MEMORIAL POULTRY LIBRARY^ "fHE GJjf OJ Pfcvwv'«v«v^irv»v»vmv»vm'v«v»wmvs«v«?«n CORNELL UNIVERSITY UBRARY .T 1QP4 DfiQ R.qn iRn Cornell University Library The original of this bool< is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31 9240895301 60 Bulletin of the Museum of Comparative Zoology AT HARVARD COLLEGE. Vol. XL VIII. No. 2. THE DEVELOPMENT OF THE OCULOMOTOR NERVE, THE CILIARY GANGLION, AND THE ABDUCENT NERVE IN THE H^Ik. By Fkederick Walton Carpenter. •With Seven ^ CAMBRIDGE, MASS., U.S.A.: PRINTED FOB THE MUSEUM. Jandary. 1906. Reports on the Scirntific Results of the Expedition to the East- kRN Tropical Pacific, in chahge of Alhxandeb Agassi?, by the U. S. Fish Commission Stbamk.r " Albatross,'' from October, 1904, to March, 1905, Lieutenant Commander L. M. Garrett, U. S. N., Commanding, published or in preparation: — A. AGASSIZ. General Report on the Expedi- tion. A. AGASBIZ. I.< Tliree Letters to Geo. M: Bowers, U. S. Fish Com, A. AGASSIZ and H. L. CLARK. The Ecliini. F. K BEDDAllD. Tiie Earthworms. H. B. BIGELOW. The Medusae. R. P. BIGELOW. The Stomatopods. S. F. CLARKE. The Hydroids. • W. R. COE; The Nemerteans. L. J. COLE. The'Pycnogonida. W. H. DAtL. The MoUuslcs. C. R. EASTMAN. The Sharks' Teeth. B. W. EVERMANN. The Fishes. W. G. FARLOW. The Algae. S. GARMAN. The Reptiles. H. J. HANSEN. The Cirripeds. H. J. HANSEN. The Schizopods. ' 8. HENSHAW. The Insects. W. E. HOYLE. The Oephalopods. C. A.KOFOID. m.s The Protozoa. P. KRUMBACH. The Sagittae. R. VON LBNDBNFELD. The Sponges, H. LUDWIG. The Holothurians. H. LUDWIG. The Starfishes. H. LUDWIG. The Ophiurans. J. P. McMURRlCH. The Actinaria. G. W. MiJLLER. The Ostracods. JOHN MURRAY. The Bottom Specimens. MARY J. RATHBUN; The Crustacea. HARRIET RICHARDSON. Il.= The Isopods. W. E. RITTER. The Tunfeates. ALICE ROBERTSON. The Bryozoa. B. L. ROBINSON The Plants. O. 0. BARS. The Copepods. H. R. SIMROTH. The Pteropods and Hetero- pods. TH. BTUDER.' The Aloyonaria. T W. VAUGHAN. The Corals. R. WOLTERECK. The Amphipods. W. MoM. WOODWORTH. The Annelids. • Bull. M. C. Z., Vol. XL VI., No. 4, April, 1905, 22 pp. 2 Bull. M. 0. Z., Vol. XL VI., No. 6, July, 1905, 4 pp., 1 pi. s Bull. M. C. Z., Vol. XL VI., No. 9, September, 1905, 5 pp., 1 pi. Bulletin of the Museum of Comparative Zoology at harvard college. Vol. XLVIII. No. 2. THE DEVELOPMENT OF THE OCULOMOTOR NERVE, THE CILIARY GANGLION, AND THE ABDUCENT NERVE IN THE CHICK. Bt Fredebicc Walton Cakpentes. With Seven Plates. CAMBRIDGE, MASS., U.S.A.: PRINTED FOR THE MUSEUM. January, 1906. No. 2. — CONTRIBUTIONS FEOM THE ZOOLOGICAL LABORATORT OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE, UNDER THE DIRECTION OF E. L. MARK, No. 172. The Development of the Oculomotor Nerve, the Ciliary Ganglion, and the Abducent Nerve in the Chick. By Frederic Walton Carpenter. TABLE OF CONTENTS. Introduction 142 Part I. — Anatomy and Histology 143 A. Historical Survey . . . 143 a. Anatomy 143 1. Eye-Muscle Nerves . . 143 2. Ciliary Ganglion . . 144 b. Histology 147 1. Oculomotor Nerve . . 147 2. Abducent Nerve . . . 147 3. Ciliary Ganglion and Short Ciliary Nerves . 147 B. Observations 161 a. Metliods 151 b. Anatomy 152 1. Eye-Muscle Nerves and Ciliary Ganglion . . 152 0. Histology 155 1. Oculomotor Nerve . . 155 2. Abducent Nerve . . . 155 3. Ciliary Ganglion . . 156 Part II. — Development .... 169 A. Historical Survey . . . 159 1. Fishes 160 2. Amphibians 167 3. Reptiles 167 4. Birds 169 5. Mammals 171 B. Observations 176 176 1. Methods 2. Development of the Ocu- lomotor Nerve, the Cil- iary Ganglion, and the Abducent Nerve ; de- scribed by Stages 177 Stage I 177 1. Oculomotor Nerve . 177 2. Eye Muscles . . . 178 Stage II 178 1. Oculomotor Nerve . 178 2. Ophthalmic Branch of the Trigeminal Nerve 179 3. Eye Muscles . . . 180 Stage III 180 1. Oculomotor Nerve and Ciliary Gan- glion 180 2. Ophthalmic Branch of the Trigeminal Nerve 182 3. Abducent Nerve . . 183 4. Eye Muscles . . . 184 Stage IV 185 1. Oculomotor Nerve . 185 2. Ophthalmic Branch of the Trigeminal Nerve 187 3. Ciliary Ganglion . . 190 4. Abducent Nerve . . 193 6. Eye Muscles . . . 193 Stage V 193 1. Oculomotor Nerve . 194 2. Ophthalmic Branch of the Trigeminal Nerve 196 3. Ciliary Ganglion . . 198 4. Abducent Nerve . . 198 5. Eye Muscles . . . 199 142 bulletin: museum of compakative zoology. Notes on Later Develop- ment 200 1. Oculomotor Nerve . 200 2. Ophthalmic Branch of the Trigeminal Nerve 201 3. Ciliary Ganglion . . 201 4. Abducent Nerve . . 202 . 5. Eye Muscles ... 202 6. Trochlear Nerve . . 202 1 PAQB Discussion of Results .... 202 Migration of Medullary Cells . 202 Histogenesis of the Neuraxons 205 Nature of the Ciliary Ganglion 205 Homologies of the Oculomotor and Abducent Nerves . . . 209 Summary of Besults 210 Bibliography . 214 Explanation of Plates 228 Introduction. "Von alien motorisohen Nerven ist mit Ausnahme vielleicht des Hypoglossus kein anderer zum Gegenstand so widerspruchsvoller An- gabeh und Deiitungen geworden, wie der Oculomotorius. Er ist als dor- saler, als ventraler und als gemischter Nerv in Anspruch genommen worden ; man hat ihm metaraerischen Werth zu- und abgesprochen ; er ist als Theilsttiek des Trigeminus definirt, und ihm sind alle Beziehungeu zum Trigeminus geleugnet worden. Man hat Ganglion an ihm ent- deokt, deren Ursprungsort man in der Ganglienleiste sah ; man hielt sie dann fiir eine Abspaltung des G. ciliare ; man schrieb sie einem eigenen O. oculomotorii zu, das nichts mit dem G. ciliare zu thun habe ; man leugnete die Ganglion ganz und gar — kurz es war nicht mit ihm fertig zu werden." — Dohrn ('91, p. 2). Since Dohrn commented thus in 1891 upon the diversity of opinion ■which exists concerning the oculomotor nerve and the ciliary ganglion, three investigators have added still another to the already large number 'of conflicting statements. They have asserted that in selachians the oculomotor nerve grows from the mesocephalic ganglion to the ventral iace of the mid-brain, and not in the opposite direction, as had pre- viously been supposed. Dohrn himself, in the article from which the quotation is taken, described an entirely new mode of origin for the cells of the ciliary ganglion, namely, migration from the neural tube into the Toot of the oculomotor nerve. This Jack of agreement in regard to the developmental history of the oculomotor nerve, and particularly of the ciliary ganglion, seemed a suffi- cient justification for a renewed study of the •subject. In the case of the abducent nerve, opinions of observers being more in accord, a study of its development might be expected to result in little more than a confirmation of generally accepted views. Though primarily concerned with the oculomotor nerve and ciliary ganglion, I have, nevertheless, CARPENTEE: DEVELOPMENT OF THE OCULOMOTOR NERVE. 143 included in this paper my observations on the histogenesis of tlie abdu- cens. Its study has been made easy by its presence in the series of sections in which the later development of the oculomotor has been fol- lowed, and as a typical ventral cranial nerve with motor functions it has proved interesting for the purposes of comparison with the oculomotor. The remaining eye-muscle nerve, the trochlear, first appears at a com- paratively late stage, and my observations have not been extended to it. Several considerations led to the selection of the chick as the subject for investigation. First, closely connected stages in the development of the eye-muscle nerves have been studied in but few of the Amniota, the greater part of the observations having been confined to selachians. Secondly, no investigator has directly concerned himself with the genesis of these nerves in birds since Marshall published the first accounts of their development in 1877 and 1878. Marshall's descriptions are ad- mittedly incomplete, and certain of his interpretations are questionable in the light of more recent studies in other classes of vertebrates. Since his time observations on the nerves in question have been fragmentary and incidental. Thirdly, in the case of the chick it is possible to control incubation, and obtain embryos in the required stages of development. It is a pleasure to acknowledge here my sense of obligation to Professor E. L. Mark, under whose guidance the present work was carried on in the Zoological Laboratory of Harvard University. The constant interest, the helpful suggestions and the conservative judgments of Professor Mark have been of the greatest value to me. I am also indebted to Professor H. V. Neal, o^ Knox College, for advice as to the use of the vom Rath fluid in the early stages of the development of nerves. PART I. -ANATOMY AND HISTOLOGY. A. Historical Survey. a. Anatomy, 1. Eye-Muscle Nerves. The older anatomists (Muck, '15 ; Bonsdorff, '52 ; Budge, '55 ; and others) who first investigated the eye-muscle nerves of birds found them homologous with nerves of similar function in ocher classes of verte- brates. The third, or oculomotor, nerve arises from the ventral face of the mesencephalon, and is distributed to the dorsal, ventral and anterior rectus muscles and to the ventral oblique muscle. The fourth, or troch- 144 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. lear, nerve emerges from the dorsal aspect of the brain, at the posterior boundary of the mesencephalon, and innervates the dorsal oblique muscle. The sixth, or abducent, nerve takes its origin from the ventral side of the metenoephalon, and passes to the posterior rectus muscle. A slender branch of this nerve is given off to the muscles of the nicti- tating membrane (quadratus and pyramidalis). In Bronn's Thierreich is mentioned on the authority of Bonsdorff an anastomosis between the sixth nerve and the ramus ciliaris trigemini in Corvus cornix. In this bird fibres are also said to pass to the ramus ciliaris externus of the cili- ary ganglion ; while in Grus cinerea a fine branch of the abducens passes partly to the ramus ciliaris internus of the ciliary ganglion, and partly to the ganglion itself. The distribution of abducent fibres to the eyeball in birds has also been recorded by Jegorow ('86-87), who considers it possible that these may be sympathetic fibres, which join with the sixth nerve as it passes through the cavernous sinus and proceed cephalad in its trunk. 2. Ciliary Ganglion. A well-marked ganglion is always found in connection with the oculo- motor nerve of birds. This ganglion corresponds to the ciliary ganglion of human anatomy, which was first described by Schacher in 1701 (Jegorow, '86-87). In man it occurs in the posterior region of tlie orbit as a small, laterally compressed, somewhat four-sided body, measuring about 2 mm. in an antero-posterior direction. From behind it receives branches from three different sources : a short or motor root (radix bre- vis) from the oculomotor nerve, a long or sensorji root (radix longa) from the nasal branch of the ophthalmic division of the trigeminus, and a sympathetic root (radix sympathica) from the sympathetic plexus bf the cavernous sinus. It gives off in front six to eight ciliary nerves, which proceed to the sclerotic and choroid coats, ciliary muscle, iris and cornea of the eyeball. These nerves are distinguished as the short ciliary nerves from the so-called long ciliary nerves, which emanate from the nasal branch of the trigeminus, and have the same distribution as the short ciliaries (Qnain's Anatomy, Thane, '95). The ganglion in question has received various names from different authors (ciliary, ophthalmic, lenticular, oculomotor, Schacher's). It is commonly described in the text-books of human anatomy in connection with the trigeminal nerve. It is almost invariably stated to be sympa- thetic in nature (as first suggested by Arnold, '31), although it differs from typical sympathetic ganglia in giving origin to medullated periph- eral nerves (the short ciliaries) instead of non-medullated fibres. cakpenter: development of the oculomotor nerve. 145 Comparative studies of the anatomical connections of the ciliary gan- glion have been made by Schwalbe ('79), Jegorow ('86-87), Holtz- mann ('96) and Onodi (:0l). Schwalbe, in his extensive and much-quoted work on " Das Ganglion oculomotorii," shows that the ciliary ganglion is represented in the lower vertebrates by groups of ganglion cells distributed along the course of the oculomotor nerve. Passing upward toward the higher forms, the cells become more closely associated into a compact body, this change being accompanied by a gradual withdrawal of the ganglion from the trunk of the oculomotor nerve through the formation of a radix bre- vis. However, not all the higher vertebrates possess short roots, since in many mammals (sheep, calf, dog, rabbit) none exists, the ganglion being placed directly on the trunk of the oculomotor. Schwalbe denies the existence of a connection between the ciliary ganglion and the tri- geminal nerve in several species, and believes the sympathetic root to be confined to mammals. He, therefore, asserts that the ciliary ganglion belongs primarily to the oculomotor, which he considers entitled to the rank of an independent segmental nerve. Jegorow's researches on " le ganglion ophthalmique " were, like those of Schwalbe, very comprehensive. As far as the anatomical relations of the ganglion are concerned, he differs from the latter writer chiefly in regard to the importance of the connection with the trigeminus. This he regards as constant, and necessary for the existence of the ganglion, throughout the vertebrate series. Holtzmann found the ciliaiy ganglion in amphibians, birds and mam- mals more intimately connected with the oculomotor than with the trigeminus. Although he found neuraxons joining the ciliary ganglion with the fifth nerve where Schwalbe believed no connection existed, he does not regard these neuraxons, in certain cases, as constituting a phys- iological radix longa. The examination of many selachians as well as several bony fishes and mammals convinced Onodi that the connection between the ciliary ganglion and the trigeminus is more intimate than Schwalbe's researches show. In selachians he frequently found a macroscopic ciliary ganglion external to the .trunk of the oculomotor. The ganglionic groups con- nected distally with the third and fifth nerves he considers sympathetic in nature, and in support of this view calls attention to nerve fibres extending from them to form a plexus about the wall of a neighboring blood vessel. The observations which have already been made on the anatomical vol,. XLVIII. NO. 2 10 146 bulletin: museum of comparative zoology. relations of the ciliary ganglion in the group of birds will now be sum- rnarizfid. Connection with the Oculomotor Nerve. A radix brevis is present iu Strix flammea, in various species of the genus Corvus, in Falco tinnun- culus and Sterna hirundo. The ciliary ganglion is placed directly on the trunk of the third nerve in the goose, Falco palumbarius, Aquila ieuco- cephala, Meleagris gallopavo, Ardea cinerea, Vanellus cristatus and Gal- linula pusilla (Gadow und Selenka, '91). To this list should be added Gallus doinesticus, the pigeon and the duck (Holtzmann, '96). Connection with the Ophthalmic Branch of the Trigeminal Nerve. Sohwalbe ('79) describes a large ciliary nerve passing cephalad from the distal extremity of the ciliary ganglion of the goose. This nerve re- ceives, a short distance from its origin from the ganglion, a slender ramus from the ophthalmic branch of the trigeminus, but no direct fibrous connection appears to exist between the last-named nerve and the gan- glion. The same conditions were observed by Holtzmann ('96) in the hen, duck and pigeon, as well as in the goose. Schwalbe concluded from the appearances that no long root could be said to be present in birds, but Holtzmann has ascertained by microscopical examination that in the hen and goose (the only forms examined in this way) about one-fourth of the neuraxons of the communicating branch from the fifth nerve turn centrad, and, running parallel with those of the ciliary nerve, enter the ganglion. This connection he believes to be a survival of an embryonic union between the fundaments of the ciliary and Gasserian ganglia, and to possess a developmental rather than a physiological significance. A few cases of direct connection between the fifth nerve and the cil- iary ganglion have been recorded. Jegorow ('86-87) asserts that such a condition obtains in the pigeon and vultjire. In Bronn's Thierreich, Muck ('15) is cited as authority for the statement tliat in several birds the communicating branch enters the anterior part of the ganglion. Bonsdorff ('52) describes for the crane two rami from the trigeminus which have the typical relations of long roots of the ganglion. Connection mth the Sympathetic System. Neither Schwalbe ('79), Holtzmann ('96) nor the older investigators discovered any evidence of a connection between the sympathetic system and the ciliary ganglion in birds. Jegorow ('86-87), while admitting that the distribution of sym- pathetic fibres has not been proved anatomically, infers, nevertheless, the presence of sympathetic neuraxons in the ciliary ganglion from the occurrence of certain fibres which pass from the latter to the walls of neighboring arteries. He considers it possible that sympathetic neu- CAEPENTER: DEVELOPMENT OF THE OCULOMOTOR NERVE. 147 raxons may enter the third nerve in the cavernous sinus, where it comes into close relation with the cephalic extension of the cervical sympathetic system. The only description of a sympathetic root of the avian ciliary ganglion to be found in the literature is that of Eochas ('85), who detected in the goose several fine fibres extending to the ciliary ganglion from the sym- pathetic plexus about the ophthalmic artery (Weber's plexus). Ciliary Nerves. There is much variation in the number of ciliary nerves given off by the ciliary ganglia of different species of birds. Va- riations may also occur among individuals of the same species. Schwalbe ('79) states that the number may vary from one in many birds, includ- ing the hen, owl and goose, to seven in parrots. Schwalbe figures for the goose a ciliary nerve (ramus ciliaris trigemini) emerging from the ophthalmic branch of the trigemimis distal to the ori- gin of the communicating branch passing to the ramus ciliaris ooulomo- torii. Holtzmann ('96) shows that in the hen the communicating branch gives off an independent ciliary nerve to the eyeball. b. Histology. 