arteries (after Tandler):
A, 5 mm. human embryo; B, 9 mm. human embryo.
three large arteries are derived, the celiac artery, the superior mesenteric, and the
inferior mesenteric (Fig. 271).
The primitive cceliac axis arises opposite the seventh intersegmental artery. Together
with the mesenteric arteries, it migrates caudalward until eventually its origin is opposite
the twelfth thoracic segment (Mall). This migration, according to Evans, is due to the un-
equal growth of the dorsal and ventral walls of the aorta. Similarly, the superior mesenteric
artery is displaced caudad ten segments, the inferior mesenteric artery three segments.
The Umbilical and Iliac Arteries.—As previously described, the umbilical
arteries arise in young human embryos of 2 to 2.5 mm. from the primitive aorte
opposite the fourth cervical segment. They take origin from a plexus of ventral
268 THE DEVELOPMENT OF THE VASCULAR SYSTEM
vessels of the vitelline series (Fig. 270), and are gradually shifted caudalward until
they arise from the dorsal aorta opposite the twenty-third segment (fourth lum-
bar). In 5 mm. embryos the umbilical arteries develop secondary Jateral con-
nections with the aorta (Fig. 278 A). The new vessels pass lateral to the mes-
onephric ducts, and, in 7 mm. embryos, the primitive ventral stem-artery has
disappeared. The segment of this new trunk, proximal to the origin of the exter-
nal iliac artery which soon arises from it, becomes the common iliac. The re-
mainder of the umbilical trunk constitutes the hypogastric artery. When the
placental circulation ceases at birth, the distal portion of the hypogastric arteries,
from pelvis to umbilicus, atrophy, forming the solid obliterated hypogastric arteries
of adult anatomy.
The middle sacral artery is the direct caudal continuation of the aorta. Its
dorsal position is the result of secondary growth changes.
Arteries of the Extremities.—It is assumed that in man, as in observed birds and
mammals, the first vessels of the limb buds form a capillary plexus.
Upper Extremity.—The capillary plexus takes origin by several lateral branches from
the aorta. In human embryos of 5 mm. but one connecting vessel remains and this takes its
origin secondarily from the seventh dorsal intersegmental artery, forming the ventral ramus
of this artery and its lateral offset (Fig. 274). The portion of this vessel in what will become
the free arm is plexiform at first, but later becomes a single stem which forms successively
the subclavian, axillary, brachial, and interosseous arteries. Subsequently the median, radial,
and ulnar arteries of the arm are formed.
Arteries of the Lower Extremity.—In embryos of 7 mm. there is given off from the second-
ary lateral trunk of the umbilical artery (i. e. from the future common iliac) a small branch
which forms the chief arterial stem of the lower extremity, the future popliteal and peroneal
arteries. This, the arteria ischiadica, is superseded in embryos of 15.5 mm. by the external
iliac and femoral arteries, of which the latter annexes the branches of the ischiadic distal to
the middle of the thigh. The arteria ischiadica persists proximally as the inferior gluteal
artery.
DEVELOPMENT OF THE VEINS
We have seen that in embryos of 23 somites three systems of paired veins
are present, the umbilical veins from the chorion, the vitelline veins from the yolk
sac, and the cardinal veins, anterior and posterior, which unite in the common
cardinal veins, from the body of the embryo. Thus three veins open into the
right horn, and three into the left horn, of the sinus venosus (Fig. 270).
Changes in the Vitelline and Umbilical Veins —Vena porte.—With the in-
crease in size of the liver anlages there is an intercrescence of the hepatic cords
and the endothelium of the vitelline veins. As a result, these veins form in the
liver a network of sinusoids (Fig. 279), and each vein is divided into a distal portion
which passes from the yolk sac to the liver, and into a proximal portion which
DEVELOPMENT OF THE VEINS 269
carries blood from the liver sinusoids to the sinus venosus. The proximal portion
of the left vitelline vein soon is largely absorbed into the sinusoids of the liver
and shifts its blood flow into the right horn of the sinus venosus. In the mean-
time the liver tissue grows laterally, comes into contact with the umbilical veins,
and taps them so that their blood flows more directly to the heart through the
sinusoids of the liver (Fig. 280). As the channel of the right proximal vitelline is
larger, the blood from the left umbilical vein flows diagonally to the right horn of
the sinus venosus. When all the umbilical blood enters the liver, as in embryos of
Atrium.
Common cardinal vein
Left umbilical vein
Sinusoids of liver.
Left vitelline vein
Right vitelline vein:
Fic. 279.—Reconstruction of the blood vessels of a 4.2 mm. human embryo in ventral view (His).
5 to 6mm., the proximal portions of the umbilical veins atrophy and disappear
(Fig. 281). In 5 mm. embryos the vitelline veins have formed three cross anas-
tomoses with each other (Figs. 280 and 281): (1) a cranial transverse connec-
tion in the liver, ventral to the duodenum; (2) a middle one, dorsal to the duode-
num; and (3) a caudal one, ventral to it. There are thus formed about the gut
a cranial and a caudal venous ring. In embryos of 7 mm. the left umbilical vein
has enlarged, while the corresponding right vein has degenerated. Of the two
venous loops, only the right limb of the cranial ring and the left limb of the
270 THE DEVELOPMENT OF THE VASCULAR SYSTEM
caudal ring, together with the median dorsal anastomosis, persist. A new vein
the superior mesenteric, develops in the mesentery of the intestinal loop and joins
the left vitelline vein just caudal to its dorsal middle anastomosis with the right
vitelline vein. Subsequently, with the atrophy of the yolk sac, the left vitelline
vein degenerates caudal to its junction with the superior mesenteric vein. The
persisting trunk between the superior mesenteric vein and the liver is the vena
porte, and thus represents: (1) a portion of the left vitelline vein in the left limb
of the caudal ring; (2) the middle transverse anastomosis between the vitelline
veins; (3) the portion of the right vitelline vein which forms the right limb of the
cranial ring.
Ductus venosus v
Left horn sinus venosus
Right horn sinus venosus
Left vitelline vein
Cranial anastomosis of vitelline
veins
Middle anastomosis of vitelline
veins
Right umbilical vein Left umbilical vein
Caudal anastomosis of vitelline veins
Right vitelline vein
Frc. 280.—Reconstruction of the veins of the liver in a 4.9 mm. human embryo (after Ingalls).
In the liver the portal vein through its cranial anastomosis between the vitel-
line veins is connected with the left umbilical vein. As the right lobe of the liver
grows, the course of the umbilical and portal blood through the intrahepatit
portion of the right vitelline vein becomes circuitous, and hence a new direct
channel to the sinus venosus is formed through the hepatic sinusoids. This is
the ductus venosus (Fig. 281), which is obliterated after birth and forms the
ligamentum venosum of the postnatal liver.
According to Mall, the intrahepatic portion of the right vitelline vein persists proxi-
mally as the right ramus of the hepatic vein, and distally as the ramus arcuatus of the portal
vein. The intrahepatic portion of the left vitelline vein drains secondarily into the right horn
of the sinus venosus, and proximally forms later the /eft hepatic ramus. Distally, where it is
connected with the left umbilical vein, it becomes the ramus angularis of the vena porte. In
this way two primitive portal, or supplying trunks, and two hepatic, or draining trunks,
\
DEVELOPMENT OF THE VEINS 271
originate. Later there are differentiated first four, then six, such opposed trunks within the
liver, and the six primary lobules supplied and drained by these trunks may be recognized in
the adult liver.
Of the umbilical veins the right disappears early; the left persists during
fetal life, shifts to the median line, and courses in the free edge of the falciform
ligament. After birth its lumen is closed and from the umbilicus to the liver it
forms the ligamentum teres. In early stages veins from the body wall drain into
the umbilical veins.
The Anterior Cardinal Veins and the Origin of the Superior Vena Cava.—
The anterior cardinal veins consist each of two parts (Fig. 271): (1) The true
anterior cardinals, located laterad in the segmented portion of the head and neck
Efferent Common Ductus Efferent
Portal vein fy Obliterated L. vitelline vein
4 enteric vein [
g ED Vitelli eo :
Raahilizabien a a L. umbilical vein
Fic. 281.—A diagram showing the development of the portal vein as illustrated ina human embryo of
about 7 mm. (modified after His).
and draining into the common cardinal veins; (2) the vena capitis medialis, ex-
tending into the unsegmented head proper and running ventro-lateral to the
brain wall. In embryos of 20 mm. there has formed by anastomosis a large con-
nection between the right and left anterior cardinals, which carries the blood
from the left side of the head into the right vein (Fig. 282 C). Soon the left
anterior cardinal loses its connection with the common cardinal on the left side
(Fig. 282 D). The proximal portion of the left common cardinal, with the trans-
verse portion of the sinus venosus, persists as the coronary sinus. The right com-
mon cardinal and the right anterior cardinal vein as far as its anastomosis with
the left anterior cardinal become the superior vena cava. The anastomosis itself
forms. the /eft vena anonyma, while that portion of the right anterior cardinal be-
272 THE DEVELOPMENT OF THE VASCULAR SYSTEM
R. ant. cardinal vein-
R. post. cardinal vein
R. com. cardinal vein
Inf. vena cava
Mesonephros
R. ischiadic vein
R. ischiadic vein B cantieh ae
Caudal vein
Int. jugular vein L. vena anonyma
Ext. jugular vein Ant. hemi-
Subclavian vein azygos vein
Azygos vein Sup. vena
298 cava
Ext. jugular vein Int. jugular vein
Subclavian vein Com. cardinal vein \V
Coronary sinus
aed
O
Inf. vena cava
Hepatic vein
Hemi-azygos vein 5 5
Post. card 7 =
. card. “i
eth R. suprarenal vein :
Inf. vena , R. renal vein
cava
L. renal vein
Post. cardinal _ A ) Spermatic veins
Ext. iliac vein
Com. iliac vein
Hypogastric vein
Cc Median sacral vein D
Fic. 282.—Four diagrams showing the development of the superior and inferior vene cave and the
fate of the cardinal veins (modified after Kollmann). X in A, anastomosis between hepatic and sub-
cardinal vein; *, anastomosis between subcardinal veins; X in C, anastomosis between anterior car-
dinal veins which forms the left vena anonyma; * in C, cranial anastomosis between the posterior car-
dinal veins; z, caudal anastomosis between the same veins; A, kidney; S, suprarenal gland; T, testis.
tween the left vena anonyma and the right subclavian vein is known as the right
vena anonyma. The distal portions of the anterior cardinals become the internal
DEVELOPMENT OF THE VEINS 273
jugular veins of the adult, while the external jugulars are new veins which develop
somewhat later.
The vena capitis medialis (Fig. 271) is the continuation of the anterior cardinal vein
into the head of the embryo where at first it lies mesial to the cerebral nerves. Later itis
partly shifted by anastomoses lateral to the cerebral nerves and forms the vena capitis lateralis
(Figs. 283 and 284). In 11mm. embryos this emerges with the n. facialis, and, caudal to the
n. hypoglossus, becomes the internal jugular. Cranially, in the region of the fifth nerve, the
median vein of the head persists as the sinus cavernosus and receives the ophthalmic vein from
the eye and the anterior cerebral vein from the fore- and mid-brain regions (Fig. 284 C).
Between the n. trigeminus and the facialis, the middle cerebral vein from the metencephalon
(cerebellum) joins the v. capitis lateralis before it leaves the cranium. More caudally the
Middle cerebral vein Posterior cerebral vein
:
N. hypoglossus
, V. capitis lateralis
Sinus cavernosus—_\\ =
Anterior cerebral vein
iii
Fic. 283.—Veins of the head of a 9 mm. human embryo (after Mall). x 9.
posterior cerebral vein from the myelencephalon emerges through the jugular foramen and is
drained with the others by the v. capitis lateralis into the internal jugular (Fig. 284 B).
Soon the cerebral veins reach the dorsal median line (Fig. 284 C), and longitudinal anas-
tomoses are formed: (1) between the anterior and middle cerebral veins, giving rise to the
superior sagittal sinus; and (2) between the middle and posterior cerebral veins forming the
greater part of the transverse sinuses. In embryos of 33 mm. the v. capitis lateralis disappears
and the blood from the brain passes through the superior sagittal and lateral sinuses and is
drained by way of the jugular foramen into the internal jugular vein (Fig. 284 C, D). The
middle cerebral vein becomes the superior petrosal sinus, but the inferior petrosal sinus is
formed as a new channel median to the internal ear. The anterior cerebral vein becomes the
superficial middle cerebral of adult anatomy. A more detailed account of these changés may
be found in the original work of Mall (Amer. Jour. Anat., vol. 4, 1905).
18
274 THE DEVELOPMENT OF THE VASCULAR SYSTEM
The Posterior Cardinal Veins and the Origin of the Inferior Vena Cava.—
The posterior cardinal veins course cephalad along the dorsal side of the meso-
nephroi and open into the common cardinal veins (Fig. 282 A). Each receives
an ischiadic vein from the posterior extremities, mesonephric branches from the
mid-kidney and dorsal intersegmental veins from the body wall (Fig. 282 B).
Median and ventral to the mesonephros are developed the subcardinal veins which
are connected at intervals with the posterior cardinal veins by mesonephric sinu-
soids, and with each other by anastomoses ventral to the aorta. Thus all the
blood from the mesonephroi, posterior extremities, and dorsal body wall is in early
stages drained by the posterior cardinal veins alone.
The development of the unpaired vena cava inferior begins when communica-
tion is established between the right hepatic vein of the liver and the right sub-
cardinal vein of the mesonephros, primarily a tributary of the posterior cardinal
vein (Lewis, 1902).
The liver on the right side becomes attached to the dorsal body wall and from
its point of union a ridge, the plica vene cave (Fig. 199), extends caudalward.
According to Davis (1910), capillaries from the subcardinal vein invade the plica
vene cave, and, growing cranially, meet and fuse with capillaries extending
caudad from the liver sinusoids.
Thus is formed the vein of the plica vene cave (Fig. 282 A), which is already
present in human embryos of 2.6 mm. (Kollmann). This vein rapidly enlarges,
as also do the sinusoidal connections between the subcardinals and posterior car-
dinals at one point. Thus the blood from both lower posterior cardinals is soon
carried to the heart, chiefly by way of the right subcardinal and right hepatic veins.
(Fig. 282 B). Soon the posterior cardinals just cranial to their enlarged anasto-
moses with the subcardinals become small and are interrupted (Fig. 282 C).
Cranial to their interruption these veins communicate by a cross anastomosis.
and were formerly believed to persist as the vv. azygos and hemiazygos of the
adult.
In the pig the posterior cardinal veins develop along the sides of the large
mesonephroi and completely disappear with that organ. Sabin (Carnegie Publ.
No. 223, 1915) has thus been able to confirm and extend the conclusion of Parker
and Tozier that the vv. azygos and hemiazygos are entirely new veins derived from a.
prevertebral capillary plexus. The more caudal portions of the posterior cardinal
-veins likewise atrophy completely and are replaced by new prevertebral vessels.
having essentially similar topographical relations. Caudal to their lowest trans-
verse connection (z, Fig. 274 C), the new right vessel becomes the right common iliac
DEVELOPMENT OF THE VEINS 275
vein: The corresponding portion of the new left vein with the transverse anas-
tomosis. becomes the left common iliac vein. The blood from these veins is now
drained by the unpaired inferior vena cava which is composed of the following
veins: (1) the common hepatic and right hepatic veins (primitive right vitelline) ;
(2) the vein of the plica vene cave; (3) an inter-renal portion of the right subcar-
dinal vein with its great mesial anastomosis; (4) the new vein which replaces the
right posterior cardinal.
The permanent kidneys take up their positions opposite the great anastomosis
between the subcardinals and at this point the renal veins are developed (Fig.
A
Confluence of the sinuses Middle cerebral vein
Posterior cerebral
Auditory vesicle i
= aan, bs Superior
Primitive jugular sagittal
vein
Vena capitis medialis
Trigeminal nerve
Ophthalmic vein
sinus
Superior sagit- )
tal sinus Vena capi-
tis lateralis
Vena ca pilis
Anterior cerebral vein |_| | lateralis
Ophthalmic vein | i ;
Trigeminal nerve Auditory vesicle
B D
Anterior cerebral vein Inferior sagittal sinus Straight sinus
Confluence of the Middle cerebral vein Sore ae Mpal 26k Confluence
sinuses | Auditory vesicle
Superior
Sagittal
SINUS
of the sinuses
Superior sagittal
: sinus
Posterior Superior
pelrosal
A SINUS |
Trigeminal nerve DS Jugular Anterior cere- Trans-
& vein bral vein verse
O \phihalmic vein Vena capitis S ‘D henop ari- Sinus
lateralis
etal sinus ao"! . Auditory vesicle
Ophthalmic vein | -Inf. petrosal sinus
Trigeminal nerve
Fic. 284.—Four diagrams showing the development of the veins of the head (after Mall). A, At four
weeks; B, at five weeks; C, at the beginning of the third month; D, from an older fetus.
282 B); the longer left renal vein differs from the right in that proximally it rep-
resents a left portion of the anastomosis itself (Fig. 282 D). A cephalic portion
of the left subcardinal vein persists as the left suprarenal vein, which thus opens
into the left renal instead of joining the inferior vena cava as does the right
suprarenal vein of similar origin. The left spermatic or ovarian vein early drains
into the-left caudal border of the great subcardinal anastomosis, which as we have
seen contributes to the left renal vein. The right spermatic or ovarian vein opens
into the right border of that portion of the subcardinal anastomosis which is in-
276 THE DEVELOPMENT OF THE VASCULAR SYSTEM
corporated into the inférior vena cava. The /umoar veins develop from the same
prevertebral plexus that gives rise to the caudal segment of the inferior vena cava.
. Begg (Anat. Rec., vol. 10, 1916), on the contrary, finds that in the rat, where the meso-
nephroi are diminutive, the posterior cardinal veins are not incorporated into the mesonephroi
to disappear with that organ, but the right posterior cardinal persists as the segment of the
inferior vena cava below the subcardinal contribution.
A B
= V. linguo-facialis
V. linguo-facialis
Dorsal subclavian vein, 4 . .
kK 33— Ant. cardinal vein
4—Com. cardinal vein Dorsal subclavian
vein
Vena ulnaris
prima
Niwungiasss
V .ulnaris prima
oS
V. thoraco-epigastrica Post. cardinal, vein
V. thoraco- Ventral sub-
epgastrica clavian vein
V. jugularis interna
V. linguo-facialis V. jugularis
externa
V. cephalica V Unnuo facials
V. card. ant. V. jug. anterior
V. anonyma
V. card. com.
dextra
V. card post. a V. anonyma
7 Sinistra
V. mammaria
int,
V. brachialis :
Fic. 285.—Four reconstructions of the veins of the human right arm (after F. T. Lewis). > about 15.
A, 10 mm. embryo; B, 11.5 mm. embryo; C, 16 mm. embryo; D, 22.8 mm. embryo.
The Veins of the Extremities.—The primitive capillary plexus of the upper and lower -
limb buds gives rise to a border vein (Figs. 285 and 321), which courses about the periphery of
the flattened limb buds (Hochstetter). In the upper extremity, the ulnar portion of the
border vein persists, forming at different points the subclavian, axillary, brachial, and basilic
veins. The border vein at first opens into the dorsal wall of the posterior cardinal vein (em-
bryos of 10 mm.), but, as the heart shifts its position caudalward, it finally drains by a ventral
connection into the anterior cardinal or internal jugular vein (Lewis). The cephalic vein
FETAL CIRCULATION 277
develops secondarily in connection with the ulnar border vein; later, in embryos of 23 mm.,
it anastomoses with the external jugular and finally drains into the axillary vein as in the
adult. With the development of the digits, the vv. cephalica et basilica become distinct, as in
embryos of 35 mm., but later are again connected by a plexus on the dorsum mani, as in the
adult (Evans in Keibel and Mall, vol. 2).
In the lower extremity the fibular portion of the primitive border vein persists. Later
the v. saphena magna arises separately from the posterior cardinal, gives off the wv. femoralis
and tibialis posterior, and annexes the fibular border vein at the level of the knee. Distal to
this junction the border vein persists as the v. tibialis anterior and probably the v. saphena
parva; proximally it becomes greatly reduced, forming the v. glutea inferior.
Anomalies.—Anomalous blood vessels are of common occurrence. They may be
due: (1) to the choice of unusual. paths in the primitive vascular plexuses; (2) to the persist-
ence of vessels usually obliterated, e. g., double superior vene cave; right aortic arch; per-
manent ductus arteriosus; (3) to incomplete development, e. g., double (unfused) heart or
double dorsal aorte.
FETAL CIRCULATION
During fetal life oxygenated placental blood enters the embryo by way of the
large umbilical vein and is conveyed to the liver (Fig. 286). There it mingles with
the small amount of venous blood brought in by the portal vein. It is carried
to the inferior vena cava either directly, through the ductus venosus, or indirectly
through the liver sinusoids and hepatic vein. The impure blood of the inferior
vena cava and portal vein affects but slightly the greater volume of pure placental
blood. Entering the right atrium it mingles somewhat with the venous blood
returned through the superior vena cava. It is said that the blood from the
inferior vena cava is directed by the valve of this vein through the foramen ovale
into the left atrium (following the path of the sounds in Figs. 262 to 264) which,
before birth, receives little venous blood from the lungs. This purer blood of the
left atrium enters the left ventricle, and is driven out through the aorta to be dis-
tributed chiefly to the head and upper extremities.
The venous blood of the superior vena cava, slightly mixed, is supposed to
pass from the right atrium into the right ventricle, whence it passes out by the
pulmonary artery. A small amount of this blood is conveyed to the lungs by the
pulmonary arteries, but, as the fetal lungs do not function, most of it enters the
dorsal aorta by way of the ductus arteriosus. Since the ductus is caudal to the
origin of the subclavian and carotid arteries, its less pure blood is distributed to
the trunk, viscera, and lower extremities. The placental circuit is completed by
the hypogastric, or umbilical, arteries by way of the umbilical cord.
Pohlman (Anat. Rec., vol. 2, 1908) interprets ..s experiments to indicate that, con-
trary to the generally accepted view, there is a mingling of the blood which enters the right
atrium through the two caval veins. If this occurs there would be no difference in the
quality of blood distributed to the various parts of the body.
278 THE DEVELOPMENT OF THE VASCULAR SYSTEM
Changes at Birth.—At birth the umbilical vessels are ruptured and the lungs
become functional. The umbilical arteries and veins, no longer used, contract
Fic. 286.—Diagrammatic outline of the organs of circulation in the fetus of six months (Allen
Thomson).! RA, Right atrium of the heart; RV, right ventricle; LA, left atrium; Eo, valve of inf. vena
cava; LV, left ventricle; L, liver; K, left kidney; J, small intestine; a, arch of the aorta; a’, its dorsal
part; a’’, lower end; vcs, superior vena cava; vci, inferior vena cava where it joins the right atrium; zw?"
its lower end; s, subclavian vessels; j, right jugular vein; c, common carotid arteries; four curved dotted
arrow lines are carried through the aortic and pulmonary opening and the atrio-ventricular orifices; da,
opposite to the one passing through the pulmonary artery, marks the place of the ductus arteriosus;
a similar arrow line is shown passing from the vena cava inferior through the fossa ovalis of the right
atrium and the foramen ovale into the left atrium; /v, the hepatic veins; »p, vena porte; x to vci, the duc-
tus venosus; #v, umbilical vein; va, umbilical arteries; «c, umbilical cord cut short; 7, i’, iliac vessels.
and their lumina are obliterated by the thickening of the inner coat (tunica in-
tima). The lumen of the umbilical artery is occluded after four days, that of
1Jn this diagram the arteries are conventionally colored red and the veins blue, but these colors
are not intended to indicate the nature of the blood conveyed by the respective vessels.
THE LYMPHATIC SYSTEM 279
the umbilical vein within a week. The cord-like vein is persistent as the liga-
mentum teres of the liver; the arteries become the obliterated hypogastrics.
The ductus venosus atrophies because after birth only the blood from the por-
tal vein enters the liver, and this is all drained into the liver sinusoids, forming the
portal circulation. The ductus venosus is persistent as the fibrous ligamentum
venosum, embedded in the wall of the liver.
The ductus arteriosus ceases to function after birth, as all the blood from the
pulmonary arterial trunk is conveyed to the expanded lungs. The ductus be-
comes impervious from ten to twenty days after birth and persists as a solid,
fibrous cord, the ligamentum arteriosum.
After birth, the large amount of blood now returned to the left atrium from
the functional lungs equalizes the pressure in the two atria. Asa result, both dur-
ing diastole and systole, the septum primum, or valve of the foramen ovale, is
pressed against the septum secundum, closing the foramen ovale. Eventually the
two septa fuse, though they may be incompletely united during the first year
after birth, or even longer (p. 255).
~™ ‘THE LYMPHATIC SYSTEM
The lymphatic system originates in a plexus of lymphatic capillaries distrib-
uted along the primitive main venous trunks. By the dilation and coalescence
of this network at definite regions five lymph sacs appear (Fig. 287). Paired
jugular sacs arise in 10 to 11 mm. embryos lateral to the internal jugular veins.
In embryos of 23 mm. the unpaired retroperitoneal sac develops at the root of the
mesentery adjacent to the suprarenal bodies, and the cisterna chyli also appears.
Paired posterior sacs arise in relation to the sciatic veins in embryos 24 mm. long.
These sacs at first contain blood which they soon discharge into neighboring veins,
thereupon losing their venous connections. With relation to the lymph sacs as
centers, the thoracic duct (at 30 mm.) and the peripheral lymphatics develop.
The jugular sacs alone acquire with the internal jugular veins secondary con-
nections which are later utilized by the thoracic and right lymphatic ducts. The
various sacs themselves are eventually transformed into chains of lymph nodes.
Two discordant views exist as to the origin of the lymphatics. According to
Sabin (Amer. Jour. Anat., vols. 1, 1901; 9, 1909) and Lewis (Amer. Jour, Anat., vol.
5, 1906), sprouts arising from the endothelium of veins form the single and paired
sacs already described. From these five sacs the thoracic duct and peripheral
lymphatics develop as endothelial outgrowths. Thus lymphatic vessels grow to
the head, neck, and arm from the jugular sacs; to the hip, back, and leg from the
280 THE DEVELOPMENT OF THE VASCULAR SYSTEM
posterior sacs; and to the mesentery from the retroperitoneal sac. According to
this view, then, endothelium can arise only from pre-existing endothelium.
Other investigators (Huntington, Amer. Jour. Anat., vol. 16, 1914; Mem.
Wistar Inst., 1911; and McClure) hold that the lymph sacs are formed in situ by
the fusion of discrete mesenchymal spaces which become lined with an endothe-
Fic. 287.—Flat reconstruction of the primitive lymphatic system in a human embryo 30
mm. long (Sabin). X about 3.5. C.c., Cisterna chyli; Lg., lymph gland; S.J.jug., jugular lymph sac;
S.l.mes., retroperitoneal lymph sac; S./.p., posterior lymph sac; S./.s., subclavian lymph sac; V.c.,
cephalic vein; V.c.i., inferior vena cava; V.f., femoral vein; V.j.7., internal jugular vein; V/I.p.,
deep lymphatics; V.l.s., superficial lymphatics; V.r., renal vein; V.s., sciatic vein; V.w.(p.), primi-
tive ulnar vein.
lium of transformed, bordering mesenchymal cells. Venous connections are purely
secondary. The thoracic duct and the peripheral vessels develop similarly
by the progressive fusion of separate clefts; hence endothelium can differentiate
continually from young mesenchyma. The further growth of endothelium, already
formed, is not denied.
A
Afferent lyinphatic vessels
“A=.
Pewee
of
Peripheral lymph sinus
Network of lymphatic. eripnerat Lymph st
vessels — Capsule
‘— Trabecula
Reticular tissue cells
Lym phatic vessel Blood vessels Lymphatic vessel
B
Afferent lymphatic vessels
ee A Se
—
» :
Peripheral sinus iad
6 Secondary nodule
Pes
fa eee
Lymph sinus a ‘Si
Medullary cord
Capsule’ Ox Trabecula
Efferent lymphatic vessels
Fic. 289.—Diagrams representing four stages in the development of lymph glands. The earlier stages
: are shown on the left side of each figure (Lewis and Stohr).
THE LYMPHATIC SYSTEM 281
Rapidly accumulating evidence seems to favor the latter view. The method
of injection, upon which Miss Sabin and her followers have largely relied, has its
limitations, for it obviously can furnish no information regarding discrete spaces
prior to their linkage into continuous channels. Concise summaries of these views
are given by Sabin (Anat. Rec., vol. 5, 1911) and by McClure (Anat. Rec., vols. 5,
1912; 9, 1915).
Lymph Glands.—Paired lymph glands appear during the third month, first
in the axillary, iliac, and maxillary regions. Those from the lymph sacs develop
later. Plexuses of lymphatics first form either as ordinary networks of peripheral
vessels or as secondary networks produced by a connective tissue invasion of the
primitive lymph sacs. In either case a capillary plexus, with simple connective
ay
-- M esogastrium
: yw --Spleen anlage
A B
Fic. 288.—Two stages in the early development of the human spleen (dorsal is below). A, from an em-
bryo of 10.5 mm. (Kollmann); B, from a 20 mm. embryo (Tonkoff).
tissue septa, marks the first stage of development. Next (Fig. 289 A), lympho-
cytes collect in the connective tissue, forming follicles which become associated
with blood capillaries. Finally, the lymphoid tissue is channeled by sinuses
formed from lymphatic capillaries. The peripheral sinus develops afferent and
efferent vessels. The central sinuses cut the lymphoid tissue into cords. In the
larger lymph glands (Fig. 289 B) the connective tissue forms a definite capsule
from which trabecule dip into the gland.
Hemolymph glands, according to Schumacher (Arch. f. mik. Anat., Bd. 81,
1912), begin their development like lymph glands, but soon after the formation
of the peripheral sinus the lymphatic connections degenerate and blood escapes
from the blood capillaries into what are henceforth blood sinuses.
Spleen.—This appears in embryos of about 10 mm. as a swelling on the left
282 THE DEVELOPMENT OF THE VASCULAR SYSTEM
side of the dorsal mesogastrium near the dorsal pancreas (Fig. 288 A). The
thickening is due to a temporary proliferation and invasion of mesothelial cells
into the underlying mesenchyme, which, meanwhile, has also undergone local
enlargement and vascularization. Contrary to the older view, the cells from the
peritoneal epithelium probably give rise to a large part, if not all, of the future
spleen. The splenic anlage becomes pinched off from the mesogastrium (Fig. 288
B) with which it is ultimately joined by a narrow band only.
At first the blood vessels constitute a closed system. The peculiar adult
circulation is acquired relatively late. Lifschitz has shown that, in human fetuses
between 150 and 300 mm. long, red blood cells are actively formed in the splenic
pulp as clusters around the giant cells. The lymphoid tissue of the spleen first
appears as ellipsoids about the smallest arteries in fetuses of four months. At
seven months the ovoid splenic corpuscles appear as lymphoid nodules about the
larger arteries.
Glomus Coccygeum.—The coccygeal gland is present in 150 mm. (C H)
fetuses as an encapsulated cluster of polyhedral cells at the apex of the coccyx.
Later it becomes lobulated by the ingrowth of connective tissue trabecule and
receives a rich vascular supply. According to Stoerck (1906) its tissue at no time
resembles the chromaffin bodies, although this has commonly been believed.
CHAPTER X
HISTOGENESIS
THE primitive cells of the embryo are alike in structure. The protoplasm of
each exhibits the fundamental properties of irritability, contractility, reproduc-
tion, and metabolism (the absorption, digestion, and assimilation of nutritive sub-
stances and the excretion of waste products, processes through which growth
and reproduction are made possible). As development proceeds, there is a
gradual differentiation of the cells into tissues, each tissue being composed of like
cells, the structure of which has been adapted to the performance of a certain
special function. In other words, there is division of labor and adaptation of cell
structure to the function which each cell performs. The differentiation of tissue
cells from the primitive cells of the embryo is known as histogenesis. On page 54
the derivatives of the germ layers are given. We shall take up briefly the histo-
genesis of the tissues derived from the entoderm, mesoderm, and ectoderm in the
order named.
THE HISTOGENESIS OF THE ENTODERMAL EPITHELIUM
The cells of the entoderm are little modified from their primitive structure.
From the first they are concerned with the processes of absorption, digestion,
assimilation, and excretion. They form always epithelial layers, lining the di-
gestive and respiratory canals and the glandular derivatives of these. In the
pharynx, esophagus, and trachea the cells are early of columnar form and ciliated.
The epithelium of the pharynx and esophagus becomes stratified and the surface
layers flatten to form squamous cells. The stratified epithelium is developed
from a basal germinal layer like the epidermis of the integument (see p. 294).
Throughout the rest of the digestive canal the simple columnar epithelium of the
embryo persists. At the free ends of the majority of the cells a cuticular mem-
brane develops. Other cells are converted into unicellular mucous glands or goblet
cells. As outgrowths of the intestinal epithelium, are developed the simple tu-
bular glands of the stomach and intestine, and the liver and pancreas.
In the respiratory tract the entoderm forms at first a simple columnar epi-
thelium. Later, in the trachea and bronchi this is differentiated into a pseudo-
stratified, ciliated epithelium. The columnar epithelium of the alveoli and alveo-
283
284 HISTOGENESIS
lar ducts of the lungs is converted into the flattened respiratory epithelium. The
development of the thymus and thyreoid glands, liver and pancreas has been
described in Chapter VII.
HISTOGENESIS OF THE MESODERMAL ‘TISSUES
The differentiation of the mesoderm has been described on p. 51, Fig. 53.
It gives rise to the mesodermal segments, intermediate cell masses, somatic and
splanchnic layers, all of which are epithelia, and to the diffuse mesenchyme. The
somatic and splanchnic layers of the mesoderm form on their coelomic surfaces a
single layer of squamous cells termed the mesothelium. This is the covering layer
Mesodermal segment
Central cells of segment
Intermediate
cell mass
Urogenital ridge. -
Somatic mesoderm
Splanchnic
mesoderm
3 4 Hy Splanchnic
ee a og mesoderm
_ Fic. 290.—Transverse section of a 4.5 mm. human embryo showing the development of the sclerotomes
(Kollmann). X about 300.
of the pericardium, pleure, peritoneum, mesenteries, serous layer of the viscera,
and lining of the vaginal sac in the scrotum. From this mesothelium is derived
the spleen and also the epithelia of the genital glands and the Miillerian ducts.
The intermediate cell masses or nephrotomes are the anlages of the pro-
nephros, mesonephros, metanephros, and their ducts (p. 195).
The Sclerotomes and Mesenchyme.—The cavities of the mesodermal seg-
ments become filled with diffuse, spindle-shaped cells derived from the adjacent
walls; their median walls are next converted into similar tissue and the whole
migrates mesially towards the neural tube and notochord, and eventually sur-
rounds these structures (Figs. 290 and 323). This diffuse tissue is mesenchyme (see
p. 53), and that derived from a single mesodermal segment constitutes a sclerotome.
THE SUPPORTING TISSUES 285
The sclerotomes ultimately are converted into connective tissue, into the ver-
tebre, and into the basal portion of the cranium. The persisting lateral plate
of the mesodermal segment becomes a dermo-myotome, from which the voluntary
muscle is differentiated and probably the corium of the integument.
In the head region, cranial to the otocysts, no mesodermal segments are
formed, but the primitive mesoderm is converted directly into mesenchyme.
Mesenchyme is derived also from the somatic and splanchnic mesoderm and from
the primitive streak tissue. From the mesenchyme a number of tissues are
developed (see p. 54). The origin of the blood and primitive blood vessels and
lymphatics has been described; it remains to trace the development of the sup-
porting tissues (connective tissue, fat, cartilage, and bone) and of the muscle
fibers.
THE SUPPORTING TISSUES
The supporting tissues are peculiar in that during their development from
the mesenchyme a fibrous, hyaline, or calcified matrix is formed which becomes
greater in amount than the persisting cellular elements of the tissue.
Connective Tissue.—Different views are held as to the differentiation of con-
nective tissue fibers. According to Laguess and Merkel, the fibers arise in an
intercellular matrix derived from the cytoplasm of mesenchymal cells. Szily
holds that fibers are first formed as processes of epithelial cells and that into this
fibrous network mesenchymal cells later migrate. The view generally accepted,
that of Fleming, Mall, Spalteholz, and Meves, is that the primitive connective
tissue fibers are developed as part of the cell, z. e., are zntracellular in origin.
The mésenchyme is at first compact, the cell nuclei predominating. Soon a
syncytium is developed, the cytoplasm increasing in amount and forming an open
network. Next the cytoplasm is differentiated into a perinuclear granular
endoplasm and an outer distinct hyaline layer of ectoplasm (Fig. 291 A) (Mall,
Amer. Jour. Anat., vol. 1, 1902). In the ectoplasm fibrils appear, derived from
coarse filaments known as chondrioconta (Meves, 1910).
Reticular Tissue.—Fine fibers arise in the ectoplasm of the mesenchymal
syncytium. The nuclei and endoplasm persist as reticular cells. According to
Mall, reticular fibers differ chemically from white connective tissue fibers.