1. Oculomotor Nerve. In birds, as well as in man (Barratt, :0l) and in teleosts (Herriclt, '99), both large and small meduUated neuraxons are present in the oculo- motor nerve. This has been shown to be the case in the pigeon by Langendorff (:00), who found the main portion of the nerve composed of neuraxons of large calibre, while smaller ones occurred near the periphery. In all forms the majority of the small neuraxons pass into the ciliary ganglion. 2. Abducent Nerve. I have not been able to find any description of the finer structure of the abducent nerve in birds. In man it is made up of large and small meduUated neuraxons (Barratt :0l). 3. Ciliary Ganglion and Short Ciliary Nerves. Anatomical Evidence. The character and connections of the cells of the ciliary ganglion of vertebrates have long been favorite topics of inves- tigation among neurologists. Before the silver impregnation process of Golgi had come into general use, Eetzius ('81) had already demonstrated by other methods the multi- 148 bulletin: museum of comparative zoology. polarity of the ciliary-ganglion cells of mammals. In a later paper (Retzius, '94, '94°) he confirmed his former observations by the aid of the Golgi process. Using the latter method, Kolliker ('94) and Michel ('94) obtained like results, and showed, furthermore, that the ganglion cells are surrounded by pericellular baskets of nerve fibrils. These pericel- lular baskets have also been demonstrated, and proved to be intracap- sular, by the -niethylen-blue introrvitam stain (Huber, '97). Inasmuch as all these conditions are characteristic of the cells of sympathetic gan- glia, the investigators cited above are unanimous in declaring the mam- malian ciliary ganglion to be sympathetic in nature. On the other hand, Schwalbe ('79), a " partisan ardent," to quote Jegorow, of the cerebro-spinal character of the ciliary ganglion, found in the ciliary ganglion of the sheep and calf unipolar cells, such as are char- acteristic of cerebro-spiual ganglia. The crudeness of Schwalbe's methods, however, leaves his results open to question. D'Erchia ('94) discovered among the numerous multipolar cells of the cat's ciliary ganglion a few bipolar cells. Such cells were seen in both the cat and dog by Holtz- manu ('96), who, furthermore, found the comparatively small ciliary ganglion of the rabbit to be composed mainly, if liot wholly, of cells of the cerebro-spinal type, many being bipolar. Jegbrovi' ('86-87) figures, in a colored plate, spinal, sympathetic and ciliary ganglion cells of the cat, prepared by Boukhaloff according to a special differential method. The cells from the spinal and ciliary ganglia present the same appear- ance, whereas the sympathetic cells differ in staining qualities from the others. It is interesting to compare with these figures those given by His, Jun. ('91) of the same three kinds of ganglion cells taken from an embryo cat. Cells from the ciliary and from a sympathetic ganglion closely resemble each other, being small and unipolar. Those from the vagus ganglion (which belongs to the cerebro-spinal ' series) are, on the contrary, larger and bipolar in character. Haller ('98) believes that the conditions which obtain in the central nervous system of the dog-fish and trout point to the cerebro-spinal character of the representatives of ciliary-ganglion cells found in these fishes. Golgi preparations of the mid-brain show, in addition to oculo- motor neuraxons proceeding centrifugally from ganglion cells in motor niduli, other oculomotor neuraxons, which have no direct connection with central ganglion cells. These neuraxons he regards as centripetal processes from ganglion cells on the oculomotor nerve (i. e., ciliary gan- glion cells). Such cells are accordingly to be looked upon as homologous with spinal-ganglion cells. CARPENTER: DEVELOPMENT OF THE OCULOMOTOR NERVE. 149 A considerable amount of evidence as to the nature of the ciliary- ganglion in mammals has been derived from the employment of degener- ation methods. To appreciate the significance of the results obtained, it must be borne in mind that in a typical sympathetic ganglion (one of the gangliated sympathetic cord) the motor neuraxons of the white ramus (" pre-ganglionic fibres"), originating from cells within the central nervous system, pass into the sympathetic ganglia, and end in peri- cellular baskets of tine fibrils about the sympathetic cells. From the latter are given off, peripherally, non-raedullated neuraxons (" post- ganglionic fibres "), which make up the pale sympathetic nerves. These neuraxons, together with the sympathetic ganglion cells with which they are connected, form, consequently, the terminal link in a chain of neurons. The investigations of Bach ('96) on the rabbit show that removal of the iris and ciliary body, to which the short ciliary nerves are distrib- uted, results in a modification of the cells of the ciliary ganglion, while those of the nidulus of the oculomotor nerve remain normal. Apolant ('96, '96") cut the oculomotor nerve of young cats near its root. The fine medullated neuraxons passing to the ciliary ganglion degenerated periph- erally as far as the cells of that ganglion, while these cells, together with the short ciliary nerves, remained unaltered. Bumm (:00) con- firmed Apolant's results. After injury of the intrinsic eye muscles and the nerves distributed to them, nearly all the cells of the ciliary ganglion undergo changes (Marina, '98, '99), but, as shown by tlie further experi- ments of Marina, and by those of Fritz ('99), destruction of the cornea is also followed by a slight degeneration of some of the cells of the ciliary ganglion (one-eighth of the entire number, according to Marina). From this, both writers conclude that the ganglion is a mixed one, being largely motor, but also to some extent sensory in function. Fritz ('99) infers that the ciliary ganglion is connected with the sym- pathetic system from the fact that changes in the cells of the ganglion occur upon extirpation of the cervical sympathetic. Bumm (:00) is also of this opinion, since the cutting of the ciliary nerves in the cat results in the atrophy of only four-fifths of the cells of the ciliary gan- glion. He considers the cells affected to be those of peripheral neurons, while the cells which remain unaltered are probably connected with the sympathetic system. The study of degeneration preparations shows, then, that at least the majority of the mammalian ciliary-ganglion cells and their processes, the short ciliary nerves, may be considered the terminal neuraxons of a 150 bulletin: museum of compakative zoology. motor chain, and consequently sympathetic in their relations. The muscles innervated are of the unstriped variety. Stefani (:0l) was led to the conclusion that the short ciliary nerves have their centres, i. e., their ganglion cells, in the ciliary ganglion, by observing the effect on the cells of that ganglion when atropin is applied to the eye. Histologically, the ciliary ganglion of birds differs from that of mam- mals, lu the hen certain of its cells, as was first shown by Eetzius ('8l), are bipolar in character, each sending out two processes, which arise close together, and run either in the same or in opposite directions. Near their origins from the cell the processes are pale, but soon acquire medullary sheaths. Holtzmann ('96) examined the elements of the ciliary ganglion in the hen, duck, goose, and pigeon, finding in each of the four species both large and small ganglion cells. These were usually bipolar, but an occasional unipolar cell was observed, the single process of which soon divided into two. Holtzmann is of the opinion that while in many animals (amphibians, mammals) the ciliary ganglion contains both sympathetic and spinal cells, in birds a one-sided development, that of spinal elements, takes place. These do not, as a rule, become fully differentiated into true unipolar spinal ganglion cells, but remain in an embryonic bipolar condition. From the foregoing, it is apparent that those cells of the ciliary gan- glion of birds which have been described by investigators do not resemble histologically the sympathetic cells found in the same group. The great majority of the latter are well known to be multipolar, and, in general, to resemble the sympathetic cells of mammals (Ramon y Cajal, '91, '94; Timofeew, '98; Huber, '99). Physiological Evidence. From a physiological point of view the ciliary ganglion of mammals is undoubtedly sympathetic. The experimental researches of Langley and Dickinson ('89) have demonstrated the fact that a moderate dose of nicotin, which has little, if any, effect on spinal ganglia, prevents the passage of efferent nervous impulses through sym- pathetic ganglia. These authors considered this result due to a paralysis of the sympathetic cells, but Huber ('97) has shown that it is more probable that the nicotin paralyzes the pericellular baskets of the pre- ganglionic neuraxons about the cells, rather than the cells themselves. The physiological effect of nicotin has afforded, therefore, a valuable cri- terion for determining the character of the cells of the ciliary ganglion. After an injection of nicotin, Langley and Anderson ('92) found that the ciliary ganglion of the rabbit no longer transmitted nervous impulses. caepenter; development of the oculomotor nerve. 151 Direct stimulation of the short ciliary nerves, however, still caused con- traction of the ciliary body and the iris. The results of Laiigley and Anderson were confirmed for the dog and monkey by Marina ('98, '99). These investigators consequently regard it as proved that tlie neuraxons which innervate the sphincter iridis and the ciliary muscle are connected ■with the cells of the ciliary ganglion. The physiological experiments of Langendorff ('94) and Bernheimer ('97) point to the same conclusion. The former found that some time after death, when stimulation of the third nerve proximal to the ciliary ganglion was without result, excita- tion of the short ciliary nerves produced contraction of the iris. Bern- heimer demonstrated that lesion of the oculomotor nerve in the monkey leaves the iris still active. The physiological behavior of the ciliary ganglion of birds affords ad- ditional proof of the lack of similarity between its cells and those of the ciliary ganglion of mammals. Langendorff (:00) states, on the author- ity of Consiglio (:00), that, after the ciliary ganglion of birds has been subjected to the action of nicotin, stimulation of the third nerve is still followed by constriction of the pupil. He himself found that, in birds which have been bled to death, the neuraxons of the third nerve, the stimulation of which causes closure of the pupil, retained their irritabil- ity considerably longer than did tliose of mammals subjected to the same treatment. For these reasons he regards it improbable tliat an intercalation of sympathetic cells occurs, as in mammals, within the ciliary ganglion. The fact that fibres emanating from the cervical sympathetic ganglia have no effect on the movements of the pupil in birds was established by Jegorow ('87), and has recently been confirmed by Langley (:03). In mammals, several observers have shown that stimulation of the cer- vical sympathetic nerve causes dilatation of the pupil (Hensen und Volkers, '68; Nawrocki und Przybylski, '91; Anderson, :03). The radially placed dilator muscle of the iris of birds is exceptionally well developed (Geberg, '83 ; Koganei, '85 ; and others). B. Observations, a. Methods. For the purpose of examining the main trunks of the eye-muscle nerves and their larger branches, dissections of the heads of adult fowls were made. Soon after death, the orbital cavities were opened by cut- ting through the conjunctiva and connective tissue, and the whole head placed in the picro-aceto-platino-osniic mixture of von Rath, the formula 152 bulletin: museum of comparative zoology. for which is given on page 175. After immersion in this fluid for from three to five days, the material was carried through several changes of 70 per cent alcohol, in which the excess of picric acid was, to a large extent, removed. The head was then allowed to stand until needed in a mixture of alcohol and glycerin. It was found that this^ treatment, which was suggested to the writer by Mr. W. A. Willard, differentiates well the nervous from the muscular tissues, and, aided by the use of the dissecting microscope, makes possible the tracing out of the distri- bution of veiy slender bundles of neuraxons. In preparation for the detailed study of the more important and complicated relations of the nerves under consideration another method was adopted. Certain portions of the contents of the orbit were re- moved entire, care being taken to avoid straining or breaking the parts, and placed for fixation in either Zenker's fluid, osmic acid or the vom Eath mixture mentioned above. After dehydrating in alcohol, and im- bedding in paraflSn, serial sections of suitable thickness -were made. The sections were cut in planes either at right angles to, or parallel with, the axis of the main trunk of the oculomotor nerve (see Plate 1, Figs. 1 and 2). The sections of the material fixed in Zenker's fluid were stained in acid-fuchsin. The osmic acid and vom Rath preparations needed no subsequent treatment for the differentiation of the medullated neuraxons. Cells of the ciliary, the Gasserian (cerebro-spinal) and sympathetic ganglia were studied in the following manner, with a view to deter- mining the number and character of their processes. The ganglia were removed from a freshly killed fowl and immersed in a 0.05 per cent solution of chromic acid, in which they were allowed to stand and slightly macerate for two or three days. They were then carefully teased apart with fine needles, and stained in acid-fuchsin. In this way a certain number of cells, retaining longer or shorter portions of their processes, were isolated from the rest of the ganglion, and prepared for examination. Other ganglia were fixed in the vom Eath mixture and studied in the form of serial sections. b. Anatomy. 1. Eye-Musde Nerves and Ciliary Oanglion, Oculomotor Nerve and Ciliary Ganglion. The nidulus of the oculo- motor nerve lies in the ventral portion of the mesencephalon, near the mesocoele or aqueduct of Sylvius, in relation to which it occupies a CAfiPENTEE: DEVELOPMENT OF THE OCUIiOMOTOR NERVE. 153 ventro-lateral position. The situation of the nidulus in the cerebro- spinal axis corresponds to that of the somatic motor column of ganglion cells (Gaskell) of the spinal cord. The neuraxons of the cells pass ventrad to emerge from the ventral face of the mesencephalon as the third uerve, which runs ventrad and cephalad through the oculomotor foramen, and then horizontally forward (Plate 1, Fig. 1, n. oc'mot.). Passing ventrad of the posterior rectus muscle, it sends a small branch (rm. mu. rt. d.) dorsad and cephalad to the posterior edge of the base of the dorsal rectus muscle. Just distal to this branch, a large ventral ramus (rm. v.} is given o£^ on the opposite or ventral side of the nerve trunk. This ventral ramus passes beneath the ventral rectus muscle, and runs cephalad along the floor of the orbit to terminate, as a brush of fine fibres, on the ocular face of the ventral oblique muscle, about mid- way of its length. A slender bundle of neuraxons (rm. m,u. rt. v.) arises from the main trunk of the nerve in close connection with the ventral ramus, and innervates the lower face of the ventral rectus muscle, near the proximal end of the latter. From the ventral ramus, soon after it passes the anterior border of the ventral rectus muscle, a small branch (rm. mu. rt. a.) is given off to the adjacent edge of the anterior rectus muscle. Immediately distal to the origin of the ventral ramus, the remainder of the neuraxons of tlie third nerve enter the spindle-shaped ciliary ganglion (gn. cil.), which measures approximately two mm. in length, and has a gi'eatest diameter of a little less than one mm. No radix brevis can be said to exist in the hen, since it is possible, in serial sections, to trace the cells of the ciliiiry ganglion back as far as the level of the ventral ramus (Plate 2, Fig. 3, 0). From the distal end of the ciliary ganglion, a comparatively large ciliary nerve (Plate 1, Fig. 1. n. cil. oc'niot.) is given off. This runs parallel with the optic nerve, and penetrates the sclerotic coat of the eyeball. Ou its way, the ciliary nerve gives rise to a variable number of branches of microscopical size (Plate 1, Fig 2, rm. n. cil. oc'mot.), which accompany it to the eye. The ciliary nerve receives, about one mm. distal to the region of the cells of the ciliary ganglion, a slender communicating ramus (Figs. 1 and 2, rm. comn.) from the ophthalmic branch of the fifth nerve. Trochlear Nerve. The nidulus of the trochlear nerve is found in the somatic motor column in the ventral part of the mesencephalon, pos- terior to the nidulus of the oculomotor. The nerve (Fig. 1, n. trch.) takes its superficial origin from the dorsal surface of the brain, between 154 bulletin: museum of compaeative zoology. the optic lobes and the base of the epencephalon. Turning ventrad and cephalad, it passes through tl\e orbit, running raediad and dorsad of the posterior and dorsal rectus muscles, and ends on the ocular face of the dorsal oblique muscle (Fig 1, mn, db. cZ.). Abducent Nerve. Lilie the other eye-muscle nerves, the abducens has its nidulus in the somatic motor column. It lies in the ventral part of tlie metencephalon, and from it the abducent neuraxous run ventrad to emerge from the ventral face of this division of the brain, not far from the median plane. The trunk of the nerve (Fig. 1, n. abd.) pro- ceeds cephalad, and, crossing dorsad of the oculomotor nerve, divides into several terminal ramifications, which are distributed to the portion of the posterior rectus muscle lying laterad of the ophthalmic branch of the trigeminus {rm. opth. trig.), v^hioh passes througli the proximal part of tlie muscle. Shortly before terminating in this way, the abducens sends cephalad a branch (Plate 1, Fig. 2, rm. mu. qd. + pyr.) to the muscles of the nictitating membrane. Tliis branch soon bifurcates, and between its forks the communicating ramus connecting the trigeminal and oculomotor nerves often passes. Ophthalmic Branch, of the Trigeminal Nerve. The Gasserian ganglion presents two well-marked divisions, an ophthalmic portion (Fig. 1, gn. Gas.') extending cephalad, and a maxillo-mandibular portion directed ventrad. The first becomes gradually narrowed into the ophthalmic branch of the trigeminal nerve (rm. opth. trig.) which, running ceph- alad, usually penetrates the posterior rectus muscle, and then passes, ventrad of the proximal ends of the dorsal rectus and dorsal oblique muscles, to the anterior boundary of the orbit. Here it divides, one large nasal ramus (rm. na.) entering the nasal chamber, while several small branches (rm. f.) extend dorsally to a more superficial distribution, and, taken together, correspond to a frontal ramus. From a point opposite the distal extremity of the ciliary ganglion, tlie ophthalmic branch sends out a communicating ramus (Figs. 1 and 2, rm. comn.), which unites with the oculomotor ciliary nerve about one mm. distad of the ciliary ganglion. However, not all the nenraxons which here leave the ophthalmic branch reach the oculomotor ciliary nerve, since a slender bundle of them emerges from the communicating ramus about midway in its course, and proceeds toward the eyeball as an independent trigeminal ciliary nerve (Fig. 2, n, cil. trig.'). In some cases, another trigeminal ciliary nerve (n, dl. trig.") leaves the com- municating ramus near its union with the oculomotor ciliary nerve, and likewise passes as a separate fibre to the eyeball. CAEPEXTEE: DEVELOPMENT OF THE OCULOMOTOE NEETE. 