White Fibrous Connective Tissue.—The differentiation of this tissue may be
divided into two stages: (1) a prefibrous stage during which the ectoplasm is
formed rapidly by the endoplasm of the cells, and fibrils resembling those of
reticular tissue appear in the ectoplasm (Fig. 291 A); (2) the anastomosing fibers
take the form of parallel bundles and are converted through a chemical change
286
Fibrille in ectoplasmic
matrix
Cell of syncytium
Elastic fiber
HISTOGENESIS
Cc
Mesenchymal cell
Cartilage matrix
Cartilage cell
Fic. 291.—The differentiation of the supporting tissues (after Mall). 270. 4, White fibers
forming in the corium of a 5 cm. pig embryo; B, elastic fibers forming in the syncytium of the umbilical
cord from a 7 cm. embryo; C, developing cartilage from the occipital bone of a 20 mm. pig embryo.
into typical white fibers. The spindle-shaped cells are transformed into the
connective tissue cells characteristic of the adult. In tendons, the bundles of
Fic. 292.—Develop-
ing fat cells, the fat black-
ened with osmic acid (after
Ranvier). n., Nucleus; g.,
fat globules. é
plasmic granules, but
white fibers are arranged in compact parallel fascicles, in
areolar tissue they are interwoven to form a meshwork.
The cells of the tendons are compressed between the
bundles of fibers and this accounts for their peculiar form
and arrangement. In the cornea of the eye the cells retain
their processes. The corneal tissue is thus embryonic
in character and is without elastic fibers or blood vessels.
Elastic Tissue.—With the exception of the cornea
and tendon, yellow elastic fibers develop in connection
with all white fibrous connective tissue. Like the
white fibers they are produced in the ectoplasm of the
mesenchymal syncytium (Fig. 291 B). They are de-
veloped as single fibers, but may coalesce to form the
fenestrated membranes of the arteries. According to
Ranvier, elastic fibers are produced by the union of ecto-
this view is not supported by either Mall or Spalteholz.
CARTILAGE 2847
Adipose Tissue.—Certain of the mesenchymal cells give rise not to fibro-
blasts but to fat cells. They secrete within their cytoplasm droplets of fat which
increase in size and become confluent (Fig. 292). Finally, a single fat globule fills
the cell of which the nucleus-and cytoplasm are pressed to the periphery. The
fat cells are most numerous along the course of the blood vessels in areolar con-
nective tissue and appear first during the fourth month.
CARTILAGE
Cartilage has been described as developing in two ways: (1) The mesen-
chymal cells increase in size and form a compact cellular precartilage. Later the
hyaline matrix is developed be-
tween the cells from their cyto- ‘
plasm (Fig. 293 A). The matrix
may in this case be regarded as
Precartilage Cartilage --
5 ONY cise
the ectoplasm of the cartilage
cells. (2) According‘to Mall, mes-
enchymal cells give rise first to an
ectoplasm in which fibrille de-
velop. Next, the cells increase in
size and are gradually extruded
Fic. 293.—Diagrams of the development of car-
until they lie in the spaces of the _ tilage from mesenchyma (Lewis and Stéhr). A, Based
upon Studnicka’s studies of fish; B, upon Mall’s study
of mammals. Mes., mesenchyma.
ectoplasmic matrix (Figs. 291 C
and 293 B). Simultaneously, the
ectoplasm is converted into the hyaline matrix peculiar to cartilage, undergoing
both a chemical and structural change. About the cartilage cells the endo-
plasm produces capsules of hyaline substance.
The interstitial growth of cartilage is due: (1) to the direct production of new hyaline
matrix; (2) to the formation of capsules about the cells and their transformation into mattix;
(3) to the proliferation of the cartilage cells, which may separate or occur iri clusters within
a single capsule. ‘
Perichondral growth also takes place about the periphery of the cartilage and is due to
the activity of persisting mesenchymal cells, which, with an outer sheath of connective tissue,
constitute the perichondrium. When cartilage is replaced by bone the perichondrium
becomes the periosteum.
In hyaline cartilage the matrix remains hyaline. In fibro-cartilage the fibrillations of the
primitive ectoplasm are converted into white fibers. In elastic cartilage yellow elastic fibers
are formed in the hyaline matrix, according to Mall; before the hyaline matrix is differentiated,
according to Spalteholz. Most of the bones of the skeleton are laid down first in the form of
cartilage. Later, this is gradually replaced by the development of bone tissue.
288 HISTOGENESIS
BONE
Bone is a tissue appearing relatively late in the embryo. There are de-
veloped two types, the membrane bones of the face and cranium and the cartilage
bones which replace the cartilaginous skeleton. Cartilage bones are not simply
cartilage transformed into bone by the deposition of calcium salts, but represent
a new tissue which is developed as the cartilage is destroyed.
Membrane Bone.—The flat bones of the face and skull are not preformed as
cartilage. The form of a membrane bone is determined by the development of a
Osteoblast
A
Bone matrix\
Katharine Wilh
Bone cell
Bone Osteoclast
Fibrille in bone matrix
Fic. 294.—Two stages in the development of bone. A, Section through the frontal bone of a 20
mm. pig embryo (after Mall). > 270. B, Section through the periosteum and bone lamelle of the
mandible of a 65 mm. human fetus. X 325.
periosteal membrane from the mesenchyma. ‘The bone matrix is differentiated
within the periosteum from enlarged cells, the osteoblasts (bone formers). Osteo-
blasts appear in clusters and from their cytoplasm is differentiated a fibrillated
ectoplasmic matrix like that which precedes the formation of connective tissue and
cartilage (Fig. 294 A). This fibrillated matrix, by a chemical change apparently,
is converted into a homogeneous bone matrix, which first takes the form of
spicules. Others view the fibrillated matrix as an intercellular product and the
bone matrix as an interfibrillar deposit. Howéver this may be, the spicules
Fic. 295.—A longitudinal section of the two distal phalanges from the finger of a five-months’ hu-
man fetus (Sobotta). XX 15. Kn, Cartilage showing calcification and resorption; eK, endochrondral
bone; M, marrow cavity; pK, periosteal bone.
BONE 289
coalesce, form a network of bony plates, and constitute the bone matrix upon the
surfaces of which osteoblasts are arranged in a single layer like the cells of an
epithelium (Fig. 294 B). These cells may be cuboidal, columnar, or may flatten
out as bone formation ceases. As the matrix of the bone is laid down, osteoblasts
Lecome engulfed and form bone cells. The bone cells are lodged in spaces termed
lacune. These are connected by microscopic canals, the canaliculi, in which
delicate cell processes course and anastomose with those of neighboring cells.
The plates of the spongy membrane bone are formed about blood vessels
as centers. As the bone grows at the periphery, the bone matrix is resorbed
centrally. At this time large multinucleated cells (43 to 91 « long) appear upon
the surfaces of the bone matrix. These cells are known as osteoclasts (bone
destroyers). There is, however, no positive evidence that the osteoclasts are
active in dissolving the bone. They may be interpreted also as degenerating, fused
osteoblasts (Arey, Anat. Rec., vol. 11, 1917). The cavities in which they are
frequently lodged are known as Howship’s lacune. The bone lamelle of the cen-
tral portion of the membrane bone are gradually resorbed and this portion of the
bone is of a spongy texture. Some time after birth, compact bone lamella are
laid down by the inner osteoblast cells of the periosteum. In the case of flat
bones, compact inner and outer plates or tables are thus developed with spongy
bone between them. The spaces in the spongy bone are filled by derivatives of
the mesenchyme: reticular tissue, blood vessels, fat cells, and developing blood
cells. These together constitute the red bone marrow. The ossification of mem-
brane bone begins at the middle of the bone and proceeds in all directions from
this primary center.
Cartilage Bone.—The form of the cartilage bone is determined by the pre-
formed cartilage and its surrounding membrane, the perichondrium (Fig. 296).
Bone tissue is developed as in membrane bones, save that the cartilage is first
destroyed and the new bone tissue develops (1) in, and (2) about it. In the first
case, the process is known as endochondral bone formation. In the second case,
it is known as perichondral or periosteal bone formation.
Endochondral Bone Formation.—The cartilage cells enlarge, become ar-
ranged in characteristic rows, and lime is deposited in the matrix (Fig. 295). The
perichondrium becomes the periosteum. From its inner or osteogenic layer,
which is densely cellular, ingrowths invade and resorb the cartilage and fill the
primary marrow cavities. The invading osteogenic tissue gives rise to osteoblasts
and bone marrow. By the osteoblasts bone is differentiated directly upon
persisting portions of the cartilage. As new bone is developed peripherally, it is
19
290 HISTOGENESIS
resorbed centrally to form large marrow spaces. Eventually, all of the cartilage
matrix, and probably the cartilage cells as well, are destroyed.
Perichondral Ossification——Compact bone is developed after birth by the
osteogenic layer of the periosteum and thus are produced the periosteal lamelle.
In the ribs this is said to be the only method of ossification. The bone lamelle
deposited about a blood vessel are concentrically arranged and form the concentric
lamelle of a Haversian system. The Haversian canal of adult bone is merely the
space occupied by a blood vessel.
Growth of Cartilage Bones.—In cartilage bones there is no interstitial growth
as in cartilage. Most of the cartilage bones have more than one center of ossi-
fication and growth is due to
the expansion of the interven-
ing cartilage. Flat bones grow
at the periphery; ring like
bones, such as the vertebre,
have three primary centers
of ossification, between which
the cartilage continues to grow
(Fig. 296 A). In the case of
the numerous long bones of
the skeleton, the primitive ossi-
fication center forms the shaft
or diaphysis (Fig. 296 C-F).
The cartilage at either end of
the diaphysis grows rapidly
and thus the bone increases
Frc. 296.—Diagrams to show the method of growth of in length. Eventually, osteo-
A, a vertebra; B, of sacrum; C-F, of a long bone (the
2 genic tissue invades these car-
tibia).
tilages and new ossification
centers, the epiphyses, are formed, one at either end. When the growth of the
bone in length is completed, the epiphyses, by the ossification of the intervening
cartilage, are united to the diaphysis.
The shaft of the long bones grows in diameter by the peripheral deposition
of bone lamella and the central resorption of the bone. In the larger long bones
spongy, or cancellated, bone tissue persists at the ends, but in the middle portion
a large medullary, or marrow cavity, is developed. This is filled chiefly with fat
cells and constitutes the yellow bone marrow.
THE HISTOGENESIS OF MUSCLE 2g1
Regeneration of Bone.—If bone is injured or fractured, new bone is developed by
osteoblasts derived either from the periosteum or from the bone marrow. The repair of a
fracture is usually preceded by the formation of cartilage which unites the ends of the bones
and is later changed to bone. In adults, the periosteum is regarded as especially important
in the regeneration of bone tissue. Macewen (1912), however, rejects this view.
Joints.—In joints of the synarthrosis type in which little movement is allowed
the mesenchyma between the ends of the bones differentiates into connective
tissue or cartilage. This persists in the adult.
In joints of the dzarthrosis type the bones are freely movable. The mesen-
chyma between the bones develops into an open connective tissue in which a cleft
appears, the joint cavity. The cells lining this cavity flatten out and form a more
or less continuous layer of epithelium, the synovial membrane. From the con-
nective tissue surrounding the joint cavity are developed the various fibrous
ligaments typical of the different joints. Ligaments or tendons apparently cours-
ing through the adult joint cavities represent secondary invasions, which are
covered with reflexed synovial membrane, and hence are really external to the
cavity.
THE HISTOGENESIS OF MUSCLE
The muscular system is composed of muscle fibers which form a tissue in
which contractility has become the predominating function. The fibers are of
three types: (1) smooth muscle cells found principally in the walls of the viscera and
blood vessels; (2) striated skeletal muscle, chiefly attached to the elements of the
skeleton and producing voluntary movements; (3) striated cardiac muscle, form-
ing the myocardium of the heart. All three types are derived from the meso-
derm. The only exceptions are the smooth muscle of the iris, and the smooth
muscle of the sweat glands, which are derived from the ectoderm.
Smooth Muscle in general may be said to arise from the mesenchyme, or
from embryonal connective tissue. Its development has been studied by McGill
(Internat. Monatschr. f. Anat. u. Physiol., vol. 24, 1907) in the esophagus of
pig embryos. The stellate cells of the mesenchyma enlarge, elongate, and their
cytoplasm becomes more abundant. The resulting spindle-shaped cells remain
attached to each other by cytoplasmic bridges and develop in the superficial
layer of their cytoplasm coarse non-contractile myoglia fibrils (Fig. 297) similar
to the primitive fibrille of connective tissue. The myoglia fibrils may extend
from cell to cell, thus connecting them. These fibrils are the products of coa-
lesced granules found within the cytoplasm of the myoblasts. In embryos
of 30 mm. fine myofibrille are differentiated in the cytoplasm of the myoblasts
292 HISTOGENESIS
and give it a longitudinally striated appearance. The cytoplasmic processes of
the muscle cells, the cytoplasmic bridges, later give rise to white connective
tissue fibers which envelop the muscle fibers and bind them together. Smooth
muscle increases in amount: (1) by the formation of new fibers from the mesen-
chyme of the embryo; (2) by the transformation into muscle fibers of interstitial
Fic. 297.—Two stages in the development of smooth muscle fibers (after McGill). A, from the
esophagus of a 13 mm. pig (X 550); coalescing granules give rise to coarse myoglia fibrils. B, from the
esophagus of a 27 mm. pig (X 850); both coarse myoglia fibrils and fine myofibrils are present.
cells; (3) by the multiplication of their nuclei by mitosis in the more advanced
fetal stages.
Striated Skeletal Muscle.—All striated voluntary muscle is derived from
the mesoderm, either from the myotomes of the segments (muscles of the trunk)
or from the mesenchyma (muscles of the head). According to Bardeen (in
Keibel and Mall, vol. 1), after the formation of the sclerotome (Fig. 290), which
Fic. 298.—Stages in the histogenesis of skeletal muscle (after Godlewski). A, from 13 mm. sheep
embryo; B, homogeneous myofibrils in myoblast from 10 mm. guinea pig embryo; C, myoblast from 8.5
mm. rabbit embryo with longitudinally splitting striated myofibrils.
gives rise to skeletal tissue, the remaining portion of the primitive segment con-
stitutes the myotome. All the cells of the myotome give rise to myoblasts. Wil-
liams (Amer. Jour. Anat., vol. 11, 1910), working on the mesodermal segments of
the chick, finds that only the dorsal and mesial cells are myoblasts. By multi-
plication they form a mesial myotome, while the lateral cells of the original
THE HISTOGENESIS OF MUSCLE 293
mesodermal segment persist as a dermatome and give rise only to the connective
tissue of the corium (Fig. 323). The dermatome lies lateral to the myotome
(Fig. 47) and the two together constitute the dermo-myotome (Williams).
As to the origin of the striated voluntary muscle fibers, there is also a differ-
ence of opinion. It is generally believed that the myoblasts elongate, and, by
the repeated mitotic division of their nuclei, become multinucleated. (God-
lewski, however, holds that several myoblasts unite to form a single muscle fiber.)
The nuclei lie at first centrally, surrounded by the granular sarcoplasm (Fig.
298 A). The sarcoplasmic granules become arranged in rows and constitute the
myofibrille which increase in number by longitudinal splitting (Fig. 298 B, C).
The myofibrille soon differentiate alternating dark and light bands, due to differ-
ences in density, and the individual fibrilla become so grouped that their dark
and light bands coincide (Fig. 298 C). During development the muscle fibers
increase enormously in size, the nuclei migrate to the surface, and the myofibrille
are arranged in bundles or muscle columns (sarcostyles). The fibrils of each col-
umn are said to arise by the longitudinal splitting of single primitive myofibrils.
According to Baldwin (Zeitschr. f. allg. Physiol., vol. 14, 1912), the nucleus and perinu-
clear sarcoplasm is separated from the rest of the muscle fiber by the sarcolemma. With
Apathy, he would therefore regard the myofibrille as a differentiated product of the muscle
cells and to be homologized with connective tissue fibers. The extrusion of the muscle cell
from the muscle fiber may be compared to the extrusion of cartilage cells from the pre-
cartilage matrix, as described by Mall (see p. 287).
During the later stages in the development of striated voluntary muscle there is, accord-
ing to many observers, an active degeneration of the muscle fibers.
While smooth muscle fibers form a syncytium and the enveloping connective
tissue is developed directly from the muscle cells, in the case of striated skeletal
muscle each fiber is a multinucleated entity which is bound together with others
by connective tissue of independent origin.
Striated Cardiac Muscle.—This is developed from the splanchnic mesoderm
which forms both the epicardium and the myocardium (Fig. 255). The cells of
the myocardium at first form a syncytium in which myofibrille develop from
chondrioconta, or cytoplasmic granules. The myofibrille are developed at the
periphery of the syncytial strands of cytoplasm and extend long distances in
the syncytium. They multiply rapidly and form dark and light bands as in
skeletal muscle. The syncytial character of cardiac muscle persists in the adult
and the nuclei remain central in position. The intercalated discs, typical of adult
cardiac muscle, appear relatively late, just before birth in the guinea pig (Jordan
and Steele, 1912).
2904 HISTOGENESIS
‘THE HISTOGENESIS OF THE ECTODERMAL DERIVATIVES
Besides forming the enamel of the teeth and the salivary glands (cf. p. 161),
the ectoderm gives rise: (1) to the epidermis and its derivatives (subcutaneous
glands, nails, hair, and the lens and conjunctiva of the eye); (2) to the nervous sys-
tem and sensory epithelia; (3) to parts of certain glands producing internal secre-
tions such as the pituitary body, suprarenal glands, and chromaffin bodies. We
shall describe here the histogenesis of the epidermis, the development of its
derivatives, and the histogenesis of the nervous tissues, reserving for final chap-
ters the development of the nervous organs and the glands formed in part from
them.
THE EPIDERMIS
The single-layered ectoderm of the early embryo by the division of its cells
becomes differentiated into a two-layered epidermis composed of an inner layer
of cuboidal or columnar cells, the stratum germinativum, and an outer layer of
flattened cells, the epitrichium or periderm (Fig. 299 A).
The stratum germinativum is the reproducing layer of the epidermis. As
development proceeds, its cells by division gradually give rise to new layers above
it until the epidermis becomes a many-layered or stratified epithelium. The
periderm is always the outermost layer of the epidermis. In embryos of 25 to 121
mm. (C R) the epidermis is typically three-layered, the outer flattened layer
forming the periderm, a middle layer of polygonai cells, the intermediate layer,
and the inner columnar layer, the stratum germinativum (Fig. 299 B). After
the fourth month the epidermis becomes many layered. The inner layers of cells
now form the stratum germinativum and are actively dividing cells united with
each other by cytoplasmic bridges. The outer layers of cells become cornified,
the cornification of the cells proceeding from the stratum germinativum toward the
surface. Thus, next the germinal layer are cells containing keratohyalin, which
constitute the double-layered stratum granulosum. A thicker layer above the
stratum granulosum shows cells in which drops of a substance called eleidin are
formed. These droplets, which are supposed to represent softened keratohyalin,
give these cells a clear appearance when examined unstained. Hence the layer is
termed the stratum lucidum. In the outer layers of the epidermis the thickened
walls of the cells become cornified and in the cells themselves a fatty substance
collects. These layers of cells constitute the stratum corneum. The cells of this
layer are also greatly flattened, especially at the surface.
When the hairs develop they do not penetrate the outer periderm layer of
the epidermis, but, as they grow out, lift it off (sixth month). Hence this layer
THE HAIR 205
is known also as the epitrichium (layer upon the hair). Desquamated epitrichial
and epidermal cells mix with the secretion of the sebaceous glands to form the
cheesy vernix caseosa which covers the fetal skin. Pigment granules appear soon
after birth in the cells of the stratum germinativum. ‘These granules are prob-
ably formed im situ. Negro children are quite light in color at birth, but within
six weeks their integument has reached the normal degree of pigmentation.
The derma or cortum of the integument is developed from mesenchyme,
perhaps from that of the dermatomes (Fig. 323) of the mesodermal segments (p.
292). At about the end of the third month a differentiation into the compact
Epitrichium
=e pec
= ae % 2 @ —S= SSS =
Sore Se oS S Saye
Corium = Se See = Beas
SS <8 oS Ss
se = 2S SSS SS rt
Stratum germinatioum—=
Epitrichium
Intermediate layer
Stratum germinativum =
=~ es ae Wa oS ie =e Hi
Fic. 299.—Sections of the integument from a 65 mm. human fetus. > 440. A, Section through
the integument of the neck showing a two-layered epidermis and the beginning of a third intermediate
layer; B, section from the integument of the chin in which three layers are well developed in the epidermis.
corium proper and the areolar subcutaneous tissue occurs. From the corium
papille project into the stratum germinativum.
Anomalies.—Dermoid cysts, resulting from epidermal inclusions, are not infrequent
along the lines of fusion of embryonic structures, e. g., branchial clefts, mid-dorsal and mid-
ventral body wall.
THE HAIR
Hairs are derived from thickenings of the epidermis and begin to develop at
the end of the second month on the eyebrows, upper lip, and chin. The hair of
the general body integument appears at the beginning of the fourth month.
The first evidence of a hair anlage is the elongation of a cluster of epidermal
cells in the inner germinal layer (Fig. 300 A). The bases of these cells project into
the corium, and, above them, cells of the epidermis are arranged parallel to the
296 HISTOGENESIS
surface. The elongated cells continue to grow downward until a cylindrical hair
anlage is produced (Fig. 300 B, C). This consists of an outer wall formed of a
single layer of columnar cells, continuous with the basal layer of the epidermis.
This wall bounds a central mass of irregularly polygonal epidermal cells. About
the hair anlage the mesenchyma forms a sheath, and at its base a condensation of
mesenchyme produces the anlage of the hair papilla, which projects into the
enlarged base of the hair anlage. As development proceeds, the hair anlage grows
deeper into the corium and its base enlarges to form the hair bulb (Fig. 300 C).
The hair differentiates from the basal epidermal cells surrounding the hair
papilla. These cells give rise to a central core which grows toward the surface,
Epitrichium
“Central cells
Epidermal anlage of.
hair A
Epidermal anlage of
hair B
EEE ONGC
BIIBO LLY
Epidermal anlage of hair C
Mesenchymal sheath
Hair bulb
Yer et
Hair papilla
Fic. 300.—Section through the integument of the face of a 65 mm. human fetus showing three stages in
the early development of the hair. X 330.
distinct from the peripheral cells which form the outer sheath of the hair (Fig. 301).
The central core of cells becomes the inner hair sheath and the shaft of the hair.
At the sides of the outer hair sheath two swellings appear on the lower side of the
obliquely directed hair anlage. The more superficial of these is the anlage of the
sebaceous gland (Fig. 301). The deeper swelling is the “epithelial bed,” a region
where the cells by rapid division contribute to the growth of the hair follicle.
Superficial to the bulb, the cells of the hair shaft become cornified and
differentiated into an outer cuticle, middle cortex, and central medulla. The
hair grows at the base and is pushed out through the central cavity of the anlage,
the cells of which degenerate. When the hair projects above the surface of the
epidermis it breaks and carries with it the epitrichial layer. The mesenchymal
MAMMARY GLANDS 207
tissue which surrounds the hair follicle in the neighborhood of the epithelial bed
gives rise to the smooth fibers of the arrector pili muscle. Pigment granules de-
velop in the basal cells of the hair and give it its characteristic color.
— Sebaceous gland
Epithelial bed
Root of hair
Frc. 301.—Longitudinal section through a developing hair from a five and one-half months’ human
fetus (after Stéhr). -xX 220 ;
The first generation of hairs are short-lived, all except those covering the face
being cast off soon after birth. The coarser replacing hairs develop, at least in
part, from new follicles. Thereafter hair is shed periodically throughout life.
SWEAT GLANDS
The sweat or sudoriparous glands begin to develop in the fourth month from
the epidermis of the finger tips, the palms of the hands, and the soles of the feet.
They are formed as solid downgrowths from the epidermis, but differ from hair
anlages in having no mesenchymal papilla at their bases. During the sixth
month the tubular anlages of the gland begin to coil and in the seventh month
their lumina appear. The inner layer of cells forms the gland cells, while the outer
cells become transformed into smooth muscle fibers which here arise from the
ectoderm. In the axillary region sweat glands occur which are large and branched.
MAMMARY GLANDS
The tubular mammary glands peculiar to mammais are regarded as modified
sweat glands. In embryos of 9 mm. (Figs. 94 and 118) an ectodermal thickening
298 HISTOGENESIS
extends ventrolaterally between the bases of the limb buds on either side. This
linear epidermal thickening is the milk line. In the future pectoral region of this
line, by the thickening and downgrowth of the epidermis there is formed the
papilla-like anlage of the mammary gland (Fig. 302 A). From this epithelial
anlage buds appear (B) which elongate and form solid cords 15 to 20 in number,
the anlages of the milk ducts (C). These branch in the mesenchymal tissue of the
corium and eventually produce the alveolar end pieces of the mammary glands.
In the region where the milk ducts open on the surface the epidermis is evagi-
Gland anlage Epidermis
- !
* Nipple Ea
A 4
Epidermis Gland anlage : ON" of areola
4 Z \
Panniculus
~ adiposus
aw). _ Pectoral
muscle
Fic. 302.—Sections representing three successive stages of development of the human mammary
gland (Tourneux). A, fetus of 32 mm.; B, of 102 mm.; C, of 244 mm. *, Groove limiting glandular
area.
nated to form the nipple. The glands enlarge rapidly at puberty and are further
augmented during pregnancy, while after parturition they become functionally
active.
The mammary glands are homologised with sweat glands because their development is
similar, and because in the lower mammals their structure is the same. Rudimentary mam-
mary glands (of Montgomery), which also resemble sweat glands, occur in the areola about
the nipple. In many mammals numerous pairs of mammary glands are developed along the
milk line (pig, dog, etc.); in some a pair of glands is developed in the pectoral region (primates,
elephants); in others glands are confined to the inguinal region (sheep, cow, horse). In man
supernumerary mammary glands developed along the milk line are of not infrequent occur-
rence.
THE NAILS
The anlages of the nails proper are derived from the epidermis and may be
recognized in fetuses of 45 mm. (CR). A nail anlage forms on the dorsum of each
digit and extends from the tip of the digit almost to the articulation of the terminal
phalanx. At the base of the anlage, that is, proximally, the epidermis is folded
inward to form the proximal nail fold (posterior nail fold of the adult) (Fig. 303
THE NAILS 299
C). The nail fold also extends laterally on either side of the nail anlage and forms
the lateral nail fold of the adult (A, B).
The material of the nail is developed in the lower layer of the proximal nail
fold (C). In certain of the epidermal cells, which according to Bowen represent a
modified stratum lucidum, there are developed keratin or horn fibrils during the
fifth month of fetal life. These appear without the previous formation of kerato-
hyalin granules as is the case in the cornification of the stratum corneum. The
cells flatten and form the plate-like structure of which the solid substance of the
nail is composed. Thus the nail substance is formed in the proximal nail fold as
Sole plate
~ Nail
A -~ Lunula
eo E hiv
Eponychiwm vn = ape
“Nail plate
Sole plate... ae Nail fold
fm Nail bed
Fic. 303.—Figures showing the development of the nail. A, From a 40 mm. human fetus (X 20);
B, from a 100 mm. fetus (X 13); C, longitudinal section through the nail anlage of a 100 mm. fetus
(X 24). (Kollmann.)
far distad as the outer edge of the Junula (the whitish crescent at the base of the
adult nail). The underlying epidermis distal to the lunula takes no part in the
development of the nail substance. The corium throws its surface of contact with
the nail into parallel longitudinal folds which produce the longitudinal ridges of
the nail. The nail is pushed toward the tip of the digit by the development of
new nail substance in the region of the nail fold. The stratum corneum and the
epitrichium of the epidermis for a time completely cover the nail matrix and are
termed the eponychium (Fig. 303 C). Later, this is thrown off, but a portion of
the stratum corneum persists during life as the curved fold of epidermis which
adheres to the base of the adult nail. During life the nail constantly grows at its
300 HISTOGENESIS
base (proximally), is shifted distally over the nail bed, and projects at the tip
of the digit.
The nails of man are the homologues of the claws and hoofs of other mammals. During
the third month thickenings of the integument over the distal ends of the metacarpals and
metatarsals become prominent. These correspond to the touch-pads on the feet of clawed
mammals. Similar pads are developed on the under sides of the distal phalanges.
THE HISTOGENESIS OF THE NERVOUS TISSUES
The primitive anlage of the nervous system consists of the thickened layer of
ectoderm along the mid-dorsal line of the embryo. This is the neural plate (Fig.
304 A, B) which is folded to form the newral groove (Figs. 77 A and 78). The edges
Neural groove Neural plate
Neural groove Neural plate 1
Ectoderm Neural groove
Neural tube Neural cavity
D
Fic. 304.—Four sections showing the development of the neural tube in human embryos. 4, An early
embryo (Keibel); B, at 2 mm. (Graf Spee); C, at 2 mm. (Mall); D, at 2.7 mm. (Kollmann).
of the neural plate come together and form the neural tube (Fig. 304 C, D). The
cranial portion of this tube enlarges and is constricted into the three primary
vesicles of the brain (Fig. 324). Its caudal portion remains tubular and con-
stitutes the spinal cord. From the cells of this tube, and the ganglion crest con-
nected with it, are differentiated the nervous tissues, with the single exception of
the nerve cells and fibers of the olfactory epithelium.
The Differentiation of the Neural Tube.—The cells of the neural tube dif-
ferentiate along two lines. There are formed: (1) nerve cells and fibers, in which
irritability and conductivity have become the predominant functions; (2) meu-
roglia cells and fibers which constitute the supporting or skeletal tissue peculiar to
THE HISTOGENESIS OF THE NERVOUS TISSUES 301
the nervous system. The differentiation of these tissues has been studied by
Hardesty in pig embryos (Amer. Jour. Anat., vol. 3, 1904). The wall of the
neural tube, consisting at first of a single layer of columnar cells, becomes many-
layered and finally three zones are differentiated (Fig. 305 A-D). As the wall
becomes many-layered the cells lose their sharp outlines and form a compact,
Marginal layer Manile layer Ependymal layer
4 he
Marginal layer ‘Ependymal layer Internal limiting membrane
M esoderm M arginal layer Ependymal layer
} \Germinal
-“ cell
External limiting membrane Mantle layer Internal limiting membrane
External limiting membrane Germinal cell , nternal limiting membrane
’ 4
Ay?
vie
D
pe ee Ves ee oe si
Mesoderm Marginal layer = Manile layer Ependymal layer
Fic. 305.—Three stages in the early development of the neural tube showing the origin ot the syn-
cytial framework (after Hardesty). > 690. A, From rabbit before the closure of neural tube; B,
from 5 mm. pig after closure of tube; C, froma 7mm. pig embryo; D, from a 10 mm. pig embryo.
* Boundary between nuclear layer and marginal layer.
cellular syncytium which is bounded, on its outer and inner surfaces, by an exter-
nal and internal limiting membrane (B). Ina10mm. embryo the cellular strands
of the syncytium are arranged radially and nearly parallel (D). The nuclei are
now so grouped that there may be distinguished three layers: (1) an inner epen-
dymal zone with cells abutting on the internal limiting membrane, their processes
302 HISTOGENESIS
extending peripherally; (2) a middle mantle or nuclear zone, and (3) an outer or
marginal zone, non-cellular, into which nerve fibers grow. The ependymal zone
contributes cells for the development of the mantle layer (D). The cellular
mantle layer forms the gray substance of the central nervous system, while the
fibrous marginal layer constitutes the white substance »f the spinal cord.
The primitive germinal cells of the neural tube divide by mitosis and give
rise to the ependymal cells of the ependymal zone and to indifferent cells of the
mantle layer. From these latter arise spongioblasts and neuroblasts (Fig. 306).
The spongioblasts are transformed into ewroglia cells and fibers, which form the
supporting tissue of the central nervous system; the neuroblasts are primitive
nerve cells, which, by developing cell processes, are converted into neurones. The
neurones are the structural units of the nervous tissue.
A
Germinal cells
Ependymal cells Indifferent cells
Mitotic indifferent cells
oO
Pn eco SAP
242 805 Seg tS
09 0
Neuroblasts Neuroglia cells
Fic. 306.—Diagrams showing the differentiation of the cells in the wall of the neural tube and the theo-
retical derivation of the ependymal cells, neuroglia cells and neuroblasts (after Schaper).
The Differentiation of the Neuroblasts into Neurones.—The nerve fibers are
developed as outgrowths from the neuroblasts, and a nerve cell with all its pro-
cesses constitutes a neurone or cellular unit of the nervous system. The origin
of the nerve fibers as processes of the neuroblasts is best seen in the development
of the root fibers of the spinal nerves.
The Efferent or Ventral Root Fibers of the Spinal Nerves.—At the end of
the first month clusters of neuroblasts separate themselves from the syncytium
in the mantle layer of the neural tube. The neuroblasts become pear-shaped
and from the small end of the cell a slender primary process grows out (Figs. 307
and 308). The process becomes the axis cylinder of a nerve fiber. The primary
processes may course in the marginal layer of the neural tube, or, converging, may
penetrate the marginal layer ventro-laterally and form the ventral roots of the
THE UHISTOGENESIS OF THE NERVOUS TISSUES 303
Fic. 307.—A, Transverse section through the spinal cord of a chick embryo of the third day show-
ing neuraxons (F) developing from neuroblasts of the neural tube and from the bipolar ganglion cells, d.
B, Neuroblasts from the spinal cord of a seventy-two-hour chick. The three to the right show neuro-
fibrils; C, incremental cone (Cajal).
‘Ventral root
Fic. 308.—Transverse section of the spinal cord from human fetus of five weeks showing pear-
shaped neuroblasts giving rise to ventral root fibers (His in Marshall). X 150. NC, Central canal
of spinal cord.
304 HISTOGENESIS
spinal nerves. Similarly, the efferent fibers of the cerebral nerves grow out from
neuroblasts of the brain wall. Within the cytoplasm of the nerve cells and their
primary processes strands of fine fibrils are early differentiated. These, the
neurofibrille, are the conducting elements of the neurones. The cell bodies of the
efferent neurones soon become multipolar by the development of branched
secondary processes, the dendrites.
The Development of the Spinal Ganglia.—A{ter the formation of the neural
plate and groove a longitudinal ridge of cells appears on each side where the ecto-
derm and neural plate are continuous (Fig. 309 A). This ridge of ectodermal
Neural crest
a
O7 oS) ~ Neural crest
a: Sad
Ono ae
SS 8 & Mesodermal segment
Fic. 309.—Three stages in the development of the ganglion crest in human embryos (after von Len-
hossek in Cajal).
cells is the neural or ganglion crest. When the neural tube is formed and the
ectoderm separates from it, the cells of the ganglion crest overlie the neural tube
dorso-laterally (Fig. 309 C). As development continues they separate into right
and left linear crests, distinct from the neural tube, and migrate ventro-laterally
to a position between the neural tube and myotomes. In this position the
ganglion crest forms a band of cells extending the whole length of the spinal cord
and as far cranially as the otic vesicles. At regular intervals in its course along
the spinal cord the proliferating cells of the crest give rise to enlargements, the
spinal ganglia (Fig. 358). The spinal ganglia are arranged segmentally and are
THE HISTOGENESIS OF THE NERVOUS TISSUES 305
connected at first by bridges of cells which later disappear. In the hind-brain
region certain ganglia of the cerebral nerves develop from the crest but are not seg-
mentally arranged.
The Differentiation of the Afferent Neurones.—The cells of the spinal ganglia
differentiate into (1) ganglion cells, and (2) supporting cells, groups which are
comparable to the neuroblasts and spongioblasts of the neural tube. The neuro-
blasts of the ganglia become fusiform and develop a primary process at either pole;
thus these neurones are of the bipolar type (Fig. 307 d). The centrally directed
processes of the ganglion cells converge and by elongation form the dorsal roots.
They penetrate the dorso-lateral wall
of the neural tube, bifurcate, and course
cranially and caudally in the marginal
layer of the spinal cord. By means of
branched processes they come in con-
tact with the neurones of the mantle
layer. The peripheral processes of the
ganglion cells, as the dorsal spinal roots,
join the ventral roots, and, together
with them, constitute the trunks of the
spinal nerves (Fig. 325).
The Differentiation of the Unipolar
Ganglion Cells.—At first bipolar, the h ee
majority of the ganglion cells become yy, Me
Uy
Ur
unipolar either by the fusion of the two
primary processes or by the bifurcation My
Fic. 310.—A portion of a spinal ganglion from a
of a single process (Fig. 310). The pro- ye Ge tsar Cale mated (Ga.
cess of the unipolar ganglion is now
T-shaped. Many of the bipolar ganglion cells persist in the adult, while others
develop several secondary processes and thus become multipolar in form. In
addition to forming the spinal ganglion cells, neuroblasts of the ganglion crest
are believed to migrate ventrally and form the sympathetic ganglia (Fig. 325).