155 c. Histology. 1. Oculomotor Nerve. The trunk of the oculomotor nerve is made up of both large and small meduUated neuraxons. Some of the former may reach the size of 15 micra in diameter, while some of the latter may measure only 3 micra. Between these two extremes all intermediate sizes are to be found. The trunk of the nerve is composed mainly of comparatively large neu- raxons, among which a few small ones are interspersed, but at its lateral periphery a zone of small neuraxons occurs (Plate 7, Fig. 21). This zone is represented by the shaded portions of the diagrams shown in Plate 2, Figure 3, which represent cross-sections of the oculomotor at various levels along its course. The unshaded portion of the nerve trunk is that in which large neuraxons predominate. At A is shown the conditions which obtain in the nerve proximal to its branches. Dia- grams B and C represent, respectively, sections through the origin of the branch to the dorsal rectus, and through the origin of the ventral ramus. It will be noticed that both draw their neuraxons from the un- shaded portion of the nerve trunk. A photomicrograph of a cross-sec- tion of the branch to the dorsal rectus muscle is given in Plate 7, Figure 22. While the branch is mainly made up of neuraxons of large size, a certain number of smaller ones is also present. It is probable that the differences in size of the neuraxons correspond with the differ- ences in the degree of development of the muscle fibres to which they are distributed. It has been pointed out by C. J. Herrick ('99) that large neuraxons of the eye-muscle nerves of Menidia are connected with large muscle fibres, and those of lesser calibre with small muscle fibres. In the eye muscles of the hen, fibres of varying sizes also occur. In a later paper (C. J. Herrick, :02), the writer just cited has advanced the opinion that differences in the calibre and niedullation of neuraxons frequently signify nothing more than a correlation with the degree of functional development of the peripheral end-organs. The neuraxons which pass into the ciliary ganglion are those which form the peripheral zone, shown by the shading in the diagrams of Plate 2, Figure 3. They are of small calibre. Distal to the ganglion the ciliary nerve is likewise entirely made up of small neuraxons (D), the medullary sheaths of which are, however, well developed. 2. Abducent Nerve. The elements of the abducent nerve, when seen in cross-section, appear, for the most part, as large meduUated neuraxons. As in the oculomotor 156 bulletin: museum of compaeative zoology. nerve, a few small neuraxons occur among the larger ones. The nerve trunk closely resembles in appearance that of the oculomotor, except that a peripheral zone of small neuraxons is not present. The branch given off to the muscles of the nictitating membrane is composed almost entirely of neuraxons of large size. 3. Ciliary Ganglion. The cells obtained from the ciliary ganglion by maceration and iso- lation answer to the description of the ciliary cells of the hen already given by Retzius ('8l). They are large bipolar cells, the processes of which become heavily meduUated a short distance from the cell body (Plate 2, Fig. 4). In a few instances the two neuraxons were seen to arise by a common stem, so that the ganglion cell may be said to be unipolar in character. With the object of comparing the finer structure of the ciliary gan- glion with that of cerebro-spinal and sympathetic ganglia, attempts were made to obtain an intra-'vitam methylen-blue stain. Three trials were made, but in only one of these were the cells of the ciliary ganglion affected. The cells in this instance were not, however, deeply stained, and no pericellular baskets of fibrils were differentiated about them, such as have been demonstrated about the sympathetic cells of birds by Huber ('99) after injection of methylen-blue into the blood system. Such evidence as I have been able to obtain as to the sympathetic or cerebro-spinal nature of the ciliary ganglion has resulted from the use of the vom Rath mixture. After fixation in this reagent, sections of spinal ganglia can with ease and certainty be distinguished from sections of sym- pathetic ganglia. Spinal cells average larger, some measuring as much as 60 micra in diameter. Cells of this size are never found in sympathetic ganglia. Large meduUated neuraxons are given off by spinal cells, and portions of these occur in every section through the ganglia. The periph- eral neuraxons of the sympathetic cells, though also meduUated, are never of as large calibre as those of the spinal cells. But the most convincing/ and characteristic peculiarity of the sympathetic ganglia is the mass of fine fibrils which occurs in them, filling in the interstices between the cells, and obscuring, to some extent, their boundaries and cytoplasm. Though their relations to the cells are not well brought out by the vom Eath stain, these fibrils undoubtedly form the pericellular baskets which are known to be present in sympathetic ganglia. Owing to the absence of such an abundance of fibrous elements the spinal gan- carpenter: development of the oculomotor nerve. 157 glia present a quite different appearance. Tlie cells of these are unob- scured, and their boundaries are sharply defined. When the ciliary ganglion of the hen is prepared according to the vom Eath process, and sectioned longitudinally, it is seen, under the micro- scope, to be divisible into two regions (compare A and B, text Figure). Approximately two-thirds of tlie ganglion {B) is composed niaiuly of large, well-defined cells, around which very few pericellular fibrils occur. Some of these cells reach the dimensions of the largest of the spinal- ganglion cells. In this portion of the ganglion are found small, but heavily niedullated, neuraxons. Of these, a part are evidently continu- ous with the neuraxons entering the ganglion from the oculomotor nerve. Others are plainly seen to leave the ciliary ganglion by the ciliary nerve, the greater part of which arises from this portion of the ganglion, and is made up of small, well-medullated neuraxons. In the dorsal region of the ciliary ganglion, on the side toward the ophthalmic branch of the fifth nerve, qccurs an accumulation (^) of small cells, which makes up approximately one-third of the entire volume of the ganglion. Here are found fine neuraxons, showing little evidence of medullation, and a quantity of delicate fibrils resembling those of the pericellular baskets of sympathetic ganglia, although not present in such profusion as in the sympathetic ganglia. A communicating ramus between the ophthalmic branch of the trigeminus and the oculomotor ciliary nerve has been mentioned. Longitudinal sections of the ciliary nerves show that the slender, very slightly niedullated neuraxons which compose the communicating ramus divide into two sets upon reaching the ciliary nerve. One of these bundles turns toward the eyeball, and accompanies the ciliary nerve to its peripheral distribution. The other bundle is recurrent, being deflected toward the ciliary ganglion. Its neuraxons run parallel with those of the ciliary nerve, and enter that portion of the ganglion which is characterized by small cells and peri- cellular fibrils. From this region there is given off to the eyeball a bundle (a) of fine neuraxons with but slight traces of medullation. These accompany the medullated neuraxons from the remaining two- thirds of the ganglion {fi) as component elements of the ciliary nerve. In certain cases they may be found occurring in the form of a distinct, non-meduUated bundle, running close beside the larger group of medul- lated neuraxons, but separated from the latter by perineurium. The relations of the two parts of the ciliary ganglion are shown in a diagrammatic way in the accompanying figure. In any attempt to assign the ciliary ganglio:i, on histological grounds. 158 bulletin: museum of compaeativb zoologt. to either the oerebro-spinal or sympathetic systems, it is plain that the two regions described above must be separately considered. In respect to the first or ventral region {B), it can be said that the size of its cells, the heavy medullation of both its central and peripheral neuraxons, and the comparatively small volume of the pericellular fibrils, agree virith the conditions found in cerebro-spinal ganglia. The central and peripheral neuraxons are, however, of smaller calibre. It is probable that the large bipolar ganglion cells obtained by maceration methods, Diagram of a longitudinal section through the ciliary ganglion. A, region of small cells, non-meduUated neuraxons, and pericellular fibrils; j, small, non-medullated ciliary neuraxons; B, region of large cells and small medullated neuraxons; /3, small, meduUated ciliary neuraxons; n, ciL, ciliary nerve; ». oc'mo(., oculomotor nerve (large and small medullated neuraxons); rm. comn., communicating ramus (small non-medullated neu- raxons); rm. comn.\ distal neuraxons of communicating ramus; rm. comn.*\ recurrent neuraxons of communicating ramus; rm. opth. trig., ophthalmic branch of trigeminal nerve ; rm. v., ventral ramus (large and small medullated neuraxons). as already described, were from this region. These resemble cerebro- spinal elements, except that two medullated processes instead of one are given off by each cell ; and even where a tendency toward uni- polarity occurs, the typical T-shaped condition of a cerebro-spinal neuron is not attained. In the second or dorsal region (A) of the ciliary ganglion, the small- ness of the cells, the absence of ,heavily medullated processes, and the comparative abundance of pericellular fibrils, suggest a sympathetic ganglion. Moreover, the resemblance is strengthened by the entrance CAKPENTEE: DEVELOPMENT OF THE OCDLOMOTOE KERVE. 159 into this region of fine, very slightly medulluted neuraxons from a gangliated dorsal nerve, the ophthalmic branch of the trigeminus. These neuraxons are identical in appearance with those of a typical ramus commuuicans passing, in the thoracic region of the fowl, to a sympathetic ganglion. A lack of correspondence between this portion of the ciliary ganglion and a typical sympathetic ganglion is to be recognized, however, in the comparative absence of meduUation around the fine ciliaiy neuraxons given off distally. The post-gangliouio neuraxons of a sympathetic ganglion are also of small calibre, but in the hen they are well' medullated. This lack of correspondence, due merely to differences in the degree of medullation, seems comparatively unimportant. It should be remembered, too, that in other classes of vertebrates non-medullated neuraxons are characteristic of post-ganglionic sympathetic nerves. PART II. - DEVELOPMENT. A. Historical Survey. In reviewing the literature which deals with the development of the ciliary ganglion, it must be kept in mind that the name " ciliary " has been applied by various authors to two entirely distinct ganglia. One of these is found at the base of the first or ophthalmic branch of the trigeminal nerve, while the other is connected with the oculomotor nerve, and is the true ciliary ganglion of the adult. The first of these ganglia arises in the following manner. In early stages of development, at least in sharks and birds, the most anterior portion of the neural crest becomes diflFerentiated, in the region of the eye muscles, into an enlargement resembling the fundament of a cerebro- spinal ganglion. In sharks this has been observed to fuse later with the anterior part of the Gasserian ganglion, and the same fusion doubt- less takes place in birds. From the ganglion originating thus, the first branch of the trigeminus proceeds forward (van Wijhe, '82 ; Beard, '87 ; Neal, '98). For this ganglion of the ophthalmicus profundus in sharks Beard, in 1887, proposed the name " mesocephalic." Following Dohrn and Neal, I have adopted this designation, since all names previously applied to the ganglion in question have been used as synonyms for the ciliary. Indeed, as already stated, the name ciliary itself has often been applied to it. The separate development of the ganglion of the ophthalmic branch of the trigeminus, and its subsequent fusion with the rest of the 160 BULLETIN : MUSEUM OF COMPAllATIVE ZOOLOGY. Gasserian ganglion, have been observed in mammals and amphibians, as well as in fishes. Chiarugi ('94, '97) makes such an assertion for the embryos of guinea-pigs, and Evvart ('90), mentions the discovery, in a five-months' human embryo, of vestiges of an "ophthalmicus profundus ganglion" lying under cover of the inner portion of the Gasserian. Brauer (:04) describes the development of the " ganglion ophthalmicum '' and . of the " ganglion maxillo-mandibulare " in the Gymnophiona as independent of each other. Hoffmann ('85) considers the Gasserian ganglion in both embryonic and adult reptiles as divisible into two parts. In some of the lower verteftrates, as cyclostomes (Dohrn, '83; von Kupffer, '95) and ganoids (AlJis, '97), these two ganglia, tlie mesocephalic and the maxillo-mandibular, retain complete independence throughout life. There is good evidence, then, that a distinct ganglion, the meso- cephalic, is developed throughout the vertebrate series in connection with the ophthalmic division of the fifth nerve. This ganglion, except in the cases of certain low forms, soon becomes fused with the common ganglion of the maxillary and mandibular divisions to form the Gasserian ganglion of the adult. The true ciliary ganglion appears much later than the mesocephalic, and is always more or less directly connected witli the third cranial nerve. The various vcays in which its development has been said to take place will be outlined in the following pages. It will be sufficient, at present, to say that all writers whose statements are based on actual observation agree that it does not arise, like the mesocephalic and other cerebro-spinal ganglia, through direct differentiation from the cells of the neural crest. In the reviews which follow I shall summarize in chronological order the observations already made on the development of the oculomotor and abducent nerves and the ciliary ganglion in the five classes of vertebrates. As far as possible, I shall distinguish between the mesocephalic and the ciliary ganglia. Each author's nomenclature will first be given, and then, if the identities of the ganglia are clear from his description, the terminology adopted above will be substituted for the sake of clearness and uniformity. 