The Neurone Theory.—The above account of the development of the nerve fibers is
the one generally accepted at the present time. It assumes that the axis cylinders of all
nerve fibers are formed as outgrowths, each from a single cell, an hypothesis first promulgated
by His. The embryological evidence is supported by experiment. It has long been known
from the work of Waller that if nerves are severed, the fibers distal to the point of section,
and thus isolated from their nerve cells, will degenerate; also, that regeneration will take
place from the central stumps of cut nerves, the fibers of which are still connected with their
20
306 HISTOGENESIS
cells. More recently Harrison (Amer. Jour. Anat., vol. 5, 1906), experimenting on amphibian
larve, has shown: (1) that no peripheral nerves develop if the neural tube and crest are re-
moved; (2) that isolated ganglion cells growing in clotted lymph will give rise to long axis
cylinder processes in the course of four or five hours.
A second theory, supported by Schwann, Balfour, Dohrn, and Bethe, assumes that the
nerve fibers are in part differentiated from a chain of cells, so that the neurone would represent
a multicellular, not a unicellular structure. Apathy and O. Schulze modified this cell-chain
theory by assuming that the nerve fibers differentiate in a syncytium which intervenes between
the neural tube and the peripheral end organs. Held further modified this theory by assum-
ing that the proximal portions of the nerve fibers are derived from the neuroblasts and
ganglion cells and that these grow into a syncytium which by differentiation gives rise to the
peripheral portion of the fiber.
The Differentiation of the Supporting Cells of the Ganglia and Neural Tube.
—The supporting cells of the spinal ganglia at first form a syncytium in the
meshes of which are found the neuroblasts. They differentiate (1) into flattened
capsule cells which form capsules about the ganglion cells, and (2) into sheath cells
which ensheath the axis cylinder processes and are continuous with the capsules
of the ganglion. It is probable that many of the sheath cells migrate peripherally
along with the developing nerve fibers (Harrison). They are at first spindle-
shaped, and, as primary. sheaths, enclose bundles of nerve fibers. Later, by the
proliferation of the sheath cells, the bundles are separated into single fibers, each
with its sheath (of Schwann), or newrilemma. Each sheath cell forms a segment
of the neurilemma, the limits of contiguous sheath cells being indicated by con-
strictions, the nodes of Ranvier.
The Myelin or Medullary Sheath.—During the fourth month an inner mvelin
sheath appears about many nerve fibers. This consists of a spongy framework of
neurokeratin in the interstices of which a fatty substance, mvelin, is deposited.
The origin of the myelin sheath is in doubt. By some (Ranvier) it is believed to
be a differentiation of the neurilemma, the myelin being deposited in the substance
of the nucleated sheath cell. By others (Kolliker, Bardeen) the myelin is regarded
as a direct or indirect product of the axis cylinder. Its integrity is dependent at
least upon the nerve cell and axis cylinder, for, when a nerve is cut, the myelin
very soon shows degenerative changes. Furthermore, it may form where the
sheath is absent.
In the central nervous system there is no distinct neurilemma sheath invest-
ing the fibers. Sheath cells are said to be present and most numerous during the
period when myelin is developed. Hardesty derives the sheath cells in the central
nervous system of the pig from a portion of the supporting cells, or spongioblasts,
of the neural tube, and finds that these cells give rise to the myelin of the fibers.
THE HISTOGENESIS OF THE NERVOUS TISSUES 307
Those fibers which are first functional receive their myelin sheaths first.
The myelination of nerve fibers is only completed between the second and third
year (Westphal). Many of the peripheral fibers, especially those of the sympa-
thetic system, remain unmyelinated and supplied only with a neurilemma sheath.
The myelinated fibers, those with a myelin sheath, have a glistening white ap-
pearance and give the characteristic color to the white substance of the central
nervous system and to the peripheral nerves. Ranson (Amer. Jour. Anat., vol.
12, 1911) has shown that large numbers of unmyelinated fibers occur in the peri-
pheral nerves and spinal cord of adult mammals and man. Those found in the
spinal nerves arise from the small cells of the spinal ganglia.
B
ot
Fic. 311 —Ependymal cells from the neural tube of chick embryos. A, of first day; B, of third day.
Golgi method (Cajal).
The Development of the Supporting Cells—The spongioblasts of the
neural tube (p. 302) differentiate into the supporting tissue of the central nervous
system. ‘This includes the ependymal cells, which line the neural cavity, forming
one of the primary layers of the neural tube, and neuroglia cells and their fibers.
We have described how the strands of the syncytium formed by the spongio-
blasts become arranged radially in the neural tube of early embryos (Fig. 305 D).
As the wall of the neural tube thickens, the strands elongate pari passu and form a
radiating branched framework (Fig. 302). The group of spongioblasts which
line the neural cavity constitutes the ependymal layer. Processes from these
308 HISTOGENESIS
cells radiate and extend through the whole thickness of the neural tube to its
periphery. The cell bodies are columnar and persist as the lining of the central
canal and ventricles of the spinal cord and brain (Fig. 312).
Near the median line of the spinal cord, both dorsally and ventrally, the
supporting tissue retains its primitive ependymal structure in the adult. Else-
where the supporting framework is differentiated into mewroglia cells and fibers.
The neuroglia cells form part of the spongioblastic syncytium and are scattered
through the mantle and marginal layers of the neural tube. By proliferation
wl
ae
Fic. 312.—Ependymal cells of the lumbar cord from a human fetus of 44 mm. (Golgi method,
Cajal). A, Floor plate; B, central canal; C, line of future fusion of walls of neural cavity; E, ependy-
mal cells; *, neuroglia cells and fibers.
they increase in number and their form depends upon the pressure of the nerve
cells and fibers which develop around them.
Neuroglia fibers are differentiated (in a manner comparable to the formation
of connective tissue fibers, Fig. 291) from the cytoplasm and cytoplasmic proc-
esses of the neuroglia cells, and, as the latter primarily form a syncytium, the
neuroglia fibers may extend from cell to cell. The neuroglia fibers develop late
in fetal life and undergo a chemical transformation into meurokeratin, the same
myelinated substance which is found in the sheaths of medullated fibers.
CHAPTER XI
THE MORPHOGENESIS OF THE SKELETON AND MUSCLES
I. THE SKELETAL SYSTEM
THE skeleton comprises (1) the axial skeleton (skull, vertebre, ribs, and ster-
num), and (2) the appendicular skeleton (pectoral and pelvic girdles and the limb
bones). Except for the flat bones of the face and skull, which develop directly in
membrane, the bones of the skeleton exhibit first a blastemal or membranous
stage, next a cartilaginous stage, and finalty a permanent osseous stage.
For a detailed account of the development of the various bones of the skele-
ton the student is referred to Bardeen, Keibel and Mall, vol. 1.
AXIAL SKELETON
The primitive axial skeleton of all vertebrates is the notochord or chorda dor-
salis, the origin of which has been traced on pp. 33 and 35. The notochord con-
stitutes the only skeleton of Amphioxus, whereas in fishes and amphibians it is
replaced in part, and in higher animals almost entirely, by the permanent axial
M yotome--$- aM otochord
4 Sclevol i> Anlage of vertebra
=> Sclerotome
_Intersegmental Ectoderm..§ § 4 ...Intervertebral fissure
artery ;
--. Notochord 4 — I ntersegmental artery
A
Z
FAR
A B
Fic. 313.—Frontal sections through the mesodermal segments of the left side of human embryos.
A, at about 4 mm. showing the differentiation of the sclerotomes into less dense and denser regions; B,
at about 5 mm. illustrating the union of the halves of successive sclerotomes to form the anlages of the
vertebre.
skeleton. In the development of mammals, this transient elastic rod disappears
early except in the intervertebral discs where it persists as the nuclei pulposi.
Vertebra and Ribs.—The mesenchyme derived from the sclerotomes grows
mesad (Figs. 290 and 323) and comes to lie in paired segmental masses on either
side of the notochord, separated from similar masses before and behind by the
intersegmental arteries. In embryos of about 4 mm. each sclerotome soon differ-
entiates into a caudal compact portion and a cranial less dense half (Fig. 313 A).
309
310 THE MORPHOGENESIS OF THE SKELETON AND MUSCLES
From the caudal portions, horizontal tissue masses now grow toward the median
line and enclose the notochord, thus establishing the body of each vertebra.
Similarly, dorsal extensions form the vertebral arch, and ventro-lateral outgrowths,
the costal processes. The looser tissue of the cranial halves also grows mesad and
fills in the intervals between successive denser regions.
The denser caudal half of each sclerotomic mass presently unites with the less
dense cranial half of the sclerotome next caudad to form the anlages of the
definitive vertebre (Fig. 313 B). Tissue bordering the cranial and caudal portion
of the original sclerotome gives rise to the intervertebral discs. Since a vertebra is
formed from parts of two adjacent sclerotomes, it is evident that the interseg-
mental artery must now pass over the body of a vertebra and the myotomes and
vertebre alternate in position.
Following this blastemal stage centers of chondrification appear, two centers
in the vertebral body, one in each half of the vertebral arch, and one in each costal
process. These centers enlarge and fuse to form a cartilaginous vertebra; the
union of the costal processes, which will give rise to ribs, with the body is, how-
ever, temporary, an articulation forming later. Transverse and articular processes
grow out from the vertebral arch, and the rib cartilages, having in the meantime
formed tubercles, articulate with the transverse processes somewhat later. The
various ligaments of the vertebral column arise from mesenchyme surrounding
the vertebre.
Finally, at the end of the eighth week, the stage of ossification sets in. A
single center appears in the body, one in each half of the arch, and one near the
angle of each rib (Fig. 296 A). The replacement of cartilage to form a solid mass
is not completed until several years after birth. At about the seventeenth year
secondary centers appear in the cartilage still covering the cranial and caudal ends
of the vertebral body and form the disc-like, bony epiphysis. These unite with
the vertebra proper to constitute a single mass at about the twentieth year.
While the foregoing account holds for vertebre in general, a few deviations
occur. When the ailas is formed a body differentiates as well, but it is appropri-
ated by the body of the epistropheus (axis), thereby forming the tooth-like dens
of the latter. The sacral and coccygeal vertebra represent reduced types. At
about the twenty-fifth year the sacral vertebre unite to form a single bony mass,
and a similar fusion occurs between the rudimentary coccygeal vertebrz.
The ribs, originating as ventro-lateral outgrowths from the vertebral bodies,
reach their highest development in the thoracic region. In the cervical region
they are short; their tips fuse with the transverse processes and their heads with
AXIAL SKELETON 311
the vertebral bodies, thus leaving intervals—the transverse foramina—through
which the vertebral vessels course. In the lumbar region the ribs are again dimin-
utive and are fused to the transverse processes. The rudimentary ribs of the
sacral vertebra are represented by flat plates which unite on each side to form a
pars lateralis of the sacrum. With the exception of the first coccygeal vertebra,
ribs are absent in the most caudal vertebre.
Sternum.—The sternal anlages arise as paired mesenchymal bands, with
which the first eight or nine thoracic ribs fuse secondarily (Whitehead and Wad-
dell, Amer. Jour. Anat., vol. 12, 1911). After the heart descends into the thorax,
these cartilaginous sternal bars, as they may now be termed, unite in a cranio-cau-
dal direction to form the sternum, at the same time incorporating a smaller mesial
sternal anlage (Fig. 314). Ultimately one or two pairs of the most caudal ribs
. Episternal
Clavicle cartilage
Fic. 314.—Formation of the sternum in a Fic. 315.—Sternum of a child, showing centers of
human fetus during the third month (modified ossification.
after Ruge).
lose their sternal connections, the corresponding portion of the sternum consti-
tuting the xiphoid process in part. At the cranial end of the sternum there are
two imperfectly separated episternal cartilages with which the clavicles articulate.
These usually unite with the longitudinal bars and contribute to the formation
of the manubrium. Variations in the ossification centers are not uncommon, al-
though a primitive, bilateral, segmental arrangement is evident (Fig. 315). In
the two cranial segments, however, unpaired centers occur.
The Skull.—The earliest anlage of the skull consists in a mass of dense mesen-
chyme which envelops the cranial end of the notochord and extends cephalad into
the nasal region. Laterally it forms wings which enclose the neural tube. Except
in the occipital region, where there are indications of the incorporation into the
skull of three or four vertebree, the skull is from the first devoid of segmentation.
312 THE MORPHOGENESIS OF THE SKELETON AND MUSCLES
Chondrification begins in the future occipital and sphenoidal regions, in the
median line, and extends cephalad and to a slight extent dorsad. At the same
time, the internal ear becomes invested with a cartilaginous periotic capsule
which eventually unites with the occipital and sphenoidal cartilages (Fig. 316).
The chondrocranium, as it is termed, is thus confined chiefly to the base of the
skull, the bones of the sides, roof, and the face being of membranous origin.
Chondrification also occurs more or less extensively in the branchial arches, and,
as will appear presently, the first two pairs contribute substantially to the forma-
tion of the skull.
In the period of ossification, which now ensues, it becomes evident that some
bones which are separate in adult lower animals fuse to form compound bones in
- Inter parietal
Supra-occi pital
~-- Exocci pital
--Cond yle
{
\ of ~Basi-occi pital
Fic. 316.—Reconstruction of the chondro- Fic. 317.—Occipital bone of a human fetus
cranium of a human embryo of 14 mm. (Levi in of four months (after Sappey). The portions still
McMurrich). as, Alisphenoid; 60, basi-occipital; cartilaginous are shown as a background.
_bs, basisphenoid; eo, exoccipital; m, Meckel’s car-
tilage; os, orbitosphenoid; ~, periotic; ps, pre-
sphenoid; so, sella turcica; s, supra-occipital.
the human skull. The sphenoid and temporal bone} for example, represent five
primitive pairs each. As such components may arise either in membrane or
cartilage the mixed origin of certain adult bones is explained.
Ossification of the Chondrocranium.—Occipital Bone.—Ossification begins i in
the occipital region during the third month. Four centers appear at right angles
about the foramen magnum (Fig. 317). From the ventral center arises the basilar
(basi-occipital) part of the future bone; from the lateral centers the lateral (exoc-
cipital) parts which bear the condyles, and from the dorsal, originally paired
center the squamous (supra-occipital) part below the superior nuchal line. The
squamous (interparietal) part above that line is an addition of intramembranous
origin. These several components do not fuse completely until about the seventh
year.
AXIAL SKELETON 313
Sphenoid Bone.—Ten principal centers arise in the cartilage that corre-
sponds to this bone (Fig. 318): (1 and 2) in each ala magna (alisphenoid) ; (3 and
4) in each ala parva (orbitosphenoid); (5 and 6) in the corpus between the ale
magne (basisphenoid); (7 and 8) in each lingula; (9 and 10) in the corpus between
the ale parvee (presphenoid). Intramembranous bone also enters into its compo-
“t= Nasal septum
3\.... Perpendicular plate
caeeCrista gallt
Ala magna Ala parva
(Alisphenoid) Presphenoid (Orbitosphenoid)
My Le yy wi
Lingula “Bi KC -- Pterygoid
Basis phenoid process
Fic. 318.—Sphenoid bone of a human fetus Frc. 319.—Ethmoid bone of a human fetus of
of nearly four months (after Sappey). Parts still four months (modified after Kollmann).
cartilaginous are represented in stipple.
? Cribriform plate
A--~ Labyrinth
sition, forming the orbital and temporal portion of each ala magna and the mesial
lamine (Fawcett) of each pterygoid process (except the hamulus). Fusion of the
various parts is completed during the first year.
Ethmoid Bone.--The ethmoid cartilage consists of a mesial mass, which
extends from the sphenoid to the tip of the nasal process, and of paired masses
lateral to the olfactory fossa. The lower part
Squamosum
of the mesial mass persists as the cartilaginous
nasal septum, but ossification of the upper por-
tion produces the lamina perpendicularis and
the crista galli (Fig. 319). The lateral masses
ossify at first into the spongy bone of the eth- e ;
moidal labyrinths. From this the definitive 7”?7"""" (A.g}
honeycomb structure (ethmoidal cells) and the Big: 900. Gila et deapoeal Bone
conche are formed through evaginations of the at birth. The portion of intracarti-
laginous origin is represented in
nasal mucous membrane and the coincident re- sGcéle
sorption of bone. (Similar invasions of the mu-
cous membrane and dissolution of bone produce the frontal, sphenoidal, and
maxillary sinuses.) Fibers of the olfactory nerve at first course between the
unjoined mesial and lateral masses. Later cartilaginous, and finally bony
trabeculae surround these bundles of nerve fibers, and, as the cribriform plates,
interconnect the three masses.
Temporal Bone.—Several centers of ossification in the periotic capsule unite
314 THE MORPHOGENESIS OF THE SKELETON AND MUSCLES
to form a single center from which the whole cartilage is transformed into the
petrous and mastoid portions of the temporal bone (Fig. 320). The mastoid
process is formed after birth by a bulging of the petrous bone, and its internal cavi-
ties, the mastoid cells, are formed and lined by the evaginated epithelial lining of
the middle ear. The squamosal and tympanic portions of the temporal bone are
of intramembranous origin, while the styloid process originates from the proximal
end of the second, or hyoid, branchial arch.
Membrane Bones of the Skull.—From the preceding account it is evident
that although the bones forming the base of the skull arise chiefly in cartilage, they
receive substantial contributions from membrane bones. The remainder of the
sides and roof of the skull is wholly of intramembranous origin, each of the parietals
Incus
Styloid process
Tympanic ring
Stylo-hyoid lig.
Cricoid cartilage
By Hyoid cartilage (greater horn)
‘Thyreoid cartilage
Fic. 321.—Lateral dissection of the head of a human fetus, showing the derivatives of the branchial
arches (after Kollmann).
forming from a single center, the frontal from paired centers. At the incomplete
angles between the parietals and their adjacent bones union is delayed for some
time after birth, These membrane-covered spaces constitute the fontanelles.
The vomer forms from two centers in the connective tissue flanking the lower
border of the lamina perpendicularis of the ethmoid. The cartilage of the eth-
moid thus invested undergoes resorption.
Single centers of ossification in the mesenchyme of the facial region give rise
to the nasal, lacrimal, and zygomatic, all pure membrane bones.
Branchial Arch Skeleton.—The first branchial arch forks into an upper mawil-
lary and a lower mandibular process (Fig. 119). Cartilage fails to appear in the
maxillary processes, due to accelerated development, hence the palate bones and
APPENDICULAR SKELETON 315
the mawille arise directly in membrane. Each palate bone develops from a single
center of ossification. According to one view five centers contribute to the for-
mation of each maxilla; Mall, however, maintains that there are but two centers,
one giving rise to the portion bearing the incisor teeth, the other to the remainder
of the maxilla.
The entire core of the mandibular process becomes a cartilaginous bar,
Meckel’s cartilage, which extends proximally into the tympanic cavity of the ear
(Fig. 321). Membrane bone developing distally in the future body encloses
Meckel’s cartilage and the inferior alveolar nerve, whereas proximally in the
ramus the membrane bone merely lies lateral to these structures—hence the posi-
tion of the adult mandibular foramen. The portion of Meckel’s cartilage enclosed
in bone disappears, while the cartilage proximal to the mandibular foramen be-
comes in order, the spheno-mandibular ligament, the malleus, and the incus (p.
389 and Fig. 387).
Each second branchial arch comes into relation proximally with the periotic
capsule. This upper segment of the cartilage becomes the stapes and the styloid
process of the temporal bone (Figs. 321 and 387). The succeeding distal portion
is transformed into the stylo-hyoid ligament and connects the styloid process with
the distal end of the arch, which also undergoes intracartilaginous ossification to
form the lesser horn of the hyoid bone.
The cartilage of the third branchial arches ossifies and gives origin to the
greater horns of the hyoid bone, while a plate connecting the two arches becomes
its body.
The fourth and fifth branchial arches co-operate in the formation of the
thyreoid cartilage of the larynx.
APPENDICULAR SKELETON
Whereas the axial skeleton originates chiefly from the sclerotomes of the
mesodermal segments, the appendicular skeleton is apparently derived from the
unsegmented somatic mesenchyme. In embryos of 9 mm. mesenchymal conden-
sations have formed definite blastemal cores in the primitive limb buds (Fig. 323).
Following this blastemal stage the various bones next pass through a cartilagi-
nous stage and finally an osseous one.
Upper Extremity.—The clavicle is the first bone of the skeleton to ossify,
centers appearing at each end. Prior to ossification it is compased of a peculiar
tissue which makes it difficult to decide whether the bone is intramembranous
or intracartilaginous in origin.
316 THE MORPHOGENESIS OF THE SKELETON AND MUSCLES
The scapula arises as a single plate in which there are two chief centers of
ossification. One center early forms the body and spine. The other, after birth,
gives rise to the rudimentary coracoid process, which in lower vertebrates extends
from the scapula to the sternum. Union between the coracoid process and the
body does not occur until about the fifteenth year.
The humerus, radius, and ulna ossify from single primary centers and two
or more epiphyseal centers (Fig. 296 C-F).
In the cartilaginous carpus there is a proximal row of three, and a distal row
of four elements. Other inconstant cartilage may appear and subsequently dis-
appear or become incorporated in other carpal bones. The pisiform is regarded as
a sesamoid bone which develops in the tendon of the flexor carpi ulnaris; in the
same category is the patella which forms in the tendon of the quadriceps extensor
cruris.
Lower Extremity.—The cartilaginous plate of the os coxe is at first so placed
that its long axis is perpendicular to the vertebral column (Fig. 322). Later it
rotates to a position parallel with the vertebral column and shifts slightly caudad
to come into relation with the first three sacral vertebra. A retention of the
membranous condition in the lower half of each primitive cartilaginous plate
accounts for the obturator membrane which closes the foramen of the same name.
Three centers of ossification appear, forming the ilium, ischium, and pubis. The
three bones do not fuse completely until about puberty.
The general development of the femur, tibia, fibula, tarsus, metatarsus, and
phalanges is quite similar to that of the corresponding bones of the upper extremity.
Anomalies.—Variations in the number of vertebra (except cervical) are not infre-
quent. The last cervical and first lumbar vertebre occasionally bear ribs, due to the con-
tinued development of the primitive costal processes. Cleft sternum or cleft xiphoid process
represents an incomplete fusion of the sternal bars. Additional fingers or toes (polydactyly)
may occur; the cause is obscure. Hare lip and cleft palate have already been mentioned
(pp. 146, 149).
II. THE MuscULAR SYSTEM
The skeletal muscles, with the exception of those attached to the branchial
arches, originate from the myotomes of the mesodermal segments (pp. 51, 292
and Fig. 323). Although the primitive segmental arrangement of the myotomes
is, for the most part, soon lost, their original innervation by the segmental spinal
nerves is retained throughout life. For this reason the history of adult muscles
formed by fusion, splitting, or other modifications may be traced with consider-
able certainty.
THE MUSCULAR SYSTEM 317
The development of the human musculature is fully described by W. H. Lewis
in Keibel and Mall, vol. 1.
Fundamental Processes.—The changes occurring in the myotomes during
the formation of adult muscles are referable to the operation of the following fun-
damental processes:
(1) A change in direction of the muscle fibers from their original cranio-caudal
orientation in the myotome. The fibers of but few muscles retain this initial
orientation.
(2) A migration of myotomes, wholly or in part, to more or less remote regions.
Thus the /atissimus dorsi originates from cervical myotomes, but finally attaches
to the lower thoracic and lumbar vertebre and to the crest of the ilium. Other
examples are the serratus anterior and the trapezius.
(3) A fusion of portions of successive myotomes. The rectus abdominis illus-
trates this process.
(4) A longitudinal splitting of myotomes into several portions. Examples are
found in the sterno- and omo-hyoid and in the trapezius and sterno-mastoid.
(5) A tangential splitting into two or more layers. The oblique and the trans-
verse muscles of the abdomen are formed by this common process.
(6) A degeneration of myotomes, wholly or in part. In this way fascias, liga-
ments, and aponeuroses may be produced.
Muscles of the Trunk.—Ventral extensions grow out from the cervical and
thoracic myotomes and a fusion that is well advanced superficially occurs between
all the myotomes in embryos of 10 mm. A dorsal, longitudinal column of fused
myotomes, however, can still be distinguished from the sheet formed from the
combined ventral prolongations (Fig. 322).
From the superficial portions of the dorsal column there arise by longitudinal
and tangential splitting the various long muscles of the back which are innervated
by the dorsal rami of the spinal nerves. The deep portions of the myotomes do
not fuse, but give rise to the several intervertebral muscles, which thus retain their
primitive segmental arrangement.
The muscles of the neck, other than those innervated by the dorsal rami and
those arising from the branchial arches, differentiate from ventral extensions of
the cervical myotomes. Reference has already been made to the probable con-
tribution from cervical myotomes to the formation of the diaphragm (p. 188).
In the same way the thoraco-abdominal muscles arise from the more pronounced
ventral prolongations of the thoracic myotomes which grow into the body wall
along with the ribs (Fig. 322).
318 THE MORPHOGENESIS OF THE SKELETON AND MUSCLES
The ventral extensions of the lumbar myotomes (except the first) and of the
first two sacral myotomes do not participate in the formation of the body wall.
If they persist at all, it is possible that they contribute to the formation of the
lower limb. The ventral portions of the third and fourth sacral myotomes give
rise to the muscles of the perineal region.
Fic. 322.—Reconstruction of a 9 mm. human embryo to show the partially fused myotomes and
the premuscle masses of the limbs (Bardeen and Lewis). X 13. Distally, in the upper extremity, the
radius, ulna and hand plate are disclosed; in the lower extremity the os coxe anlage and the border vein
show.
Muscles of the Limbs.—It has generally been believed that the muscles of
the extremities are developed from buds of the myotomes which grow into the
anlages of the limbs. In sharks this is clearly the case, and in man the segmental
nerve supply is suggestive, but not proof, of a myotomic origin. According to
THE MUSCULAR SYSTEM 319
Lewis, “there are no observations of distinct myotome buds extending into the
limbs.” A diffuse migration of cells from the ventral portion of the myotomes
has been recorded by various observers, recently by Ingalls. These cells soon
lose their epithelial character and blend with the undifferentiated mesenchyma of
the limb buds (Fig. 323). From this diffuse tissue, which at about 10 mm. forms
premuscle masses, the limb muscles are differentiated, the proximal muscles being
the first to appear (Fig. 322).
Spinal ganglion
Dermatome
Ventral rovt
Myotome
Spinal nerve
Arm bud
Proliferating cells of
myotome
oF
Mesonephric duct
Mesonephric tubule and
glomerulus
Celom
Somatic mesoderm
Frs. 323.—Transverse section of a 10.3 mm. monkey embryo showing the myotome and the mesenchyma
of the arm bud (Kollmann). A, aorta; *, sclerotome.
Muscles of the Head.—Distinct mesodermal segments do not occur in the
head region. It is possible, however, that a premuscle mass, from which the eye
muscles of man are developed, is comparable to three myotomic segments having
a similar fate in the shark (cf. p. 366).
The remaining muscles of the head differ from all other skeletal muscles in
that they arise from the splanchnic mesoderm of the branchial arches and are
innervated by nerves of a different category than those which supply myotomic
muscles. The mesoderm of the first branchial arch gives rise to the muscles of
mastication and to all other muscles innervated by the trigeminal nerve. Simi-
320 THE MORPHOGENESIS OF THE SKELETON AND MUSCLES
larly the muscles of expression, and other muscles supplied by the facial nerve,
originate from the second, or hyoid arch. The third arch probably gives origin to
the pharyngeal muscles, and the third and fourth arches to the intrinsic muscles of
the larynx.
The muscles of the tongue are supplied by the n. hypoglossus, and therefore it
has been assumed that they are derived from myotomes of the occipital region.
According to Lewis, “there is no evidence whatever for this statement, and we are
inclined to believe from our studies that the tongue musculature is derived from
the mesoderm of the floor of the mouth.”
Anomalies.—Variations in the form, position, and attachments of the muscles are
common. Most of these anomalies are referable to the variable action of the several develop-
mental factors listed on p. 317.
CHAPTER XII
THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
In discussing the histogenesis of the nervous tissue the early development of
the neural tube has been described as an infolding of the neural plate (Fig. 78)
and a closure of the neural groove (Fig. 304). The groove begins to close along
the mid-dorsal line, near the middle of the body, in embryos of 2 mm. and the
Mesencephalon
Rhombencephalon
Myelencephalon
Amnion (cut)
Mesodermal segment I4
Neural tube (not closed)
“Ho Body stalk
Fic. 324.—Human embryo of 2.4 mm. showing a partially closed neural tube and the brain vesicles (after
Kollmann). X 36.
closure extends both cranially and caudally (Fig. 324). Until the end of the third
week there still persists an opening at either end of the neural tube, somewhat dor-
sad. These openings are the neuropores (Fig. 330). Before the closure of the
neuropores, in embryos of 2 to 2.5 mm. the cranial end of the neural tube has en-
2z 321
322 THE MORPHOGENESIS OF THE CENTRAI NERVOUS SYSTEM
larged and is constricted at two points to form the three primary brain vesicles.
The caudal two-thirds of the neural tube, which remains smaller in diameter, con-
stitutes the anlage of the spinal cord.
THE SPINAL CORD
The spinal portion of the neural tube is at first nearly straight, but is bent
with the flexure of the embryo into a curve which is convex dorsally. Its wall
gradually thickens during the first month and the diameter of its cavity is di-
Neural cavity
Dorsal root
Ependymal layer
Spinal ganglion
Manile layer
Dorsal ramus
Ventral
root
?
Roe! Syoss
Nerve trunk Sympathetic ganglion
Fic. 325.—Transverse section through a 10 mm. human embryo at the level of the arm buds showing the
spinal cord and a spinal nerve of the right side. X 44.
minished from side to side. By the end of the first month three layers have been
developed in its wall as described in Chapter X, p. 301 (Fig. 325). These layers
are the inner ependymal layer, which forms a narrow zone about the neural cavity,
the middle mantle layer, cellular, and the outer marginal laver, fibrous.
The Ependymal Layer is differentiated into a dorsal roof plate and a ventral
floor plate (Fig. 326). Laterally, its proliferating cells contribute neuroblasts and
neuroglia cells to the mantle layer. The proliferation of cells ceases first in the
THE SPINAL CORD 323
ventral portion of the layer, which is thus narrower than the dorsal portion in
10 to 20 mm. embryos (Figs. 325 and 326). Consequently, the ventral portion
Roof plate Dorsal funiculus
Dorsal column
Dorsal root Neural cavity
Mantle layer +
Ventral column Marginal layer
Ependymal layer
Floor plate Ventral median fissure
Fig. 326.—Transverse section of the spinal cord from a 20 mm. human embryo. X 44.
of the mantle layer is differentiated first. The neural cavity is at first somewhat
rhomboidal in transverse section, wider dorsally than ventrally. Its lateral angle
Dorsal funiculus
Dorsal column
Dorsal median Dorsal root
septum
Lat. funiculus
Dura mater
Central canal
Ventral
column
Spinal
ganglion
Ventral funiculus Ventral median fissure
Fic. 327.—Transverse section of the spinal cord from a 34 mm. human embryo, showing also the spinal
ganglion and dura mater on the left side. X 44.
forms the sulcus limitans (Fig. 334) which marks the subdivision of the lateral
walls of the neural tube into the dorsal alar plate (sensory) and ventral basal plate
324 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
(motor). When the ependymal layer ceases to contribute new cells to the mantle
layer its walls are approximated dorsally. As a result, in 20 mm. embryos the
neural cavity is wider ventrally (Fig. 326). In the next stage, 34 mm., these walls
fuse and the dorsal portion of the neural cavity is obliterated (Fig. 327). Ina
65 mm. (C R) fetus the persisting cavity is becoming rounded (Fig. 328). It forms
the central canal of the adult spinal cord. The cells lining the central canal are
ependymal cells proper. ‘Those in the floor of the canal form the persistent floor
plate. Their fibers extend ventrad, reaching the surface of the cord in the depres-
sion of the ventral median fissure.
When the right and left walls of the ependymal layer fuse, the ependymal
cells of the roof plate no longer radiate, but form a median septum (Fig. 327).
Dorsal median septum Fasciculus gracilis
Fasciculus cuneatus
Dorsal root
Dorsal column Substantia gelatinosa
Lat. funiculus
Central canal
Lat. column
\ a
Ken
Ventral funiculus Ventral median fissure
Fic. 328.—Transverse section of the spinal cord from a 65 mm. human fetus. X 44.
Later, as the marginal layers of either gide thicken and are approximated, the
median septum is extended dorsally. Thus the roof plate is converted into part
of the dorsal median septum of the adult spinal cord (Fig. 328).
The Mantle Layer, as we have seen, is contributed to by the proliferating
cells of the ependymal layer. A ventro-lateral thickening first becomes promi-
nent in embryos of 10 to 15 mm. (Fig. 325). This is the ventral (anterior) gray
column, or horn, which in later stages is subdivided, forming also a lateral gray
column (Fig. 328). It is a derivative of the basal plate. In embryos of 20 mm.
a dorso-lateral thickening of the mantle layer is seen, the cells of which constitute
the dorsal (posterior) gray column, or horn (Figs. 327 and 328); about these cells
the collaterals of the dorsal root fibers end. The cells of the dorsal gray column
THE SPINAL CORD 325
thus form terminal nuclei for the afferent spinal nerve fibers and they are deriv-
atives of the alar plate of the cord. Dorsal and ventral to the central canal the
mantle layer forms the dorsal and ventral gray commissures. In the ventral floor
plate nerve fibers cross from both sides of the cord and form the ventral (anterior)
white commissure.
The Marginal Layer is composed primarily of a framework of neuroglia and
ependymal cell processes. Into this framework grow the axis cylinder processes
of nerve cells, so that the thickening of this layer is due to the increasing number
of nerve fibers contributed to it by extrinsic ganglion cells and neuroblasts.
When their myelin develops, these fibers form the white substance of the spinal
cord. The fibers have three sources (Fig. 360): (1) they may arise from the
spinal ganglion cells, entering as dorsal root fibers and coursing cranially and
caudally in the marginal layer; (2) they may arise from neuroblasts in the mantle
layer of the spinal cord (a) as fibers which connect adjacent nuclei of the cord
(fasciculi proprii or ground bundles), (0) as fibers which extend cranially to the
brain; (3) they may arise from neuroblasts of the brain (a) as descending tracts
from the brain stem, (0) as long descending cerebrospinal tracts from the cortex
of the cerebrum.
Of these fiber tracts (1) and (2 a) appear during the first month; (2 6) and
(3 a) during the third month; (3 5) at the end of the fifth month.
The dorsal root fibers from the spinal ganglion cells, entering the cord dorso-
laterally, subdivide the white substance of the marginal layer into a dorsal funic-
ulus and lateral funiculus. The lateral funiculus is marked off by the ventral
root fibers from the ventral funiculus (Fig. 327). The ventral root fibers, as we
have seen, take their origin from the neuroblasts of the ventral gray column in the
mantle layer. They are thus derivatives of the basal plate.
The dorsal funiculus is formed chiefly by the dorsal root fibers of the ganglion
cells and is subdivided into two distinct bundles, the fasciculus gracilis, median,
and the fasciculus cuneatus, lateralin position. The dorsal funiculi are separated
only by the dorsal median septum (Fig. 328).
The lateral and ventral funiculi are composed of fasciculi propria or ground
bundles, originating in the spinal cord, of ascending tracts from the cord to the
brain, and of the descending fiber tracts from the brain. The fibers of these
fasciculi intermingle and the fasciculi are thus without sharp boundaries. The
floor plate of ependymal cells lags behind in its development, and, as it is inter-
posed between the thickening right and left walls of the ventral funiculi, these do
not meet and the ventral median fissure is produced (cf. Figs. 325 and 328).
326 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
The development of myelin in the nerve fibers of the cord begins late in the fourth
month of fetal life and is completed between the fifteenth and twentieth years (Flechsig,
Bechterew). Myelin appears first in the root fibers of the spinal nerves and in those of the
ventral commissure, next in the ground bundles and dorsal funiculi. The cerebrospinal
(pyramidal) fasciculi are the last in which myelin is developed; they are myelinated during
the first and second years. As myelin appears in the various fiber tracts at different periods,
this condition has been utilized in tracing the extent and origin of the various fasciculi in
the central nervous system.
The Cervical and Lumbar Enlargements.—At the levels of the two nerve
plexuses, supplying the upper and lower extremities, the spinal cord enlarges.
As the fibers to the muscles of the extremities arise
from nerve cells in the ventral gray column, the num-
ber of these cells and the mass of the gray substance
is increased; since larger numbers of fibers from the
Cerebrum ..
Mesencephalon..
Cerebellum-
Cervical
< integument of the limbs also enter the cord at this
enlargement f
level, there are likewise present more cells about
which sensory fibers terminate. There is formed con-
sequently at the level of the origin of the nerves of
Lumbar
enlargement the brachial plexus the cervical enlargement, opposite
the origins of the nerves of the lumbo-sacral plexus
the lumbar enlargement (Fig. 329).
At the caudal end of the neural tube in a 110 mm.
f (C R) fetus an epithelial sac is formed which is ad-
Fic. 320.—Dissection of herent to the integument. Cranial to the sac the
the brain and cord of a three central canal is obliterated, this part of the neural
months’ fetus, showing the cer- : ;
vical and lumbar enlargements tube forming the filwm terminale. The caudal end
(after Koélliker in Marshall).
Manat aoe of the central canal is irregularly expanded and is
known as the terminal ventricle.
After the third month the vertebral column grows faster than the spinal cord.