1. Fishes, The first investigations upon the development of the oculomotor and abducent nerves in fishes were made by Marshall in 1881. In the embryo shark (Scyllium canicula) Marshall ('81) found that, in Balfour's cakpenter: development of the oculomotoe nerve. ]61 stage K, the third nerve arises from the ventral face of the mid-hrain by a root triangular in shape, containing many " nerve cells." Eunning caudad and laterad, the nerve reaches the interval between the dorsal ends of the first and second head cavities, vrhere it expands into a small ganglion. At this ganglion, the nerve divides into two main branches, one running cephalad along the top of the first head cavity to the extreme anterior end of the head, the other passing ventrad between the first and second cavities. A short branch, coming directly from the Gasserian ganglion, enters this small ganglion and later, uniting with the first branch of the' oculomotor described above, forms the ramus ophthalmicus profundus of the adult ; tliis ramus has, in most cases, the appearance of being a branch of the trigeminal nerve. From the posterior wall of the first head cavity are derived those muscles of the eyeball which later are innervated by the oculomotor. The author considers this small ganglion between the tops of the first two head cavities to be the ciliary ganglion of the adult, and agrees with Schwalbe that it belongs exclusively to the third nerve. It is evident from later investigations by others that Marshall is here dealing with the mesocephalic ganglion, and that his first branch of the oculomotor has only an apparent connection with that ilerve, being, as a matter of fact, the ramus ophthalmicus profundus of the trigeminus, with which the mesocephalic ganglion is primitively connected. The close contact into which the two nerves come in the region of the mesocephalic ganglion accounts for the author's failure to separate them. (Comp. van Wijhe, '82.) The sixth or abducent nerve was first observed in stage 0, some time after the oculomotor had made its appearance. It springs from the ventral face of the hind-brain by a large number of slender roots, and runs to the fundament of the posterior rectus muscle. The roots as well as the trunk of the nerve contain many more or less elongated, fusi- form cells, but none of these are ganglion cells. Marshall and Spencer ('81) confirm Marshall ('8l) without adding anything of importance to the latter's account. In 1882 van Wijhe published his excellent description of the develop- ment of cranial nerves in selachians. In Balfour's stage I he found the fundament of a ganglion connected with the ophthalmic branch of the trigeminus, and representing the anterior extremity of the neural crest. This ganglion — called the ciliary by van Wijhe, but evidently the mesocephalic of our nomenclature — at first lies immediately under the epidermis, but soon moves away from it in the direction of the gan- VOL. XLVIII. — NO. 2 11 162 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. glion of the second and third branches of the trigeminus. Not until the next stage J, does the oculomotor appear ; it then arises by a broad proxi- mal end from the mid-brain. Passing ventrad the third nerve crosses the ophthalmic branch of the fifth on its median side at the level of the mesocephalic ganglion, to which it becomes closely applied ; but, accord- ing to van AVijhe's view, only a close contact, not an actual union, occurs. Later, the oculomotor nerve and the mesocephalic ganglion draw away from each other, although a slender communicating nerve con- tinues to connect the two. But van Wijhe's most important contribution to the subject was the discovery of a second ganglion, which, in stage 0, appears as a dumb- bell-shaped mass of cells placed on the branch of the third nerve which supplies the ventral oblique muscle, in the position occupied by the ganglion oculomotorii described by Schwalbe ('79) in adult selachians. This ganglion oculomotorii (which is our ciliary) is comparatively remote from the mesocephalic ganglion, and the author emphasizes the lack of connection between the two. On account of its late appearance, and the presence of a small branch from it to the arteria ophthalmica, he considers the ganglion as belonging to the sympathetic system, but gives no ac- count of its actual development. The mesocephalic ganglion he regards as homologous with a spinal ganglion. The origin of the oculomotor from the base of the brain, the time of its appearance, its histological structure, its lack of a true ganglion in early stages, and its crossing, if not union, with a dorsal root distal to the ganglion of the latter, seem to the author to prove that the nerve in question is a purely ventral one. To Marshall's account of the comparatively simple development of the sixth nerve van Wijhe added practically nothing. Beard ('85) describes the development in elasmobranchs of what he then called the ciliary ganglion, but later termed the mesocephalic. This account was repeated and supplemented in his notable paper of 1887, to which reference has been made. In this paper he clears up the existing confusion caused by the various names given to the ganglia developing in connection with the oculomotor nerve and the ophthalmic branch of the trigeminus. The first he shows to be the true ciliary ganglion of the adult, and, therefore, entitled to that name ; and for the second he proposes, as already stated, the name mesocephalic. He describes the way in which, in elasmobranchs, the mesocephalic ganglion, deriving its cells partly from the neural crest and partly from the ecto- derm in the region of a primitive branchial sense organ, gradually re- cedes from the skin, and fuses with the maxillo-mandibular ganglion. CARPENTER: DEVELOPMENT OF THE OCULOMOTOR NERVE. 163 As this change of position takes place, a nerve — the ophthalmicus pro- fundus — is developed, connecting the mesocephalic ganglion with the branchial sense organ. The oculomotor appears later than the ophthal- micus profundus, and never has any direct connection with the meso- cephalic ganglion, although, during development, it is for a time closely applied to the latter. The true ciliary ganglion is not present until much later. When it does appear, the mesocephalic ganglion and the oculomotor nerve are connected by a small communicating branch, prob- ably corresponding to the radix longa of higlier animals ; and it is near the entrance of this branch into the third nerve that the ciliary ganglion is first to be seen. Beard did not follow the development of the ganglion step by step, but calls attention to the assertion of Hoff- mann ('85), that it aiises in reptiles as an outgrowth of the ophthalmic (mesocephalic) ganglion. He favors the view that it belongs to the sympathetic system. Phisalix ('88, '88'^), mistaking the mesocephalic for the ciliary ganglion in skate embryos, asserts that at first the oculomotor nerve and its gan- glion are independent of each other. The ganglion is said to result from the dividing into two of the ganglion of the trigeminal nerve before the oculomotor has appeared. Ewart ('90) gives us the first account of actual observations on the de- velopment of the ciliary ganglion in fishes. He finds that, at a certain stage in skate embryos, a slender outgrowth from the inferior border of the ophthalmicus profundus (mesocephalic) ganglion meets and blends with the descending (ventral) branch of the oculomotor. This out- growth is crowded with cells, while the fibres of the descending branch of the oculomotor, as well as its root and trunk, are " absolutely destitute of cells." Later, cells accumulate at the junction between the outgrowth and the oculomotor, as if the intermingling of the two sets of fibres formed a network which resisted the further migration of cells from the mesocephalic ganglion. At a still later stage, in typical cases, all the ganglion cells are seen to have left the outgrowth, and to have accumu- lated on the oculomotor as a rounded mass, from which ciliary nerves take their origin. The ganglion thus arising, plainly the ciliary, stands, therefore, in the relation of a sympathetic ganglion to a dorsal cranial nerve, the ophthalmicus profundus. Dohrn ('91) takes up the matter of the histogenesis of the oculomotor more fully than previous writers on the eye-muscle nerves, and also offers an entirely new explanation of the origin of the ciliary ganglion. He observed (p. 3) that in the embryos of selachians the third nerve 164 bulletin: museum oe comparative zoology. first makes its appearance as a number of crowded psile cells in the mar- ginal veil of the mid-brain. The plasma of these cells emerges from the ventral side of the neural tube as fine processes, which unite to form an irregular network. The meshes of this network are extended by the fusion of the processes of large cells, the nuclei of which lie in the plasma mass. The network stretches out through the mesenchyme, and gives rise, in the vicinity of the front end of the chorda dorsalis, to the small trunk of the oculomotor nerve. Dohrn's view of the structure of the growing nerve is in general accordance with the opinions of Balfour, Marshall, Beard, von Kupfifer, and others, who support the "chain theory" of nerve formation. To prove that the medullary tube is the source of these cells whose processes make up the nerve trunk, Dohm devotes much space and many figures. He shows that cells may be ob- served in the root of the oculomotor nerve, half in and half out of the medullary tube. Although he is obliged to admit that with present staining methods it is not possible, in early embryonic stages, to dis- tinguish emigrating medullary cells from the surrounding mesodermal cells, he calls attention to the fact that the nuclei of the nervous net- work are larger than the nuclei of nearly all the neighboring mesoder- mal cells. Numbers of rounded and oval nuclei are to be seen in the course of the oculomotor before this grows down and connects with the mesocephalic ganglion, the long axes of these niiclei being perpendicular to those of the nuclei of the ganglion. The nuclei lying along the third nerve cannot, therefore, be considered derivatives of the mesocephalic ganglion. In older embryos there occur, in the course of the third nerve, groups of differentiating ganglion cells, corresponding in position to the ganglia found along the oculomotor in the fully grown animal ; and the develop- ment of these cells can be followed with certainty until the adult condi- tion is reached. The ganglion cells arising in this way Dohm believes to have been originally migrant medullary cells. After considering the definitions which have been given to cerebro- spinal and sympathetic ganglia, Dohm reaches tlie conclusion that in the diffuse ciliary ganglion of selachians we have a ganglion which, because of its unique origin from emigrant medullary cells, belongs neither to the cerebro-spinal nor to the sympathetic systems. In its histogenesis, the abducent nerve was found to resemble closely the oculomotor. Cells were discovered, wandering out into its roots from the ventral wall of the hind-brain, but in the case of this nerve none of these become ganglion cells in the adult. carpenter: development of the oculomotor nerve. 165 Following the article of Dohrn just cited, three investigators published in rapid succession accounts of the development of the oculomotor nerve in selachians. These accounts were remarkable for the fact that they agreed in ascribing to the third nerve an extraordinary origin, the possi- bility of which had apparently never occurred to other investigators. Piatt ("91) describes, in embryos of Acanthias vulgaris, a line of nerve cells extending cephalad from the trigeminal ganglion, and soon enlarg- ing into the fundament of a ganglion, which she terms the ciliary. This ganglionic fundament, first meeting an anterior prolongation from the neural crest (which develops into the transitory " thalamic nerve "), finally ends in a mass of 'cells connected with the primary nasal epithelium. This line of cells is later represented by tlie ramus ophthalmicus pro- fundus trigemini. (From the foregoing description I consider it prob- able that Piatt's " ciliary " ganglion is the same as Beard's mesocephalic.) From the inner (median) cells of this ganglion, the oculomotor nerve takes its origin as a cellular proliferation, which grows from the ganglion toward the brain, with wliich it becomes united in the floor of the mid- brain. Two figures are given showing the oculomotor when it consists of but a single cell. The author concludes that the third nerve is, there- fore, primarily sensory, tlie mesocephalic ganglion being at this timo connected with a patch of thickened epitlielium, and no muscle cells having as yet appeared in the walls of the premandibular cavity. Mitrophauow ('93) followed with a confirmation of Piatt's account of the origin of the oculomotor. He observed this peculiar development of the nerve in embryos of Raja, Torpedo, and Pristiurus. The ganglion from which the third nerve grows, as a cordon of cells, to tlie brain is plainly the mesocephalic, but the author prefers to call it the ciliary, although he indicates his familiarity with the name mesocephalic by mentioning it several times as a synonym for ciliary. Finally, Sedgwick ('94) maintains that nerves do not develop as pro- cesses from central cells, according to the view of His, but arise tlirough the differentiation of a reticular substance already in position. The oculomotor is formed in elasmobranchs as a differentiation of this retic- ulum, resulting from the breaking up of the neural crest, and first appears as a forward projection of nuclei from tlie ciliary (niesoceplialio) ganglion. In the author's words (p. 96), "The third nerve, therefore, presents this interesting and remarkable peculiarity in Scyllium and Acanthias; it grows or is differentiated from the ciliary ganglion to the floor of the mid-brain and not in the opposite direction, as has hitherto been sup- posed." Sedgwick publishes no figures in support of his contention. 166 bulletin: museum of compakative zoology. In his study of tlie development of the cranial nerves of Ammocoetes, von Kupffer ('9X) assigned to the nerve he believed homologous with the oculomotor of higher vertebrates both dorsal and ventral roots and spinal as well as sympathetic ganglia. In a later paper (von Kupffer, '95) he states that the oculomotor in Ammocoetes appears to be a ventral nerve with which is related dorsally the anterior half of the first trigemi- nal ganglion. The third nerve and the ciliary ganglion in embryos of Amia calva are described by Allis ('97). This writer has, however, no positive informa- tion to offer as to the early stages of their development. Hoffmann ('97) states that the oculomotor, when first detected in Acanthias vulgaris, appears as a fibrous ventral root. At this time there is no ganglion at the proximal end of the nerve. His observa- tions on the abducens confirm those of Marshall and van Wijhe. Chiarugi ('97) declares, in opposition to Mitrophanow, that the third nerve in selachians grows centrifugally from the base of the mid-brain. By the use of his modification of the vom Eath method, Neal ('98) demonstrated in so convincing a manner the neuroblasts of the oculo- motor and their processes in Squalus acanthias that he removed all doubt as to the origin of the nerve from the ventral wall of the mid- brain. The neuroblasts showed the characteristics of those described by His in the spinal cord, and their darkly staining processes could be followed partly into the mesenchyme, where they were grouped to form the nerve trunk, and partly in a posterior directicm within and parallel to the medullary wall, where they took part in the formation of the ventral fibre tract. The nerve growing out in this fashion from the mid-brain exhibits many nuclei lying peripherally along its fibres. It soon connects with the cells of the mesocephalic ganglion. Whereas the nerve is several cells in thickness near the ganglion, its calibre grows less toward the brain wall, a condition which, if one were unac- quainted with its earlier history, might lead to the supposition that the growth of the nerve takes place from the ganglion toward the bniin. Cells were observed migrating out from the mesocephalic ganglion and adhering closely to the oculomotor fibres. The fate of these cells was not determined, nor was the develojiment of the ciliary ganglion followed. No entirely satisfactory evidence of migration of medullary elements was observed. The abducent nerve was found by Neal to arise in the form of a slender bundle of neuraxons from neuroblasts situated in the ventral horn of the medulla. The number of its roots increases during devel- CARPENTER: DEVELOPMENT OF THE OCULOMOTOR NERVE. 167 opment from one to three or four. The nuclei seen along its course are distinctly peripheral iii relation to its fibres. There were no convincing indications of tlie migration of cells from the neural tube. The development of the third nerve in selachians was described by Hoffmann ('99), but he added nothing new to the subject. The nerve in question grows down from the mid-brain and anastomoses with the ganglion ophthalmicus. This is the mesocephalic ganglion of Beard ('87), with whose article the author in the main agrees ; but he con- siders the name ophthalmicus preferable to mesocephalic. Later, the oculomotor nerve and the mesocephalic ganglion draw apart, remaining connected, however, by a ramus anastoraoticus. Presently, two ganglia appear on the third nerve, corresponding in position to those described by Schwalbe ('79) for adult selachians. Although the author was not able to work out the development of the ganglionic groups appearing on the oculomotor, he believes the account of Ewart ('90) to be correct, and considers the ciliary the most anterior sympathetic ganglion of the head. Allis (:01, p. 131) mentions the presence of small and large cells in the ciliary ganglion of an embryo of Mustelus Isevis. The large cells resemble cerebro-spinal ganglion cells, and the author suggests the probability that both spinal and sympathetic elements enter into the composition of the ciliary ganglion. Long and short roots were distin- guished, but no extra-cranial sympathetic root could be made out. 2. Amphibians. Almost nothing is known of the development of the third and sixth cranial nerves and the ciliary ganglion in amphibians. Johnson and Sheldon ('86, p. 94) state that in the embryo newt the oculomotor arises, like certain other of the cranial nerves, as an outgrowth of the neural crest, but no details of the process are given. According to Mar- shall ('93, p. 133) the oculomotor is present as a slender nerve in tad- poles of the frog at the time of tlie opening of the mouth. It arises from the lower part of the side of the mid-brain, not far from the me- dian plane, and has already the course and relations of the nerve in the adult. Its early development has not been ascertained. 3. RBPTILEa. Our knovvledge of the development of the eye-muscle nerves and the ciliary ganglion in reptiles is derived almost entirely from the extended accounts of the two investigators, B^raneck and Hoffmann, both of whom studied embryos of Lacerta agilis. 168 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. B^raneck ('84) found the oculomotor, in an early stage of develop- ment, arising from the ventral face of the mid-brain by a triangular root crowded with cells possessing round nuclei, " et par touts leurs caract^resserapprochent beaucoup des cellules meduUaires." Whether this cellular accumulation at the root of the nerve is really ganglionic, or whether its cells, like those distributed along the nerve, later form the sheaths of the fibres, could not be determined. At the proximal termination of the oculomotor occurs a little cellular mass, which the author believes to represent the ciliary ganglion. At this stage no com- municating branch exists between this or any other part of the third nerve and the ophthalmic branch of the fifth. In later stages, numerous cells are to be seen distributed along the whole nerve, those at the broad root being rounded and closely re- sembling the medullary cells, while those more distally situated are fusiform, with their long axes parallel to the nerve fibres. These dif- ferences in shape among the cells of the oculomotor are more apparent the older the embryo. The cells are more abundant in the proximal than in the distal part of the nerve trunk. The approximately spherical ciliary ganglion incloses round cells with distinct nuclei and fine gran- ules. It is now connected by a slender ramus with the ophthalmic division of the trigeminal nerve. From the anterior face of the ciliary ganglion there runs cephalad, for an undetermined distance, a fine bundle of fibres, which the author believes to represent the oph- thalmic branch of the oculomotor described in sharks by Marshall ('81). In old embryos, the ciliary ganwHon shows the relations to the third nerve found in the adult : it is situated a little on one side of the nerve trunk, to which it is attached by a very short and thick bundle of nerve fibres. Nowhere in his account does B^raneok advance the theory that the cells along the third nerve may have been derived through migration from the neural tube, nor does he express an opinion as to the source of the cells which differentiate in situ, in the oculomotor, into the cells of the ciliary ganglion. The abducens appears somewhat later than the oculomotor, springing as a slender nerve from the ventral face of the hind-brain. During its development it presents but a single root, fibrillar in character, and des- titute of cells, except for a few mesodermal elements which surround it externally. The same conditions are found along the course of the nerve trunk, the only cells connected with it being of mesodermal ori- gin, arranged in a single layer about its periphery. CARPENTER: DEVELOPMENT OF THE OCULOMOTOR NERVE. 