As the cord is fixed to the brain the vertebra and the associated roots and ganglia
of the spinal nerves shift caudally along the cord. In the adult the origin of the
coccygeal nerves is oppogite the first lumbar vertebra and the nerves course ob-
liquely downward nearly parallel to the spinal cord. As the tip of the neural
tube is attached to the coccyx, its caudal portion becomes stretched into the
slender, solid cord known as the filum terminale. The obliquely coursing spinal
nerves, with the filum terminale, constitute the cauda equina.
THE BRAIN 327
THE BRAIN
We have seen that in embryos of 2 to 2.5 mm. the neural tube is nearly
straight, but that its cranial end is enlarged to form the anlage of the brain (Fig.
324). The appearance of two constrictions in the wall of the anlage subdivides
it into the three primary brain vesicles—the fore-brain or prosencephalon, mid-brain
or mesencephalon, and hind-brain or rhombencephalon.
In embryos of 3.2 mm., estimated age four weeks, three important changes
have taken place (Fig. 330): (1) the neural tube is bent sharply in the mid-brain
region (the cephalic flexure) so that the axis of the fore-brain now forms a right
: Cor pus striatum
Anterior : Pallium
neuropore Pallium of telencephalon
Diencephalon — Anterior
neuro pore
ATesence phalon
~ My SY
oN :
“Cephalic flexure
Mesencephalon Optic
recess
Isthmus Future pontine
flexure
Optic vesicle
Rhombencephalon
Rhombencephalon
Wu
Future pontine_|{
flexure
ia
|
Kataarin®
Fic. 330.—Reconstructions of the brain of a 3.2 mm. human embryo (after His). X about 35. A, Lateral
surface; B, sectioned in the median sagittal plane.
angle with the axis of the hind-brain; (2) the fore-brain shows indication dorsally
of a fold, the margo thalamicus, which subdivides it into the telencephalon and the
diencephalon; (3) the lateral walls of the fore-brain show distinct evaginations, the
optic vesicles, which project laterad and caudad. A ventral bulging of the wall
of the hind-brain indicates the position of the future pontine flexure.
In embryos of 7 mm. (five weeks) the neuropores have closed (Fig. 331).
The cephalic flexure, now more marked, forms an acute angle, and the pontine
flexure, just indicated in the previous stage, is now a prominent ventral bend in
the ventro-lateral walls of the hind-brain. This flexure forms the boundary
line which subdivides the rhombencephalon into a cranial portion, the meten-
328 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
cephalon, and into a caudal portion, the myelencephalon. At a third bend the
whole brain is flexed ventrally at an angle with the axis of the spinal cord. This
bend is the cervical flexure and is the line of demarcation between the brain and
spinal cord (cf. Fig. 333 A). The telencephalon and diencephalon are more dis-
tinctly subdivided, and the evaginated optic vesicle forms the optic cup attached
to the brain wall by a hollow stalk, in which later grows the optic nerve. The
walls of the brain show a distinct differentiation in certain regions. This is
especially marked in the myelencephalon, which has a thicker ventro-lateral wall
and thinner dorsal wall.
Embryos of 10.2 mm. show the structure of the brain at the beginning of the
second month (Figs. 341 and 344). The five brain regions are now sharply dif-
A B
Diencephalon Mesencephalon Thalamus
Pallium Mesencephalon
Cephalic ,
flexure
AMeten-
cephalon
Corpus striatum
Optic recess
Hy poolamus’’
Medulla oblongata
Tharine Hil
Fic. 331.—Reconstructions of the brain of a 7 mm. human embryo (His). A, Lateral view; B, in median
sagittal section.
ferentiated externally, but the boundary line between the telencephalon and dien-
cephalon is still indistinct. The telencephalon consists of paired, lateral out-
growths, the anlages of the cerebral hemispheres and rhinencephalon (olfactory
brain). In Fig. 359 the external form of the brain is seen with the origins of the
cerebral nerves. It will be noted that, with the exception of the first four (the
olfactory, optic, oculomotor, and trochlear), the cerebral nerves take their super-
ficial origin from the myelencephalon.
The cephalic flexure forms a very acute angle, and, as a result, the long axis
of the fore-brain is nearly parallel to that of the hind-brain (Fig. 359). The ocu-
lomotor nerve takes its origin from the ventral wall of the mesencephalon. Dor-
sally there is a constriction, the isthmus, between the mesencephalon and meten-
THE BRAIN 329
cephalon, and here the fibers of the trochlear nerve take their superficial origin.
The dorsal wall of the myelencephalon is an exceedingly thin ependymal layer
which becomes the tela chorioidea. The ventro-lateral walls of this same region,
on the other hand, are very thick.
A median sagittal section of a brain at a somewhat later stage shows the
cervical, pontine, and cephalic flexures well marked (Fig. 332). The thin dorso-
lateral roof of the myelencephalon has been removed. The telencephalon is a
paired structure. In the figure its right half projects cranial to the primitive
median wall of the fore-brain which persists as the lamina terminalis (cf. Fig. 342).
The floor of the telencephalon is greatly thickened caudally as the anlage of the
o
Cerebral aqueduct
Cerebral peduncle, ., Mesencephalon
_ ae
Hypothalamus
Epithalamus , +4
ss. Ahombence phalic isthmus
' Cerebellum
- Metence phalon
Ying é ine —~y,
i234
| Corpus striatum Pons a
Lamina terminalis yu
; : ve F
SY Spinal cord
Rhinence phal on
Fic. 332.—Brain of a 13.6 mm. human embryo in median sagittal section (after His in Sobotta).
1, Optic recess; 2, ridge formed by optic chiasma, 3; 4, infundibular recess.
corpus striatum. A slight evagination of the ventral wall of the telencephalon
just cranial to the corpus striatum marks the anlage of the rhinencephalon. The
remaining portion of the telencephalon forms the pallium or cortex of the cerebral
hemispheres. The paired cavities of the telencephalon are the Jateral (first and
second) ventricles, and these communicate through the interventricular foramina
(Monroi) with the cavity of the diencephalon, the third ventricle. The cavities
of the olfactory lobes communicate during fetal life with the lateral ventricles
and were formerly called the first ventricles.
The crossing of a portion of the optic nerve fibers in the floor of the brain
forms the optic chiasma, and this, with the transverse ridge produced by it inter-
nally, is taken as the ventral boundary line between the telencephalon and dien-
330 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
cephalon (Fig. 332). A dorsal depression separates the latter from the mesenceph-
alon. The lateral wall of the diencephalon is thickened to form the thalamus,
the caudal and lateral portion of which constitutes the metathalamus. From the
metathalamus are derived the geniculate bodies. In the median dorsal wall, near
the caudal boundary line of the diencephalon, an outpocketing begins to appear
in embryos of five weeks (Fig. 332). This is the epithalamus, which later gives rise
to the pineal body, or epiphysis.
The thalamus is marked off from the more ventral portion of the diencephalic
wall, termed the hypothalamus, by the obliquely directed sulcus hypothalamicus
(Fig. 341). Cranial to the optic chiasma is the optic recess, regarded as belonging
to the telencephalon (Fig. 332). Caudal to it is the pouch-like infundibulum, an
extension from which during the fourth week forms the posterior lobe of the
hypophysis. Caudal to the infundibulum the floor of the diencephalon forms the
tuber cinereum and the mammillary recess; the walls of the latter thicken later and
give rise to the mammillary bodies. An oblique transverse section through the
telencephalon and hypothalamic portion of the diencephalon (Fig. 343) shows the
relation of the optic recess to the optic stalk, the infundibulum, and Rathke’s
pocket, and the extension of the third ventricle, the proper cavity of the dien-
cephalon, into the telencephalon between the corpora striata.
The mesencephalon in 13.6 mm. embryos (Fig. 332) is distinctly marked off
from the metencephalon by the constriction which is termed the isthmus. Dorso-
lateral thickenings form the corpora quadrigemina. Ventrally, the mesencephalic
wall is thickened to form the tegmentum and crura cerebri. In the tegmentum are
located the nuclei of origin for the oculomotor and trochlear nerves. The former,
as we have seen, takes its superficial origin ventrally, while the trochlear nerve
fibers bend dorsad, cross at the isthmus, and emerge on the opposite side. As
the walls of the mesencephalon thicken, its cavity later is narrowed to a canal,
the cerebral aqueduct (of Sylvius).
The walls of the metencephalon are thickened dorsally and laterally to form
the anlage of the cerebellum. Its thickened ventral wall becomes the pois
(Varolii). Its cavity constitutes the cranial portion of the fourth ventricle.
The caudal border of the pons is taken as the ventral boundary line between
the metencephalon and myelencephalon. The myelencephalon forms the medulla
oblongata. Its dorsal wall is a thin, non-nervous ependymal layer, which later
becomes the posterior medullary velum. From its thickened ventro-lateral
walls the last eight cerebral nerves take their origin. Its cavity forms the greater
part of the fourth ventricle which opens caudally into the central canal of the
THE BRAIN ’ 331
spinal cord, cranially into the cerebral aqueduct. The increase in the flexures of
the brain and the relative growth of its different regions may be seen by comparing
the brains of embryos of four, five, and seven weeks (Fig. 333).
A
Mesencephalon I sthmus
Diencephaton Metencephalon
Telencephalon. —Olic vesicle
~-M yelencephalon
Cervical flexure
A yelence phalon
Metencephalon
\- M yelencephalon
Fic. 333.—Brains of human embryos, from reconstructions by His: A, 4.2 mm. embryo (X 20);
B, 6.9 mm. embryo (X 16); C, 18.5 mm. embryo (X 4). vu, Optic vesicle; im, infundibulum; m, mam-
millary body; f, pontine flexure; ol, olfactory lobe; 6, basilar artery; », Rathke’s pouch (American
Text-Book of Obstetrics).
In the table on page 332 are given the primitive subdivisions of the neural
tube and the parts derived from them:
332 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
THE DERIVATIVES OF THE NEURAL TUBE
Primary VESICLES. SUBDIVISIONS. DERIVATIVES. CAVITIES.
Telencephalon Cerebral cortex Lateral ventricles
Corpora striata Cranial portion of
Rhinencephalon third ventricles
Diencephalon Epithalamus Third ventricle
(Pineal body)
Thalamus
Prosencephalon Optic tract
Hypothalamus
Hypophysis
Tuber cinereum
Mammillary bodies
Mesencephalon Mesencephalon Corpora quadrigemina | Aqueductus cerebri
Tegmentum .
Crura cerebri
Rhombencephalon Metencephalon Cerebellum Fourth ventricle
Pons
Myelencephalon Medulla oblongata
Spinal cord Spinal cord Central canal.
7
THE LATER DIFFERENTIATION OF THE SUBDIVISIONS OF THE BRAIN
Myelencephalon.—We have seen that the wall of the spinal cord differen-
tiates dorsally and ventrally into roof plate and floor plate, laterally into the alar
A B
- Roof plate Rae,
Mantle layer.
ye
Sulcus limitans
Ependymal
la yer
Marginal Basal plate
laycr
Spinal
ganglion
x.
Y Ventral spinal root
Fic. 334.—Transverse sections. X 44. A, Through the upper cervical region of the spinal cord in a
10 mm. human embryo; B, through the caudal end of the myelencephalon.
plate and basal plate. The boundary line between the alar and basal plates is the
sulcus limitans (Fig. 334 A). The same subdivisions may be recognized in the
myelencephalon. It differs from the spinal cord, however, in that the roof plate
THE BRAIN 333
is broad, thin, and flattened to form the ependymal layer (Figs. 334 B and 335).
In the alar and basal plates of the myelencephalon the marginal, mantle, and
Alar plate
Sulcus
limitans
Basal
plate
MN Ganglion
W jugulare
a N. hypoglossus
NV. accessorius
Fic. 335.—Transverse sections through the myelencephalon of a 10.2 mm. embryo (His). X 37.
A, Through the nuclei of origin of the spinal accessory and hypoglossal nerves; B, through the vagus
and hypoglossal nerves.
ependymal zones are differentiated as in the spinal cord (Fig. 335). Owing to the
formation of the pontine flexure at the beginning of the second month, the roof
Inner layer Roof plate Tractus solitarius
y. | Spinal tract of
NV. trigeminus
“6 Neuroblasts from
alar plate
Marginal layer
N. hypoglossus Septum medulle Neuroblasts from alar plate
(Rudiment of accessory olive)
Fic. 336.—Transverse section through the myelencephalon of a 22 mm. embryo (His). X 10.
plate is broadened} especially in the cranial portion of the myelencephalon, and
the alar plates bulge laterally (Figs. 336 and 337 A). The cavity of the myelen-
334 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
cephalon is thus widened from side to side and flattened dorso-ventrally. This
is most marked cranially where, between the alar plates of the myelencephalon
and metencephalon, are formed the lateral recesses of the fourth ventricle (Figs.
337 and 353). Into the ependymal roof of the myelencephalon blood vessels
grow, and, invading the lateral recesses, form there the chorioid plexus of the
fourth ventricle. The plexus consists of small, finger-like folds of the ependymal
layer and its covering mesenchymal layer. The line of attachment of the epen-
dymal layer to the alar plate is known as the rhombic lip and later becomes the
tenia and obex of the fourth ventricle (Fig. 337 B).
Cc
Mid-brain Lateral lobe of cerebellum Lobules of vermis
\ Medulla Flocculus
Rhombic lip oblongata
B Lateral lobe of Pyramis
cerebellum _
Cor pora quadiizemnnia.
Cerebrum f
Anlage of
vermis
f
Lateral lobe of *
cerebellum
Rhombic lip Obex Flocculus Z
: Nodulus
Fic. 337.—Dorsal views of four stages in the development of the cerebellum. 4, of a 13.6 mm. em-
bryo (His); B, of a 24 mm. embryo; C, of a 110 mm. fetus; D, of a 150 mm. fetus.
In early stages the floor of the myelencephalon is constricted transversely by
the so-called rhombic grooves, six in number; the intervals between successive
grooves are neuromeres (cf. Figs. 96 and 122). Some have viewed these as evi-
dential of a former segmentation of the head similar to that of the trunk. It is
more probable, however, that they merely stand in relation to certain cerebral
nerves and hence their segmental arrangement is secondary.
The further growth of the myelencephalon is due: (1) to the rapid formation
of neuroblasts, derived from the ependymal and mantle layers; (2) to the devel-
opment of nerve fibers from these neuroblasts; (3) to the development and
THE BRAIN 335
growth into it of fibers from neuroblasts in the spinal cord and in other parts of
the brain.
The neuroblasts of the basal plates early give rise chiefly to the efferent fibers
of the cerebral nerves (Fig. 335). They thus constitute motor nuclei of origin
of the trigeminal, abducens, facial, glossopharyngeal, vagus complex, and hypo-
glossal nerves, nuclei corresponding to the ventral and lateral gray columns of the
spinal cord. The basal plate likewise produces part of the reticular formation
which is derived in part also from the neuroblasts of the alar plate (Fig. 336).
The axons partly cross as external and internal arcuate fibers and form a portion of
the median longitudinal bundle, a fasciculus corresponding to the ventral ground
bundles of the spinal cord. Other axons grow into the marginal zone of the same
side and form intersegmental fiber tracts. The reticular formation is thus differ-
entiated into a gray portion, situated in the mantle zone, and into a white portion,
located in the marginal zone (Fig. 336). The marginal zone is further added to
by the ascending fiber tracts from the spinal cord and the descending pyramidal
tracts from the brain. As in the cord, the marginal layers of each side remain
distinct, being separated by the cells of the floor plate.
The alar plates differentiate later than the basal plates. The afferent fibers
of the cerebral nerves first enter the mantle layer of the alar plates, and, coursing
upward and downward, form definite tracts (fractus solitarius, descending tract
of fifth nerve). To these are added tracts from the spinal cord so that an inner
gray and an outer white substance is formed. Soon, however, the cells of the
mantle layer proliferate, migrate into the marginal zone, and surround the tracts.
These neuroblasts of the alar plate form groups of cells along the terminal tracts of
the afferent cerebral nerves (which correspond to the dorsal root fibers of the
spinal nerves) and constitute the receptive or terminal nuclei of the fifth, seventh,
eighth, ninth, and tenth cerebral nerves. Caudally, the nucleus gracilis and
nucleus cuneatus are developed from the alar plates as the terminal nuclei for the
afferent fibers which ascend from the dorsal funiculi of the spinal cord. The
axons of the neuroblasts forming these receptive nuclei decussate through the
reticulor formation chiefly as internal arcuate fibers and ascend to the thalamus as
the median lemniscus.
There are developed from neuroblasts of the alar plate other nuclei, the
axons of which connect the brain stem, cerebellum, and fore-brain. Of these the
most conspicuous is the inferior olivary nucleus.
The characteristic form of the adult myelencephalon is determined by the
further growth of the above-mentioned structures. The nuclei of origin of the
336 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
cerebral nerves, derived from the basal plate, produce swellings in the floor of the
fourth ventricle which are bounded laterally by the sulcus limitans. The terminal
nuclei of the mixed and sensory cerebral nerves lie lateral to this sulcus. The
enlarged cuneate and gracile nuclei bound the ventricle caudally and laterally
as the cuneus and clava. The inferior olivary nuclei produce lateral rounded
prominences and ventral to these are the large cerebrospinal tracts or pyramids.
The Metencephalon.—Cranial to the lateral recesses of the fourth ventricle
the cells of the alar plate proliferate ventrally and form the numerous rela-
tively large.nuclei of the pons. The axons from the cells of these nuclei mostly
cross to the opposite side and form the brachium pontis of the cerebellum. Cere-
bral fibers from the cerebral peduncles end about the cells of the pontine nuclei.
Others pass through the pons as fascicles of the pyramidal tracts.
B
A
Mesencephalon
Mesencephalon
Cerebellum t=
Posterior medullary velum Aaa edblaks) velum
Fic. 338.—Median sagittal section of the cerebellum and part of mid-brain. A, from a 24 mm.
embryo; B, from a 150 mm. fetus.
Cerebellum.—When the alar plates of the cranial end of the myelencephalon
are bent out laterally the caudal portions of their continuations into the meten-
cephalic region are carried laterally also. As a result, the alar plate of the meten-
cephalon takes up a transverse position and forms the anlages of the cerebellum
(Fig. 337 A). During the second month the paired cerebellar plates thicken and
bulge into the ventricle (Fig. 338 A). Near the mid-line paired thickenings indi-
cate the anlages of the vermis, while the remainder of the alar plates form the
anlages of the lateral lobes or cerebellar hemispheres (Figs. 337 B and 353).
The cerebellar anlages grow rapidly both laterally and in length, so that their
surfaces are folded transversely. During the third month their walls bulge out-
ward and form on either side a convex lateral lobe connected with the pons by the
brachium pontis (Fig. 337 C). In the meantime the anlages of the vermis have
THE BRAIN 337
fused in the mid-line, producing a single structure marked by transverse fissures.
The rhombic lip gives rise to the flocculus and nodulus. Between the third and
fifth months the cortex cerebelli grows more rapidly than the deeper layers of the
cerebellum and its principal lobes, folds and fissures are formed (Fig. 337 C, D).
The hemispheres derived from the lateral lobes are the last to be differentiated.
Their fissures do not appear until the fifth month.
Cranial to the cerebellum the wall of the neural tube remains thin dorsally
and constitutes the anterior medullary velum of the adult (Fig. 338 B). Caudally,
the ependymal roof of the fourth ventricle becomes the posterior medullary velum.
The points of attachment of the vela remain approximately fixed, while the cere-
bellar cortex grows enormously. As a result, the vela are folded in under the
expanding cerebellum (Fig. 338).
The anlages of the cerebellum show at first differentiation into the same three layers
which are typical for the neural tube. During the second and third months, cells from the
ependymal, and perhaps from the mantle layer of the rhombic lip migrate to the surface of
the cerebellar cortex and give rise to the molecular and granular layers which are character-
istic of the adult cerebellar cortex (Schafer). The later differentiation of the cortex is only
completed at, or after, birth. The cells of the granular layer become unipolar by a process of
unilateral growth. The Purkinje cells differentiate later. Their axons and those of enter-
ing afferent fibers form the deep medullary layer of the cerebelium.
The cells of the mantle layer may take little part in the development of the cerebellar
cortex, but give rise to neuroglia cells and fibers and to the internal nuclei. Of these the
dentate nucleus may be seen at the end of the third month; later, its cellular layer becomes
folded, producing its characteristic convolutions. The fibers arising from its cells form the
greater part of the brachium conjunctivum. (For a detailed account of the development of
the cerebellum see Streeter, in Keibel and Mall, vol. 2).
Mesencephalon.—-The basal and alar plates can be recognized in this sub-
division of the brain and each differentiates into the three primitive layers (Fig.
339). In the basal plate the neuroblasts give rise to the axons of motor nerves—
the oculomotor cranial in position, the trochlear caudal (Fig. 339 B). In ad-
dition to these nuclei of origin, the nucleus ruber (red nucleus) is developed in
the basal plates ventral and somewhat cranial to the nucleus of the oculomotor
nerve. The origin of the cells forming the red nucleus is not definitely known.
The alay plates form the paired superior and inferior colliculi which together
constitute the corpora quadrigemina (Figs. 337 B and 349). The plates thicken
and neuroblasts migrate to their surfaces, forming stratified ganglionic layers com-
parable to the cortical layers of the cerebellum and the cerebellar nuclei. With
the development of the superior and inferior colliculi the cavity of the mesen-
cephalic region decreases in size and becomes the cerebral aqueduct.
22
338 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
The mantle layer of the basal plate region is enclosed ventrally and laterally
by the fiber tracts which develop in the marginal zone. Ventro-laterally appear
the median and lateral lemnisci and ventrally develop later the descending tracts
from the cerebral cortex, which together constitute the peduncles of the cerebrum.
DIV.
-Marginal layer
_..Nucleus
N.III.
----Root fibers N. ITI.
1
AS a
i
Frc. 339.—Transverse sections through the mesencephalon of a 10.2 mm. embryo (His). A,
Through the isthmus and origin of the trochlear nerve; B, through the nucleus of origin of the oculomotor
nerve; D. IV, decussation of oculomotor nerve; M.l., mantle layer.
The Diencephalon.—In the wall of the diencephalon we may recognize
laterally the alar and basal plates, dorsally the roof plate, and ventrally the floor
plate (Fig. 340). The roof plate expands, folds as seen in the figure, and into the
folds extend blood capillaries. The roof plate thus forms the ependymal lining
Roof plate (with chorioid plexus) .
Alar plate or Thalamus
‘Sulcus limitans or S. hypothalamicus
Basal plate or Hypothalamus
Mammillary recess
Fic. 340.—Transverse section through the diencephalon of a 13.8 mm embryo (His). X 29.
of the éela chorioidea of the third ventricle. The vessels and ingrowing mesenchy-
mal tissue form the chorioid plexus. Cranially, the tela chorioidea roofs over the
median portion of the telencephalon and is folded laterally into the hemispheres
as the chorioid plexus of the lateral ventricles. Laterally, the roof plate is attached
to the alar plates and at their point of union are developed the ganglia habenule.
THE BRAIN 339
The pineal body, or epiphysis, is developed caudally as an evagination of the
roof plate. It appears at the fifth week (Fig. 335) and is well developed by the
third month (Fig. 342). Into the thickened wall of the anlage is incorporated
a certain amount of mesenchymal tissue and thus the pineal body proper is
formed.
The alar plate is greatly thickened and becomes the anlage of the thalamus
and metathalamus. ‘The latter, really a part of the thalamus, gives rise to the
lateral and median geniculate bodies.
The saulcus hypothalamicus (Fig. 341) forms the boundary line between the
thalamus (alar plate) and the hypothalamus (basal plate plus the floor plate).
Hypothalamus
Sulcus hypothalamicus
Pallium
Mammillary recess
Cor pus striatum Tnfundibulum
Optic ridge
Fic. 341.—Median sagittal section of the fore- and mid-brain regions of a brain from a 10.2 mm. embryo
(after His).
» This sulcus thus corresponds to the salcus limitans of the spinal cord and brain
stem. The basal plate is comparatively unimportant in the diencephalic region,
as no nuclei of origin for motor nerves are developed here. In the floor plate
the ridge formed by the optic chiasma constitutes the pars optica hypothalamica.
The Hypophysis —The infundibulum develops as a recess caudal to the
pars optica hypothalamica (Figs. 342 and 343). At its extremity is the sac-like
anlage of the posterior lobe of the hypophysts or pituitary body. During the fourth
week the infundibular anlage comes into contact with Rathke’s pouch, the epi-
thelial anlage of the anterior lobe of the hypophysis (Fig. 343). The epithelial
340 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
anlage is at first flattened and soon is detached from its epithelial stalk. Later,
it grows laterally and caudally about the anlage of the posterior lobe, and, during
Thalamus
i Pineal body (epithalamus)
; Cer ebral peduncle
' Cerebral aqueduct
" Mesence phalon
Dience phalon_,
Chorioid plexus
Cor pus striatum~
b,
| Isthmus
38— Cerebellum
'~ Metencephalon
~~ — Rhomboid fossa
~~ Myelencephalon
Telencephalon A
> Pons ~ ~~
é Optic Hypo- =
+ Johiasma physis Medulla ~
Lamina lena J Hypothalamus ones
Rhinence phalon
Lf. :
/]~~- Spinal cord
t
: f ~~ Central canal
Fic. 342.—Median sagittal section of the brain from a fetus of the third month (His in Sobotta).
Foramen Monroi
Third ventricle
Optic vesicle Es
Lens vesicle gD
Infundibulum Z
Rathke’s pocket—Gare
Fic. 343.—Oblique transverse section through the diencephalon and telencephalon of a 10 mm. embryo.
X 61.
the second month, its wall is differentiated into convoluted tubules which obliter-
ate its cavity. The tubules become closed glandular follicles surrounded by a rich
network of blood vessels and produce an important internal secretion. Coinci-
THE BRAIN 341
dent with the differentiation of the anterior lobe the infundibular anlage of the
posterior lobe loses its cavity, but the walls of the infundibulum persist as its
solid, permanent stalk. The lobe enlarges and its cells are differentiated into a
diffuse tissue resembling neuroglia. About the two lobes of the hypophysis the
surrounding mesenchyme develops a connective tissue capsule.
Caudal to the infundibulum in the floor plate are developed in order the tuber
cinereum and the mammillary recess (Figs. 341, 344 and 346). The lateral walls
of the latter thicken and give rise to the paired mammillary bodies.
The third ventricle lies largely in the diencephalon and is at first relatively
broad. Owing to the thickening of its lateral walls it is compressed until it forms
Mesencephalon Diencephalon Pallium
: |
Mammillary body ‘i | #
ot \
Hy pophysis ne - \
Optic stalk Lobus olfactorius
Fic. 344.—Lateral view of the fore- and mid-brains of a 10.2 mm. embryo (His).
a narrow, vertical cleft. In a majority of adults the thalami are approximated,
fuse, and form the massa intermedia or commissura mollis, which is encircled by
the cavity of the ventricle.
The Telencephalon.—This is the most highly differentiated division of the
brain (Fig. 344). The primitive structures of the neural tube can no longer be
recognized, but the telencephalon is regarded as representing greatly expanded
alar plates and is, therefore, essentially a paired structure. Each of the paired out-
growths expands cranially, dorsally, and caudally, and eventually overlies the
rest of the brain (Figs. 344, 345 and 346). The telencephalon is differentiated
into the corpus striatum, rhinencephalon, and pallium (primitive cortex of cerebral
342 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
hemisphere). The median lamina between the hemispheres lags behind in its
development and thus there is formed the great longitudinal fissure between the
hemispheres. The lamina is continuous caudally with the roof plate of the dien-
cephalon; cranially it becomes the lamina terminalis, the cranial boundary of the
third ventricle (Figs. 332 and 342).
Chorioid Plexus of the Lateral Ventricles—It will be remembered that the
chorioid plexus of the third ventricle develops in the folds of the roof plate of the
diencephalon. Similarly the thin, median wall of the pallium at its junction
with the wall of the diencephalon is folded into the lateral ventricle. A vascular
Fissura prima
Chorioid plexus of lat. ventricle — Corpus striatum
if Hippocampus
Pallium —\ Me
Roof plate
Pineal body
Superior colliculus Mesencephalon
Fic. 345.—The fore-brain and mid-brain of an embryo 13.6 mm. long seen from the dorsal surface.
The pallium of the telencephalon is cut away, exposing the lateral ventricle (His).
plexus, continuous with that of the third ventricle, grows into this fold, and pro-
jects into the lateral ventricle of either side (Figs. 345 and 347). The fold of the
pallial wall forms the chorioidal fissure and the vascular plexus is the chorioid
plexus of the lateral ventricle. This is a paired structure and with the plexus
of the third ventricle forms a T-shaped figure, the stem of the Y overlying the
third ventricle, its curved arms projecting into the lateral ventricles just caudal
to the interventricular foramen. Later, as the pallium extends, the chorioid
plexus of the lateral ventricles and the chorioidal fissures are extensively elongated
into the temporal lobe and inferior horn of the lateral ventricle (Fig. 348).
THE BRAIN 343
The interventricular foramen (of Monro) is at first a wide opening (Fig.
343), but is later narrowed to a slit, not by constriction but because its boundaries
grow more slowly than the rest of the telencephalon (Fig. 347).
Diencephalon
Mesencephalon Pallium
poy
Corpus mammillare
Pars ant. olf. lobe
aa
Tuber cinereum
Pars post. olf. lobe
Infundibulum Optic stalk
Fic. 346.—Lateral view of the fore-brain and mid-brain of a 13.6 mm. embryo (His).
Lateral ventricle
Chorioid plexus of lateral
ventricle
Thalamus
Corpus striatum +
Third ventricle
Fic. 347.—Transverse section through the fore-brain of a 16 mm. embryo showing the early develop-
ment of the chorioid plexus and fissure (His).
The third ventricle extends some distance into the caudal end of the telen-
cephalon and laterally in this region the optic vesicles develop. Into each optic
stalk extends the optic recess (Fig. 343).
344 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
Falx
Fic. 348.—A transverse section through the telencephalon of an 83 mm. fetus (aiter His). Th,
Thalamus; Cs, corpus striatum; /,f., hippocampal fissure; fa, marginal gray seam; fi, edge of white
substance.
Chorioid fissure
Ili p pocam pus
Sup. colliculus Nucleus
caudalus
Inf. colliculus Internal
capsule
Cerebellum ——— Olfactory lobe
Medulla oblongata
Fic. 349.—Lateral view of the brain of a 53 mm. fetus. The greater part of the pallium of the
right cerebral hemisphere has been removed, leaving only that covering the lenticular nucleus, and
exposing the internal capsule, caudate nucleus and hippocampus (His).
THE BRAIN 345
The corpus striatum is developed as a thickening in the floor of each cerebral
hemisphere. It is already prominent in embryos of six weeks (13.6 mm.) bulg-
ing into the lateral ventricle (Figs. 345 and 347). It is in line caudally with the
thalamus of the diencephalon and in development is closely connected with it,
although the thalamus always forms a separate structure. The corpus striatum
elongates as the cerebral hemisphere lengthens, its caudal portion curving around
to the tip of the inferior horn of the lateral ventricle and forming the slender tail
of the caudate nucleus (Fig. 349). The thickening of the corpus striatum is due
Anterior horn
~— Lenticular nucleus
Ant. columns of
fornix
Internal capsule
Nucleus caudatus
Interventricular
foramen
Third ventricle
Thalamus
Chorioid plexus of ; Hip pocampus
lat. ventricle
Posterior horn
Fic. 350.—Horizontal (coronal) section through the fore-brain of a 160 mm. fetus (His).
to the active proliferation of cells in the ependymal layer which form a prominent
mass of mantle layer cells. Nerve fibers to and from the thalamus to the cere-
bral cortex course through the corpus striatum as lamine which are arranged in
the form of a wide V, open laterally, when seen in horizontal sections. This V-
shaped tract of white fibers is the znternal capsule, the cranial limb of which partly
separates the corpus striatum into the caudate and lenticular nuclei (Fig. 350).
The caudal limb of the capsule extends between the lenticular nucleus and the
thalamus.
346 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
The thalamus and corpus striatum are separated by a deep groove until the
end of the third month (Fig. 347). As the structures enlarge, the groove between
them disappears and they form one continuous mass (Fig. 350). According to
some investigators, there is direct fusion between the two.
The Rhinencephalon or Olfactory Apparatus.—This is divided into a basal
portion and a pallial portion. The basal portion consists: (1) in a ventral and
cranial evagination (pars anterior), formed mesial to the corpus striatum, which
is the anlage of the olfactory lobe and stalk (Fig. 346). This receives the olfactory
fibers and its cells give rise to olfactory tracts. The tubular stalk connecting the
olfactory lobe with the cerebrum loses its lumen. (2) Caudal to the anlage of the
olfactory lobe a thickening of the brain wall develops (pars posterior) which ex-
tends mesially along the lamina terminalis and laterally becomes continuous with
the tip of the temporal lobe (Fig. 346). This thickening constitutes the anterior
perforated space and the parolfactory area of the adult brain (Fig. 356).
The pallial portion of the rhinencephalon is termed the archipallium because
it forms the entire primitive wall of the cerebrum, a condition which is permanent
in fishes and amphibia. Later, when the neopallium, or adult cortex, arises, the
archipallium forms a median strip of the pallial wall curving along the dorsal edge
of the chorioidal fissure from the anterior perforated space around to the tip of
the temporal lobe, where it is again connected with the basal portion of the rhinen-
cephalon. The archipallium differentiates into the hippocampus (Figs. 345 and
349), a portion of the gyrus hippocampi, and into the gyrus dentatus. It resembles
the rest of the cerebral cortex in the arrangement of its cells. The infolding of the
hippocampus produces the hippocampal fissure. :
The Commissures of the Telencephalon.—The important commissures are
the corpus callosum, fornix, and anterior commissure. The first is the great trans-
verse commissure of the neopallium, or cerebral cortex, while the fornix and an-
terior commissure, smaller in size, are connected with the archipallium of the
rhinencephalon. The commissures develop in relation to the lamina terminalis,
crossing partly in its wall and partly in fused adjacent portions of the median
pallial walls. Owing to the fusion of the pallial walls dorsal and cranial to it, the
lamina terminalis thickens rapidly in stages between 80 and 150 mm. (C R)
(Streeter). “It [the lamina terminalis] is distended dorsalward and antero-
lateralward through the growth of the corpus callosum, the shape of which is
determined by the expanding pallium.” Between the curve of the corpus callo-
sum and the fornix the median pallial walls remain thin and membranous, and
constitute the sepium pellucidum of the adult. The walls of this septum
THE BRAIN 347
enclose a cavity, the so-called fifth ventricle, or space of the septum pellucidum
(Fig. 351).
The fornix takes its origin early, chiefly from cells in the hippocampus.
The fibers course along the chorioidal side of the hippocampus cranially, pass-
ing dorsal to the foramen of Monro (Fig. 351 A). In the cranial portion of the
Corpus callosum Body of fornix
Hippocampal commissure
Anterior commissure Chorioid fissure
Ant. pillar of fornix Thalamus
B
Body of fornix Hippocampal commissure
Septum pellucidum
p p Corpus callosum
Ant. com missure
Thalamus
Ant. pillar of fornix
Fic. 351.—Two stages in the development of the cerebral commissure. (Based on reconstructions
by His and Streeter.) A, Median view of the right hemisphere of an 83 mm. fetus; B, the same of a 120
mum. fetus.
lamina terminalis fibers are given off to, and received from, the basal portion of the
rhinencephalon. In this region, fibers crossing the midline form the hippocampal
commissure. Other fibers, as the diverging anterior pillars of the fornix, curve
ventrally and end in the mammiilary body of the hypothalamus. The commissure
of the hippocampus, originally cranial in position, is carried caudalward with the
caudal extension of the corpus callosum (Fig. 351 B).
348 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
The fibers of the anterior commissure cross in the lamina terminalis ventral
to the hippocampal commissure. They arise in a cranial and a caudal division.
The fibers of the former take their origin from the olfactory stalk and the adjacent
cortex. The fibers of the caudal division pass ventrally about the corpus striatum,
between it and the cortex, and may be derived from one or both of these regions.
The corpus callosum appears cranial and dorsal to the hippocampal com-
missure in the roof of the thickened lamina terminalis (Fig. 351 A). Through
Lobus parietalis
Lateral
jissure
Lobus
frontalis
Lobus
occipitalis
Lobus
temporalis Cerebellum
Pons
M yelenceph-
alon
4 ae ees Spinal cord
é \
Fic. 352.—Lateral view of the brain of a 90 mm. fetus (His).
its fibers, which arise from neuroblasts in the wall of the neopallium (cerebral cor-
tex), nearly all regions of one hemisphere are associated with corresponding regions
of the other. With the expansion of the pallium, the corpus callosum is extended
cranially and caudally by the development of interstitial fibers. The fibers first
found in the corpus callosum arise in the median wall of the hemispheres. In
fetuses of 150 mm. (C R) (five months) this great commissure is a conspicuous
structure and shows the form which is characteristic of the adult (Fig. 351 B).