169 The account of the development of the ciliary ganglion given by Hoffmann ('85) is at variance with that of Beraneck, although both authors used as material the same species of lizard, namely, Lacerta agilis. While Hoffmann saw the younger stages of the development of the ganglion in snake embryos, the entire process was worked out in lizards only. According to his observations, the anterior part of the neural crest gives rise to two ganglia, that of the first branch of the trigeminus — the ophthalmic (mesocephalic) — and the ganglion com- mon to the second and third branches — the Gasserian. The oculomotor develops later than the trigeminus, springing by a broad base from the ventral surface of the mid-brain. It is composed of a small amount of finely striated protoplasm, containing many nuclei closely crowded together. Passing on the median side of the mesocephalic ganglion, the nerve sends out to the anterior face of the latter a communicating ramus. By the aid of several drawings and a series of diagrams, the author shows that a large mass of cells is proliferated from the distal end of the mesocephalic ganglion, that this mass separates from the parent ganglion, and, guided by the communicating ramus, makes its way to a point close to the third nerve, with which it becomes united by a very short and thick bundle of fibres. This mass of cells becomes the ciliary ganglion of the adult, and the bundle of fibres binding it to the third nerve, the radix brevis. The ganglion retains connection with the fifth nerve through a slender radix longa, which, however, does not end in the mesocephalic ganglion, but in the ramus nasalis, a branch of the ophthalmic nerve, which grows out from the distal extremity of the mesocephalic ganglion while the ciliary ganglion is undergoing development. Hoifmann is convinced of the sympathetic nature of the ciliary ganglion, basing his opinion on the late appearance of the ganglion, its origin from the homologue of a spinal ganglion, and its development through the participation of both sensory and motor nerves, one, the ophthalmic, arising by a true dorsal root, the other, the oculomotor, by a true ventral root. C. L. Herrick ('93) gives a figure of the developing oculomotor nerve in a snake embryo, showing migration of nuclei from the mid-brain into the root of the nerve. Tliese nuclei he holds to be those of cells which, outside the neural tube, produce the nerve fibres. 4. BlEDS. Eemak ('51) and His ('68, '79) describe and figure in chick embryos, between the third and fifth days, a cellular prolongation extending 170 BULLETIN: MUSEUM OF OOMPAEATIVE ZOOLOGY. ceplialad from the Gasserian ganglion, and swelling into a crescentio en- largement near the eye vesicle. This structure is regarded by His as the persistent neural crest in the anterior head region. Both authors, without tracing its fate, assume that the crescentio terminal enlargement is the ciliary ganglion. Kolliker ('79) expresses himself as also of this opinion, although he acknowledges the insufftciency of the evidence on which the assumption is based. That the ganglion in question is the mesoccphalic, and not the ciliary of the adult, is beyond doubt. The first study of the development of the eye-muscle nerves of verte- brates was made by Marshall ('77, '78) on chick embryos. He found that the neural crest forms, at the twenty-ninth hour of incubation, a prominent outgrowth above the mid-brain. At forty-three hours this outgrowth is directed ventrad and lies in close contact with the walls of the mid-brain ; and at fifty-three hours a large mass of cells is to be found connected with the mid-brain, and about half-way down its side. In a sixty-hours' chick, the oculomotor nerve arises from the ventral surface of the mid-brain, but is farther from the median plane than at later stages. From this evidence the author is "led to the belief that the third nerve is developed directly out of the outgrowth from the top of the mid-brain " seen at the twenty-ninth hour, and " that, at some period between the forty-third and sixtieth hours, its attachment shifts down from the top of the mid-brain to the lower part of its sides " (p. 25). Eabl ('89) accepts, on theoretical grounds, this view of the origin of the third nerve. Longitudinal sections of a ninety-six-hours' chick show the oculomotor as a large nerve, arising from the ventral face of the mid-brain. Its base is "ganglionic," and it terminates a little posterior to the optic nerve, in a "ganglionic" swelling. From its enlarged distal end two branches are given off, one passing ceplialad and dorsad to the fundament of the posterior rectus muscle, the other continuing the course of the main trunk, crossing the ophthalmic nerve nearly at right angles, and passing caudad and ventrad of the optic nerve. No mention is made of a connection between any part of the third nerve and the ophthalmic division of the fifth. In a subsequent paper (Marshall, '8l) the author accepts the view advanced by Schwalbe ('79) that this terminal swelling of the trunk of the oculomotor represents the ciliary ganglion of the adult. He ad- vances no opinion as to the source of the cells of the ganglion. Marshall's observations on the sixth nerve were incomplete. It was first detected in an embryo of ninety-three hours. Unlike the ooulo- CARPENTER: DEVELOPMENT OP THE OCULOMOTOR NERVE. 171 motor, it arises by a series of slender roots instead of a single, large "ganglionic" root. It is a very slender, cfellular nerve and does not branch. Foster and Balfour ('83, p. 128) make brief mention of a connection between the ophthalmic branch of the trigeminus and the oculomotor in the chick, soon after the third day of incubation. They merely state that, near the eye, the ophthalmic branch of the fifth nerve "meets and unites with the third nerve, where the ciliary ganglion is developed." The view of Remak and His in regard to the development of the cil- iary ganglion from the neural crest is shared by Goldberg ('91). Like them, he mistakes the mesocephalic ganglion for the ciliary. G(;ronowitsch ('93), on the other hand, declares that the primary neural crest completely disappears in bird embryos during an early period of development. The ciliary ganglion arises later, and quite independently of the neural crest. The writer did not observe the actual process by which it is developed, but is inclined to accept the explanation given by Dohrn of its origin in selachians. He states that he himself has observed in teleosts abundant evidence of the migration of cells from the medullary tube into the root of the third nerve. D'Erchia ('95) gives a description and a figure of a communicating nerve between the ophthalmic branch of the trigeminus and the ciliary ganglion in an embryo chick of twelve days' incubation. He, therefore, denies Schwalbe's assertion that no sensory root of the ciliary ganglion exists in birds. This difference of opinion is easily explained by the fact that D'Erchia made his observations on embryonic, and Schwalbe on adult material. While present in the embryo, this direct connection between the trigeminal nerve and the ciliary ganglion does not persist in the adult. (See Plate 1, Figs. 1 and 2.) Rex (:00) observed that in embryos of the duck the ciliary ganglion first makes its appearance as a distinct thickening in the course of the oculomotor nerve, due to the presence of an accumulation of ganglion cells. He did not follow the development of the ganglion. 5. Mammals. The observations on the genesis of the eye-muscle nerves and the ciliary ganglion in mammals have been fragmentary. In sections of rabbit embryos, Kolliker ('79) discovered the oculo- motor arising at the earliest stage observed from the lateral, not the ventral, face of the brain. It showed no evidence of a ganglionic swell- 172 bulletin: museum of comparative zoology. ing at its base, and no cells were intermingled with its fibres. About its periphery was a thin envelope, formed by a single layer of mesoder- mal cells. Later, the nerve was observed to have descended to the ventral face of the brain. His ('80, '88, '88^ ) maintains that the ciliary ganglion develops in man from the anterior portion of the first ganglionic or trigeminal com- plex of the head, which is a direct descendant of the neural crest. He does not accept the distinction made by Beard ('87) between a raeso- cephalio and a ciliary ganglion, but asserts that the ganglion which Eemak, himself, and so many others Jiave seen at the anterior extremity of the neural crest, is identical with the long-known ciliary ganglion. In opposition to Schwalbe, he assigns this ganglion to the fifth rather than to the third nerve, since it develops over the fore-brain, while the third nerve grows out from the ventral face of the pid-hrain. Furthermore, the oculomotor arises as a purely motor nerve, and, as isuch, is not entitled to a ganglion. Both the third and sixth nerves arise in human embryos as fibrous 'outgrowtlis of neuroblasts situated in the ventral zone of the medullary wall, not far from the median plane. It is stated in Quain's Anatomy ('95, Thane, p. 388) that Martin i('90) found a dorsal root of the oculomotor nerve in an embryo cat. The original article has not been accessible. His, Jun. ('91) compared, in an embryo cat, the cells of a cerebro- spinal, a sympathetic and the ciliary ganglion. In the first, he found large, bipolar cells, and in the sympathetic and ciliary ganglia, small, unipolar cells. Chiarugi ('94, '97) observed the oculomotor nerve, in the youngest guinea-pig embryos in which it was present, springing from the ventral side of the mid-brain, near the median plane. On the trunk of the nerve, close to the root, he discovered a rudimentary ganglion. Since no ganglion cells were to be seen along the further course of the nerve, between the ganglion and the place of connection with the ophthalmic branch of the trigeminus, he considers it improbable that the ganglion owes its origin to nervous elements which have passed from the trigem- inus to the oculomotor. In the wall of the brain there were to be seen, however, several neuroblasts, lying along the root fibres of the third nerve, and, apparently, advancing to the free surface. This leads him to the supposition that the cells of tlie ganglion are derived through migration from the brain wall. He believes that this ganglion has no connection with the ciliary ganglion, which develops later in close re- caepentee: development of the oculomotoe neeve. 173 lation with both the inferior branch of the third nerve and a communi- cating ramus passing to it from the ophthalmic branch of the fifth. He states that he found the origin of the ciliary ganglion difiScult to trace, but is inclined to think that its cells come from the oplithalmic branch of the trigeminus. At the origin of the commimicutiug branch from the latter nerve, a small cluster of ganglion cells was found. Throughout the whole length of the third nerve, there could be seen, disseminated among the nerve fibres, nuclei of cells, the interpretation of which the writer found very difiicult. The ciliary ganglion was recognized by Dixon ('95) as a distinct cellu- lar mass in a human embryo of the sixth week. It appears at first to be more closely connected with the frontal and fourth nerves than with the nasal and third nerves. It later shifts its position, and, by tlie eighth week, has established connections as in the adult. (Comp. Eeuter, '97.) Renter ('97), though concerned chiefly with the development of the eye muscles in the pig, furnishes some interesting information in regard to the early stages of the ciliary ganglion. He discovered, in an em- bryo measuring 14 mm. from nape to rump, dift'erentiatiug ganglion cells lying in the oculomotor nerve, both at the place where it divides into its terminal branches and in the course of its long branch to the ventral oblique muscle. In a later stage these cells are accumulated in one mass in the form of a distinct ciliary ganglion. At the time of the first appearance of the cells, no connection exists between the oculomo- tor and the first branch of the trigeminus. Later, a radix longa is de- veloped. The writer obtained no clew as to the source of the cells of the ciliary ganglion, but, in view of the latter's late development, he considers it very unlikely that its cells have any genetic connection with the neural crest or the Gasserian ganglion. He is of the opinion that His ('88") and Dixon ('95) have mistaken for the ciliary ganglion the fundament of the dorsal oblique muscle during the period between the disintegration of the neural crest and the first appearance of the ganglion. It is evident from the foregoing reviews that, while observers agree in the main as to the development of the abducent nerve, there is a wide diversity of opinion in the cases of the oculomotor nerve and the ciliary ganglion. Consequently, as far as the latter structures are con- cerned, it is difficult to draw from the existing literature satisfactorily supported generalizations. Especially is this true of the ciliary ganglion. 174 bulletin: museum of compakative zoology. which appears to vary in its manner of development not only among animals belonging to different classes of vertebrates, but also among animals belonging to the same class. Even in the same species the accounts of its origin, in one instance, disagree (comp. Beraneck, '84, and Hoffmann, '85). The weight of the evidence, however, seems to jus- tify the following general statements : — 1. The oculomotor nerve develops after the manner of a ventral spinal nerve from the floor of the mid-brain. Its histogenesis has been described both in accordance with the " process theory," i. e., formation by neuraxons growing out from centrally situated neuroblasts (Neal) ; and in accordance with the " chain theory," i. e., formation by chains of cells which anastomose in the mesenchyme (Dohrn). Marshall's theory of a primary connection between the third nerve and the neural crest is not supported by the facts, he himself being unable to trace satisfactorily the intermediate steps in the shifting of the nerve from a dorsal to a ventral position. Kolliker, it is true, finds the nerve in rabbit embryos at first half-way up the side of the neural tube, but his observation in this respect stands alone. Other investi- gators have so convincingly disproved the statements of Piatt, Mitro- phanow and Sedgwick, who hold that the third nerve grows from the mesocephalic ganglion toward the bi-ain, that their erroneous conclusions must be set down to inadequate methods and mistaken interpretations. 2. The developing oculomotor nerve exhibits throughout its course numerous cells distributed among its fibres. Its proximal extremity is enlarged and crowded with cells. With these statements all writers except Ewart and Kolliker agree. Ewart ('90) asserts that in skates at the time of the formation of the ciliary ganglion the oculomotor fibres are free from cells ; and Kolliker ('79) finds no cells connected with the oculomotor in rabbit embryos, except a single peripheral layer which is of mesodermal origin. 3. The abducent nerve develops after the manner of a ventral spina! nerve, usually by several roots, from the ventral wall of the hind-brain. Numerous cells are associated with the nerve fibres in fishes and birds, but not in reptiles (Beraneck), and probably not in mammals. The comparatively few observations on the genesis of the sixth nerve are all in general agreement. 4. The ciliary ganglion may originate either (a, sympathetic type of development) by the migration of ganglion cells from the mesocephalic ganglion into the oculomotor nerve, either directly or by way of the ophthalmic branch of the trigeminus (Ewart, Hoffmann, Chiarugi) ; or CARPENTER: DEVELOPMEKT OF THE OCULOMOTOR NERVE. 175 (h, unclassifiable type of development) by the development of ganglion cells in situ in the third nerve (Dohrn, Beraneck, Rex, Reuter). None of the investigators who have observed the formation of the ganglion in situ have advanced an opinion as to the source of its cells with the exception of Dohrn, who gives evidence of their derivation through migration from the wall of the neural tube. 5. In all vertebrates, at an early stage in the development of the of the ciliary ganglion, a connection, in the form of a communicating ramus, is established between it and the ophthalmic branch of the trigeminus. B. Observations. 1. Methods. The two most satisfactory staining methods for the purposes of my study proved to be the mixture devised by vom Rath for fixing tissues and the Heidenhain iron haematoxylin stain. The vom Rath fluid used according to Neal's procedure (see Neal, :03) has the very desirable effect of differentiating neuroblasts and their growing processes. It colors but slightly other cells of the neural tube (ependymal cells, spongio- blasts and indifferent cells), and the same is true of its effect on the cells of the mesenchyme. In preiiariug the fluid the formula used was that given in 1895 by vom Rath~('95, p. 283) : — 200 c.c. saturated solution of picric acid. 1 grm. platinic chloride, dissolved in 10 c.c. water. 2 c.c. glacial acetic acid. 25 c.c. 2 per cent osmic acid. In this mixture, embryo chicks were allowed to remain for three days or more, during which time the fluid was once changed. They were then washed for a minute in two changes of methyl alcohol, and placed for from twenty-four to forty-eight hours in a 0.5 per cent solution of pyrogallio acid, which intensified the stain. From this reagent the em- bryos were brought up slowly through the different strengths of alcohol to absolute, then cleared in xylol, and embedded in paraffin. This treat- ment rendered the material very brittle, and careful handling was neces- sary in all operations subsequent to immersion in the fixing fluid. After serial sectioning and fixation to the slides, no treatment for the further staining of the tissues followed. The paraffin was dissolved in xylol, and the sections were immediately mounted in xylol-balsam. The prep- arations were allowed to dry uncovered, since the use of cover glasses 176 bulletin: museum of compakative zoology. is likely to be followed by an alteration in the stain, resulting iu a faded or yellowish appearance, and loss of good differentiation. Heidenhain's iron haematoxylin stain was employed in the usual way after fixation either in Zenker's fluid, or in a saturated aqueous solution of corrosive sublimate to which had been added 1 per cent glacial acetic acid. Besides giving its well-known sharp nuclear stain, iron haematoxylin differentiates clearly the primitive fibrils as soon as these begin to appear in developing nerves. It is less favorable than vom' Bath's mixture for the earliest stages of nerve formation, as it is not a selective stain for the processes of the neuroblasts. After fixation in Zenker's fluid or the corrosive-acetic mixture, it is seen that these early neuraxons appear frequently to approach, and to unite longitudinally with one another, thus giving to the nerve the structure of a coarse reticulum. Among the other general stains tried Brazilin and Delafield's haema- toxylin gave the most satisfactory results. Golgi impregnation and intra-viiam staining with methylen-blue were attempted, but repeated trials failed to produce the desired effect on the eye-nmscle nerves of tlie embryos. The method of van Gieson was used in advanced embryos for the pur- pose of studying the first stages in the formation of the sheaths of Schwann. By following Heidenhain's iron haematoxylin with the van Gieson mixture of acid fuohsin and picric acid, a good plasma stain was ob- tained, which brought out distinctly the cytoplasmic processes of the cells accompanying the nerve fibres, as well as those of the mesodermal cells. Serial sections of embryos of various ages were made in the following planes : — 1. Parasagittal. Such series gave nearly longitudinal sections of the third and sixth nerves, and were best adapted to the study of their roots and the cell migration from the neural tube, since the roots of these nerves are spread out in a longitudinal but not in a transverse direction. Obliquely longitudinal sections of the ophthalmic branch of the fifth nerve were obtained in the series cut in parasagittal planes. 2. Transverse to the longitudinal axis of the mid-brain. Owing to the cephalic flexure, cutting in this plane gave longitudinal sections of the third nerve, obliquely longitudinal sections of the sixth, and nearly transverse sections of the ophthalmic branch of the fifth. 3. Frontal to the mid-brain ; resulting in transverse sections of the third nerve, obliquely transverse sections of the sixth, and nearly longi- tudinal sections of the ophthalmic branch of the fifth. carpenter: development of the oculomotor nerve. 177 2. Development of the Oculomotor Nerve, the Ciliary Ganglion AND the Abducent Nerve j described by Stages. Over fifty series of sections were made from chick embryos of various ages between the sixtieth hour and the seventh day of incubation, the period during which the third and sixth nerves and the ciliary ganglion assume both their distinctive histological characters and the most of their adult anatomical relations. From these series have been selected, for the purposes of the descriptions which follow, those best illustrating the successive steps in the development of the nervous structures with which we are dealing. For convenience in description I have divided the first five days of this period into five stages, which will be described in chronological order. Stage I. 1. Oculomotor Nerve. The earliest indication of the origin of the oculomotor nerve was observed in an embryo of seventy-two and one-half hours' incubation. Tliat the development of this embryo had been ab- normally retarded cannot be doubted, since the third nerve in a more ad- vanced stage was freq\iently met with in embryos of seventy hours and in one case in an embryo of sixty hours. Parasagittal sections of the mid-brain at this stage show the thickness of its ventral wall to be about one-eighth the height of the neural canal. Entering into the composition of the medullary wall may be observed the elements first described by His and later studied in such detail by Schaper ('97). Near the internal limiting membrane lie numerous germinative cells in process of division, while the mantle layer, making up the greater part of the wall, is composed of ependymal cells which have assumed a supporting function by developing into a medullary framework, and of the products of the proliferating activity of the germ- inative cells, namely, indifferent cells, which later differentiate into either nervous or supporting elements. The structure of the medullary framework is not well brought out by the vom Eath method. Near the external limiting membrane there is a narrow zone free from nuclei, the marginal veil (" Randschleier "). In addition, at a distance of about 100 micra from the median plane on either side, is to be seen a small group of cells which tlie vom Eath stain renders distinguishable from the other elements of the medullary wall. Each cell shows, at one side of its nucleus, a variable amount of darkly colored cytoplasm, the characteristic feature of a neuroblast. Inasmuch as these neuroblasts VOL. XLViii. — No. 2 12 178 bulletin: museum of compakative zoology. occupy the position of those from which, at a slightly more advanced stage, the neuraxons of the oculomotor nerv£ take their origin, and since there is, even at this stage, evidence of the outgrowth of their pro- cesses from the neural tube, it follows that each group of cells is to be looked upon as the developing iiidulus of the oculomotor nerve of that side. The niduli, then, at the very beginning of their development, are in a decidedly ventral position. The neuroblasts on the right side of the median plane are more closely grouped together than those on the left, and even under low powers of the microscope stand out clearly from the rest of the medullary wall because of their darkly staining cytoplasm. Each group lies close to the external limiting membrane, projecting into the region of the marginal veil. On the right, the external limiting membrane remains intact, no processes of the neuroblasts having forced their way through it. On the left (Plate 3, Fig. 8), however, the external limiting membrane has been ruptured, and a part of the substance of the marginal veil protrudes. In the parasagittal section of the ventral mid-brain wall shown in Fig- ure 8 there are included only a few of the several neuroblasts making up the nidulus. The letters n'hV designate a neuroblast with its cyto- plasm drawn out, aud directed toward the break in the medullary wall. Near it lie two cells, with cytoplasm tending in the same direction. The neuroblast marked n'bl." shows a well-defined cytoplasmic process, narrowing toward its peripheral extremity, which has evidently pushed its way through the external limiting membrane. Of interest is a nu- cleus (d. med. mig.), plainly medullary, lying in the material which is escaping from the neural wall through the aperture in the external limit- ing membrane to which reference has been made. This medullary nu- cleus appears to be making its way out of the neural tube at a very early stage in the development of the oculomotor nerve. A little posterior to the oculomotor nidulus, at this stage, a few of the first fibres of the ventral fibre-tract can be seen running caudad toward the hind-brain. 2. Hye Muscles. As early as this stage, the fundament of the poste- rior rectus muscle has made its appearance, but since the series repre- senting Stage II is more favorable for its study, description is deferred until then. Its character and relations in Stage I have not been altered in Stage II. Stage //, This stage is found in a series of seventy hours* incubation. 1. Oculomotor Nerve. Cross-sections of the mid-brain show the oculo- CARPENTER : DEVELOPMENT OF THe' OCULOMOTOR NERVE. 179 motor niduli, lying in the ventral mid-brain wall, with their cefitres about 120 micra from the median plain. The niduli invade the marginal veil, which is now well developed. From the neuroblasts, cytoplasmic pro- cesses ruu out into the mesenchyme, as may be seen in Plate 3, Figure 9. These appear to blend more or less, and, outside the neural tube, make a network with many nuclei lying along the threads of the net. This reticulated appearance of the nerve during its early development always follows fixation in Zenker's fluid or the corrosive-acetic mixture. In the present instance, the material was fixed in Zenker's fluid, and stained with Brazilin. Vom Rath preparations, on the contrary, exhibit the neuraxons of the growing nerve as separate elements, approximately parallel to one another, and not connected to form a network (comp. Plate 6, Fig. 20). The cells lying along the nerve strands are composed of rounded nuclei with which very little cytoplasm appears to be associated. For the sake of convenience, these cells will hereafter be called accompanying cells. Their nuclei resemble closely in form, size and staining qualities, the nuclei of the indifferent cells which lie inside the neural tube. Certain of the indifferent cells of the nidulus may be seen in the process of di- vision (Plate 3, Fig. 9, clJ), and many lie near, and indeed in some cases (cl.") immediately in contact with, the external limiting membrane. The oculomotor may now be traced for some distance by its cytoplasmic threads and " accompanying " cells. It pursues a straight course through the mesenchyme in a ventral direction from the mid-brain. On account of the cephalic flexure, it lies nearly parallel to the axis of the hind-brain. After proceeding a short distance, it terminates in a slightly expanded distal extremity. 2. Ophthalmic Branch of the Trigeminal Nerve. In this stage the Gasserian ganglion, which has of course been present since its differen- tiation, much earlier, from the neural crest, shows plainly a partial divi- sion into a mesocephalic ganglion, giving rise to the ophthalmic branch of the trigeminus, and the ganglion of the maxillary and mandibular branches. The mesocephalic division is directed cephalad, and from its distal extremity the ophthalmic branch proceeds for a short distance ftlong the outer wall of the anterior cardinal vein in the direction of the eye- ball, but cannot be traced as far as the level of that organ. The maxillo- niandibular division of the Gasserian ganglion is directed ventrad and laterad, making approximately a right angle with the mesocephalic portion. It extends as far as the ectoderm, with a thickened patch of which (Plate 7, Fig. 23, gn. mx-md. Gas.) it is directly connected. 180 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. The maxillary and mandibular branches of the trigeminus are poorly de- veloped at this stage, 3. Eye Mmclen. An isolated and compact accumulation of cells, staining deeply with haematoxylin, lies in the mesenchyme along the inner border of the anterior cardinal vein, not far from the ventro-lateral face of the anterior portion of the hind-brain (Fig. 23, mu. rt. p.). The dis- tal extremity of the second division of the Gasserian ganglion lies lat- erad to this cell group, separated from it by the lumen of the anterior cardinal vein. The subsequent development of the mass proves it to be the fundament of the posterior rectus eye-muscle. No other eye-muscle fundaments are present, and not even the nidulus of the abducent nerve, which later innervates the posterior rectus muscle, has as yet appeared. Stage III. This stage is represented by two series of preparations, one of eighty- eight hours', the other of ninety-three hours', incubation. The two em- bryos had attained practically the same degree of development. 1. Oculomotor Nerve and Ciliary Ganglion. A fortunate section from the eighty-eight-hours' series, taken tran.sveraely to the axis of the mid- brain, shows the oculomotor cut longitudinally throughout its whole length (Plate 7, Fig. 24, n. oc^mot.). On either side of the median plane, and lying in the ventral wall of the mid-brain, the oculomotor uiduli {nidi, oc'mot.) are seen as elliptical groups of cells, delimited by narrow areas comparatively free from medullary elements. These niduli present an elliptical outline whether viewed in transverse or parasagittal sections, but the ellipse seen in the latter plane has the longer principal diameter. Each nidulus measures approximately as follows : longitudinal diameter, 450 mi era ; transverse diameter, 135 mi era ; ver- tical diameter, 78 micra, the latter being more than half the thickness of the mid-brain wall, which here measures 117 micra. Tlie centres of these niduli are slightly over 150 micra from the median plane. Allowing for increase in size due to growth, it is apparent that little, if any, change has taken place in their positions since their first appearance in Stage I. They then lay at the distance of 100 micra from the median plane. The marginal veil is now invaded by the developing ventral fibre-tract, which forms a comparatively bi'oad band of separation between the ordi- nary cells of the medullary wall and the external limiting membrane. The cells of the nidulus, however, project into this region, and extend to the very margin of the wall. Not all the neuraxons given off by the neuroblasts of the oculomotor nidulus pass out into the root of the carpenter: development of the oculomotor nerve. 181 nerve. Some are directed caudad, and contribute to the formation of the ventral fibre tract, as can easily be observed in vom Eath prepara- tions (Plate 6, Fig. 20, trt.fbr. v.). No fibres were observed running cephalad from tlie nidulus. The nerve, consisting of fibres densely crowded with nuclei, arises by a root which is spread out in a longitudinal direction into a- fan-shaped form (Fig. 20). Seen in a plane at right angles to this, the root does not show this enlargement (Plate 7, Fig. 24). The trunk of the nerve pursues a straight course ventrad and laterad, terminating at the side of the ventral extremity of the infundibulum (wliioh is brought by the cephalic flex\ire into a position ventral to the mid-brain). The distal end of the oculomotor is conspicuously enlarged, so that the whole nerve, seen in cross-sections of the mid-brain, may be said to have a clavate form (Fig. 24, n. oc'mot.) In parasagittal sections, tlie enlarge- ment has the shape of an unsymmetrical spindle, the posterior side of which has the sharper curvature. This terminal swelling is the funda- ment of the cihary' ganglion (gn. cil.). Examination of the root of the oculomotor nerve with high powers re- veals conditions strongly suggesting the migration of medullary cells. Figure 11 (Plate 4) represents a section through the root of tlie third nerve shown in Figure 24 (Plate 7). In this preparation, which was fixed in the corrosive-acetic mixture and stained with Heidenhain's iron haematoxylin, the processes projecting out from the neuroblasts run to- gether and blend in such a way as to lose their identities as neuraxons. Again, as in Stage II, we find nuclei lying on the nerve fibres, both within and witliout the brain wall. Not all of those within are the nu- clei of neuroblasts, for some (as dJ) are without cytoplasmic processes, and others appear to be lying peripherally on processes apparently origi- nating from neuroblasts more centrally placed. These nuclei, with which very little cytoplasm is connected, answer to the description of the indifferent cells of Schaper, wliich are known to possess the power of locomotion, since, within the boundaries of the medullary wall, they pass from the region of the germinative cells, where they originate, into the mantle layer. In the present instance they can be traced beyond the mantle layer to the free surface of the neural tube. Indeed, now and then, one can be detected in the position of cl." lying half within and half without the neural wall, in the same position in which Dohrn ('9l) has figured emigrating cells in the root of the developing oculo- motor nerve in selachians. Although there is little evidence of cell divi- sion within the nidulus, the germinative cells at the inner border of 182 bulletin: museum of compakative zoology. the medullary wall are numerous, and actively engaged in the production of indifferent cells. Along the course of the nerve the " accompanying " cells may fre- quently be seen in process of mitosis, so that in case these are, as I be- lieve, medullary and not mesodermal elements, it does not follow that the whole number present at this time have migrated from the neural tube, since their numbers are constantly increasing through cell division. A vom Rath preparation of the root of the oculomotor, in an embryo ninety-three hours old, shows a distinct difference in staining qualities between the "accompanying" nuclei and the adjacent mesodermal nuclei. While the "accompanying" nuclei take the stain readily, and are sharply defined, the surrounding mesodermal nuclei are, owing to their paleness, much less conspicuous (Plate 6, Fig. 20). On the other hand, the nuclei of the medullary wall show in their affinity for the stain a striking resemblance to the " accompanying " nuclei. It will be noticed that both the nuclei within the neural tube and those lying in the root of the oculomotor are more or less rounded in form. At least few, if any, exhibit a pronounced elongation. However, as we pass distally along the nerve, we find the most of the nuclei be- coming more and more elongated, until the great majority are distinctly spindle-shaped. A few cells, distributed along the entire length of the nerve, retain the rounded form. Taking into consideration the whole course of the nerve, the greatest amount of cell division occurs in the enlarged terminal (distal) portion (Plate 5, Fig. 15), the fate of which proves it to be the fundament of the ciliary ganglion. Here, the majority of the nuclei are not as elongated as those lying among the fibres of the trunk of the nerve, many show- ing approximately circular outlines. All are nearly destitute of cyto- plasm at this stage. It is evidently owing to the proliferation of these nuclei lying among the terminal nerve fibres that the enlargement in this region has taken place. 