THE BRAIN 349
The Form of the Cerebral Hemispheres.—When the telencephalon expands
cranially, caudally, and at the same time ventrally, four lobes may be distin-
guished (Fig. 352): (1) a cranial frontal lobe; (2) a dorsal parietal lobe ; (3) a caudal
occipital lobe; and (4) a ventro-lateral temporal lobe. The ventricle extends
into each of these regions and'forms respectively the anterior horn, the body, the
posterior horn, and the inferior horn of the lateral ventricle. The surface extent
of the cerebral wall, the thin, gray cortex, increases more rapidly than the un-
derlying, white medullary layer. As a result the cortex is folded, producing con-
volutions between which are depressions, the fissures and sulci. The chorioidal
Jjissure is formed, as we have seen (p. 342), by the ingrowth of the chorioid plexus.
cerebrum
Corpora
quadrigemina
Impression of
thalamus
Hemisphere of Temporal lobe
cerebellum
Vermis cerebelli Lateral recess of
ventricle fourth
XN 7 Fasciculus gracilis
ial Medulla oblongata
Fic. 353.—Dorsal view of the brain from a 100 mm. fetus (Kollmann).
During the third month the hippocampal fissure develops as a curved infolding
along the median wall of the temporal lobe. Internally the infolded cortex forms
the hippocampus (Figs. 345 and 349). The lateral fissure (of Sylvius) makes its
appearance also in the third month (Fig. 352), but its development is not com-
pleted until after birth. The cortex overlying the corpus striatum laterally de-
velops more slowly than the surrounding areas and is thus gradually overgrown
by folds of the parietal and frontal lobes (fronto-parietal operculum) and of the
temporal lobe (temporal operculum). The area thus overgrown is the imsula
(island of Reil) and the depression so formed is the lateral fissure (of Sylvius) (Fig.
355). Later, frontal and orbital opercula are developed ventro-laterally from the
350 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
Insula
Median olfactory Lat. olfactory gyrus
gyrus
Middle olfactory
gyrus
Diagonal gyrus
Gyrus ambiens
Gyrus semilunaris
Cerebellum
Oliva
Fic. 354.—Ventral view of the brain of a 100 mm. fetus, showing development of the rhinencephalon
(Kollmann),
Sulcus postcentralis Sulcus centralis
|
Lobus
parietalis
superior Inferior
Supra- frontal
marginal sulcus
and an-
ae Posi. Ascend-
ramus ing
of lateral ramus
Jissure Lateral
fissure
(Sylviz)
Middle _f
temporal }
sulcus
Temporal
Occipital lobe
pole
Superior temporal gyrus Middle temporal gyrus
Fic. 355.—Lateral view of the right cerebral hemisphere from a seven months’ fetus (Kollmann).
frontal lobe. These are not approximated over the insula until after birth. The
frontal operculum is included between the anterior limbs of the Sylvian fissure, and
the extent of its development, which is variable, determines the form of these limbs.
THE BRAIN 351
In fetuses of six to seven months four other depressions appear which later
form important landmarks in the cerebral topography. These are: (1) the cen-
tral sulcus, or fissure of Rolando, which forms the dorso-lateral boundary line
between the frontal and parietal lobes (Fig. 355); (2) the parieto-occi pital fissure,
which, on the median wall of the cerebrum, is the line of separation between the
occipital and parietal lobes (Fig. 356); (3) the calcarine fissure, which includes
between it and the parieto-occipital fissure the cuneus and marks the position of
the visual area of the cerebrum; (4) the collateral fissure on the ventral surface
of the temporal lobe, which produces the inward bulging on the floor of the
Corpus callosum Sulc. corp. callost
Gyrus cinguli Splenium
Paricto-occipital fissure
Space of
septum
pellu-
cidum
Rostral
lamina
Parol-
factory
area
Cuneus
Cal-
- carine
eo jissure
Olfactory lobe Fissura rhinica
Optic nerve Temporal lobe
Fic. 356.—Median surface of the right cerebral hemisphere from a seven months’ fetus (Kollmann).
posterior horn of the ventricle known as the collateral eminence. The calcarine
fissure also affects the internal wall of the ventricle, causing the convexity termed
the calcar avis (hippocampus minor).
Simultaneously with the development of the collateral fissure appear other
shallower depressions known as sulci. These have a definite arrangement and
with the fissures mark off from each other the various functional areas of the
cerebrum. The surface convolutions between the depressions constitute the gyri
and lobules of the adult cerebrum.
Histogenesis of the Cerebral Cortex.—In the wall of the pallium are differ-
entiated the three primitive zones typical of the neural tube: the ependymal,
352 THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM
mantle, and marginal layers. During the first two months the cortex remains
thin and differentiation is slow. At eight weeks neuroblasts migrate from the
ependymal and mantle zones into the marginal zone and give rise to layers of
pyramidal and other cells typical of the cerebrum. The differentiation of these
layers is most active during the third and fourth months, but probably continues
until after birth (Mellus, Amer. Jour. Anat., vol. 14, 1912). From the fourth
month on, the cerebral wall thickens rapidly, owing to the development of (1) the
fibers from the thalamus and corpus striatum; (2) of endogenous fibers from the
neuroblasts of the cortex. The fibers form a white, inner, medullary layer sur-
rounded by the gray cortex. Myelination begins shortly before birth (Flechsig),
but some fibers may not acquire their sheaths until after the twentieth year. As
the cerebral wall increases in thickness the size of the lateral ventricle becomes
relatively less, its lateral diameter especially being decreased.
Anomalies.—Defects involving the neural tube may give rise to various monstrous
conditions, e. g., cyclopia, acrania. In spina bifida, a sac-like protrusion of the cord, or its
membranes, extends through a cleft in the vertebral column.
se
CHAPTER XIII
THE PERIPHERAL NERVOUS SYSTEM
THE nerves, ganglia, and sense organs constitute the peripheral nervous
system. The peripheral nerves consist of bundles of myelinated and unmyelin-
ated nerve fibers and aggregations of nerve cells, the ganglia. The fibers are of
two types: afferent fibers, which carry sensory impulses to the central nervous
system, and efferent fibers, which carry effective impulses away from the nervous
centers. The peripheral efferent fibers of both brain and spinal cord take their
origin from neuroblasts of the basal plate. Typically they emerge ventro-later-
ally from the neural tube. Those arising from the spinal cord take origin in the
mantle layer, converge, and form the ventral roots of the spinal nerves. The
efferent fibers of the brain take origin from more definite nuclei and constitute
the motor or effector portions of the cerebral nerves. The peripheral afferent fibers
originate from nerve cells which lie outside the neural tube. Those sensory
nerve cells related to the spinal cord
. IX XxX-XI . t
and to the brain stem caudal to the \ eee
otic vesicle are derived from the gang-
lion crest, the origin of which has
been described (Chapter X, p. 304).
Bridge
A. SPINAL NERVES
The spinal nerves are segment-
ally arranged and each consists of :
y 8 : Fic. 357.—Reconstruction of an embryo of 4
dorsal and ventral roots, spinal gang- mm., showing the development of the cerebrospinal
nerves (Streeter), XX 17. Cr., z., etc., cervical
lion, and nerve trunks. In embryos :
spinal nerves.
of 4 mm. the ventral roots are already
developing as outgrowths of neuroblasts in the mantle layer of the spinal cord
(Fig. 357). The spinal ganglia are represented as enlargements along the gang-
lion crest and are connected by cellular bridges.
In 7 mm. embryos (five weeks old) the cells of the spinal ganglia begin to
develop centrally directed processes which enter the marginal zone of the cord as
the dorsal root fibers (Fig. 358). These fibers course in the dorsal funiculi and
23 353
354 THE PERIPHERAL NERVOUS SYSTEM
eventually form the greater part of them. Perpiheral processes of the ganglion
cells join the ventral root fibers in the trunk of the nerve (Fig. 360). At 10mm.
(Fig. 359) the dorsal root fibers have elongated and the cellular bridges of the
ganglion crest between the spinal ganglia have begun to disappear. In transverse
sections at this stage (Fig. 325 and 350) the different parts of a spinal nerve may
be seen. The trunk of the nerve just ventral to the union of the dorsal and
IV
IX-X-XI gang. crest
Ophthal. div.
Sup. max. div.
M. masticatorius
1 te] |
Cy. petros.
fe NV. laryng. sup.
Gang. nodos.
Inf. max. div.
S. I.
Fic. 358.—Reconstruction of a 6.9 mm. embryo, showing the development of the dorsal root fibers from
the spinal and cerebral ganglia (Streeter). X 16.7.
ventral roots gives off laterally the dorsal, or posterior ramus, the fibers of which
supply the dorsal muscles. The ventral ramus continuing gives off mesially the
ramus communicans to the sympathetic ganglion and divides into the lateral and
ventral (anterior) terminal rami. The efferent fibers of these rami supply the
muscles of the lateral and ventral body wall, and the afferent fibers end in the
integument of the same regions.
SPINAL NERVES 355
At the points where the anterior and lateral terminal rami arise, connecting
loops may extend from one spinal nerve to another. Thus in the cervical region
superficial and deep nerve plexuses are formed. The deep cervical plexus forms
the ansa hypoglossi and the phrenic nerve (Fig. 359).
Gang. Ba oe p pte ed IX
Cerebellum | N.VI ' of | 'Gang. petrosum
H eee ae Gang. radicis n.X
N.III~ NV. hypoglossus
N.IV--
a en, LEG.
N. fronialis~ _ DAS on 4 é -~ N.XI
—Gang. nodos.
____N. desc. cerv.
__ Rami hyoid.
(Ansa hypoglossi)
N. Musculocutan.
N. axillaris
sei phrenicus
- iN. medianus
N. radialis
NV. nasociliaris ~~
N. maxillaris ~
NV. mandibularis “
Gang. geniculalum ~~
N. chorda tympani’
Cor" ~~ N, ulnaris
a ITh
Diaphragma ~~
He par
I Co,
NV. tibialis
NV. peroneus ~
R. posterior
Tubus digest. “4 : oster p
R. terminalis lateralis
IS. ! eae R. terminalis anterior
N. femoral : i ETL.
N. obturator : Mesonephros
Nn. ilioing. et hypogastr.
Fic. 359.—Reconstruction of the nervous system of a 10 mm. embryo (Streeter). XX 12.
The Brachial and Lumbo-sacral Plexuses-—The nerves supplying the arm
and leg also unite to form plexuses. In embryos of 10 mm. (Fig. 359) the trunks
of the last four cervical nerves and of the first thoracic are united to form a flat-
tened plate, the anlage of the brachial plexus. From this plate nervous cords
extend into the intermuscular spaces and end in the premuscle masses. The
developing skeleton of the shoulder splits the brachial plexus into dorsal and
356 THE PERIPHERAL NERVOUS SYSTEM
ventral lamine. From the dorsal lamina arise the musculocutaneous, median, and
ulna nerves; from the ventral lamina, the axillary and radial nerves.
In 10 mm. embryos the lumbar and sacral nerves which supply the leg unite
in a plate-like structure, the anlage of the lumbo-sacral plexus (Fig. 359). The
plate is divided by the skeletal elements of the pelvis and femur into two lateral
and two median trunks. Of the cranial pair the lateral becomes the femoral
nerve; the median, the obturator nerve. The caudal pair constitute the sciatic
nerve; the lateral trunk is the peroneal nerve, and the median trunk is the “bial.
Dorsal root Marginal layer
Ependymal layer
VC) Mantle layer
Somatic sensory neurone
Visceral sensory neurone.
Spinal cor
Visceral motor neurone
Somatic motor neurone
Dorsal ramus.
Lat. terminal
division
Ventral terminal division of
spinal nerve e
»
Ramus communicans Sympathetic ganglion
Fic. 360.—Transverse section of a 10 mm. embryo showing the spinal cord, spinal nerves and their
functional nervous components. Diagrammatic.
Save for the neurones from the special sense organs (nose, eye, and ear) which
form a special sensory group, the neurones of the peripheral nerves, both spinal
and cerebral, fall into four functional groups (Fig. 360).
(1) Somatic afferent, or general sensory, with fibers ending in the integument
of the body wall.
(2) Visceral afferent or sensory, with fibers ending in the walls of the viscera.
(3) Somatic efferent or motor, with fibers ending on voluntary muscle fibers.
(4) Visceral efferent or motor: (a) with fibers ending about sympathetic
ganglion cells, which in turn control the smooth muscle fibers of the viscera and
blood vessels (spinal nerves); or (b) with fibers ending directly on visceral muscle.
fibers (mixed cerebral nerves).
THE CEREBRAL NERVES 357
B. THE CEREBRAL NERVES
The cerebral nerves of the human brain are twelve in number. They differ
from the spinal nerves: (1) in that they are not segmentally arranged, and (2)
in that they do not all contain the same types of nervous components. Classed
according to the functions of their neurones they fall into three groups:
SPECIAL SoMATIC SENSORY. SomATIcC Motor oR EFFERENT. VISCERAL SENSORY AND Motor.
I. Olfactory. III. Oculomotor. V. Trigeminal.
II. Optic. IV. Trochlear. VII. Facial.
VIII. Acoustic. VI. Abducens. IX. Glossopharyngeal.
XII. Hypoglossal. X. Vagus complex, including
XI. Spinal Accessory.
It will be seen (1) that the nerves of the first group are purely sensory,
corresponding to the general somatic afferent neurones of the spinal nerves; (2)
that the nerves of the somatic motor group are purely motor and correspond to the
somatic efferent or motor neurones of the spinal nerves; (3) that the nerves of the
third group are of mixed function and correspond to the visceral components of
the spinal nerves.
I. THE SPECIAL SOMATIC SENSORY NERVES
1. The Olfactory Nerve, though purely sensory, has no ganglion. Its nerve
cells lie at first in the olfactory epithelium of the nose and are of the bipolar type
(fourth week). From these cells periph-
eral processes develop and end directly :
- “Olfactory tract
1 ithe-
at the surface of the olfactory epi abe at ol
lium (Fig. 361). Central processes grow
-—Glomerulus
toward the olfactory lobe and form the
\\Cribriform plate
- Olfactory nerve fiber
: - - 4 ri ebydtt -- Olfactory epithelium
contact with the dendrites of the mitral = fe f ?
Fic. 361—Diagram of the relations of the
fibers in the olfactory nerve.
strands of the olfactory nerve. They end Lb,
in the glomeruli of the olfactory bulb in
cells, or olfactory neurones of the second
order. Some olfactory cells migrate
from the epithelium, with which, however, they retain peripheral connections.
Such bipolar cells, found along the entire course of the nerve, resemble ordinary
dorsal ganglion cells. The olfactory nerve fibers are peculiar in that they remain
unmyelinated. Nerve fibers from the epithelium of the vestigial vomero-nasal
organ (of Jacobson) also end in the olfactory bulb.
When the ethmoidal bone of the cranium is developed, its cartilage, as the
cribriform plate, forms around the strands of the olfactory nerve.
358 THE PERIPHERAL NERVOUS SYSTEM
The ganglionated 2. terminalis courses in close association with the olfactory
nerve. Its fibers end in the epithelium of the vomero-nasal organ and of the
nose. Although evidently a distinct nerve its significance is obscure.
2. The Optic Nerve is formed by fibers which take their origin from neuro-
blasts in the nervous layer of the retina. The retina is differentiated from the
wall of the fore-brain and remains attached to it by the optic stalk (Fig. 343),
hence the optic nerve is not a true peripheral nerve, but belongs to the central
system of tracts. The neuroblasts from which the optic nerve fibers develop
constitute the ganglion cell layer of the retina (Fig. 381). During the sixth and
seventh weeks these cells give rise to central processes which form a nerve fiber
layer on the inner side of the retina. The optic fibers converge to the optic stalk
and grow through its wall back to the brain. The cells of the optic stalk are
converted into a neuroglia framework and the cavity is obliterated. In the floor
of the fore-brain, at the boundary between telencephalon and diencephalon,
the fibers from the median half of each retina at about the end of the second
month cross to the opposite side, and this decussation constitutes the optic chiasma
(from Greek letter ¥ or “‘chi”). The crossed and uncrossed fibers constitute the
optic tract which rounds the cerebral peduncles laterally and dorsally (Fig. 354).
Eventually the optic fibers end in the lateral geniculate body, thalamus, and
superior colliculus.
Efferent fibers, terminating in the inner reticular layer of the retina, are also
present. In certain fishes where their function has been studied these fibers
resemble visceral efferent components (Arey, Jour. Comp. Neurol., vol. 26, 1916).
8. The Auditory Nerve, or N. Acusticus, is formed by fibers which grow
from the cells of the acoustic ganglion. The origin of these cells is unknown,
though they appear in 4 mm. embryos just cranial to the otic vesicle (Fig. 358).
The cells become bipolar, central processes uniting the ganglion to the tuberculum
acusticum of the myelencephalon and peripheral fibers connecting it with the
wall of the otocyst. The acoustic ganglion is differentiated into the vestibular and
spiral ganglia (Fig. 362). Its development has been studied by Streeter (Amer.
Jour. Anat., vol. 6, 1907). The ganglion elongates and is subdivided into superior
and inferior portions in 7 mm. embryos. The superior part supplies nerves to
the utriculus and to the ampulle of the anterior and lateral semicircular canals.
It forms part of the vestibular ganglion of the adult. Part of the inferior portion
supplies nerves to the sacculus and to the ampulla of the posterior semicircular
canal, and this portion, together with the pars superior, constitutes the vestibular
ganglion. The greater part of the pars inferior is, however, differentiated into the
THE CEREBRAL NERVES 359
aN. vestib..--
-
-
>-=.Pars. sup.
4mm. 7mm...
--Pars. inf.
Pars. sup. Pars. sup.
; ;
sf -- N. vestib. - Pars. inf.
ae
N
Ret
Pars. inf.
nm
t
t ‘
SS
9mm. ——
“SN. coch.
-- Gang. spirale
R. amp. sup.
-R. amp. lat.
“R. utric.
R. amp. sup.
4
6 -R. amp. lat.
30 mm.
MEDIAN VIEW LATERAL VIEW
Fic. 362.—The development of the acoustic ganglia and nerves. The vestibular ganglion is finely
stippled, the spiral ganglion coarsely stippled (Streeter).
Spiral ganglion, the peripheral fibers of which innervate the hair cells of the spiral
organ (of Corti) in the cochlea. The spiral ganglion appears in 9 mm. embryos
and conforms to the spiral turns of the cochlea, hence its name. Its central nerve
360 THE PERIPHERAL NERVOUS. SYSTEM
fibers form the cochlear division of the acoustic nerve. This is distinctly sepa-
rated from the central fibers of the vestibular ganglion which constitute the ves-
tibular division of the acoustic nerve, the fibers of which are not auditory in func-
tion. The pars inferior of the vestibular ganglion becomes closely connected
with the n. cochlearis, and thus in the adult it appears as though the sacculus and
posterior ampulla were supplied by the cochlear nerve.
Il. THE SOMATIC MOTOR NERVES
The nerves of this group, consisting of the three nerves to the eye muscles
and the n. hypoglossus, are purely motor nerves, the fibers of which take origin
from the neuroblasts of the basal plate of the brain stem, near the midline. They
are regarded as the homologues of the ventral motor roots of the spinal cord, but
have lost their segmental arrangement and are otherwise modified. The nuclei
of origin of these nerves are shown in Fig. 364.
12. N. Hypoglossus.—This nerve is formed by the fusion of the ventral root
fibers of three to five precervical nerves. Its fibers take origin from neuroblasts
of the basal plate and emerge from the ventral wall of the myelencephalon in
several groups (Fig. 357). In embryos of five weeks (7 mm.) the fibers have
converged ventrally to form the trunk of the nerve (Fig. 358). Later they grow
cranially, lateral to the ganglion nodosum, and eventually end in the muscle
fibers of the tongue (Fig. 359). The nerve in its development unites with the first
three cervical nerves to form the ansa hypoglossi. Its nucleus of origin is shown in
Fig. 364.
That the hypoglossal is a composite nerve homologous with the ventral roots of the
spinal nerves is shown: (1) by the segmental origin of its fibers; (2) from the fact that its
nucleus of origin is a cranial continuation of the ventral gray column, or nucleus of origin for
the ventral spinal roots; (3) from the fact that in mammalian embryos (pig, sheep, cat, etc.)
rudimentary dorsal ganglia are developed, one of which at least (Froriep’s ganglion) sends a
dorsal root to the hypoglossal. In human embryos Froriep’s ganglion may be present as a
rudimentary structure (Figs. 359 and 363), or it may be absent and the ganglion of the first
cervical nerve may also degenerate and disappear. In pig embryos Prentiss (Jour. Comp.
Neurol., vol. 20, 1910) has found two and three accessory ganglia (including Froriep’s) from
which dorsal roots extended to the root fascicles of the hypoglossal nerve (Fig. 121).
3. The Oculomotor Nerve originates from neuroblasts in the basal plate of
the mesencephalon (Fig. 339 B). The fibers emerge as small fascicles on the
ventral surface of the mid-brain in the concavity due to the cephalic flexure (Figs.
359 and 364). The fascicles converge, form the trunk of the nerve, and end in the
premuscle masses of the eye. The nerve eventually supplies all of the extrinsic
THE CEREBRAL NERVES 361
muscles of the eye save the superior oblique and external rectus. A branch is also
supplied to the ciliary ganglion. In the chick embryo, bipolar cells migrate along
the fibers of the oculomotor nerve to take part in the development of the ganglion.
The ciliary ganglion of human embryos is derived entirely from the semilunar
ganglion of the trigeminal nerve.
4. The Trochlear Nerve fibers take their origin from neuroblasts of the basal
plate, located just caudal to the nucleus of origin of the oculomotor nerve. They
are directed dorsally, curve around the cerebral aqueduct, and, crossing in its
roof, emerge at the isthmus (Fig. 339 A). From their superficial origin each is
directed ventrally as a slender nerve which connects with the anlage of the
superior oblique muscle of the eye (Fig. 359).
6. The N. Abducens takes origin from a nucleus of cells in the basal plate of
the myelencephalon, located directly beneath the fourth neuromere of the floor of
the fourth ventricle (Figs. 359 and 364). The converging fibers emerge ventrally
at a point caudal to the future pons, and, asa single trunk, course cranially, mesial
to the semilunar ganglion, finally ending in the anlage of the external rectus
muscle of the eye. Vestigial rootlets of the abducens and hypoglossal nerve
tend to fill in the gap between these two nerves, according to Bremer and Elze.
Il. THE VISCERAL MIXED NERVES
The nerves of this group, the trigeminal, facial, glossopharyngeal, and vagus
complex (vagus plus the spinal accessory), are mixed in function. The trigem-
inal nerve, beside its visceral nerve components, contains also numerous somatic
sensory neurones which supply the integument of the head and face.
5. The Trigeminal Nerve is largely sensory. Its semilunar ganglion is the
largest of the whole nervous system and is a derivative of the ganglion crest, but
very early is distinct from the other cerebral ganglia (Fig. 358). It arises later-
ally at the extreme cranial end of the hind-brain. Central processes from
its cells form the large sensory root of the nerve which enters the wall of the hind-
brain at the level of the pontine flexure (Fig. 359). These fibers fork and course
cranially and caudally in the alar plate of the myelencephalon. The caudal
fibers constitute the descending spinal tract of the trigeminal nerve, which extends
as far caudad as the spinal cord (Fig. 364). The peripheral processes separate
into three large divisions, the ophthalmic, maxillary, and mandibular rami, and
supply the integument of the head and face and the epithelium of the mouth and
tongue.
The motor fibers of the trigeminal nerve arise chiefly from a dorsal motor
362 THE PERIPHERAL NERVOUS SYSTEM
nucleus which lies opposite the point at which the sensory fibers enter the brain
wall (Fig. 364). In the embryo these fibers emerge as a separate motor root,
course along the mesial side of the semilunar ganglion, and, as a distinct trunk,
supply the premuscle masses which later form the muscles of mastication. From
the chief motor nucleus, a line of cells extending cranially into the mesencephalon
constitutes a second source of origin for motor fibers. In the adult, the motor
fibers form a part of the mandibular division of the nerve.
The facial, glossopharyngeal, and vagus nerves are essentially visceral in func-
tion. Their sensory fibers, chiefly of the visceral type, supply the sense organs
Vagus root ganglion (jugular)
Accessory root ganglia
Gang. nodos.
ih iesan. aap: —Inter-gang. bridge
XIT
Fic. 363.—Reconstruction of the cerebral nerves of an embryo of 10.2 mm. (Streeter). X 16.7.
of the branchial arches and viscera. These fibers originate in the ganglia of their
respective nerves, and, entering the alar plate of the myelencephalon, course
caudally as the solitary tract (Fig. 364). A few somatic sensory fibers, having
the same origin and course in the myelencephalon, supply the adjacent integu-
ment.
In aquatic vertebrates, special somatic sensory fibers from the /afcral line organs join the
facial, glossopharyngeal, and vagus nerves, and their ganglion cells form part of the genicu-
late, petrosal, and nodose ganglia. In human embryos the organs of the lateral line are repre-
sented by ectodermal thickenings or placodes which occur temporarily over these ganglia.
The nervous elements supplying these vestigial organs have completely disappeared.
THE CEREBRAL NERVES 363
7. The Facial Nerve is largely composed of efferent motor fibers which supply
the facial muscles of expression. In 10 mm. embryos these fibers arise from a
cluster of neuroblasts in the basal plate of the myelencephalon located beneath
the third rhombic groove or neuromere (Fig. 364). The fibers from these cells
course laterally, and emerge just mesial to the acoustic ganglion. The motor trunk
then courses caudally and is lost in the tissue of the hyoid visceral arch, tissue
which later gives rise to the muscles of expression (Fig. 359). The sensory fibers
of the facial nerve arise from the cells of the geniculate ganglion, which are in turn
derived from the ganglion crest (Streeter). This ganglion is present in 7 mm.
Nucl motor. n X (ambiguus)
Tractus solitarius Nucl motor n, trigemini
' Gang radicis n.X Tractus spinalis pV . MO ERGenleAuiaeeel
1
I
! ‘
' I ‘Gang ¢ are
H ‘Radix sens n.1X ! 1 Gang semilunare
1 ' ent
' ' Gang geniculatum |! Tr cerebell ob V.
: ' Radix sens n VII |
| (pars Intermed) +
'
1
'
1
1
Radix sens. n.X i
1 '
'
'
Rr. mot. n.X
Nucl. n. hypoglossi
N. accessorius «------~--
s --N. frontalis
B.-----N nasoclliatis
tos
| Nucl n.VI 1}
' | N abducens
'
'
1
'
1
'
'
'
1
t
'
Lau ; t
Nucl o.VII
Cane spetive sas, N. maxillaris
t : N. facialis
Funie. ze j N. hypoglossus Portlo minor
posterior =
Gang. nodos. 1
andibularis
1
'
'
)
'
t
m:
Zz
“~~ R, mot. ventr.
“ R mot. lat.
R. hyold
* R. posterlor
N. vagus
Fic. 364.—Reconstruction of the nuclei of origin and termination of the cerebral nerves in an embryo
of 10 mm. The somatic motor nuclei are colored red (Streeter). X 30.
embryos (Fig. 358), located cranial to the acoustic ganglion. The centrally
directed processes of the geniculate ganglion enter the alar plate and form part
of the solitary tract. The peripheral fibers in part course with motor fibers in
the chorda tympani, join the mandibular branch of the trigeminal nerve, and end
in the sense organs of the tongue. Other sensory fibers form later the great
superficial petrosal nerve, which extends to the spheno-palatine ganglion.
The motor fibers of the facialis at first course straight laterad passing cra-
nial to the nucleus of the abducens. The nuclei of the two nerves later gradually
shift their positions, that of the facial nerve moving caudad and laterad, while the
nucleus of the abducens shifts cephalad. As a result, the motor root of the
N. oculomotorus
364 THE PERIPHERAL NERVOUS SYSTEM
facial nerve in the adult bends around the nucleus of the abducens producing the
genu or knee, of the former. The two together produce the rounded eminence in
the floor of the fourth ventricle known as the facial colliculus.
9. The Glossopharyngeal Nerve takes its superficial origin just caudal to the
otic vesicle (Figs. 358, 363 and 365). Its few motor fibers arise from neuroblasts
in the basal plate beneath the fifth neuromeric groove. These neuroblasts form
part of the nucleus ambiguus, a nucleus of origin which the glossopharyngeal
shares with the vagus (Fig. 364). The motor fibers course laterally beneath the
spinal tract of the trigeminal nerve and emerge to form the trunk of the nerve.
These fibers later supply the muscles of the pharynx.
The sensory fibers of the glossopharyngeal nerve arise from two ganglia, a
superior, or root ganglion, and a peirosal, or trunk ganglion (Figs. 359 and 365).
These fibers constitute the greater part of the nerve and divide peripherally to
form the tympanic and lingual rami to the second and third branchial arches.
Centrally, these fibers enter the alar plate of the myelencephalon and join the
sensory fibers of the facial nerve coursing caudally in the solitary tract.
10, 11. The Vagus and Spinal Accessory.—The vagus, like the hypoglossal,
is composite, representing the union of several nerves, which, in aquatic animals,
supply the branchial arches (Figs. 359 and 365). The more caudal fascicles of
motor fibers take their origin in the lateral gray column of the cervical cord as far
back as the fourth cervical segment. These fibers emerge laterally, and, as the
spinal accessory trunk (in anatomy a distinct nerve), course cephalad along
the line of the neural crest (Figs. 358, 359 and 365). Other motor fibers take their
origin from the neuroblasts of the nucleus ambiguus of the myelencephalon (Fig.
364). Still others arise from a dorsal motor nucleus which lies median in position.
The fibers from these two sources emerge laterally as separate fascicles and join
the fibers of the spinal accessory in the trunk of the vagus nerve. The accessory
fibers soon leave the trunk of the vagus and are distributed laterally and caudally
to the visceral premuscle masses which later form the sterno-cleido-mastoid and
trapezius muscles of the shoulder (Fig. 359). Other motor fibers of the vagus
supply muscle fibers of the pharynx and larynx.
As the vagus is a composite nerve it has several root ganglia which arise as
enlargements along the course of the ganglion crest (Figs. 359 and 365). The
more cranial of these ganglia is the ganglion jugulare. The others, termed
accessory ganglia, are vestigial structures and not segmentally arranged. In
addition to the root ganglia of the vagus the ganglion nodosum forms a ganglion
of the trunk (Fig. 365). The trunk ganglia of both the vagus and glossopharyn-
THE CEREBRAL NERVES 365
geal nerves are believed to be derivatives of the ganglion crest, their cells migrat-
ing ventrally in early stages.
The central processes from the neuroblasts of the vagus ganglia enter the
wall of the myelencephalon, turn caudalward, and, with the sensory fibers of the
Vagus root ganglion (Jugular) Accessory root ganglia
IX. root gang. (Superior)
Gang. petros.
N. tymp.
XII
N. laryng. sup.
Gang. nodos.
Sympathetic
Fic. 365.—A reconstruction of the peripheral nerves in an embryo of 17.5 mm. (Streeter). X 16.7.
facial and glossopharyngeal nerves, complete the formation of the solitary tract.
The peripheral processes of the ganglion cells form the greater part of the vagus
trunks after the separation from it of the spinal accessory fibers.
The Segmentation of the Vertebrate Head.—The vertebrate head undoubtedly
consists of fused segments. This was suggested to the earlier workers by the arrangement of
366 THE PERIPHERAL NERVOUS SYSTEM
the branchial arches (branchiomerism), and by the discovery, in the embryos of lower ver-
tebrates, of so-called head cavities, homologous with mesodermal segments. (Note also the
presence of neuromeres, p. 334.)
Assuming that the branchiomeres are portions of the primary head segments—and
there are recent observations which tend to disprove this—their segmentation is still not com-
parable to that of the trunk, for the branchial arches are formed by the segmentation of
splanchnic mesoderm, tissue which in the trunk never segments. The branchial arches,
therefore, represent a different sort of metamerism.
Only the first three head cavities persist. These form the eye muscles, innervated by
the third, fourth, and sixth cranial nerves respectively. All the remaining muscles of the
head are derived from the branchiomeres. From what has been said it is evident that one
cannot compare the relation of the cranial nerves to the branchiomeric muscles with the
relation of a spinal nerve to its myotomic muscles. For this reason the cranial nerves furnish
unreliable evidence as to the primitive number of cephalic segments. Various investigators
have set this number between eight and nineteen.
C. THE SYMPATHETIC NERVOUS SYSTEM
The sympathetic nervous system is composed of a series of ganglia and
peripheral nerves, the fibers of which supply gland cells and the smooth muscle
fibers of the viscera and blood vessels. It may function independently of the
central nervous system and is hence known as the eutonomic system.
The sympathetic ganglion cells are derived from the cells of the ganglion
crest. In fishes discrete cellular masses become detached from the spinal ganglia.
At an early stage (6 to 7 mm.) in human development, on the contrary, certain
cells of the ganglion crest migrate ventrally and give rise to a series of ganglia,
which, in the region of the trunk, are segmentally arranged (Figs. 139 and 360).
According to Kuntz (Jour. Comp. Neurol., vol. 20, 1910), the primary source of
these errant cells is the neural tube, from which they migrate along both dorsal
and ventral nerve roots. At 9 mm. the ganglionated cord is formed and fibers
connecting the sympathetic ganglia with the spinal nerves constitute the rami
communicantes (Streeter). The more peripheral ganglia (cardiac and cceliac)
_and the sympathetic ganglia of the head may be found in 16 mm. embryos (Fig.
366).
The cells which are to form the ganglia of the sympathetic chain migrate
ventrally in advance of the ventral root fibers and take up a position lateral to the
aorta. These ganglionic anlages are at first distinct, but soon unite with each
other from segment to segment, forming a longitudinal cord of cells. After the
formation of the primitive rami communicantes by the root fibers from the spinal
nerves, centripetal processes from the sympathetic cells grow back and join the
trunks of the spinal nerves. The visceral, spinal fibers later become myelinated
and constitute the white rami; the sympathetic, centripetal fibers remain unmye-
THE SYMPHATHETIC NERVOUS SYSTEM 367
linated and form separately the gray rami. Nerve fibers appear in the paired
longitudinal cords, which were at first purely cellular, in such a manner that seg-
mental masses of cells (sympathetic ganglia) become linked by fibrous, commissural
cords.
In the head region the sympathetic ganglia are not segmentally arranged, but
are derived from cells of the cerebrospinal ganglia which migrate to a ventral
oor”_A& :
1 2 ~ oe
RAI
rime AE aes
tal
iid LO “yee
Fic. 366.—The sympathetic system in a 16 mm. human embryo (Streeter in Lewis and Stdhr).
X 7. The ganglionated trunk is heavily shaded. The first and last cervical, thoracic, lumbar, sacral
and coccygeal spinal ganglia are numbered. a., Aorta; acc., accessory nerve; car., carotid artery; cil.,
ciliary ganglion; coe., coeliac artery; Ht., heart; nod., nodose ganglion; of., otic ganglion; pet., petrosal
ganglion; s-m., submaxillary ganglion; s.mes., superior mesenteric artery; sph.-p., sphenopalatine
ganglion; s#l., splanchnic nerve; Sé., stomach.
position (Fig. 365). These cells likewise give rise to nerve fibers which constitute
longitudinal commissures connecting the various ganglia of the head with the
ganglionated cord of the trunk region. The small cranial sympathetic ganglia
are probably all derived from the anlage of the semilunar ganglion (Fig. 366).
368 THE PERIPHERAL NERVOUS SYSTEM
The ciliary ganglion is related by a ramus communicans to the ophthalmic division
of the trigeminal nerve and receives fibers from the oculomotor nerve. Its cells
are probably derived entirely from the semilunar ganglion. The sphenopalatine,
submaxillary, and otic ganglia probably take their origin from migrating cells of
the semilunar ganglion, but as they are connected with the geniculate ganglion of
the facial nerve, the latter may contribute to their formation. The sphenopala-
tine ganglion is connected directly with the semilunar ganglion by two com-
municating rami. The submaxillary ganglion is intimately related through the
mandibular division of the trigeminal nerve to the semilunar ganglion, while the
otic ganglion is united to the latter by a plexus and is related to the glossopharyn-
geal nerve through its tympanic branch.
The cervical ganglia lose their segmental arrangement and represent the fusion
of from two to five ganglia of the cervical and upper thoracic region. The more
distally located prevertebral ganglia (of the cardiac, coeliac, hypogastric, and pelvic
plexuses) are derived from cells of the neural crest which migrate to a greater dis-
tance ventrally (Fig. 366). The visceral ganglia (of the myenteric and submucous
plexuses), and the prevertebral cardiac plexus as well, are derived by Kuntz
chiefly from migratory cells from the hind-brain and from the vagus ganglia.
The sympathetic nerve cells give rise to axons and dendrites, and are thus
typically multipolar cells. Their axons possess a neurilemma sheath, but remain
unmyelinated.
D. CHROMAFFIN BODIES: SUPRARENAL GLAND
Certain cells of the sympathetic ganglia do not form nerve cells, but are
transformed into peculiar gland cells which produce an internal secretion. The
secretion formed by these cells causes them to stain brown when treated with
chrome salts, hence they are called chromaffin cells. Cells of this type derived
from the ganglionated cord of the sympathetic system give rise to structures known
as chromaffin bodies. Chromaffin derivatives of the cceliac plexus, together with
mesenchymal tissue, form the anlage of the swprarenal gland, an organ which
reaches a relatively large size in human embryos (Fig. 232).
The Chromaffin Bodies of the ganglionated cords are rounded cellular
masses partly embedded in the dorsal surfaces of the ganglia (Fig. 367). At birth
they may attain a diameter of 1 to 1.5mm. In number they vary from one to
several for each ganglion.
Similar chromaffin bodies may occur in all the larger sympathetic plexuses.
The largest of these structures found in the abdominal sympathetic plexuses are
CHROMAFFIN BODIES: SUPRARENAL GLAND 369
the aortic chromaffin bodies (of Zuckerkandl). These occur on either side of the
inferior mesenteric artery, ventral to the aorta and mesial to the metanephros.
At birth they attain a length of 9 to 12 mm. and are composed of cords of chromaf-
fin cells intermingled with strands of connective tissue, the whole being sur-
rounded by a connective tissue capsule. After birth the chromaffin bodies de-
generate, but do not disappear entirely.
Glomus Carolicum.—Associated with the intercarotid sympathetic plexus is a
highly vascular chromaffin body known as the carotid gland. Its anlage has been
first observed in 20 mm. embryos.
The Suprarenal Gland is de-
veloped from chromaffin tissue which
becomes its medulla, and from meso-
dermal tissue which gives rise to its
cortex. In an embryo of 6 mm. the
anlage of the cortex is present, ac-
cording to Soulié, and is derived from
ingrowing buds of the ccelomic epi-
thelium. At 8 mm. the glands are
definite organs and at 9 mm. their
vascular structure is evident. The
cellular elements of the cortex are at
first larger than the chromaffin cells
which give rise tou the medulla. The
anlages of the glands form projections
in the dorsal wall of the coclom be-
tween the mesonephros and mesen-
z Fic. 367.—Section through a chromaffin body
tery (Figs. 221, 232 and 233). in a 44 mm. human fetus (after Kohn). X 450.
The chromaffin cells of the med- __ p, Mother chromaffin cells; sy, sympathetic cells;
b, blood vessel.
ulla are derived from the cceliac plexus
of the sympathetic system. In embryos of 15 to 19 mm. (Fig. 368) masses of these
cells begin to migrate from the median side of the suprarenal anlage to a central
position, and later surround the central vein which is present in embryos of 23
mm. The primitive chromaffin cells are small and stain intensely. They con-
tinue their immigration until after birth. The differentiation of the cortex into
its three characteristic layers is not completed until between the second and third
years. The inner reticular zone is formed first, next the fasciculate zone, and last
the glomerular zone.
24
370 THE PERIPHERAL NERVOUS SYSTEM
When the cells of the medulla begin to produce an internal secretion they
give the chrome reaction. By using extract of the aortic bodies, which are en-
tirely composed of chromaffin cells, Bied] and Wiesel have proved that its effect,
like that of adrenalin, is to increase the blood pressure. The logical conclusion is
that the effect of adrenalin, an extract of the suprarenal glands, is due to an
internal secretion produced by the chromaffin cells of the suprarenal medulla.
Fic. 368.—Transverse section through right suprarenal gland of a 15.5 mm. human embryo (after
Bryce). sy, Sympathetic cells; sy’, groups of chromaffin sympathetic cells migrating into the suprarenal
gland.
Portions of the suprarenal anlage may be separated from the parent gland and form
accessory supravenals. As a rule, such accessory glands are composed only of cortical sub-
stance; they may migrate some distance from their original position, accompanying the
genital glands. In fishes the cortex and medulla persist normally as separate organs.
E. DEVELOPMENT OF THE SENSE ORGANS
The sense cells of primitive animals (e. g., worms) are ectodermal in origin
and position. Only those of the vertebrate olfactory organ have retained this
primitive relation. During phylogeny the cell-bodies of all other such primary
sensory neurones migrated inward to form the dorsal ganglion (Parker), hence
‘
DEVELOPMENT OF THE SENSE ORGANS 371
their peripheral processes either end freely in the epithelium or appropriate new
cells to serve as sensory receptors (taste, hearing).
The nervous structures of the sense organs consist of the general sense organs
of the integument, muscles, tendons, and viscera, and of the special sense organs
which include the taste buds of the tongue, the olfactory epithelium, the retina,
optic nerve and lens of the eye, and the epithelial lining of the ear labyrinth.
I, GENERAL SENSORY ORGANS
Free nerve terminations form the great majority of all the general sensory
organs. When no sensory corpuscle is developed, the neurofibrils of the sensory
nerve fibers separate and end among the cells of the epithelia.
Lamellated corpuscles first arise during the fifth month as masses of meso-
dermal cells clustered around a nerve termination. These cells increase in num-
ber, flatten out, and give rise to the concentric lamelle of these peculiar structures.
In the cat these corpuscles increase in number by budding.
The tactile corpuscles, according to Ranvier, are developed from mesenchymal
cells and branching nerve fibrils during the first six months after birth.
I. TASTE BuDS
The anlages of the taste buds appear as thickenings of the lingual epithelium
in 110 mm. (C H) fetuses (Graberg). The cells of the taste bud anlage lengthen
and later extend to the surface of the epithelium. They are differentiated into the
sensory taste cells, with modified cuticular tips, and into supporting cells. The
taste buds are supplied by nerve fibers of the seventh, ninth, and tenth cerebral
nerves; the fibers branch and end in contact with the periphery of the taste cells.
In the fetus of five to seven months taste buds are more widely distributed
than in the adult. They are found in the walls of the vallate, fungiform, and
foliate papille of the tongue, on the under surface of the tongue, on both surfaces
of the epiglottis, on the palatine tonsils and arches, and on the soft palate. After
birth many of the taste buds degenerate, only those on the lateral walls of the
vallate and foliate papille, on a few fungiform papillz, and on the laryngeal sur-
face of the epiglottis persisting.
Il. THE OLFACTORY ORGAN
The olfactory epithelium arises as paired thickenings or placodes of the
cranial ectoderm (Fig. 369 A). The placodes are depressed to form the olfactory
pits, or fosse, about which the nose develops (Fig. 89).
372 THE PERIPHERAL NERVOUS SYSTEM
In embryos of 4 to 5 mm. (Fig. 369) the placodes are sharply marked off from
the surrounding ectoderm as ventro-lateral thickenings near the top of the head.
They are flattened and begin to invaginate in embryos of 6to7mm. In 8mm.
embryos the invagination has produced a distinct pit, or fossa, surrounded every-
where save ventrally by a marginal swelling.
The later development of the olfactory organ is associated with that of the
face. It will be remembered (cf. p. 145) that the first branchial arch forks into
the maxillary and mandibular processes. Dorsal to the oral cavity is the fronto-
Fore-brain
Olfactory placode Olfactory placode Vomero-nasal organ
Telencephalon
Nasal fossa
Lat. nasal process
Median nasal process Med. nasal process
Nasal fossa 4 Maxillary process
Epithelial plate
Frc. 369.—Sections through the olfactory anlages of human embryos. A, 4.9 mm. (X 20); B, 6.5 mm.
(X 13); C, 8.8mm. (X 13); Dand £,10mm. (A, Band C from Kiebel and Elze.)
nasal process of the head, lateral to it the maxillary processes, and ventral to it
are the mandibular processes (Fig. 97). With the development of the nasal pits
the fronto-nasal process is divided into paired lateral nasal processes and a single
median frontal process, from which are differentiated later the median nasal proc-
esses, or processus glotuilares (Fig. 370). The nasal pits are at first grooves, each
bounded mesially by the median frontal process and laterally by the lateral nasal
process and the maxillary process (Fig. 370 A). The fusion of the maxillary
processes with the ventro-lateral ends of the median frontal process converts the
DEVELOPMENT OF THE SENSE ORGANS 373
nasal grooves into blind pits or fosse, shutting them off from the mouth cavity
(Fig. 370). Thus in embryos of 10 to 12 mm. the nasal fossa has but one opening,
the external naris, and is separated from the mouth cavity by an ectodermal plate
(Fig. 369 D, E).
The ventro-lateral ends of the median frontal process enlarge and become
the median nasal processes which fuse with the lateral nasal processes and re-
duce the size of the external nares (Fig. 370 B). Externally, the nares are now
bounded ventrally by the fused nasal processes. The epithelial plates which
separate the nasal fossee from the primitive mouth cavity become thin membra-
Nasal septum
Ext. naris:
Ext. naris Lat. nasal.
Lat. nasal process
process Ate
Med. nasal +
process
Maxillary
process i
Mandible Oral cavity
Maxillary
process
Mandible’ :
A Med. nasal process Oral cavity
Fic. 370.—Two stages in the development of the jaws and nose. A, Ventral view of the end of the head
of a 10.5 mm. human embryo (after Peter); B, of an 11.3 mm. embryo (after Rabl).
nous structures caudally, and, rupturing, produce two internal nasal openings, the
primitive choane (Fig. 153). Cranially, the epithelial plate is destroyed by in-
growing mesoderm of the maxillary process and median nasal process which
replaces it, thereby forming the primitive palate (Fig. 369 D). The primitive
palate forms the ip and the premaxillary palate. The nasal fosse now open ex-
ternally through the external nares and internally into the roof of the mouth
cavity through the primitive choane.
Coincident with these changes the median frontal process has become rela-
tively smaller and that portion of it between the external nares and the nasal
fossee becomes the nasal septum (Fig. 370). As the facial region grows and elon-
374 THE PERIPHERAL NERVOUS SYSTEM
gates, the primitive choanze become longer and form slit-like openings in the
roof of the mouth cavity. By the development and fusion of the palatine proc-
esses (described on p. 147) the dorsal portion of the mouth cavity is separated
off and constitutes the nasal passages (cf. Figs. 371 and 372). The nasal pass-
ages of the two sides for a time communicate through the space between the
hard palate and the nasal septum. Later, the ventral border of the septum fuses
with the hard palate and completely separates the nasal passages (Fig. 372). The
nasal passages of the adult thus consist of the primitive nasal fossz plus a portion
Olfactory epithelium Corie wf ail
es
Vomero-nasal organ
Cartilage of vomero-
i nasal organ.
Naso-larcimal duct
Dental lamina
Fic. 371—Transverse section through the nasal passages and palatine processes of a 20 mm. human
embryo. In the nasal septum is seen a section of the vomero-nasal organ (of Jacobson). X 30.
of the primitive mouth cavity which has been appropriated secondarily by the
development of the hard palate. The passages of the adult thus open caudally
by secondary choane into the cavity of the pharynx.
Part of the epithelium which lines the nasal fossz is transformed into the
sensory olfactory epithelium (Fig. 371). The remainder covers the conche and
lines the vomero-nasal organ (of Jacobson), the ethmoidal cells, and the cranial
sinuses.
The Vomero-nasal Organ (of Jacobson) is a rudimentary epithelial structure
which first appears in 8.5 to 9 mm. embryos on the median wall of the nasal fossa
DEVELOPMENT OF THE SENSE ORGANS 375
(Fig. 369 C, Z). The groove deepens and closes caudally to form a tubular struc-
ture in the cranial portion of the nasal septum (Fig. 371). During the sixth month
it attains alength of 4mm. Nerve fibers, arising from cells in its epithelium, join
; en Olfactory epithelium
Pe
ee ~— Ethmo-turbinal I
Cpseahlte Seni nee
@ {
Kelton ae Hut!
Fic. 372.—Transverse section through the nasal passages of a 65 mm. human fetus. xX 14.
the olfactory nerve, and it also receives fibers from the . terminalis. In late
fetal stages it often degenerates, but may persist in the adult (Merkel, Mangakis).
Soft palate
Fic. 373.—Right nasal passage of a fetus at term (after Killian). J, Maxillo-turbinal; I7-VI, ethmo-
turbinals. The slight elevation at the left of J and IJ is the naso-turbinal. '
Special cartilages are developed for its support (Fig. 371). The organ of Jacob-
son is not functional in man, but in many animals evidently constitutes a special
olfactory organ.
376 THE PERIPHERAI. NERVOUS SYSTEM
The Conche are structures which are poorly developed in man. They ap-
pear on the lateral and median walls of the primitive nasal fosse. The inferior
concha, or maxillo-turbinal, is developed first in human embryos (Figs. 371 and
372). It forms a ridge along the caudal two-thirds of the lateral wall and is
marked off by a ventral groove which becomes the inferior nasal meatus (Fig. 373).
The zaso-turbinal is very rudimentary and appears as a slight elevation dorsal
and cranial to the infertor concha (Fig. 373). Dorsal to the inferior concha
arise five ethmo-turbinals, which grow progressively smaller caudally. According
to Peter, the ethmo-turbinals arise on the medial wall of the nasal fossa, and, by
a process of unequal growth, are transferred to the lateral wall (Fig. 372). Acces-
sory conche are also developed (Killian).
In adult anatomy, the inferior concha forms from I (Fig. 373), the middle concha from
II, and the superior concha from III and IV.
In addition to the ridges formed by the conche, there are developed in the grooves
between the ethmo-turbinals the ethmoidal cells. After birth the frontal recess (located
between J and IJ, Fig. 373) gives rise to the frontal sinus. During the third month the maxil-
lary sinus grows out from the inferior recess of the same groove. The most caudal end of the
nasal fossa becomes the sphenoidal sinus, which, as it increases in size, invades the sphenoid
bone.
The cells of the olfactory epithelium become ciliated, but only a small area, representing
the primitive epithelial invagination, functions as an olfactory sense organ. The olfactory
cells of this area give rise to the fibers which constitute the olfactory nerve (cf. p. 357).
IV. THE DEVELOPMENT OF THE EYE
The anlage of the human eye appears in embryos of 2.5 mm. as a thickening
and evagination of the neural plate of the fore-brain. At this stage the neural
groove of the fore-brain has not closed (Figs. 324, 330 and 382). At 4 mm. the
optic vesicles are larger, but still may be connected by a wide opening with the
brain cavity (Fig. 374 A, B). In the section shown in Fig. 374 C, the optic
vesicle is attached to the ventral brain wall by a distinct optic stalk (cf. Fig. 343).
The thickening, flattening, and invagination of the distal and ventral wall of
the optic vesicle gives rise to the optic cup (Fig. 374 B-D). The area of invagina-
tion also extends ventrally along the optic stalk and produces a groove known as
the chorioid fissure (Figs. 331, 375 and 377).
At the same time that the optic vesicle is converted into the optic cup, the
ectoderm overlying the former thickens, as seen in Fig. 374 B, forming the lens
plate, or optic placode. This plate invaginates to form the lens pit, the external
opening of which closes in embryos of 6 to 7 mm. (Fig. 374 D), producing the
lens vesicle, which remains at first attached to the overlying ectoderm. In an
DEVELOPMENT OF THE SENSE ORGANS 377
embryo of 10 mm. (Fig. 376) the lens vesicle has separated from the ectoderm,
which will form the epithelium of the cornea. The lens vesicle in earlier stages
Anlage of lens
Optic vesicle
Retinal layer
Fic. 374.—Stages in the early development of the humaneye. A, B, at 4mm. (X 27); C, at 5mm. (X
23); D, at 6.25 mm. (X 18) (after Keibel and Elze).
(Fig. 374 D) is closely applied to the inner wall of the optic cup, but now it has
separated from it, leaving a space in which the vitreous body is developing.
The inner retinal layer of the
optic cup has become very thick
and is applied to the outer layer,
so that the cavity of the primi-
tive optic vesicle is nearly ob-
literated (Fig. 376). Pigment
granules have begun to appear in
the outer cells which form the
pigment layer of the retina. Mes-
enchymal tissue surrounds the
optic cup and is beginning to
make its way between the lens
vesicle and the ectoderm. Here
Crystalline lens
Chorioid fissure
Fic. 375.—The optic stalk, cup and lens of a human
embryo of 12.5 mm. The chorioid fissure has not yet ex-
tended along the optic stalk (from Fuchs, after Hoch-
stetter). xX 90.
the anterior chamber of the eye develops later as a cleft in the mesoderm. The
distal mesenchymal tissue (next the ectoderm) forms the substantia propria of the
THE PERIPHERAL NERVOUS SYSTEM
Optic recess of brain
378
Vitreous body Optic stalk
Mesenchyma Lens vesicle
SHY tan
Psi.
2
Se foam
2n28h
722
isso a s
$ Oe
28
/ 5 \ ' 3
Epithelium of cornea Pigment layer of retina Nervous layer of retina
Frc. 376.—A transverse section through the optic cup, stalk and lens of a 10 mm. human embryo.
x 100. ;
Epithelial layer of lens Pigment layer of the retina
Central artery
SN
=
ss on
Mr &
wel tas
Serih Pre
Be an Vitreous body
Bio. ef
BN (
; Layer of lens fibers
Mesenchyme
Fic. 377.—Transverse section passing through the optic cup at the level of the chorioid fissure.
The central artery of the retina is seen entering the fissure and sending a branch to the proximal surface
of the lens; from a 12.5 mm. humanembryo. X 105.
cornea and its posterior epithelium, while the proximal mesenchyma (next the
lens) differentiates into the vascular capsule of the lens. The mesenchyme sur-
rounding the optic cup is continuous with that which forms the cornea and later
DEVELOPMENT OF THE SENSE ORGANS 379
gives rise to the sclerotic layer, to the chorioid layer, and to the anterior layers
of the ciliary body and iris.
Both the inner and outer layers of the optic cup are continued into the optic
stalk, as seen in Fig. 376. This is due to the trough-like invagination of the
ventral wall of the optic stalk} the chorioid fissure, when the optic vesicle is trans-
formed into the optic cup (Fig. 375). Into the chorioid fissure grows the central
artery of the retina, carrying with it into the posterior cavity of the eye a small
amount of mesenchyme (Fig. 377). Branches from this vessel extend to the
posterior surface of the lens and supply it with nutriment for its growth. Ata
later stage the chorioid fissure Epithelial layer
closes, so that the distal rim of =
the optic cup forms a complete
circle.
If the chorioid fissure fails to
close, the optic cup remains open at
one point and this results in the de-
Capsule
fective development of the iris, ciliary
body, and chorioid layer. Such a Vascular
defect is known as coloboma. S membrane
The old view that the develop-
ment of the lens vesicle causes the Lens fibers
formation of the optic cup by push-
ing in its distal wall has been dis-
proved by W. H. Lewis, for if an
anlage of the optic vesicle from an
amphibian embryo be transplanted
to some other part of the embryo,
it can develop into an optic cup in
the absence of a lens. Furthermore,
it is the contact of optic vesicle with
ectoderm that furnishes the stimulus for lens formation, both normally and after trans-
plantation to foreign regions, e. g., abdomen.
Ectoderm
Fic. 378.—Section through the lens and corneal ecto-
derm of a16 mm. pigembryo. X 140.
The lens vesicle, and its early development from the ectoderm, have been de-
scribed. Its proximal wall is much thickened in 10 mm. embryos, and these
cells form the Jens fibers (Fig. 376) which will soon obliterate the cavity of the
vesicle, as in embryos of 15 to 17 mm. (Fig. 378). The cells of the distal layer
remain of a low columnar type and constitute the epithelial layer of the lens.
When the lens fibers attain a length of 0.18 mm. they cease forming new fibers by
cell division. New fibers thereafter arise from the cells of the epithelial layer at
its line of union with the lens fibers. The nuclei are arranged in a layer convex
toward the outer surface of the eye and later degenerate, the degeneration begin-
380 THE PERIPHERAL NERVOUS SYSTEM
ning centrally. Lens sutures are formed on the proximal and distal faces of the
lens when the longer newly formed peripheral fibers overlap the ends of the
shorter central fibers. By an intricate but orderly arrangement of fibers these
sutures are later transformed into lens-stars of three, and finally of six or nine
rays (Fig. 379). The structureless capsule of the lens is probably derived from
the lens cells. The lens, at first somewhat triangular in cross section, becomes
nearly spherical at three months (Fig. 379).
Raphe between Posterior epithelium
palpebra of cornea
Anterior epiihelium
of corned
Anterior
chamber
Cornea
Epithelium
of lens
Pars iridica
retn@
Pigment layer
of retina
Pars optica
reting
Lens fibers
Lens capsule
Vitreous
: a body
& te : = é FAM Xervarne Hin:
Fic. 379.—Section through the distal half of the eyeball and through the eyelids of a 65 mm. human
fetus. X 35.
The origin of the vitreous body has been in doubt, one view deriving it from
the mesenchyma which enters the optic cup through the chorioid fissure and
about the edge of lens, another view holding that it arises from cytoplasmic
processes of cells in the retinal layer.
It is certain that the vitreous tissue is formed before mesenchyma is present in the cavity
of the optic cup. Szily (Anat. Hefte, Bd. 35, 1908) regards this primitive vitreous body as a
DEVELOPMENT OF THE SENSE ORGANS 381
derivative of both retinal and lens cells, it forming a non-cellular network of cytoplasmic
processes which are continuous with the cells of the lens and retina. With the ingrowth of
the central artery of the retina, from which the artery of the lens passes to the proximal sur-
face of the lens and branches on it, a certain amount of mesenchymal tissue invades the optic
cup, and this tissue probably contributes to the development of the vitreous body (Tig. 377).
The vitreous body may therefore be regarded as a derivative both of the ectoderm and
of the mesoderm.
The mesenchyma accompanying the vessels to the proximal surface of the
lens, and that on its distal surface, give rise to the vascular capsule of the lens
(Fig. 377). On the distal surface of the lens this is supplied by branches of the
anterior ciliary arteries and is known as the pupillary membrane; the vessels
disappear and the membrane degenerates just before birth. The artery of the
<—External limiting
A ‘i & DCEO Ae Ate ee membrane
Cone a | , A ieee me nes ee
Ree JO a “= TLayer of rod and cone
cells
Fiber of Miiller
Amacrine cell
Internal limiting
membrane
Frc. 380.—Section of the nervous layer of the retina from a 65 mm. human fetus. At the left is shown
diagrammatically the cellular elements of the retina according to Cajal. 440.
lens also degenerates, its wall persisting as the transparent hyaloid canal. Fibrille
extending in the vitreous humor from the pars ciliata of the retinal layer to the cap-
sule of the lens persist as the zonula ciliaris or suspensory ligament of the lens.
Differentiation of the Optic Cup.—We have seen that of the two layers of the
optic cup the outer becomes the pigment layer of the retina. Pigment granules
appear in its cells in embryos of 7 mm. and the pigmentation of this layer is
marked in 12 mm. embryos (Fig. 377).
The inner, thicker layer of the optic cup, the retinal layer, is subdivided into
a distal zone, the pars ceca, which is non-nervous, and into the pars optica, or the
nervous retina proper. The line of demarcation between the pars optica and the
pars caca is a serrated circle, the ora serrata. The blind portion of the retinal
layer, the pars czeca, with the development of the ciliary bodies is differentiated
382 THE PERIPHERAL NERVOUS SYSTEM
into a pars ciliaris and pars iridis retine. The former, with a corresponding zone
of the pigment layer, covers the ciliary bodies. The pars iridis forms the proxi-
mal layer of the iris and blends intimately with the pigment layer in this region,
its cells also becoming heavily pigmented (Fig. 379).
The pars optica, or nervous portion of the retina, begins to differentiate
proximally, the differentiation extending distally. An outer cellular layer and an
inner fibrous layer may be distinguished in 12 mm. embryos (Fig. 377). These
correspond to the cellular
Pigment layer layer (ependymal and mantle
Rods and Cones .
zones) and marginal layer of
ar eee ae the neural tube. In fetuses
Outer reticular. layer of 65 mm. (C R) the retina
ror WD shows three layers, large
Be ees ime nuclear layer ganglion cells having ‘tai
dé grated in from the outer
cellular layer of rods and
cones (Fig. 380). In a fetus
of the seventh month all the
layers of the adult retina
cele He Tet may be recognized (Fig. 381).
Fibers of Miiller As in the wall of the neural
tube, there are differentiated
aN
oTharine Walt
in the retina supporting tis-
ll
Fic. 381.—Section through the pars optica of the retina
from a seven months’ fetus. X 440.
Internal limiting membrane ’ ‘
= une . sue and nervous tissue. The
supporting elements, or fibers
of Miiller, resemble epen-
dymal cells and are radially arranged (Figs. 380 and 381). Their terminations
form internal and external limiting membranes.
The neuroblasts of the retina differentiate into an outer layer of rod and
cone cells, the visual cells of the retina, which are at first unipolar (Fig. 381).
Internal to this layer are layers of bipolar and multipolar cells. The inner layer
of multipolar cells constitutes the ganglion cell layer. Axons from these cells form
the inner nerve fiber layer of optic fibers. These converge to the optic stalk, and,
in embryos of 15 mm., grow back in its wall to the brain. The cells of the optic
stalk are converted into neuroglia supporting tissue and the cavity of the stalk is
gradually obliterated. The optic stalk is thus transformed into the optic nerve
(cf. p. 358).
DEVELOPMENT OF THE SENSE ORGANS 383
The Sclerotic and Chorioid Layers, and their Derivatives.—After the mes-
enchyme grows in between the ectoderm and the lens (Fig. 377), the lens and op-
tic cup are surrounded by a condensed layer of mesenchymal tissue, which gives
rise to the supporting and vascular layers of the eyeball. By condensation and
differentiation of its outer layers, a dense layer of white fibrous tissue is developed,
which forms the sclerotic layer. This corresponds to the dura mater of the brain.
In the mesenchyme of 25 mm. embryos a cavity appears distally which separates
the condensed layer of mesenchyme continuous with the sclerotic from the vascu-
lar capsule of the lens (Fig. 379). This cavity is the anterior chamber of the eye
and separates the anlage of the cornea from the lens capsule.
An inner layer of mesenchyme, between the anlage of the sclerotic and the
pigment layer of the retina, becomes highly vascular during the sixth month.
Its cells become stellate in form and pigmented, so that the tissue is loose and
reticulate. This vascular tissue constitutes the chorioid layer, in which course
the chief vessels of the eye. The chorioid layer corresponds fo the pia mater of
the brain. Distal to the ora serrata of the retinal layer the chorioid is differen-
tiated: (1) into the vascular folds of the ciliary bodies; (2) into the smooth fibers
of the ciliary muscle; (3) into the stroma of the iris. The proximal pigmented
layers of the iris are derived from the pars iridis retina and from a corresponding
zone of the pigment layer. Of these, the pigment layer cells give rise to the
sphincter and dilator muscles of the iris. These smooth muscle fibers are thus of
ectodermal origin.
The Eyelids appear as folds of the integument in 20 mm. embryos. The lids
come together and the epidermis at their edges is fused in 33 mm. embryos (Fig.
379). Later, when the epidermal cells are cornified, separation of the eyelids
takes place. A third rudimentary eyelid, corresponding to the functional nicti-
tating membrane of lower vertebrates, forms the plica semilunaris. The epi-
dermis of the eyelids forms a continuous layer on their inner surfaces known as
the conjunctiva, which in turn is continuous with the anterior epithelium of the
cornea.
The Eyelashes, or cilia, develop like ordinary hairs and are provided with
small sebaceous glands. In the tarsus, or dense connective tissue layer of the
eyelids, which lies close to the conjunctival epithelium, there are developed about
30 tarsal (Meibomian) glands. These arise as ingrowths of the epithelium at the
edges of the eyelids, while the latter are still fused.
The Lacrimal Glands appear in embryos of about 25 mm., according to
Keibel and Elze. They arise as five or six ingrowths of the conjunctiva, dorsally
384 THE PERIPHERAL NERVOUS SYSTEM
and near the external angle of the eye. The anlages are at first knob-like, but
rapidly lengthen into solid epithelial cords. They begin to branch in 30 mm.
embryos. At stages between 50 and 60 mm. (C R) additional anlages appear
which also branch.
In 38 mm. (C R) embryos a septum begins to partition the gland into orbital and pal-
pebral portions. ‘This septum is complete at 60 mm. (C R), the five or six anlages first de-
veloped constituting the peripheral orbital part. Lumina appear in the glandular cords in
fetuses of 50 mm. (C R) by the degeneration of the central cells. ‘Accessory lacrimal glands
appear in 300 mm. (C R) fetuses. The lacrimal gland is not fully differentiated at birth, being
only one-third the size of the adult gland. In old age marked degeneration occurs.
The Naso-lacrimal Duct arises in 12 mm. embryos as a ridge-like thickening
of the epithelial lining of the naso-lacrimal groove (Fig. 149), which, it will be
remembered, extends from the inner angle of the eye to the olfactory fossa. This
thickening becomes cut off, and, as a solid cord, sinks into the underlying meso-
derm (Schaeffer). Secondary sprouts growing out from this cord to the eyelids
form the lacrimal canals. A lumen, completed at birth, appears during the third
month (Fig. 372).
V. THE DEVELOPMENT OF THE EAR
The human ear consists of a sound-conducting apparatus and of a receptive
organ. The conveyance of sound is the function of the external and middle ears.
Hind-brain
Auditory ganglion Auditory placode ’ :
; a Otic vesicle
Optic vesicle
A B
Fic. 382.—Two stages in the early development of the internal ear (after Keibel and Elze). A,
Horizontal section through the head and open neural tube of a 2 mm. human embryo showing the
auditory placode and ganglion (X 27); B, section through the hind-brain and otic vesicles of a 4mm.
human embryo (X 33).
The end organ proper is the inner ear with the auditory apparatus residing in the
cochlear duct. Besides this acoustic function the labyrinthine portion of the inner
ear acts as an organ of equilibration.
DEVELOPMENT OF THE SENSE ORGANS 385
The Inner Ear.—The epithelium of the internal ear is derived from the ecto-
derm. Its first anlage appears in embryos of 2 mm. as a thickened ectodermal
Ectoderm Wall of hind-brain
— Neur. 4
Fic. 383.—Four sections through the right otic vesicle of a 4mm. human embryo (after Keibel and
Elze). X about 30. r.e., Endolymphatic recess, the anlage of the endolymph duct and sac; 0.v., otic
vesicle; Neur. 4, Neur. 5, neuromeres four and five of the myelencephalon.
plate, the auditory placode (Fig. 382 A). These are developed, dorsal to the
second branchial grooves, at the sides of the hind-brain opposite the fifth neuro-
meres (Fig. 383). The placodes are invaginated to form hollow vesicles which
close in embryos of 2.5 to 3
mm., but remain temporarily
attached to the ectoderm — qoy of myclencephalon
(Fig. 382 B).
The auditory vesicle, or Endolymph duct
otocyst, when closed and de-
tached, is nearly spherical, Vesiibsilor anlage
but approximately at the
point where it was attached
to the ectoderm a recess, the
ductus endolymphaticus, is Cag earnrnaee
formed. The point of origin
of this recess is shifted later
from a dorsal to a mesial Fic. 384.—Right half of a transverse section through
position (Figs. 384 and 3854). the hind-brain and otic vesicle, showing the position of the
The en dolymph duck coves: a duct. From a human embryo 6.9 mm. long
ponds to that of selachian
fishes, which remains permanently open to the exterior. In man, its extremity
is closed and dilated to form the endolymphatic sac (Fig. 385 e).
The differentiation of the auditory vesicle has been described by His, Jr.,
and more recently by Streeter (Amer. Jour. Anat., vol. 6, 1906). In an em-
bryo of about 7 mm. the vesicle has elongated, its narrower ventral process con-
25
386 THE PERIPHERAI, NERVOUS SYSTEM
stituting the anlage of the cochlear duct (Fig. 385 a). The wider, dorsal portion
of the otocyst is the vestibular anlage, which shows indications dorsally of the de-
veloping semicircular canals. These are formed in 11 mm. embryos as two
pouches—the anterior and posterior canals from a single pouch at the dorsal
border of the otocyst, the lateral canal later from a lateral outpocketing (Fig.
385 c). Centrally the walls of these pouches flatten and fuse to form epithelial
plates. In the three plates thus produced canals are left peripherally, communi-
cating with the cavity of the vestibule. Soon the epithelial plates are resorbed,
leaving the semicircular canals as in Fig. 385 d, e. Dorsally a notch separates
the anterior and posterior canals. Of these canals, the anterior is completed be-
fore the posterior. The lateral canal is the last to develop.
In a 20 mm. embryo (Fig. 385 e) the three canals are present and the coch-
lear duct has begun to coil like a snail shell. It will be seen that the anterior and
posterior canals have a common opening dorsally into the vestibule, while their
opposite ends and the cranial end of the lateral canal are dilated to form ampulle.
In each ampulla is located an end organ, the crista ampullaris, which will be re-
ferred to later. By a constriction of its wall the vestibule is differentiated into
a dorsal portion, the utriculus, to which are attached the semicircular canals, and
a ventral portion, the sacculus, which is connected with the cochlear duct (Fig.
385 e, f). At 30 mm. the adult condition is nearly attained. The sacculus and
utriculus are more completely separated, the canals are relatively longer, their
ampullz more prominent, and the cochlear duct is coiled about two and a half
turns (Fig. 385 f). In the adult, the sacculus and utriculus become completely
separated from each other, but each remains attached to the endolymph duct by
a slender canal which represents the prolongation of their respective walls.
Similarly, the cochlear duct is constricted from the sacculus, the basal end of the
former becomes a blind process, and a canal, the ductus reuniens, alone connects
the two.
The epithelium of the labyrinth at first is composed of a single layer of low
columnar cells. At an early stage, fibers fyom the acoustic nerve grow between
the epithelial cells in certain regions and these become modified to produce special
sense organs. These end organs are the criste ampullares in the ampulle of the
semicircular canals, the macule acustice in the utriculus and sacculus, and the
spiral organ (of Corti) in the cochlear duct.
The criste and macule are static organs, or sense organs for maintaining
equilibrium. In each ampulla, transverse to the long axis of the canal, the epi-
thelium and underlying tissue form a curved ridge, the crista. The cells of the
lat.groove Genet absorpt. veel
endolymph. vestib. p.
vestib, é - lat. groove
pouch “7
£0Ch,
pouch cochlea
&.6.6mm lateral. b omm.lareral,
@. 20mm iateral. . f. 30mm. lateral.
Fic. 385.—Six stages in the development of the internal ear (Streeter). X25. The figures show
lateral views of models of the left membranous labyrinth—e at 6.6 mm.; 6 at 9mm.; cat 11 mm.; d at
13 mm.; e at 20 mm., and f at 30 mm. The colors yellow and red are used to indicate respectively
the cochlear and vestibular divisions of the acoustic nerve and its ganglia. absorp. focus, Area of wall
where absorption is complete; crus, crus commune; c. sc. /at., ductus semicircularis lateralis; c. sc. post.,
ductus semicircularis posterior; v. sc. svp., ductus semicircularis superior or anterior; cochlea, ductus
cochlearis; coch. pouch, cochlear anlage; endolymph., appendix endolymphaticus; sacc., sacculus; sac. en-
dol., saccus endolymphaticus; simus ut. dat., sinus utriculi lateralis; wéric., utriculus.
DEVELOPMENT OF THE SENSE ORGANS 387
epithelium are differentiated: (1) into sense cells with bristle-like hairs at their
ends, and (2) into supporting cells. About the bases of the sensory cells nerve
fibers from the vestibular division of the acoustic nerve branch. The macule
resemble the criste in their development save that larger areas of the epithelium
are differentiated into cushion-like end organs. Over the maculz concretions of
lime salts may form otoconia which remain attached to the sensory bristles.
The true organ of hearing, the spiral organ, is developed in the basal epithe-
lium of the cochlear duct, basal having reference here to the base of the cochlea.
The development of the spiral organ has been studied carefully only in the lower
mammals. According to Prentiss (Amer. Jour. Anat., vol. 14, 1913) in pig em-
bryos of 5 cm. the basal epithelium is thickened, the cells becoming highly colum-
nar and the nuclei forming several layers. In later stages, 7 to 9 cm., inner and
outer epithelial thickenings are differentiated, the boundary line between them
being the future spiral tunnel (Fig. 386 A). At the free ends of the cells of the
epithelial swellings there is formed a cuticular structure, the membrana tectoria,
which appears first in embryos of 4to5 cm. The cells of the inner (axial) thick-
ening give rise to the epithelium of the spiral limbus, to the cells lining the internal
spiral sulcus, and to the supporting cells and inner hair cells of the spiral organ (Fig.
386 B,C). The outer epithelial thickening forms the pillars of Corti, the outer
hair cells, and supporting cells of the spiral organ. Differentiation begins in the
basal turn of the cochlea and proceeds toward the apex. The internal spiral
sulcus is formed by the degeneration and metamorphosis of the cells of the inner
epithelial thickening which lie between the labium vestibulare and the spiral
organ (Fig. 386 B,C). These cells become cuboidal, or flat, and line the spiral
sulcus, while the membrana tectoria loses its attachment with them. The mem-
brana tectoria becomes thickest over the spiral organ and in full term fetuses is
still attached to its outer cells (Fig. 386 C).
Hardesty (Amer. Jour. Anat., vol. 18, 1915), on the contrary, asserts that the membrana
tectoria is not attached permanently to the cells of the spinal organ.
From what is known of the development of the spiral organ in human embryos, it follows
the same lines of development as described for the pig. It must develop relatively late, how-
ever, for in the cochlear duct of a newborn child figured by Krause the spiral sulcus and the
spiral tunnel are not yet present.
The mesenchyme surrounding the labyrinth is differentiated into a fibrous
membrane directly surrounding the epithelium, and into the perichondrium of
the cartilage which develops about the whole internal ear. Between these two
is a more open mucous tissue which largely disappears, leaving the perilymph
388 THE PERIPHERAL NERVOUS SYSTEM
oo io s.c.tymp. B
st... ~%, Ay { Na cm tect.