2. Ophthalmic Branch of the Trigeminal Nerve. The first branch of the fifth nerve (Plate 7, Fig. 24, Plate 4, Fig. 12, rm. opth. trig.) passes, in this stage, straight cephalad from the mesocephalic ganglion along the lateral wall of the anterior cardinal vein {vn. crd. a.). It termin- ates dorsad of the optic stalk between the fore-brain and eyeball, and just caudad of the laterally projecting vesicle of the hemisphere of that side. Its distal extremity is marked by a transitory fusiform ganglion (Plate 4, Fig. 12, gn. t'i.), having an approximate length of 165 micra, and a greatest diameter of 75 micra. This Figure is drawn from several cakpenter: development of the oculomotok nerve. 183 sagittal sections of a vera Rath series, and represents about one-half the length of the ophthalmic branch of the trigeminus. In this particular case, the nerve, just after passing the level of the optic stalk, divides into two branches, which unite again at the ganglion ; but I do not find this condition to be a constant one. An examination of longitudinal sections of the ophthalmic branch of the trigeminus at this stage shows that, beside the elongated " accom- panying" cells, resembling those of the oculomotor, there are to be found distributed along the whole length of the nerve, as well as in the. transitory ganglion, ganglionic cells whose larger nuclei and more abun- dant and deeply staining cytoplasm make them easily distinguishable from the "accompanying" cells. The description of these ganglionic cells, and the discussion of the terminal ganglion, will be taken up under Stage IV. In the transverse series of an eighty-eight-hours' chick there can be seen at this stage, on the left side of the body, an exceedingly slender offshoot of the ophthalmic branch of the trigeminus, passing to the fundament of the ciliary ganglion. The offshoot is apparently composed of a single neuraxon, to which a few "accompanying" cells are applied, and at the place of its origin from the ophthalmic branch lies a cluster of ganglion cells. On the right side of the body, on the contrary, the most careful search has failed to reveal any fibrous connection what- ever with the fundament of the ciliary ganglion. Of interest, however, are two cells having the appearance of ganglion cells, both of which lie in the mesenchyme between the ophthalmic branch and the fundament of the ciliary ganglion, one being about midway between the two struc- tures, and the other close to the surface of the ganglionic fundament. Both these cells differ markedly from mesodermal cells, and also from those of the fundament of the ciliary ganglion, on account of the larger size of their nuclei and their deeply staining cytoplasm ; while their resem- blance to ganglion cells seen in the ophthalmic branch, in the same sec- tions, is very close. They appear to be migrating ganglion cells, and their origin from the ophthalmic branch seems yery- probable. Further evidence that ganglion cells do migrate from this nerve will be presented in Stage IV. 3. Abducent Nerve. The sixth nerve was first met with in a seventy- eight-hours' series, that is, in a stage intermediate between StagSs II and III. It was observed on both sides of the head as a very small nerve, arising by five delicate roots from the ventral surface of the hind-brain, about 135 micra from the median plane. The nerve can be traced 184 bulletin: museum of compaeative zoology. cephalad for a very short distance only. In another series of the same length of incubation the abducens was not found. It will be noted that the abducens appears several hours after the differentiation of the funda- ment of its muscle, the posterior rectus. In the ninety-three-houra' series, the nidulus of the abducens can be seen lying close to the ventral surface of the hind-brain wall. It is elongated in a longitudinal direction, and lies about 175 micra from the median plane. It is not possible, in my preparations, to make out its limits as definitely as in the case of the oculomotor nidulus. The nerve itself is a slender one, springing from the ventral face of the hind-brain by a varying number of attenuated roots, placed one behind the other, and crowded with "accompanying" cells. As many as eight roots have been counted, the number differing in different em- bryos, and even on opposites sides of the body in the same embryo. The roots unite a short distance from the brain to form the trunk of the nerve, which passes straight cephalad, running parallel to the ventral face of the hind-brain. In vom Rath preparations the nerve can easily be traced as a slender bundle of a few darkly stained neuraxous with elongated " accompanying " cells to the posterior edge of the compact cluster of cells making up the fundament of the posterior rectus muscle. It will thus be seen that the abducens, though appearing later than the oculomotor, is the first of the two nerves to become connected with a muscle fundament. The fundament of this muscle, the posterior rectus, was already recognizable in the mesenchyme of Stage I, therefore, long before any of the other eye-muscle fundaments, none of which can be made out jjrevious to the present stage. 4. Eye Muscles. The fundament of the posterior rectus eye muscle lies near the anterior portion of the hind-brain in a ventro-lateral posi- tion. It consists of a mass of modified mesodermal cells, which differ from surrounding ones in their closer association and their rather greater amount of cytoplasm. The absence of fibres in the muscle fundament, and their presence in that of the ciliary ganglion, make a notable differ- ence in the appearance of these two structures. The muscle mass is elongated in an antero-posterior direction (Plate 7, Fig. 23, mu. rt. p.}. It lies mostly caudad of the fundament of the ciliary ganglion, but its anterior end runs cephalad and laterad as a narrow prolongation, which, passing laterad of the posterior half of the ganglionic fundament, termi- nates between it and the anterior cardinal vein. The fundament of this muscle appears in Plate 7, Figure 24 (mu. rt.j).)^ where the section has passed very near its anterior extremity. caepentee: development of the oculomotoe neeve. 185 The fundaments of the dorsal rectus and dorsal oblique muscles, and the common fundament of the anterior and ventral rectus muscles, are now to be seen. These consist of small, local differentiations of meso- dermal cells, which tend to become more closely associated, and, through their activities, produce an abundance of cytoplasmic material ; this material represents the first stage in the formation of the contractile substance of the muscle cells. These muscle fundaments will be described and figured in Stage V, where my sections are in planes more favorable for showing their relations. Their positions in the later stage are practically the same as in the present one. The fundament of the ventral oblique muscle cannot be distinguished at this stage. It might here be stated that it has been impossible, in the chick, to assign the eye-muscle fundaments to their respective somites by the aid of the head cavities. These have not been present in the stages studied owing to the fact that, in the chick, they appear to be obliterated very early in development — much earlier than is the case in the embryos of ducks (van Wijhe, '86 ; Eex, :00) terns, gulls and lapwings (van Wijhe, '86). Stage IV. This stage occurs at about the one hundredth hour of incubation. It is described from two series, one of one hundred hours, the other of one hundred and one hours. 1. Oculomotor Nerve. The oculomotor nidulus retains its extreme ventro-median position. In fact it appears to have moved toward rather than away from the median plane. Although the neural tube has increased in size, the centre of the nidulus now lies nearly 50 micra nearer the median plane than in the stage last described, being distant only 105 micra from it. The ventral fibre tract has increased in thick- ness, and, while the nidulus of the oculomotor extends into it, the lower border of the nidulus does not lie as near the external limiting membrane as in Stage III. Neuraxons from the nidulus, passing through the ventral fibre tract — a I'egion free from nuclei — are frequently seen to be accompanied by nuclei which have the rounded form of the indiffer- ent elements in the nidulus dorsal to them, and of the "accompanying" cells of the root of the oculomotor ventral to them. Farther out on the trunk of the nerve, the grtat majority of the cells have become elongated) though occasional round ones are to be observed. The oculomotor nerve pursues, as in the stage last described, a 186 BULLETIN: MUSEUM OF COMPABATIVB ZOOLOGY. straight course, passing ventrad and slightly laterad (Plate 4, Fig. 13) through the mesenchyme, between the infimdibuhim on its median, and the lower half of the eyeball on its lateral side. The large accumulation of cells forming the ciliary ganglion lies mainly on the lateral, or ocular, side of the nerve trunk (Compare Plate 7, Fig. 25 gn. cil). The neu- raxons of the nerve continue for a short distance beyond the ganglion, and, bending somewhat laterad, terminate immediately mediad of the antero-ventral portion of the eyeball in the fundament of the ventral oblique muscle, which makes its first appearance in this stage. The oculomotor is as yet entirely without branches. A histological change has by this time taken place in, at least, the dis- tal two-thirds of the nerve. In, early stages, especially in vom Eath preparations, the neuraxons of the nerve are to be seen, under high powers of the microscope, as relatively thick fibres with peripherally situated " accompanying " cells. The same powers now show, especially in the more distal parts of the nerve, that the relatively thick fibres no longer appear, their places having been taken by much finer fibrils. As a consequence, the fibrous components of the nerve are now greatly in- creased in number without a corresponding increase in the calibre of the nerve. I shall hereafter speak of these fine filaments as fibrils in contradistinction to the earlier j'zftres — the coarser structures which the fibrils have replaced. The histogenesis of the fibrils is considered under Stage V. Lying at all depths within the nerve, " accompanying " cells may be seen closely applied to the fibrils. In the description of the preceding stage, evidence was brought forward to show that these "accompanying" cells have been derived through migration from the neural tube, where, it is maintained, they originate as rounded, indifferent cells, the descend- ants of the germinative cells, and where, according to Schaper, they may later diiferentiate. into either nervous or supporting elements, i. e., neu- roblasts or spongioblasts. Certain of these cells, through their power of locomotion, are capable of leaving the central nervous system, and, fol- lowing the path of the neuraxons, of reaching the peripheral nerve trunk. Lying among the fibrils of the nerve, they increase in number by divi- sion. Knowing, as we do, the subsequent history of the indifferent cells remaining within the neural tube, an analogous fate might be expected, a priori, in the case of those which migrate out into the nerve trunk. That many of these emigrant indifferent cells do eventually subserve a supporting function, not in the form of neuroglia, but as the sheaths of Schwann, can hardly be doubted. Such cells become elongated soon CAEPENTEE : DEVELOPMENT OF THE OCULOMOTOE NERVE. 187 after their escape from the medullary wall, and, during succeeding stages of development, can be observed adhering closely to the nerve fibrils, until finally, just before the hatching of the animal, they give evidence of participation in the formation of the sheaths of Schwann. The other possibility indicated by their identity with the indifferent cells of the neural tube — namely, their differentiation, in part, into ganglion cells — will be considered under the subject of the ciliary ganglion. In the one-hundred-hours' series there occurs on the oculomotor nerve midway between its root and the ciliary ganglion, a group of cells worthy of attention. As is shown in Plate 5, Figure 17 — which is a longitudinal section through a portion of the oculomotor nerve — these cells are placed at the margin of the nerve trunk, and present a striking contrast to the "accompanying" cells lying along the fibrils. While the nuclei of the latter cells are drawn out into oval, elliptical and spindle forms, and possess very little cytoplasm, those of the cells forming the group have mostly an almost circular outline, and lie embedded in an abundance of granular cytoplasmic material, which stains deeply with haematoxylin. Proximal and distal to this group the nerve exhibits only fibrils and the ordinary elongated " accom- panying" cells. I shall have occasion to refer again to this accumu- lation of differentiated cells when discussing the ciliary ganglion. The single layer of strikingly elongated cells at the periphery of the nerve is probably made up of mesodermal elements. The processes of these cells unite to form a thin envelope, which doubtless represents the connective-tissue sheath or perineurium of the adult nerve trunk. 2. Ophthalmic Branch of the Trigeminal Nerve. The transitory ganglion which appeared in Stage III still persists, but in one series it presents on the left side of the body a disorganized appearance, being represented by small clumps of ganglion cells, which lie scattered about in the mesenchyme in the immediate vicinity of the place on the ophthalmic branch where one would expect to find the ganglion. Possibly we have here the beginning of a process of disintegration. The ganglion on the right nerve in the same series is as compact and definitely limited a body as in the preceding stage, and it lies in the same position, immediately posterior to the laterally projecting vesicles of the cerebral hemispheres. The nerve continues beyond the ganglion as a slender strand, which runs ventrad of the lateral vesicle of the fore-brain, trending also laterad as it proceeds, and, before reaching the level of the anterior extremitj' of the fore-brain, terminates in a small swelling containing ganglion cells. Several small and rather indefinite 188 bulletin: museum of comparative zoology. branches are given off immediately proximal to the large transitory- ganglion. These branches can be followed for short distances into the mesenchyme. As was stated in the description of Stage III, ganglion cells can be found scattered along the nerve from the mesocephalic to the transi- tory ganglion. These cells are more numerous near the mesocephalic ganglion; in fact, the transition from ganglion to nerve is a very gradual one, since so many ganglion cells have migrated outside the true limits of the ganglion. Comparison shows that the ganglion cells to be found in the mesocephalic ganglion are precisely like those of the transitory ganglion. Both have relatively large, rounded nuclei con- taining chromatin which is concentrated, for the most part, into one or two large masses. Surrounding the nucleus, but lying mostly on one side of it, is a considerable amount of finely granular cytoplasm, which becomes drawn out to a blunt extremity, and stains a deep blue with iron haematoxylin. The nucleus itself is much less deeply stained, with the exception of the included chromatin particles, which take on ,a dense, black appearance. Such ganglion cells are shown in Plate 2, Figure 5, where o and P are cells from the transitory ganglion and y, a ■cell from the mesocephalic ganglion. These ganglion cells are plainly ;in an early stage of development, being in the condition of neuroblasts the cytoplasm of which has become drawn out to one side preparatory •to developing into neuraxons. In the mesocephalic ganglion many ganglion cells have already sent out their neuraxons, and these form the trunk of the ophthalmic division of the fifth nerve. There are also present, however, young ganglion cells which have not reached this stage of development, as the one to which attention has just been called (Fig. 5, y). In fact, such ganglion cells seem to be forming here through the activities of proliferating cells, which can be seen in every section made through the (Jasserian ganglion in this and preceding stages. Since, then, the mesocephalic ganglion is the scene of constant cell production, and since its young ganglion cells resemble exactly those of the transitory ganglion, and are connected with them by a continuous series of similar ganglion ^.ells lying scattered along the nerve, the source of the component cells of the transitory ganglion does not seem to be open to question, especially when we remember that young ganglion cells possess an extraordinary capacity for locomotion (His, Jun., '91). All the evidence points to a migration of ganglion cells distally along the nerve to form the transitory ganglion and similarly, any other smaller ganglia that may be found near it, cae?entee: development of the oculomotor nerve. 