Ze
\" \
AN A BANOS
\ RENO SON
i.pil.
Fic. 386.—Three stages in the differentiation of the basal epithelium of the cochlear duct to form
the spiral organ (of Corti), internal spiral sulcus and labium vestibulare. ., Section through the coch-
lear duct of an 8.5 cm. pig fetus (x 120); B, the same from a 20 cm. fetus (X 140); C, from a 30 cm.
fetus (near term) (X 140). ep.s.sp., Epithelium of spiral sulcus; /.c., hair cells; 7.ep.c., inner epithelial
thickening; i./.c., inner hair cells; 7.p7/., inner pillar of Corti; /ab. vest., labium vestibulare; Limb. sp.,
limbus spiralis; m. bas., basilar membrane; m. ¢ec!., membrana tectoria; m. vest., vestibular membrane;
n. coch., cochlear division of acoustic nerve; 0.ep.c., outer epithelial thickening; 0./.c., outer hair cells;
s.sp., sulcus spiralis; sc.tymp., scala tympani; st. H., stripe of Hensen; t.s., spiral tunnel.
DEVELOPMENT OF THE SENSE ORGANS 389
space. The membranous labyrinth is thus suspended in the fluid of.the peri-
lymph space. The bony labyrinth is produced by the conversion of the cartilage
capsule into bone. In the case of the cochlea, large perilymph spaces form above
and below the cochlear duct. The duct becomes triangular in section as its
lateral wall remains attached to the bony labyrinth, while its inner angle is ad-
herent to the modiolus. The upper perilymph space is formed first and is the
scala vestibuli, the lower space is the scala tympani. The thin wall separating
the cavity of the cochlear duct from that of the scala vestibuli is the vestibular
membrane (of Reissner). Beneath the basal epithelium of the cochlear duct a
fibrous structure, the basilar membrane, is differentiated by the mesenchyme.
The modiolus is not preformed as cartilage, but is developed directly from the
mesenchyme as a membrane bone. The development of the acoustic nerve has
been described on page 358 with the other cerebral nerves.
The Middle Ear—The middle ear cavity is differentiated from the first
pharyngeal pouch which appears in embryos of 3mm. The pouch enlarges rap-
idly up to the seventh week, is flattened :
horizontally, and is in contact with the = mae
° * Br. arch. I
ectoderm (Fig. 168). During the latter (Meckel’s cartilage)
part of the second month, in embryos
of 24 mm., the wall of the tympanic :
cavity is constricted to form the auditory - Tngamin| -
(Eustachian) tube. This canal lengthens ae. ae i eal
and its lumen becomes slit-like during ie Cer tee opine Jas Baal
the fourth month. The tympanic cavity arch origin of the auditory ossicles.
is surrounded by loose areolar connec-
tive tissue in which the auditory ossicles are developed and for a time are em-
bedded. Even in the adult, the ossicles, muscles, and chorda tympani nerve retain
a covering of mucous epithelium continuous with that lining the tympanic cavity.
The pneumatic cells are formed at the close of fetal life. .
The development of the auditory ossicles has been described by Broman
(Anat. Hefte, Bd. 11, 1899), with whose general conclusions most recent workers
agree. The condensed mesenchyma of the first and second branchial arches gives
rise to the ear ossicles.
The malleus and incus are differentiated from the dorsal end of the first arch
(Fig. 387). The cartilaginous anlage of the malleus is continuous ventrally with
Meckel’s cartilage of the mandible. Between the malleus and incus is an inter-
mediate disk of tissue, which later forms an articulation. When the malleus be-
390 THE PERIPHERAL NERVOUS SYSTEM
gins to ossify it separates from Meckel’s cartilage. The incus is early connected
with the anlage of the stapes, and the connected portion becomes the crus longum.
Between this and the stapes an articulation develops.
The stapes and Reichert’s cartilage are derived from the second branchial
arch (Fig. 387). The mesenchymal anlage of the stapes is perforated by the
stapedial artery, and its cartilaginous anlage is ring-shaped. This form persists
until the middle of the third month, when it assumes its adult structure and the
stapedial artery disappears.
Fic. 388.—Stages in the development of the auricle. (Adapted in part after His). A,11mm.; B,
13.6 mm.; C,15 mm.; D,adult. 1, 2, 3, elevations on the mandibular arch; 4, 5, 6, elevations on the
hyoid arch; af, auricular fold; ov, otic vesicle; 1, tragus; 2, 3, helix; 4, 5, antihelix; 6, antitragus.
The muscle of the malleus, the fensor tympani, is derived from the first branchial arch;
the stapedial muscle from the second arch. The further fact that these muscles are innervated
' by the trigeminal and facial nerves, which are the nerves of the first and second arches re-
spectively, points toward a similar origin for the ear ossicles.
Fuchs, studying rabbit embryos, on the contrary, concludes: (1) the stapes is derived
from the capsule of the labyrinth; (2) the malleus and incus arise independently of the first
branchial arch.
The External Ear.—The external ear is developed from and about the first
ectodermal branchial groove. The auricle arises from six elevations which appear,
three on the mandibular, and three on the hyoid arch (Fig. 388). Modern ac-
DEVELOPMENT OF THE SENSE ORGANS 391
counts of the transformation of these hillocks into the adult auricle agree in the
main.
Caudal to the hyoid anlages a fold of the hyoid integument is formed, the
auricular fold or hyoid helix. A similar fold forms later, dorsal to the first branch-
ial groove, and unites with the auricular fold to form with it the free margin of the
auricle. The point of fusion of these two folds marks the position of the satyr
tubercle, according to Schwalbe. Darwin’s tubercle appears at about the middle of
the margin of the free auricular fold, and corresponds to the apex of the auricle in
lower mammals. The éragus is derived from mandibular hillock 1; the helix from
mandibular hillocks 2 and 3; the antihelix from hyoid hillocks 4 and 5; the anti-
tragus from hyoid hillock 6. The Jobule represents the lower end of the auricular
fold.
The external auditory meatus is formed as an ingrowth of the first branchial
groove. In embryos of 12 to 15 mm. the wall of this groove is in contact dorsally
with the entoderm of the first pharyngeal pouch. Later, however, this contact
is lost, and during the latter part of the second month, according to Hammar,
an ingrowth takes place from the ventral portion of the groove, to form a funnel-
shaped canal.
The lumen of this tube is temporarily closed during the fourth and fifth months, but
later re-opens. During the third month a cellular plate at the extremity of the primary
auditory meatus grows in and reaches the outer end of the tympanic cavity. During the
seventh month a space is formed by the splitting of this plate, and the secondary inner por-
tion of the external meatus is thus developed.
The tympanic membrane is formed by a thinning out of the mesodermal tissue
in the region where the wall of the external auditory meatus abuts upon the wall
of the tympanic cavity. Hence it is covered externally by ectodermal, and inter-
nally by entodermal epithelium.
INDEX
ABDOMINAL pregnancy, 20
Abducens nerve, 116, 361
Accessorius nerve, 116
Acoustic nerve, 92, 116, 358
Acrania, 352
Adipose tissue, 287
Adrenalin, 370
After pains, 241
After-birth, 242
Ale nasi, 145
Alar plate, 323, 332, 335, 337
Allantoic stalk, 66, 97, 120
vessels, 67
Allantois, 57, 66, 69, 74, 83, 93, 97
derivation ‘of, 70, 74
section through, ‘1
Allelomorphs, 21
iveoli of pancreas, 179
Alveolo-lingual gland, 152
Ameloblasts, 156
Amitosis, 12
Amnion, origin of, 65, 73
bat, 74
chick, 55, 62, 65
human, 71, 73
pig, 68
Amniotic fluid, 74
Amphiaster, 13
Amphibia, cleavage of ovum in, 25
gastrulation of, 28
origin of mesoderm in, 30
Amphioxus, cleavage of ovum in, 23
gastrulation of, 28
origin of mesoderm in, 29
Ampulla of ductus deferens, 218
Anal membrane, 160, 205
Anaphase of mitosis, 13
Anchoring villi, 237
Angioblast, 39
chick, 42
human, 243
Animal pole, 23
Anlage defined, 3
Ansa hypoglossi, 355, 360
Antihelix, 391
Antitragus, 391
Anus, 97, 143, 160
Aorta, origin of, 258
chick, 40
descending, 40, 46, 48, 59, 60, 62, 83, 99, 130,
260, 261
dorsal, 46, - 85, 99, 122, 260, 264
pig, 99, 121
ventral, 46, 48, 57, 83, 99, 249, 260
Aortic arches, 59, 60, 121, 122) 263, 264
chick, 57
393
Aortic arches, human, 83, 261
transformation of, 261
Appendicular skeleton, 315
Appendix epididymidis, 218
testis, 218, 219
vermiformis, 172, 174
Aqueduct, cerebral, 330, 337
Archenteron, 28, 29, 31, 34
Archipallium, 346
Arcuate fibers, 335
Area opaca, 36
pellucida, 36
scrotal, 227
vasculosa, 243
Areolar tissue, 286
Artemia, 14
Arteries, axillary, 268
basilar, 264, 266
brachial, 268
carotid, 99, 121, 262, 263, 266
cerebral, 266
choroidal, anterior, 266
ceeliac, 99, 122, 262, 267
development of, 261
epigastric, 266
femoral, 268
gluteal, 268
hepatic, 178
hypogastric, 268
iliac, 123, 267, 268
innominate, 263
intercostal, 266
interosseous, 268
intersegmental, 99, 122, 260, 262, 265
ischiadic, 268
lumbar, 266
mammary, 266
median, 268
mesenteric, 262, 267
inferior, 122
superior, 122, 261
of extremities, 268
of heart, 83
of lower extremity, 268
of pig, 99, 121
of upper extremity, 268
ovarian, 267
peroneal, 268
phrenic, 267
popliteal, 268
pulmonary, 99, 121, 122, 168, 258, 262
radial, 268
renal, 205, 267
sacral, middle, 268
spermatic, 267
spinal, 264
394 INDEX
Arteries, stapedial, 390
subclavian, 122, 262, 263, 266, 268
suprarenal, 267
ulnar, 268
umbilical, 86, 99, 123, 135, 260, 267
ventral, 99, 122
ventro-lateral, 99, 122
vertebral, 122, 264
vitelline, 46, 58, 63, 86, 99, 122, 260, 261, 262
chick, 42
Artificial parthenogenesis, 20
Arytenoid cartilage, 166
folds, 117
ridges, 94, 151
swellings, 165
Ascaris megalocephala bivalens, reduction of
chromosomes in spermatogenesis of, 16
univalens, 13
Atrial canal, 251
foramina, 98
septa, 251
valves, 259
Atrio-ventricular bundle, 259
foramen, 252
Atrium, 57, 83, 90, 97, 249, 251
Auditory meatus, external, 90, 112, 146, 391
nerve, 358
ossicles, 389
placode, 45, 48, 385
tube, 82, 161, 389
vesicle, 385
Auricle of ear, 146, 390
Auricular fold, 391
Autonomic system, 366
Axial skeleton, 309
Axillary artery, 268
nerve, 356
vein, 276
Axis cylinder of nerve fiber, 302
Azygos vein, 274
Basy, blue, 255
Back, muscles of, 317
Bars, sternal, 311
Bartholin’s glands, 228
Basal plate, 239, 240, 323, 332
Basilar artery, 264, 266
membrane, 389
Basilic vein, 276, 277
Basophiles, 247
Bertin’s renal columns, 202
Bicuspid valves, 121, 259
Bile capillaries, 177
duct, common, 120
Biogenesis, law of, 5
Birds, cleavage of ovum in, 26
gastrulation of, 28
origin of mesoderm in, 31
Bladder, 77, 143, 205, 207
Blastoccele, 24
Blood corpuscles, red, 244, 245
white, 245. See also Leucocytes.
elements, monophyletic theory of origin, 243
polyphylectic theory of origin, 243
islands, 38, 42, 243
plates, 247
sinuses, 281
vascular system, pig, 97, 120
vessels, anomalies of, 277
changes at birth, 278
chick, 42, 46, 57
human, 243, 247
pig, 97, 120
primitive, 259
Blue baby, 255
Body cavities, 179
stalk, 71
Bone, cartilage, 288, 289
growth of, 290
cells, 289
destroyers, 289
ethmoid, ossification of, 313
formation, endochondral, 289
perichondral, 289, 290
periosteal, 289
formers, 288
growth of, 290
histogenesis of, 288
lacrimal, 314
lacune, 289
marrow, 289
red, 289
yellow, 290
membrane, 288
of skull, 314
nasal, 314
occipital, ossification of, 312
palate, 314
regeneration of, 291
sphenoid, ossification of, 313
temporal, ossification of, 313
zygomatic, 314
Border vein, 276
Bowman’s capsule, 198, 199
Brachial artery, 268
plexus, 355
vein, 276
Brachium conjunctivum, 337
pontis, 336
Brain, human, 327
of pig, 115
olfactory, 328
vesicles, primary, 322, 327
Branchial arches, 315
chick, 57
human, 82
pig, 89, 94, 112
skeleton, 314
Blastoderm, 26
Blastodermic vesicle, 26
Blastomeres, 23
Blastopore, 29, 31
Blastula, 24
Blood cells, 42, 243, 244
ichthyoid, 244
sauroid, 244
Branchiomerism, 366
Broad ligaments, 221
Bronchi, primary, of pig, 95, 119
ventral, 167
Bronchial buds, 119, 167
Brunner, duodenal glands of, 173
Bulbar limb, 249
swellings, 121
Bulbo-urethral glands, 228
Bulbus cordis, 57, 60, 83, 249
Bundle, atrio-ventricular, 259
ground, 325
median longitudinal, 335
Bursa infracardiaca, 190
omental, 189
inferior recess of, 190
pharyngeal, 162
Cacum, 120, 143, 172, 173, 174
‘Calcar avis, 351
Calcarine fissure, 351
Calyces of metanephros, 120, 199
Canal, atrial, 251
atrio-ventricular, 252
central, of adult spinal cord, 324
entodermal, 159
Gartner’s, 219
Haversian, 290
hyaloid, 381
incisive (of Stenson), 148
inguinal, 221, 222
lacrimal, 384
pleuro-peritoneal, 183
Stenson’s, 148
Canaliculi, 289
aberrans, 199
Capillaries, bile, 177
Capsule, Bowman’s, 198, 199
cells, 306
internal, 345
of liver, 192
periotic, 312
vascular, of lens, 378, 381
Cardiac diverticulum, 119
glands, 170
muscle, 291, 293
Cardinal veins, 123
anterior, 49, 58, 59, 60, 101, 261, 268, 271
common, 58, 60, 97, 101, 261, 268, 271
posterior, 58, 62, 63, 102, 261, 268, 274
Carotid arteries, 99, 121
common, 263
internal, 262, 263, 266
gland, 369
‘Carpus, ossification of, 316
Cartilage, arytenoid, 166
bone, 288, 289
growth of, 290
corniculate, 166
cricoid, 166
cuneiform, 166
elastic, 287
fibro-, 287
histogenesis of, 287 *
hyaline, 287
Meckel’s, 315
of larynx, 165
Reichert’s, 390
thyreoid, 166, 315
Cauda equina, 326
‘Caudate lobe of liver, 192
nucleus, 345
Caul, 74, 242
‘Cavity, body, 179
head, 366
joint, 291
INDEX 395
Cavity, marrow, 290
medullary, 290
oral, 142
pericardial, 53, 179, 180
peritoneal, 53, 185
pleural, 53, 183, 185
pleuro-pericardial, 40
pleuro-peritoneal, 179
tympanic, 161, 389
ae theory of development of nerve fibers,
06
Cells, aggregation, 3
blood, 42, 243, 244
ichthyoid, 244
sauroid, 244
bone, 289
capsule, 306
chromaffin, 368, 369
cone, of retina, 382
decidual, 236
division of, 12
enamel, 156
ependymal, 302, 307, 324
ethmoidal, 313, 376
follicle, 8
ganglion, 305
unipolar, 305
genital, 213
germ, 7, 302
giant, 247
gland, 297
goblet, 283
hair, 295, 296, 387
interstitial, of testis, 213
mass, inner, 71
intermediate, 52
mastoid, 314
migration, 3
multiplication, 3
muscle, smooth, 291
nerve, 300
neuroglia, 300, 302, 307, 308
rod, of retina, 382
sense, 387
sex, 208
sheath (of axis cylinder), 306
sperm, 10
supporting, 371, 387
of neural tube, 307
of spinal ganglia, 305, 306
sustentacular (of Sertoli), 14, 213
taste, 371
Cement of teeth, 157
Centra of vertebre, 142
Central canal of adult spinal cord, 324
nervous system, 321
chick, 44, 55
human, 80, 321
pig, 114
sulcus, 351
Centrosome, 7
Cephalic flexure, 55, 79, 327
pig, 89
vein, 276, 277
Cerebellum, 115, 336
Cerebral aqueduct, 115, 330, 337
artery, anterior, 266
middle, 266
posterior, 266
396
Cerebral cortex, 351
hemispheres, 115, 328, 329, 349
nerves, 115, 357
veins, 273
Cervical duct, 161
enlargement, 326
flexure, 328
pig, 89
ganglion, 368
sixth, 116
sinus, 112
pig, 90-
vesicle, 161
Chamber, anterior, of eye, 377, 383
Chick embryos, 36, 43, 55
of fifty hours’ incubation, 55
of twenty hours’ duration, 36
of twenty-five hours’ incubation, 38
preservation of, 36
study of, 36, 43, 55
Chin, 145
Choane, 147
primitive, 373
secondary, 374
Chondrification of skull, 312
of vertebra, 310
Chondrioconta, 285
Chondrocranium, 312
ossification of, 312
Chorda dorsalis, 309
origin of, 35
gubernaculi, 221, 222
tympani, 363
Chordine tendinex, 259
Chorioid fissure of eye, 376, 379
layer of eye, 383
plexus, 141, 142, 334, 338, 342
Chorioidal artery, anterior, 266
fissure, 342, 349
Chorion, origin of, 65
chick, 65
frondosum, 235, 237
human, 72
leve, 235, 237
pig, 68, 70
villi of, 71, 72, 232, 237
Chromaffin bodies, 368
aortic, 369
Chromosomes, 13
accessory, 17, 22
number of, 13
Cilia, 383
Ciliary bodies, 383
ganglion, 361, 368
muscle, 383
Circulation, fetal, 277
portal, 279
vitelline, 46
Circulatory system, 83
Cisterna chyli, 279
Clava, 336
Clavicle, ossification of, 315
Cleavage of ovum, 23
amphibia, 25
Amphioxus, 23
birds, 26
frog, 25, 26
mammals, 26
primates, 27
INDEX
Cleavage of ovum, reptiles, 26
Cleft palate, 149
sternum, 316
xiphoid process, 316
Clitoris, 143, 223
Cloaca, 83, 97, 171, 205
anomaly of, 208
Cloacal membrane, 83, 160, 205
tubercle, 225
Closing plates, 57, 60, 95, 118, 160
ring, 239
Coccygeal gland, 282
Cochlear duct, 384, 386
Ceeliac artery, 99, 122, 262, 267
axis, primitive, 267
Ceelom, 29, 33, 180
chick, 40, 53
human, 179
pleural, 96
Ccelomic pouches, 29
Colic valve, 174
Collateral eminence, 351
fissure, 351
Colliculus, facial, 364
inferior, 337
seminalis, 221
superior, 337
Coloboma, 379
Colon, 120, 174
Column, gray, 324
Columns, muscle, 293
renal, 202
Commissura mollis, 341
Commissure, anterior, 346, 348
gray, 325
hippocampal, 347
of telencephalon, 346
posterior, of labia majora, 225
white, 325
Compact layer, 236, 240
Concealed testis, 224
Conche, 146, 313, 376
Concrescence, theory of, 31
Cone cells of retina, 382
Conjunctiva, 383
Connective tissue, 285
white fibrous, 285
Copula, 94, 117, 150
Coracoid process, 316
Cord, genital, 210
medullary, 217
nephrogenic, 197, 201
spermatic, 224
spinal, 322
testis, 212, 213
umbilical, human, 70
of pig, 70
Corium, 295
Cornea, substantia propria of, 377
Corneal tissue, 286
Corniculate cartilages, 166
Corona radiata, 9
Coronary appendages, 192
ligament, 192
sinus, 253, 271
sulcus, 225, 251
Corpora quadrigemina, 330, 337
Corpus albicans, 217
callosum, 346, 348
Corpus hemorrhagicum, 217
luteum, 217
spurium, 217
verum, 217
striatum, 329, 341, 345
Corpuscles, blood, red, 244, 245
white, 245. See also Leucocytes.
lamellated, 371
renal, 133, 198, 199
splenic, 282
tactile, 371
thymic, 163
Cortex, cerebral, 351
of cerebral hemispheres, 329
of metanephros, 202
primitive, of cerebral hemisphere, 341
Corti’s organ, 359, 386
pillars, 387
Costal processes, 310
Costo-cervical trunk, 266
Cotyledons of human placenta, 240
Cowper’s glands, 228
Crest, ganglion, 304, 353
neural, 304
Cribriform plates, 313
Cricoid cartilage, 166
Crista ampullaris, 386
galli, 313
inguinalis, 221
terminalis, 254
Crura cerebri, 330
Crus longum, 390
Cryptorchism, 214, 224
Cumulus odphorus, 216
Cuneiform cartilages, 166
Cuneus, 336, 351
Cutis plate, 111
Cuvier’s ducts, 58
Cyclopia, 352
Cystic duct, 120, 177
kidney, 205
Cysts, dermoid, 295
Cytoplasm of ovum, 7
Darwin’s tubercle, 391
Decidua basalis (serotina), 232, 236, 237, 239
capsularis (reflexa), 231, 236
vera (parietalis), 232, 235
Decidual cells, 236
membranes, 230, 231
separation of, 241
teeth, periods of eruption, 157
Dendrites, 304
Dental canaliculi, 157
lamina, 153
papilla, 153, 157
pulp, 157
sac, 157
Dentate nucleus, 337
Dentinal fibers of Tomes, 157
Dentine, 157
Derma, 295
Dermatome, 293
Dermoid cysts, 295
Dermo-muscular plates, 51
Dermo-myotome, 285, 293
Descending tract of fifth nerve, 335, 361
Determination of sex, 22
INDEX 307
Diaphragm, 187, 188
anlage of, 61
anomalies of, 194
Diaphragmatic hernia, 194
Diaphysis, 290
Diaster, 13
Diencephalon, 56, 80, 92, 115, 327, 338
Differentiation of embryo, 3
of tissues, 4
Digestive canal, chick, 45, 57
human, 80
pig, 6 mm., 93
glands, human, 81
Dilator muscles of iris, 383
Discs, intercalated, 293
intervertebral, 310
Dissecting instruments, 137
Dissections, lateral, of viscera, 138
median sagittal, 140
pig embryos, 137
ventral, 143
Diverticulum, cardiac, 119
hepatic, 96, 119, 175
Meckel’s, 79, 171
of ileum, 171
of Nuck, 224
of pharyngeal pouches, 95
Ducts, branchial, 161
cervical, 161
cochlear, 384, 386
common bile, 120
Cuvier’s, 58
cystic, 120, 177
Ebner’s, 152
genital, 208, 210, 218
hepatic, 120, 177
mesonephric, 120, 199, 210
milk, 208
Miillerian, 210
naso-lacrimal, 384
pancreatic, 179
papillary, 201
para-urethral, 227
periportal, 177
pronephric (primary excretory), 196, 199
thoracic, 279
thyreoglossal, 164
vitelline, 159
Ductuli efferentes, 199, 218
Ductus arteriosus, 263, 279
choledochus, 120, 177
deferens, 218
endolymphaticus, 385
epididymidis, 218
reuniens, 386
venosus, 101, 124, 270, 279
Duodenal glands (of Brunner), 173
Duodeno-hepatic ligament, 192
Duodenum, 119, 170, 173
Dyads, 16
Far, 384
auricle of, 146
external, 112, 146, 384, 390
inner, 384, 385
internal, 45
middle, 384, 389
Ebner’s ducts and glands, 152
398 INDEX
Ectoderm, 3, 28
formation of, 28
Ectodermal derivatives, histogenesis of, 294
Ectoplasm, 285
Efferent ductules of epididymis, 199
Elastic cartilage, 287
tissue, 286
Eleidin, 294
Ellipsoids of spleen, 282
Embryos, chick, 36, 43, 55
of thirty-eight hours’ duration, 43
of twenty hours’ duration, 36
of twenty-five hours’ duration, 38
preservation of, 36
study of, 36
human, 71
crown-rump length, 87
estimated age, 87
of Coste, 78
of Dandy, 76
of Eternod, 77
of His, 2.5 mm., 78
4.2 mm., 79
Normentafel, 84, 85
of Kromer, 76
of Mall, 76
of Peter, 74
of Spee, 74
of Thompson, 76
pig, 6 mm., 89
10 mm., transverse sections, 125
10 to 12 mm., 112
dissection of, 137
transverse sections of 6 mm., 104
whole, for study, 137
Eminence, collateral, 351
Enamel cells, 156
layer, 156
organs, 153
pulp, 155
Encephalon, 80
Endocardial cushions, 98, 121, 131, 251, 252
Endocardium, 131, 249
chick, 41
Endochondral bone formation, 289
Endolymphatic sac, 385
Endoplasm, 285
Endothelium, 54
Enlargement, cervical, 326
lumbar, 326
Entoderm, 3, 28, 29
formation of, 28
Entodermal canal, 159
epithelium, 170
histogenesis of, 283
Eosinophiles, 246
Ependymal cells, 302, 307, 324
layer, 59, 126, 307, 322, 333
zone, 301
Epicardium, 42, 98, 131, 249
Epidermis, 294
anomalies of, 295
Epididymis, 219
efferent ductules of, 199
Epigastric arteries, 266
Epigenesis, 2
Epiglottis, 94, 116, 143, 151, 165
Epiphysis (pineal gland), 141, 290, 310, 330, 339
Epiploic foramen (of Winslow), 134, 190
Epistropheus, 310
Epithalamus, 330
Epithelia, 3
Epithelial bodies, 82, 163
Epithelium, 54
basal, of cochlear duct, 387
entodermal, 170
histogenesis of, 283
olfactory, 128
respiratory, 168
stratified, 294
Epitrichium, 294, 295, 296
Eponychium, 299
Epoéphoron, 199, 219
Erythroblasts, 244, 245
Erythrocytes, 244, 245
Erythroplastids, 245
Esophagus, 81, 94, 95, 119, 143, 169
Ethmoid bone, ossification of, 313
Ethmoidal cells, 313, 376
Ethmo-turbinals, 376
Eustachian tube, 161, 389
valve, 254
Excretory duct, primary, 62, 196, 199
Expression, muscles of, 320
Extra-embryonic mesoderm, 71
Extremities, arteries of, 268
muscles of, 318
veins of, 276
Eye, chick, 43, 45, 47, 59
human, 80, 376
pig, 89
Eyelashes, 383
Eyelids, 383
Fact, development of, 144
Facial colliculus, 364
nerve, 92, 116, 362, 363 é
Falciform ligament, 133, 180, 192
False hermaphroditism, 228
Fasciculi proprii, 325
Fasciculus cuneatus, 325
gracilis, 325
Femoral artery, 268
nerve, 356
vein, 277
Femur, ossification of, 316
Fertilization, 19, 20
significance of, 21
Fertilizin, 20
Fetal circulation, 277
membranes, human, 71
pig, 68
Fetus, 87
relation of, to placenta, 241
Fibrils, horn, 299
Fibro-cartilage, 287
Fibula, ossification of, 316
Filament, axial, of spermatozoén, 12
spiral, of spermatozoin, 11
terminal, of spermatozoén, 11
Filiform papilla, 151
Filum terminale, 326
Fingers, supernumerary, 316
Fissure, calcarine, 351
chorioidal, 342, 349, 376, 379
collateral, 351
great longitudinal, 342
Fissure, hippocampal, 346, 349
lateral, 349
of Rolando, 351
parieto-occipital, 351
Sylvian, 349
ventral median, 325
Fixation of pig embryos, 137
Flagellum of spermatozoén, 11
Flexure, cephalic, 55, 79, 327
cervical, 328
pig, 89
iliac, 174
pontine, 327
Flocculus, 337
Floor plate, 322, 324, 332
Foliate papilla, 152
Follicle cells, 8
Follicles, Graafian, 9, 215
primordial, 215
vesicular, 215
Fontanelles, 314
Foramen, atrio-ventricular, 98
cecum, 151, 164
epiploic (of Winslow), 134, 190
interatrial, 98, 253
interventricular, 115, 258, 329, 343
closure of, 259
mandibular, 315
Monro’s, 343
of Winslow, 134, 190
ovale, 98, 120, 253, 254, 255
section through, 130
Fore-brain, chick, 40, 43, 44, 47, 59
human, 327
Fore-gut, chick, 39, 40, 41, 49, 57
human, 81, 159, 160
pig, 93
Fore-skin, 226 '
Fornices of vagina, 220
Fornix, 346, 347
Fossa, incisive, 148
olfactory, 371, 372
ovalis, 255
supratonsillar, 161
tonsillar, 161
Fovea cardiaca, 39, 40, 41, 83
chick, 49
Frenulum prepucii, 226
Frog, cleavage of ovum in, 25, 26
Frontal operculum, 349
sinus, 376
Fronto-nasal process, 372
Fronto-parietal operculum, 349
Froriep’s ganglion, 93, 116, 360
Fundus of uterus, 220
Fungiform papille, 151
Funiculi of spinal cord, 325
Furcula of His, 165
Gatt bladder, 120, 177
Ganglion, 353
accessory, 364
cell layer, 382
of retina, 358
cells, 305
unipolar, 305
cervical, 368
ciliary, 361, 368
INDEX 399
Ganglion crest, 304, 353
Froriep’s, 93, 116, 360
geniculate, 92, 116, 363
habenule, 338
jugular, 93, 116
nodose, 93, 116, 364
otic, 368
petrosal, 93, 116, 364
prevertebral, 368
root, 93, 364
semilunar, 92, 116, 361
sphenopalatine, 368
spinal, 50, 116, 304
supporting cells, 305, 306
spiral, 358, 359
submaxillary, 368
superior, 93, 116, 364
sympathetic, 305, 367
trunk, 364
vestibular, 358
vagus accessory, 116
visceral, 368
Gartner’s canals, 219
Gastric glands, 170
Gastro-hepatic ligament, 192
Gastro-lienic ligament, 191
Gastrula, 28
Gastrulation, 28
of amphibia, 28
of Amphioxus, 28
of birds, 28
of mammals, 29
of reptiles, 28
Geniculate bodies, 330
ganglion, 92, 116, 363
Genital cells, 213
cord, 210
ducts, 208, 210, 218
eminence, 114, 225
fold, 97, 120, 143, 197, 208
glands, 97, 143, 208
and mesonephric tubules, union of, 218
swellings, 225
tubercle, 225
Genitalia, external, 224
internal, ligaments of, 221
Germ cells, 7, 302
layers, 2, 3
derivatives of, 54
origin of, 23
plasm, continuity of, 4
Germinal disc, 26
Giant cells, 247
Gill slits, 55
Glands, accessory genital, 227
alveolo-lingual, 152
Bartholin’s, 228
Brunner’s, 173
bulbo-urethral, 228
cardiac, 170
carotid, 369
cells, 297
coccygeal, 282
Cowper’s, 228
duodenal (of Brunner), 173
Ebner’s, 152
gastric, 170
genital, 97, 143, 208
and mesonephric tubules, union of, 218
400
Glands, hemolymph, 281
intestinal, 173
lacrimal, 383
accessory, 384
lymph, 281
mammary, 114, 297
rudimentary, 298
supernumerary, 298
Meibomian, 383
parathyreoid, 118, 163
parotid, 152
pineal, 141, 290, 310, 330, 339
prostate, 227
salivary, 152
sebaceous, 296
sublingual, 152
submaxillary, 152
sudoriparous, 297
suprarenal, 143, 368, 369
sweat, 297
tarsal, 383
thymus, 118
thyreoid, 118, 164
urogenital, 144
uterine, of pregnancy, 236
vestibular, 228
Glans clitoridis, 225
penis, 226
Glomerulus, 133, 195, 198
Glomus caroticum, 369
coccygeum, 282
Glossopharyngeal nerve, 93, 116, 362, 364
Glottis, 94, 117, 151
Gluteal artery, 268
vein, 277
Goblet cells, 283
Gonads, 83
Graafian follicle, 9, 215
Granular layer of cerebellum, 337
leucocytes, 246
Granules, pigment, 295
Gray column, 324
commissures, 325
rami, 367
Groove, laryngo-tracheal, 164
neural, 300
primitive, 31
rhombic, 334
urethral, 225
Ground bundles, 325
Growth of embryo, 3
Gubernaculum testis, 222, 223
Gyrus dentatus, 346
hippocampi, 346
Hemopottsis, 243
Hair, 295
bulb, 296
cells, 295, 296, 387
papilla, 296
shaft of, 296
sheath, 296
Hare lip, 146
Haversian canal, 290
Head cavities, 366
fold, 39
muscles of, 319
process, 32, 33, 37, 38
INDEX
Head, vertebrate, segmentation of, 365
Heart, chick, 41, 46, 49
descent of, 259
human, 83, 247, 248
pig, 90, 97, 116, 120° 143
primitive, chick, 42
ventricle of, 57
Helix, 391
hyoid, 391
Hemiazygos vein, 274
Hemispheres, cerebral, 328, 329, 349
Hemolymph glands, 281
Henle’s loop, 203
Hensen’s knot, 31, 36, 51
Hepatic artery, 178
diverticulum, 96, 119, 175
duct, 120, 177
vein, 270
common, 98
Heredity, Mendel’s law of, 21
Hermaphroditism, 228
false, 228
Hernia, diaphragmatic, 194
inguinal, 224
umbilical, 70
Hind-brain, chick, 41, 44, 48, 59
human, 80, 327
Hind-gut, 57, 64, 83, 93, 159, 160
Hippocampal commissure, 347
fissure, 346, 349
Hippocampus, 346, 349
minor, 351
His, furcula of, 165
Histogenesis, 283
defined, 4
of bone, 288
of cartilage, 287
of ectodermal derivatives, 294
of entodermal epithelium, 283
of mesodermal tissues, 284
of muscle, 291
of nervous tissue, 300
Historical, 1
Horn fibrils, 299
gray, 324
greater, of hyoid bone, 315
lesser, of hyoid bone, 315
Horse-shoe kidney, 205
Howship’s lacune, 289
Human embryos, 71
crown-rump length, 87
estimated age, 87
of Coste, 78
of Dandy, 76
of Eternod, 77
of His, 2.5 mm., 78
4.2 mm., 79
Normentafel, 84, 85
of Krémer, 76
of Mall, 76
of Peter, 74
of Spee, 74
of Thompson, 76
Humerus, ossification of, 316
Hyaline cartilage, 287
Hyaloid canal, 381
Hydramnios, 74
Hymen, 219, 221
Hyoid arch, 90
Hyoid helix, 391
Hyomandibular cleft, 90
Hypogastric artery, 268
Hypoglossal nerve, 93, 116, 151, 360
Hypophysis, 57, 59, 330
anterior lobe of, 50
posterior lobe of, 115
Hypospadias, 227
Hypothalamus, 330
Icutuyorp blood cells of Minot, 244
Tleum, 83
diverticulum of, 171
Tliac arteries, 123, 267, 268
flexure, 174
veins, 274, 275
Tlium, ossification of, 316
Implantation of ovum, 231
Incisive canals (of Stenson), 148
fossa, 148
Incus, 315, 389, 390
Infundibulum, 330, 339
Inguinal canal, 221, 222
fold, 220, 221
hernia, 224
Inner cell mass, 71
epithelial mass of gonad, 208
Innominate artery, 263
Instruments, dissecting, 137
Insula, 349
Interatrial foramen, 98, 252
Intercalated discs, 293
Intercostal arteries, 266
Intermediate cell mass, 40, 52
Interosseous artery, 268
Intersegmental arteries, 99, 122, 260, 262, 265
fiber tracts, 335
veins, 274
Interstitial cells of testis, 213
Interventricular foramen, 115, 258, 329, 343
closure of, 259
septum, 121, 258
sulcus, 258
Intervertebral discs, 310
muscles, 317
Intestinal glands, 173
loop, 143, 170
Intestine, human, 81, 83, 169, 170
pig, 96, 120
villi of, 173
Introduction, 1
Tris, 382, 383
muscles of, 383
Ischiadiac vein, 274
artery, 268
Ischium, ossification of, 316
Island of Reil, 349
Tslands, blood, 243
of pancreas, 179
Isolecithal ova, 23
Isthmus, 115, 328, 330
Jacosson’s organ, 357, 374
Joint cavity, 291
Joints, 291
Jugular ganglion, 93, 116
sacs, 279
veins, 123, 273
26
INDEX 401
KeraTIN, 299
Keratohyalin, 294
Kidney, anomalies of, 205
calyces of, 120
cystic, 205
horse-shoe, 205
human, 199
tubules of, 120, 201, 203
Knot, primitive (of Ilensen), 36, 51
Lanta majora, 225
minora, 225, 227
Labyrinth, membranous, 45
Lacrimal bone, 314
canals, 384
glands, 383
accessory, 384
groove, 90
*Lacune, bone, 289
Howship’s, 289
Lamelle, periosteal, 290
Lamellated corpuscles, 371
Lamina perpendicularis, 313
terminalis, 329, 342, 346
Langhans’ layer, 238
Laryngeal nerves, recurrent, 264
Laryngo-tracheal groove, 164
Larynx, 164, 165
cartilage of, 165
muscles of, 320
ventricles of, 165
Law, Mendel’s, of heredity, 21
of biogenesis, 5
of recapitulation, 5
Layer, chorioid, of eye, 383
compact, 236, 240
enamel, 156
ependymal, 126, 307, 322, 333
epitrichial, 296
ganglion cell, 382
of retina, 358 =
germ, origin of, 23
granular, 337
Langhans’, 238
mantle, 126, 322, 324
marginal, 126, 322, 325
medullary, 337
molecular, 337
nerve fiber, 382
nervous, of retina, 126
of retina, 382
pigment, of retina, 126, 377, 381
retinal, 377, 381
sclerotic, of eye, 383
spongy, 236, 240
Lecithin, 7
Lemniscus, 338
median, 335
Lens of eye, chick, 45, 47
human, 376
fibers of, 379
pupillary membrane of, 381
suspensory ligament of, 381
vascular capsule of, 378, 381
pig, 89
pit, 376
plate, 376
vesicles, 59, 376, 379
402
Lens-stars, 380
Lenticular nuclei, 345
Lesser peritoneal sac, 133, 189
Leucocytes, 245
granular, 246
mast, 247
mononuclear, large, 246
non-granular, 245
polymorphonuclear, 246
Ligament, broad, 221
coronary, 192
duodeno-hepatic, 192
falciform, 133, 180, 192
gastro-hepatic, 192
gastro-lienic, 191
lieno-renal, 191
of internal genitalia, 221
of liver, 191
of testis, 221
proper, of ovary, 221
round, 220, 222
spheno-mandibular, 315
stylo-hyoid, 315
suspensory, of lens, 381
Ligamentum arteriosum, 279
labiale, 222
ovarii, 210
scroti, 222
teres, 271, 279
testis, 210, 222
umbilicale medium, 208
venosum, 270, 279
Limbus ovalis, 254
Limiting membranes of retina, 382
Line, milk, 298
Linguo-facial vein, 101
Lip, 373 ;
hare, 146
rhombic, 334
Liver, anlage of, 57, 62, 175
anomalies of, 178
caudate lobe of, 192
cords, 131
human, 81, 175
ligaments of, 191
lobules of, 178
pig, 90, 93, 96, 119
quadrate lobe of, 192
sinusoids of, 57, 62, 176
Lobes of cerebrum, 349
Lobule of ear, 391
Lobules of liver, 178
Lobuli epididymidis, 218
Lumbar arteries, 266
enlargement, 326
veins, 276
Lumbo-sacral plexus, 356
Lung buds, 82
Lungs, human, 81, 164, 167
apical buds, 167
changes at birth, 168
stem buds, 167
pig, 95, 119
Lunula, 299
Lymph glands, 281
sacs, 279, 280
Lymphatic system, 279
Lymphatics, origin of, 279
peripheral, 279
INDEX
Lymphocytes, small, 245
Lymphoid tissue of spleen, 282
MacuL acustice, 386
Magma reticulare, 71
Mall’s pulmonary ridge, 183
Malleus, 315, 389
muscles of, 390
Mammals, cleavage of ovum in, 26
gastrulation of, 29
origin of mesoderm in, 32
Mammary arteries, 266
glands, 114, 297
rudimentary, 298
supernumerary, 298
Mammillary bodies, 330, 347
recess, 330, 341
Mandibular arch, 90
foramen, 315
nerve, 116, 361
process, 79, 80, 90, 112, 314, 372
Mantle layer, 126, 322, 324
Manubrium, 311
Marginal layer, 126, 322, 325
sinus, 241
zone, 302
Margo thalamicus, 327
Marrow, bone, 289
red, 289
yellow, 290
cavity, 290
Massa intermedia, 341
Mast leucocytes, 247
Mastication, muscles of, 319
Mastoid cells, 314
process, 314
Maturation, 12, 14
of mouse ovum, 18
significance of, 21
Maxille, 315
Maxillary nerve, 116, 361
process, 79, 80, 90, 112, 314, 372
sinus, 376
Maxillo-turbinal anlage, 376
Meatus, external auditory, 90, 112, 146, 391
inferior nasal, 376
Meckel’s cartilage, 315
diverticulum, 79, 171
Meconium, 175
Median artery, 268
longitudinal bundle, 335
nerve, 356
Mediastinum, 167
of ovary, 214
testis, 213 :
Medulla oblongata, 80, 33
Medullary cavity, 290
cords, 217
layer, 337
sheath, 306
velum, 330, 337
Megakaryocytes, 247
Meibomian glands, 383
Membrana tectoria, 387
Membrane, anal, 160, 205
basilar, 389
bone, 288
bones of skull, 314
INDEX
Membrane, cloacal, 160, 205
decidual, 230, 231
separation of, at birth, 241
tympanic, 117
limiting, of retina, 382
obturator, 316
pericardial, 187
Pharyngeal, 159
pleuro-pericardial, 182, 183
pleuro-peritoneal, 182, 183
pupillary, 381
Reissner’s, 389
synovial, 291
tympanic, 391
urogenital, 160, 205
vestibular, 389
vitelline, 7
Membranous labyrinth, 45
Mendel’s law of heredity, 21
Menstruation, 10, 230
uterus during, 230
Mesameeboids, 243, 244
Mesencephalon, 80
chick, 44, 48, 60
human, 330, 327, 337
pig, 92, 115
Mesenchyma, 3
chick, 41, 53
human, 384
Mesenteric arteries, 262, 267
inferior, 122
superior, 261
veins, 270
superior, 99, 100, 122, 125
Mesentery, 93, 96, 120, 133, 179, 180, 191, 192
anomalies of, 194
dorsal, 81, 192
Mesocardium, 180
dorsal, 49, 249
Mesocolon, 180, 193, 194
Mesoderm, 3, 28
amphibian, 30
Amphioxus, 29
birds, 31
extra-embryonic, 34, 71
intra-embryonic, 34
mammal, 32
origin of, 29
primary, 28
reptiles, 30
somatic, 52, 53, 69
splanchnic, 53, 69, 169, 366
Tarsius, 34
Mesodermal segments, 2, 40, 51, 62, 64
tissues, histogenesis of, 284
Mesoduodenum, 180, 193
Mesogastrium, 180
Mesonephric duct, 49, 52, 62, 97, 120, 199, 210
fold, 197
tubules, 197
and genital glands, union of, 218
Mesonephros, 52, 83, 90, 93, 97, ‘120, 143, 195, 197
Mesorchium, 210
Mesorectum, 180, 194
Mesothelium, 54, 284
peritoneal, 170
Mesovarium, 210
Metameres, 2
Metamerism, 2
403
Metanephros, 52, 97, 120, 143, 195, 199
and umbilical arteries, section through, 136
calyces of, 199
collecting tubules, 199, 201
cortex, 202
pelvis, 199
tubules, 199, 201
ureter, 199
Metaphase of mitosis, 13
Metatarsus, ossification of, 316
Metathalamus, 330
Metencephalon, 80, 92, 115, 327, 328, 336
Methods of study, 5
Mid-brain, 80, 327
chick, 43, 44, 47
Mid-gut, 57, 159
Migration, cell, 3
Milk ducts, 298
line, 114, 298
teeth, periods of eruption, 157
Minot’s ichthyoid blood cells, 244
Mitosis, 12
phases of, 12-14
significance of, 21
Mitotic figure, 13
Mitral valve, 259
Moderator muscles, 259
Mcediolus, 389
Molecular layer, 337
Monaster, 13
Mononuclear leucocytes, large, 246
eis theory of origin of blood elements,
Monro’s foramen, 343
Mons pubis, 233
Montgomery’s rudimentary mammary glands, 298
Morula, 24
Motor nerves, somatic, 360
Mouse ovum, fertilization of, 19
maturation of, 18
Mouth, pig, 6 mm., 93
Miiller’s fibers, 382
tubercle, 221
Miillerian ducts, 210
Multiplication, cell, 3
Muscles, 316
anomalies of, 320
cardiac, 291, 293
ciliary, 383
columns, 293
dilator, of iris, 383
histogenesis of, 291
intervertebral, 317
moderator, 259
of back, 317
of expression, 320
of extremities, 318
of head, 319
of larynx, 320
of malleus, 390
of mastication, 319
of neck, 317
of pharynx, 320
of tongue, 320
of trunk, 317
papillary, 259
plate, 60, 62
skeletal, 291, 292
smooth, 291
404
Muscles, sphincter, of iris, 383
stapedial, 390
sterno-cleido-mastoid, 364
thoraco-abdominal, 317
trapezius, 364
voluntary, 52
Muscular system, 316
Musculocutaneous nerve, 356
Myelencephalon, 80, 92, 115, 328, 330, 332
Myelin, 306
development of, 326
sheath, 306
Myelocytes, 245, 246
Myoblasts, 292, 293
Myocardium, 42, 98, 249, 259
Myofibrille, 293
Myotomes, 2, 292
changes in, during formation of adult muscles,
31
chick, 51
Nait fold, 298, 299
human, 298
Naris, external, 373
Nasal bone, 314
meatus, inferior, 376
passages, 374
processes, 145, 372, 373
septum, 313, 373
Naso-lacrimal duct, 384
Naso-turbinal anlage, 376
Navel, 70
Neck, muscles of, 317
Neopallium, 346
Nephrogenic cords, 197, 201
tissue, 120, 136, 202
Nephrostome, 195
Nephrotome, 195
chick, 40, 52, 64
Nerve, abducens, 116, 361
accessorius, 116
acoustic, 92, 116, 358
auditory, 358
axillary, 356
cells, 300
cerebral, 115, 357
facial, 92, 116, 362, 363
femoral, 356
fibers, 300
cell-chain theory of development, 306
layer, 382
glossopharyngeal, 93, 116, 362, 364
hypoglossal, 93, 116, 151, 360
mandibular, 116, 361
maxillary, 116, 361
median, 356
motor somatic, 360
musculocutaneous, 356
obturator, 356
oculomotor, 92, 115, 330, 360
olfactory, 357
ophthalmic, 116, 361
optic, 115, 358, 383
peroneal, 356
petrosal, superficial, 116, 363
phrenic, 355
radial, 356
recurrent laryngeal, 264
INDEX
Nerve, sciatic, 356
sensory, somatic, 357
somatic, motor, 360
sensory, 357
spinal, 116, 353
accessory, 93, 116, 364
efferent or ventral root fibers of, 302
terminal, 358
tibial, 356
trigeminal, 92, 116, 361
trochlear, 116, 330, 361
ulna, 356
vagus, 93, 116, 362, 364
visceral mixed, 361
Nervous layer of retina, 126
system, 91, 114
central, 321
chick, 44, 55
human, 80, 321
peripheral, 353
sympathetic, 366
tissue, histogenesis of, 300
Neural crest, 50, 93, 304
chick, 41
folds, 38, 40
groove, 38, 300
plate, 300
tube, 38, 40, 64, 300, 332
anomalies of, 352
origin of, 29, 30
supporting cells of, 307
Neurenteric canal, 31, 32, 33, 76
Neurilemma, 306
Neuroblasts, 302
differentiation of, into neurones, 302
of retina, 382
Neurofibrilla, 304
Neuroglia cells, 300, 302, 307, 308
fibers, 300, 302, 307, 308
Neurokeratin, 306, 308
Neuromeres, 44, 104, 115, 334
Neurones, 302
afferent, 305
differentiation of neuroblasts into, 302
theory, 305
Neuropores, 40, 321
anterior, 43
Neutrophiles, 246
Nipple, 298
Node, primitive (of Hensen), 51
of Ranvier, 306
Nodose ganglion, 116
Nodulus cerebelli, 337
Normoblasts, 244
Nose, 371
Notochord, 33, 38, 309
chick, 40, 64
origin of, 29, 35
Notochordal canal, 33
plate, 31, 32, 37, 38
Nuclear zone, 302
Nuclei pulposi, 35, 309
Nucleolus, 7
Nucleus ambiguus, 364
caudate, 345
cuneatus, 335
dentate, 337
gracilis, 335
lenticular, 345
Nucleus of pons, 336
olivary, 335
receptive, 335
red, 337
ruber, 337
terminal, 335
Nuck’s diverticula, 224
Nymphe, 227
OBEX, 334
Obturator membrane, 316
nerve, 356
Occipital bone, ossification of, 312
Oculomotor nerve, 92, 115, 330, 360
Odontoblasts, 157
Olfactory apparatus, 346
brain, 328
epithelium, 128
fossa, 371, 372
lobe, 346
nerve, 357
organ, 371
pits, 112, 371, 372
pig, 89
placodes, 371
stalk, 346
tracts, 346
Olivary nucleus, 335
Omental bursa, 189
inferior recess of, 190
Omentum, 133, 169, 180, 191, 192
Odécyte, 18
primary, 18
Oégenesis, 8
Oégonia, 18
Operculum, 349
Ophthalmic nerve, 116, 361
vein, 273
Optic chiasma, 329, 358
cup, 56, 112, 328, 376, 381
nerve, 115, 358, 382
placode, 376
recess, 330, 343
stalks, 47, 59, 376
tract, 358
vesicles, 40, 43, 47, 59, 343
Ora serrata, 381
Oral cavity, 142
Orbital operculum, 349
Organ, Corti’s, 359, 386
Jacobson’s, 357, 374
spiral, 386, 387
vomero-nasal, 357, 374
Os coxe, 316
Ossicles, auditory, 389
Ossification of carpus, 316
of chondrocranium, 312
of ethmoid bone, 313
of femur, 316
of fibula, 316
of humerus, 316
of ilium, 316
of ischium, 316
of metatarsus, 316
of occipital bone, 312
of patella, 316
of phalanges, 316
of pisiform, 316
INDEX 405
Ossification of pubis, 316
of radius, 316
of skull, 312
of sphenoid bone, 313
of tarsus, 316
of temporal bone, 313
of tibia, 316
of ulna, 316
of vertebre, 310
Osteoblasts, 288, 289
Osteoclasts, 289
Ostium abdominale, 210
vagina, 221
Otic ganglia, 368
vesicle, 45
Otocyst, 45, 56, 60, 80, 92, 385
Ovarian arteries, 267
pregnancy, 20
vein, 275
Ovary, 214
anomalies, 218
compared with testis, 217
descent of, 222
mediastinum, 214
proper ligament of, 221
septula, 214
stroma of, 215
Ovulation, 10
Ovum, cleavage of, 23. See also Cleavage of ovum.
human, 7
fertilization of, 20
implantation of, 231
isolecithal, 23
maturation of, 17, 18
mouse, fertilization of, 19
segmentation of, 23. See also Cleavage of ovum.
structure of, amphibian, 7
bird,
monkey, 10
telolecithal, 23
PALATE bones, 314
cleft, 149
development of, 146
premaxillary, 373
primitive, 373
soft, 149
Palatine processes, lateral, 147
tonsil, 82, 118, 162
Pallium of cerebral hemispheres, 329
of cerebrum, 341
Pancreas, alveoli of, 179
human, 81, 178
accessory duct of, 179
islands of, 179
pig, 96, 120
Pancreatic duct, 179
Papille, dental, 153, 157
hair, 296
of tongue, 151, 152
renal, 202
Papillary ducts, 201
muscles, 259
Paradidymis, 199, 218
Parathyreoid gland, 82, 118, 163
Paraurethral ducts, 227
Parietal pleura, 168
Parietals, 314
406
Parieto-occipital fissure, 351
Parolfactory area, 346
Parodphoron, 199, 219
Parotid glands, 152
Pars ceca, 381
ciliaris, 382
iridis retine, 382
lateralis of sacrum, 311
optica, 381, 382
hypothalamica, 339
radiata, 202
Parthenogenesis, artificial, 20
Patella, ossification of, 316
Peduncles of cerebrum, 338
Pelvis, renal, 199, 201
Penis, 143
Perforated space, 346
Pericardial cavity, 49, 53, 179, 180
chick, 53, 60
membrane, 187
Perichondral bone formation, 289, 290
Perichondrium, 287
Periderm, 294
Perilymph space, 387, 389
Periosteal bone formation, 289
lamelle, 290
Periosteum, 287
Periotic capsule, 312
Peripheral lymphatics, 279
nervous system, 353
sinus, 281
Periportal ducts, 177
Peritoneal cavity, 53, 185
chick, 53
mesothelium, 170
sac, 133
lesser, 133, 189
Peritoneum, 133
Peroneal artery, 268
nerve, 356
Petrosal ganglion, 93, 116, 364
nerve, superficial, 116, 363
sinus, 273
Pfliiger’s tubes, 215
Phalanges, ossification of, 316
Phallus, 143, 225
Pharyngeal bursa, 162
membrane, 42, 46, 47, 80, 159
plate, 42
pouches, 46, 60, 95, 117, 160
tonsil, 162
Pharyngopalatine arches, 149
Pharynx, human, 81
muscles of, 320
pig, 93, 116
Philtrum, 146
Phrenic artery, 267
nerve, 355
Pia mater, 126
Pig embryos, 112
6 mm., 89
10 to 12 mm., 112
dissection of, 137
transverse sections of 6 mm., 104
of 10 mm., 125
fetal membranes, 68
Pigment granules, 295
layer of retina, 126, 377, 381
Pillars, anterior, of fornix, 347
INDEX
Pillars of Corti, 387
Pineal body or gland, 141, 290, 310, 330, 339
Pisiform, ossification of, 316
Pituitary body, 339
Placenta, accessory, 242
human, 73, 230, 231, 235, 237
cotyledons of, 240
intervillous spaces of, 241
position of, 242
relation of fetus to, 241
vessels of, 241
previa, 242
succenturiate, 242
Placodes, 362
auditory, 385
olfactory, 371
optic, 376
Plasm, germ, continuity of, 4
Plate, alar, 323, 332, 335, 337
basal, 239, 240, 323, 332
blood, 247
closing, 160
cribriform, 313
cutis, 111
floor, 322, 324, 332
lens, 376
muscle, 111
neural, 300
notochordal, 31
roof, 322, 332, 338
urethral, 225
Pleura, parietal, 168
visceral, 168
Pleural cavity, 53, 61, 183, 185
chick, 53
coelom, 96
Pleuro-pericardial cavity, 40
membranes, 182, 183
Pleuro-peritoneal canal, 183
cavities, 179
membranes, 182, 183
Plexus, brachial, 355
chorioid, 141, 142, 334, 338, 342
lumbo-sacral, 356
Plica semilunaris, 383
vene cave, 103, 133, 189, 192, 274
vein of, 274
Polar bodies, 18
Polocytes, 18
Polydactyly, 316
Polymorphonuclear leucocytes, 246
Rely payeuc theory of origin of blood elements,
43
Polyspermy, 20
Pons, 115, 330
nucleus of, 336
Pontine flexure, 327
Popliteal artery, 268
Portal circulation, 279
vein, 99, 125, 268, 270
Postbranchial bodies, 164
Postnasal gut, 97
Preformation, doctrine of, 2
Tregnancy, abdominal, 20
ovarian, 20
tubal, 20
uterine glands of, 236
uterus during, 230
Premaxillary palate, 373
INDEX
Premyelocytes, 244, 245, 246
Prepucium, 225, 226
Prevertebral ganglia, 368
Primary excretory ducts, 52, 196, 199
Primates, 68
cleavage of ovum in, 27
Primitive choane, 373
folds, 31
groove, 31, 36, 38
heart, chick, 42
knot or node, 31, 36, 51
palate, 373
pit, 31
segments, 2, 40
streak, 31, 36, 40, 51
Primordial follicles, 215
Proamniotic area, 39, 43
Process, coracoid, 316
costal, 310
fronto-nasal, 372
lateral nasal, 144, 145, 372
palatine, 147
mandibular, 112, 314, 372
mastoid, 314
maxillary, 112, 314, 372
median nasal, 144, 145, 372, 373
palatine, 147
nasal, 372
styloid, 314, 315
vermiform, 172, 174
xiphoid, 311
cleft, 316
Processus globulares, 372
Pronephric ducts, 196
tubules, 49
Pronephros, 52, 195
Pronucleus, 18, 19
Prophase of mitosis, 13
Prosencephalon, 327
chick, 44
Prostate gland, 227
Prostatic utricle, 218, 219
Pubis, ossification of, 316
Pulmonary arteries, 99, 121, 122, 168, 258, 262, 264
ridge, 183
vein, 121, 168, 255
Pulp, dental, 157
enamel, 155
Pupillary membrane, 381
Pyramids of kidney, 201
QuapraTE lobe of liver, 192
Rapist artery, 268
nerve, 356
Radius, ossification of, 316
Ramus angularis, 270
arcuatus, 270
communicans, 354, 366
dorsal, 354
gray, 367
lateral, 354
posterior, 354
terminal, 354
ventral, 354
white, 366
Ranvier’s nodes, 306
407
Rathke’s pocket, 57, 59, 81, 93, 126, 160
Recapitulation, law of, 5
Receptive nucleus, 335
Recess, inferior, of omental bursa, 190
lateral, 334
mammillary, 330, 341
optic, 330, 343
Rectum, 120, 135, 143, 174, 205
Red blood corpuscles, 244, 245
bone marrow, 289
nucleus, 337
Reference, titles for, 6
Regeneration of bone, 291
Reichert’s cartilage, 390
Reil’s islands, 349
Reissner’s membrane, 389
Renal artery, 205, 267
columns, 202
corpuscles, 133, 198, 199
papilla, 202
pelvis, 199, 201
tubules, 201, 203
veins, 275
Reptiles, cleavage of ovum in, 26
gastrulation of, 28
origin of mesoderm in, 30
Respiratory epithelium, 168
Rete ovarii, 214, 217
testis, 212
Reticular formation, 335
tissue, 163, 285
Retina, layers of, 382
nervous layer, 126
pigment layer of, 126, 377, 381
Retinal layer, 381
of optic cup, 377
Retroperitoneal sac, 279
Rhinencephalon, 328, 341, 346
Rhombencephalon, 80, 327
chick, 44
Rhombic grooves, 334
lip, 334
Rhomboidial sinus, 40, 44, 50
Ribs, 309, 310
Ridge, pulmonary, 183
Rod cells of retina, 382
Rolando’s fissure, 351
Roof plate, 322, 332, 338
Roots, spinal, dorsal, 305
Round ligament, 220, 222
SACCcULUS, 386
Saccus vaginalis, 223
Sacral artery, middle, 268
Sacrum, pars lateralis of, 311
Sagittal dissections, median, 140
sinus, superior, 273
Salivary glands, 152
Saphenous vein, 277
Sarcolemma, 293
Sarcoplasm, 293
Sarcostyles, 293
Satyr tubercle, 391
Sauroid blood cells, 244
Scala tympani, 389
vestibuli, 389
Scapula, ossification of, 316
Sciatic nerve, 356
408
Sclerotic layer of eye, 383
Sclerotome, 111, 284
Scrotal area, 227
Scrotum, 227
ligament of, 222
Sebaceous gland, 296
Sections, chick, fifty hours, 59
thirty-eight hours, 47
twenty-five hours, 40
pig, 6 mm., 104
10 mm., 125
Seessel’s pocket, 35, 81, 93, 160
Segmental zone, 64
Segmentation of ovum, 23.
ovum.
of vertebrate head, 365
Segments, mesodermal, 2, 40, 51, 62, 64
primitive, 2
Semilunar ganglion, 92, 116, 361
valves, 258
Seminal vesicle, 218
Sense cells, 387
organs, chick, 44, 55
human, 80, 370
Sensory nerves, somatic, 357
organs, general, 371
Septa placenta, 240
Septum, atrial, 251
interventricular, 121, 258
median, of adult spinal cord, 324, 325
membranaceum, 259
nasal, 313, 373
pellucidum, 346, 347
primum, 98, 120, 252, 279
scroti, 227
secundum, 121, 253, 279
spurium, 253
transversum, 61, 62, 83, 113, 116, 143, 175, 180,
182, 183
Sertoli, sustentacular cells of, 14, 213
Sex cells, 208
determination of, 22
Shaft of hair, 296
Sheath cells, 306
hair, 296
medullary, 306
myelin, 306
Shoulder-blade, ossification of, 316
Sinus, blood, 281
cavernosus, 273
cervical, 90, 112
coronary, 253, 271
frontal, 376
marginal, 241
maxillary, 376
peripheral, 281
petrosal, 273
rhomboidal, 40, 44, 50
sagittal, superior, 273
sphenoidal, 376
transverse, 273
urogenital, 120, 205, 212
venosus, 57, 60, 97, 120, 249, 250, 253
valves of, 121, 251
Sinusoids, 86
of liver, 57, 62, 176, 268, 269
Situs viscerum inversus, 194
Skeletal muscle, striated, 291, 292
system, 309
See also Cleavage of
INDEX
Skeleton, 309
anomalies of, 316
appendicular, 315
axial, 309
branchial arch, 314
Skull, 311
chondrification of, 312
membrane bones of, 314
ossification of, 312
Smooth muscle, 291
Solitary tract, 362
Soma, 4
Somatic mesoderm, 52, 53, 69
motor nerves, 360
sensory nerves, 357
Somatopleure, 30, 33, 52, 53, 62, 64
Somites, 2
Sperm cells, 10
Spermatic artery, 267
cord, 224
veins, 275
Spermatids, 14
Spermatocyte, primary, 14
secondary, 14
Spermatogenesis, 14
Spermatogonia, 14, 213
Spermatozoon, 10
Sphenoid bone, ossification of, 313
Sphenoidal sinus, 376
Spheno-mandibular ligament, 315
Spheno-palatine ganglia, 368
Sphincter muscle of iris, 383
Spina bifida, 352
Spinal accessory nerve, 93, 116, 364
arteries, 264
cord, 322
primitive segments, section through, 111
ganglia, 50, 116, 304
supporting cells, 305, 306
nerves, 116, 353
efferent or ventral root fibers of, 302
roots, dorsal, 305
tract, descending, of trigeminal nerve, 361
Spiral ganglia, 358, 359
limbus, 387
organ, 386, 387
sulcus, 387
tunnel, 387
Spireme, 13
Splanchnic mesoderm, 53, 69, 169, 366
Splanchnopleure, 30, 34, 52, 53, 64
Spleen, 191, 281
Splenic corpuscles, 282
Spongioblasts, 302, 306, 307
Spongy layer, 236, 240
Stapedial artery, 390
muscle, 390
Stapes, 315, 390
Stenson’s canal, 148
Sternal bars, 311
Sterno-cleido-mastoid muscle, 364
Sternum, 311
cleft, 316
Stoerck’s loop, 203
Stomach, 81, 96, 119, 169
Stomodeum, 57, 80, 161
Stratified epithelium, 294
Stratum corneum, 294
germinativum, 294
Stratum granulosum, 216, 217, 294
lucidum, 294
Stroma of ovary, 215
Study, methods of, 5
Stylo-hyoid ligament, 315
Styloid process, 314, 315
Subcardinal veins, 102, 123, 124, 274
Subclavian arteries, 99, 122, 262, 263, 266, 268
veins, 123, 276
Sublingual gland, 152
Submaxillary ganglia, 368
gland, 152
Substantia ossea, 157
propria of cornea, 377
Sudoriparous glands, 297
Sulcus, central, 351
coronary, 225, 251
hypothalamicus, 330, 339
interventricular, 258
limitans, 323, 332, 339
of cerebrum, 351
spiral, 387
Supporting cells, 371, 387
of neural tube, 307
of spinal ganglia, 305, 306
tissue, 285
Suprarenal artery, 267
gland, 143, 368, 369
accessory, 370
vein, 275
Supratonsillar fossa, 161
Suspensory ligament of lens, 381
Sustentacular cells (of Sertoli), 14, 213
Sweat glands, 297
Sylvian fissure, 349
Sympathetic ganglia, 305, 367
nervous system, 366
Synovial membrane, 291
TACTILE corpuscles, 371
Tenia, 334
Tail bud, 55
fold, 55
gut, 97
of caudate nucleus, 345
Tarsal glands, 383
Tarsius, origin of mesoderm in, 34
Tarsus, 383
ossification of, 316
Taste buds, 151, 371
cells, 371
Teeth, anlages of, 153, 154
anomalies of, 158
cement of, 157
decidual, periods of eruption, 157
dental lamina of, 153
papilla, 153, 157
pulp, 157
sac, 157
dentine, 157
development of, 153
enamel, 153, 155
milk, periods of eruption, 157
odontoblasts, 157
of vertebrates, 158
permanent, periods of eruption, 158
Tegmentum, 330
Tela chorioidea, 329, 338
INDEX 409
Telencephalon, chick, 56
commissures of, 346
human, 80, 327, 341
pig, 92, 115
Telolecithal ova, 23
Telophase of mitosis, 13
Temporal bone, ossification of, 313
operculum, 349
Tendon, 286
Tensor nerve, 358
nucleus, 335
ramus, 354
tympani, 390
ventricle, 326
Testis, 212
anomalies of, 214
compared with ovary, 217
concealed, 224
cords, 212, 213
descent of, 222
intermediate cords of, 213
interstitial cells of, 213
ligament of, 221
mediastinum, 213
tubuli contorti, 213
recti, 213
septula, 213
Tetrads, 16
Thalamus, 142, 330
Thebesian valve, 254
Theca folliculi, 217
Theory of concrescence, 31
Thoracic duct, 279
Thoraco-abdominal muscles, 317
Thymic corpuscles, 163
Thymus, 162
anlages, 82
gland, 118
Thyreo-cervical trunk, 266
Thyreoglossal duct, 117, 164
Thyreoid anlage, 60
cartilage, 166, 315
gland, 118, 164
human, 81, 82, 164
pig, 94, 95
Tibia, ossification of, 316
Tibial nerve, 356
veins, 277
Tissue, adipose, 287
areolar, 286
connective, 285
white fibrous, 285
corneal, 286
differentiation of, 4
elastic, 286
lymphoid, of spleen, 282
nervous, 300
reticuldr, 163, 285
supporting, 285
Titles for reference, 6
Toes, supernumerary, 316
Tomes, dentinal fibers of, 157
Tongue, muscles of, 320
of pig, 116, 143, 149
papille of, 151, 152
Tonsil, palatine, 82, 118, 162
pharyngeal, 162
Tonsillar fossa, 161
Touch-pads, 300
410
Trabecule carne, 259
Trachea, human, 81, 82, 164, 166
pig, 94, 95, 119, 143
Tract, descending, of fifth nerve, 335
Tractus solitarius, 335
Tragus, 391
Trapezius muscle, 364
Triangular ligaments, 192
Tricuspid valves, 121, 259
Trigeminal nerve, 92, 116, 361
Trochlear nerve, 116, 330, 361
Trophectoderm, 26, 71, 72, 232
Trophoderm, 232, 234
Tubal pregnancy, 20
Tuber cinereum, 330, 341
Tubercle, cloacal, 225
Darwin’s, 391
genital, 225
Miiller’s, 221
satyr, 391
Tuberculum acusticum, 358
impar, 82, 94, 117, 150
Tubular heart, 248
Tubules, mesonephric, 197
and genital glands, origin of, 218
renal, 120, 201, 203
Tubuli contorti, 213
recti, 213
septula, 213
Tunica albuginea, 212, 214
externa, 217
interna, 217
vaginalis, 224
Turbinate anlages, 143
Twins, development of, 21
Tympanic cavity, 161, 389
membrane, 117, 391
ULNA, ossification of, 316
Ulnar artery, 268
nerve, 356
Ultimobranchial body, 118, 164
Umbilical arteries, 86, 99, 123, 135, 260, 267
cord, human, 70
of pig, 70
hernia, 70
veins, 259, 261, 268, 271
human, 86
pig, 100, 124
vessels, 70
Umbilicus, 70
Unguiculates, 68
Ungulates, 68
Unipolar ganglion cells, 305
Urachus, 77, 208
Ureter, 120, 135, 199, 208
Urethra, 143, 205, 227
Urethral groove, 225
plate, 225
Urogenital ducts, 144
fold, 197, 208
glands, 144
membrane, 160, 205
opening, 225
organs, 83, 143
sinus, 120, 205, 212
system, 97, 120, 195
Uterine glands of pregnancy, 236
INDEX
Uterine tubes, 219
Utero-vaginal anlage, 220
Uterus, 219, 220
anomalies of, 221
bicornis, 221
during menstruation, 230
pregnancy, 230
fetalis, 221
fundus of, 220
growth of, 221
infantilis, 221
ligaments of, 221, 222
masculinus, 219
planifundus, 221
Utricle, prostatic, 218, 219
Utriculus, 386
Uvula, 149
Vacina, 219, 220
anomalies of, 221
fornices of, 220
masculina, 218, 219
Vagus ganglia, accessory, 116
nerve, 93, 116, 362, 364
Vallate papilla, 151
Valves, atrio-ventricular, 259
bicuspid, 121, 259
colic, 174
Eustachian, 254
mitral, 259
of coronary sinus, 254
of inferior vena cava, 254
of sinus venosus, 121, 251, 253, 254
semilunar, 258
Thebesian, 254
tricuspid, 121, 259
Vascular system, 243
Vegetal pole, 23
Veins, anterior cardinal, 49, 58, 59, 60, 86, 101,
123, 261, 268, 271
axillary, 276
azygos, 274
basilic, 276, 277
border, 276
brachial, 276
cardinal, 123
ane 49, 58, 59, 60, 86, 101, 123, 261, 268,
common, 58, 60, 86, 101, 123, 261, 268, 271
left, 97
right, 97
posterior, 58, 62, 63, 86, 102, 123, 261, 268, 274
cephalic, 276, 277
cerebral, 273
Ea, 58, 60, 86, 101, 123, 261, 268,
1
left, 97
right, 97
development of, 268
femoral, 277
gluteal, 277
hemiazygos, 274
hepatic, 270
common, 98
iliac, 274, 275
intersegmental, 274
ischiadic, 274
jugular, 123, 273
INDEX
Veins, linguo-facial, 101
lumbar, 276
mesenteric, superior, 99, 100, 125, 270
of extremities, 276
of heart, 86
of lower extremity, 277
of pig, 99, 123
of plica ven cave, 274
ophthalmic, 273
ovarian, 275
portal, 99, 125, 268, 270
posterior cardinal, 58, 62, 63, 86, 102, 123, 261,
268, 274
pulmonary, 121, 168, 255
renal, 275
saphenous, 277
spermatic, 275
subcardinal, 102, 123, 124, 274
subclavian, 123, 276
suprarenal, 275
tibial, 277
umbilical, 259, 261, 268, 271
human, 86
pig, 100, 124
vitelline, 49, 62, 86, 99, 124, 182, 260, 268
chick, 39, 40, 42
right, 98
Velum, medullary, 330, 337
Vena anonyma, left, 271
right, 272
capitis lateralis, 273
medialis, 271, 273
cava, inferior, 100, 103, 120, 124, 189, 274,
275
superior, 271
porte, 268,270
Ventral arteries, 99, 122
ramus, 354
Ventricle, fifth, 347 °
first, 329
fourth, 115, 330
lateral, 115, 329
of heart, 57, 83, 90, 97, 249, 251, 258
of larynx, 165
second, 329
terminal, 326
third, 329, 341
Ventricular limb, 249
Ventro-lateral arteries, 99, 122
Vermiform process, 172, 174
Vermis cerebelli, 336
Vernix caseosa, 74, 295
Vertebre, 309, 310
chondrification of, 310
ossification of, 310
pig, centra of, 142
variations in number, 316
Vertebral arch, 310
arteries, 122, 264
Vertebrate head, segmentation of, 365
Vesicle, auditory, 385
blastodermic, 26
brain, primary, 322, 327
4Il
Vesicle, cervical, 161
lens, 376, 379
optic, 40, 43, 47, 59, 343
otic, 45
seminal, 218
Vesicular follicles, 215
Vestibular anlage, 386
ganglia, 358
glands, 228
membrane, 389
Villi, anchoring, 237
of chorion, 71, 72, 232, 237
of intestine, 173
origin of, 72
Viscera, 114
lateral dissections, 138
pig, dissections of, 91
Visceral ganglia, 368
mixed nerves, 361
pleura, 168
Vitelline arteries, 46, 58, 63, 86, 99, 122, 260, 261,
262
chick, 42
circulation, 46
duct, 159
membrane, 7
veins, 49, 62, 86, 99, 124, 182, 260, 268
chick, 40
right, 98
Vitello-umbilical trunk, 86, 260
Vitreous body of eye, 380
Voluntary muscle, 52
Vomer, 314
Vomero-nasal organ, 357, 374
Wate blood corpuscles, 245. See also Leucocytes.
commissure, 325
fibrous connective tissue, 285
rami, 366
Whole embryos for study, 137
Winslow’s foramen, 134, 190
Wolffian ducts, 97, 83
XIPHOID process, 311
cleft, 316
YELLOW bone marrow, 290
Yolk, 7
sac, 65, 66, 67, 68, 77
stalk, 65, 66, 77, 79, 120, 159, 170, 171
ZONA pellucida, 7
Zone, ependymal, 301
marginal, 302
nuclear, 302
segmental, 64
Zonula ciliaris, 381
Zuckerkand!’s bodies, 369
Zygomatic bone, 314
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