1S9 If we look upoa the proliferating cells in the tnesocephalic ganglion as comparable, to a certain extent, with the germinative cells of the neural tube, and consider them, as Schaper has proved for the germi- native cells, the producers of a generation of indifferent cells capable of becoming in part nervous, in part supporting elements, then we can easily account for the presence in the ganglion of many small, rounded cells, almost destitute of cytoplasm. It seems probable that these correspond to the indifferent cells of the neural tube. While some of thenl may change to ganglion cells, others may develop into the small, somewhat elliptical cells found in the ganglion, especially near its distal end, where they pass by an easy gradation into the more elongate cells lying among the fibrils of the nerve which here takes its origin. This leads to the supposition that the "accompanying" cells of the nerve, which later subserve a supporting function by developing the sheaths of Schwann, have been derived through migration from the ganglion. In support of this ^view I have introduced a drawing (Plate 2, Fig. 6) made from sections of an embryo of Amblystoma punctatura. Group A is taken from the central part of the Gasserian ganglion. It will be noticed that here, lying within the limits of the ganglion, are to be found nuclei in all stages of transition between the rounded and the elongated forms. The nuclei of group B lie on the neuraxons at the proximal end of the ophthalmic branch of the trigeminal nerve, those designated by cl. comit.' and cl. comit." being situated at the emergence of the nerve from the ganglion. The oval one (cl. comit.') I take to represent a stage in the differentiation of a rounded cell into a long " accompanying " cell, such as those to be found in the remainder of the course of the nerve. Neither in the chick nor in Amblystoma is there at any point along the ophthalmic branch evidence of an intrusion of mesodermal cells. It is quite possible that supporting derivatives of indifferent cells remain in the ganglia, and later, through their activities, form the nucleated capsules of the ganglion cells, just as the " accompanying " cells of the peripheral nerve form, in an analogous manner, the sheaths of Schwann about the neuraxons. The similar origin of the capsule of the ganglion cell and of the Schwann's sheath of its process would account for the continuity of the two structures. Those investigators who afBrm the mesodermal derivation of the Schwann's-sheath cells have never been able, so far as I know, to obtain evidence of an invasion of cerebro-spinal ganglia by mesodermal elements destined to give rise to the envelopes of the ganglion cells. 190 bulletin: museum of comparative zoology. The r61e played by the ophthalmic branch of the trigeminus in the development of the ciliary ganglion will be considered under the follow- ing heading. 3. Ciliary Ganglion. As has been stated, the ciliary ganglion lies mainly on the lateral side of the third nerve near its distal extremity. Its form in transverse section, and its general relation to the ophthalmic branch of the fifth nerve and to the eyeball, are shown in a diagram- matic way, for the right side of the head, in Plate 4, Figure 13. The striated portion represents the fibrils of the oculomotor, while the ciliary ganglion is indicated by the evenly shaded part. The ganglion contains, now for the first time, cells so far advanced in differentiation that they can be declared without hesitation to be young ganglion cells (Plate 5, Fig. 16, E). Interspersed among them are a few cells {E, dJ, cl.") exactly like the "accompanying" cells of the nerve fibrils. The ganglion is no longer, as in the preceding stage, the scene of active cell division. A few of the young ganglion cells lie along the median border of the nerve opposite the laterally placed body of the ganglion. Figure 13 (Plate 4) also shows the ophthalmic branch of the trigem- inus (rm. opth. trig?) cut transversely. A small offshoot is indicated, running from this nerve in the direction of the ciliary ganglion, which, however, it fails to reach. The fibrils of this branch become lost in the mesenchyme, so that it is not possible to trace them all the way to the ganglion, although, on tlie opposite side of the head, where development is a little more advanced, this can be done. I have also indicated in the figure aU the ganglion cells to be found along that segment of the ophthalmic branch of the trigeminus which lies opposite the ciliary ganglion as well as all the ganglion cells that have become detached from the ophthalmic branch at this level. The number and positions of these cells were ascertained by studying the series of consecutive sections extending from the anterior to the posterior face of the ciliary ganglion, and recording the ganglion cells observed by projecting them on the plane of the diagrammatic section represented by Figure 13. A comparison of the ganglion cells of the ophthalmic branch of the trigeminus with those of the ciliary ganglion will prove instructive. In Figure 1 6 at ^ are shown two ganglion cells (a, y8), taken from the ophthalmic branch, their positions in that nerve being indicated by the same letters in Figure 1 3. The ganglion cells of the group E were taken at random from the ciliary ganglion. They exhibit the features characteristic of young ganglion cells, resembling those of the ophthalmic branch in the possession of deeply staining granular cytoplasm, accumu- carpenter: development of the oculomotor nerve. 191 lated at one side of the rounded nucleus. But an evident and con- sistent difference exists between the cells taken from the two sources. The cells of the ciliary ganglion are smaller than those of the ophthalmic branch of the trigeminus. This difference is to be seen at once in the drawings in Figure 16, in which the outlines of the cells were made with the aid of the camera lucida under precisely the same optical con- ditions for all groups. Not only is the amount of cytoplasm less in the ciliary-ganglion cells, but their nuclei are distinctly smaller than those of the ophthalmic cells. It is not an easy matter to compare accurately the two classes of cells by measurements of tlie diameters of their respective nuclei, since these are seldom exactly circular in outline. I have, however, made measurements in the cases of such nuclei as approached nearest to a circular form, and I find that while the diam- eters of the nuclei of the ciliary-ganglion cells fall between 5.2 micra and 6.5 micra, the nuclei of the ophthalmic cells show constantly a diameter of approximately 7.8 micra. The ganglion cells lying in the mesenchyme, between the ophthalmic branch and the ciliary ganglion, are very evidently emigrant ophthalmic cells. Two such cells, having the positions y and S in Figure 13, are shown in B and C, Figure 16, each surrounded by mesodermal cells. A glance shows that they belong to the ophthalmic and not to the ciliary type. Within the boundaries of the ciliary ganglion, lying close to the exterior of the cell mass, on the side toward the ophthalmic branch of the trigeminus, are to be found among the smaller ganglion cells three cells with large nuclei, two of which are shown in Figures 16, D, and 13 (e, in both figures). These appear to be ophthalmic ganglion cells, which have traversed the mesenchyme and entered the ciliary ganglion. With the foregoing evidence before us, let us inquire into the source of the cells of the ciliary ganglion. We have seen that in the early stages of the growth of the oculomotor nerve a migration of medullary cells takes place from the neural tube into its root. I believe that in Stage I we have the first migratory cell forcing its way through the external limiting membrane of the neural tube (Plate 3, Fig. 8). In succeeding stages these cells seem to be migrating in considerable num- bers. The rounded nuclei of the cells appear to be almost naked, for it is difficult to detect any cytoplasm surrounding them. These cells are as yet neither neuroblasts nor spongioblasts, but evidently the motile, indifferent cells of Schaper, i. e., they are descended from ger- minative cells, and are capable of differentiating later into either nervous 192 bulletin: museum of comparative zoology. or supporting elements. Many of these nuclei, once out on the nerve, become elongated as they move away from the neural tube. Such " ac- companying " cells maintain throughout development their close prox- imity to the" nerve fibrils, and in them we recognize, as has been pointed out, the nuclei of the future sheaths of Schwann. A large part, then, of the emigrant cells become supporting elements. Do any of the emigrant indiiferent cells become nervous elements? We have seen that at all stages rounded nuclei, resembling and continu- ous with the indifferent cells of the neural tube, occur abundantly at the root, and more sparingly along the trunk, of the oculomotor nerve, lying among the more numerous elongated supporting cells. In Stage III an accumulation of such cells was observed at the distal end of the nerve, causing at this place its enlargement into the fundament of the ciliary ganglion (Plate 7, Fig. 24, gn. eil.), the cells of which were undergoing active division (Plate 5, Fig. 15). Schaper, it will be remembered, shows that the indifferent cells of the central nervous system likewise possess the property of further propagation. In the present stage, IV, the ciliary ganglion of the right side contains undoubted ganglion cells. The right oculomotor has not yet come into connection with any other nerve, although the ophthalmic branch of the fiftii is sending fibrils in the direction of the ciliary ganglion, and toward the latter a few oph- thalmic ganglion cells are apparently making their way through the mesenchyme. Three, in fact, lie just within the borders of the ciliary ganglion, easily distinguishable by their larger size from the numberless ganglion cells about them. The vast majority of the cells of the ciliary ganglion, however, could have originated only by differentiation from the rounded, proliferating cells which are to be seen in Stage III occu- pying the site of the future ganglion, before there is the slightest trace either of a connection between the oculomotor and the ophthalmic branch of the trigeminus, or of the migration of ophthalmic ganglion cells through the mesenchyme towai-d the fundament of the ciliary ganglion. There is good evidence that the actively dividing cells of the latter ganglion had their origin in indifferent medullary cells which had escaped from the neural tube. If this be so, then a small portion of the indifferent cells migrating out from the mid-brain do become differentiated, after increase in numbers by division, into nervous ele- ments, i. e., ganglion cells of the ciliary ganglion. An accumulation of differentiating cells at the side of the third nerve, mi. On the Cranial Ganglia and Segmental Sense Organs of Fishes. Zool. Anz., Jahrg. 8, No. 192, pp. 220-223. Beard, J. '87. The Ciliary or Motoroculi Ganglion and the Ganglion of the Ophthal- micus profundus in Sharks. Anat. Anz., Jahrg. 2, Nos. 18-19, pp. 565- 575, 5 fig. Beard, J. '88. Morphological Studies. II. The Development of the Peripheral Nervous System of Vertebrates. Part 1. — Elasmobranchii and Aves. Quart. Jour. Micr. Sci., n. s., vol. 29, no. 114, pp. 153-227, pi. 16-21. Bell, C. '30. 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The Action of Nicotin oq the Ciliary Ganglion and on the Endings of the Third Cranial Nerve. Jour. Physiol., vol. 13, pp. 460-468. Langley, J. N., and Dickinson, W. L. '89. On the Local Paralysis of Peripheral Ganglia, and on the Connexion of Different Classes of Nerve Pibres with them. Proc. Roy. Soc. Lond., vol. 40, no. 284, pp. 423-431. Marina, A. '98. II neurone del ganglio ciliare ed i ceutri dei movimenti pupillari. E.iv. di. patolog. nerv. e ment., vol. 3, fasc. 12, pp. 529-546. Marina, A. '99. Das Neuron des Ganglion ciliare und die Centra der Pupillenbewe- gungen. Eine experimentelle Studie. Deutsch. Zeitschr. f. Nervenheilk., Bd. 14, Hefte 5-6, pp. 356-412, Taf. 13. Marshall, A. M. '77. On the Early Stages of Development of the Nerves in Birds. Jour. Anat. Physiol, vol. 11, pt. 3, pp. 491-515, pi. 20, 21. Marshall, A. M. '78. The Development of the Cranial Nerves in the Chick. Quart. Jour. Micr. Sci., n. s., vol. 18, no. 69, pp. 10-40, pi. 2, 3. Marshall, A. M. '81. On the Head Cavities and Associated Nerves of Elasmobrauchs. Quart. Jour. Micr. Sci., n. s., vol. 21, no. 81, pp. 72-97, pi. 5, 6. Marshall, A. M. '82. The Segmental Value of the Cranial Nerves. Jour. Anat. Physiol., vol. 16, pt. 3, pp. 305-354, pi. 10. Also in Stud. Biol. Lab., Owens College, vol. 1, pp. 125-169, Manchester, 1886. Marshall, A. M. '93. Vertebrate Embryology. London, xxiii + 640 pp., 255 fig. Marshall, A. M., and Spencer, W. B. '81. Observations on the Cranial Nerves of Soyllium. Quart. Jour. Micr. Sci., n. s., vol. 21, no. 83, pp. 469-499, pL 27. carpentek: development of the oculomotor kerve. 223 Martin, P. '90. Die erste Eutwiokelung der Kopfnerven bei der Katze. Oesterr. Monatsschr. f. Thierheilk., Jahrg. 15, No. 8, pp. 337-363 ; No. 9, pp. 885-396. Michel, [J.] '94. TJeber die feinere Anatoraie des Ganglion ciliare. Trans. 8th. Internat. Ophthal. Congr. in Edinburg, 1894, pp. 195-197. Mitrophanov, P. '92. Note sur la signification metatnerique des nerfs erSniens. Congres Internat. de Zool., Session 2, Moscou, Partie 1, pp. 104-111. Mitrophanow, P. '93. Etude embryogenique sur les Selaciens. Arcli. zool. exp. et gen. s^r. 3, torn. 1, pp. 161-220, pi. 9-14. Muck, F. '15. Dissertatio anatomica de ganglio ophthalmico et nervis ciliaribus ani- malium. Landishuti, 1815. 94 pp. 2 Tab. Nawrocki, F., und Przybylski, J. '91. Die pupillenerweilernden Nerven der Katze. Arch. f. ges. Physiol., Bd. 50, Hefte 5-6, pp. 234-277. Neal, H. V. '96. A Summary of Studies on the Segmentation of the Nervous System in Squalus aeanthias. Auat. Anz., Bd. 12, No. 17, pp. 377-391, 6 Kg. Neal, H. V. '98. The Segmentation of the Nervous System in Squalus acanthias. A Contribution to the Morphology of the Vertebrate Head. Bull. Mus. Comp. Zool. Harvard Coll., vol. 31, no. 7, pp. 145-294, 9 pi. Neal, H. V. :00. The Early Stages of Development of Ventral Nerves in Cyclostomes and Selachians. (^Abstract.) Science, n. s., vol. 11, no. 268, pp. 250-251. Neal, H. V. :03. The Development of the Ventral Nerves in Selachii. I. Spinal Ven- tral Nerves. Mark Anniversary Volume, New York, pp. 291-313, pi. 22-24. Onodi, A. D. '86. Ueber die Entwickelung des sympathischen Nervensystems. Zweiter Theil, Arch. f. mikr. Anat., Bd. 26, Heft 4, pp. 553-580, Taf. 23-27. Onodi, A. [^D.] ;01. Das Ganglion ciliare. Anat. Anz., Bd. 19, No. 5-6, pp. 118-124. Oppel, A. '90. Ueber Vorderkopfsomiten und die Kopfhohle von Anguis fragilis. Arch. f. mikr. Anat., Bd. 36, Heft. 4, pp. 603-627, Taf. 30. 224 bulletin: museum of comparative zoology. Phisalix, C. '88. Note sur le ganglion ophthalmique et la premiere cavity cepbalique che^ les poissons. Comp. rend, et mem. soc. biol., s^r. 8, torn. 5, Paris, pp, 666-667. Phisalix, C. '88». Note sur la nature des ganglions ophthalmiques et I'origine de la premiere cavity cepbalique cbez les Selaciens. Bull. Soc. Zool. France, Tom. 13, no. 7, pp. 177-180. Piatt, Julia B. '91. A Contribution to tbe Morphology of tbe Vertebrate Head, based on a Study of Acanthias vulgaris. Jour. Morpb., vol. 5, no. 1, pp. 79-112, pi. 4-6. Piatt, Julia B. '96. Ontogenetic Differentiations of the Ectoderm in Necturus. Study II. — On the Development of tbe Peripheral Nervous System. Quart. Jour. Micr. Sci., n. s., vol. 38, no. 132, pp. 485-547, pi. 36-38. Przybylski, J. See Nawrocki, F., und Przybylski, J. Quain's Anatomy. See Tbane, G. D. Rabl, C. '89. Theorie des Mesoderms. Morph. Jahrb., Bd. 15, Heft 2, pp. 113-252, 9 Fig., Taf. 7-10. R[amon y] Cajal, S. '91. Notas preventivas sobre la retina y gran simpatico de mamiferos. Gaz. Sauit., Barcelona, 1891. 16 pp., 7 fig. R[anion y] Cajal, S. '94. Les nouvelles id^es sur la structure du Systeme nerveux chez I'homnjp et chez les vert^bres. Paris, xvi + 200 pp., 49 fig. Rath, O. vom. '95. Zur Conservirungstechnik. Anat. Anz., Bd. 11, No. 9, pp. 280-2^)?. Remak, R. '51. Untersuchungen iiber die Entwickelung der Wirbelthiere. I. I/pber die Entwickelung des Hiihnchens im Eie. Berlin, 194 + xxix pp., 7 Taf, Retzius, G. '81. Untersuchungen fiber die Nervenzellen der cerebrospinalen Gftngli/m und der ubrigen peripherischen Kopfganglien mit besonderer Riicksysht auf die Zellenauslaufer. Arch. f. Anat. u. Physiol., Jahrg. 1880, qpat Abt., pp. 369-402, Taf. 17-22. Retzius, G. '94. Ueber das Ganglion ciliare. Anat. Anz., Bd. 9, No. 21, pp. 633-637, 1 Fig. CARPENTEE: DEVELOPMENT OF THE OCULOMOTOR NERVE. 225 Retzius, G. '94 . Ganglion ciliare. Biol. Unters., n. f., Bd. 6, pp. 37-40, Taf. 20. Reuter, K. '97. Ueber die Bntwickelung der Augenmuskulatur beim Scliwein. Anat. Hefte, Bd. 9, Abt. 1, pp. 365-387, Taf. 27-28. Rex, H. :00. Ziir Entwicklung der Augeamuskeln der Ente. Arch. f. mikr. Anta., Bd. 57, pp. 229-271, 2 Fig., Taf. 13, 14. [Separate issued iii 1900.] Rochas, F. '85. Du mode de distribution da quelques filets sympatliiqaes intra-craniens, et de I'existenoe d'une racine syrapathique da ganglion ciliare chez I'Oie. Comp. Rend., Paris, Tom. 101, pp. 829-831. Rombergf, E. See His, Jun., W., und Romberg, E. Rubaschkin, W. :03. Ueber die Beziehungeu des Nervus trigeminus zur Rieolisclileimhaut. Anat. Anz., Bd. 22, No. 19, pp. 407-415, 4 Fig. Schacher. 1701. De cataraota. Schafer, A. E. '81. Note on tlie Occurrence of Ganglion Cells in tbe Anterior Roots of the Gat's Spinal Nerves. Proc. Boy. Soc. Loud., vol. 31, no. 209, p. 348. Schaper, A. '97. Die friihesten Differenzirungsvorgange im Centralnervensystera. Kriti- sche Studie und Versuch einer Gescliiohte der Butwiokelung nervoser Substanz. Arch. f. Entwick.-mech., Bd. 5, Heft 1, pp. 81-132, 17 Fig. Abstract in, Science, n. s., vol. 5, no. 115, pp. 430-431. Schneider, A. '79. Beitrage zur vergleichenden Anatomie und Entwickhmgsgeschiohte der Wirbelthiere. Berlin, viii + 164 pp., 16 Taf., 3 Fig. Schwalbe, G. '79. Das Ganglion oculomotorii. Ein Beitrag zur vergleichenden Anatomie der Kopfnerven, Jena. Zeitschr. f. Naturwiss., Bd. 13, pp. 173-268, Taf. 12-14. Also separate, Jena, 1879. 96 pp., Taf. 12-14. Schwalbe, G. '79". Ueber die morphologische Bedeutung des Gfanglion ciliare. Sitzungsb. Jena. Gesell. f. Med. a. Naturwiss. f. 1878, pp. xc-xciii. Schwalbe, G. '81. . Lehrbuch der Neurologic. Hoffmann's Lehrbuch der Anatomie des Menschen, Aufl. 2, Bd. 2, Theil 2, Erlangen, pp. i-vi + 287-1026, Fig. 187-504. VOL. XLViii. — No. 2 15 226 bulletin: museum of comparative zoology. Sedgwick, A. '94. On the Inadequacy of the Cellular Theory of Development, and on the Early Development of Nerves, particularly of the Third Nerve, and of the Sympathetic in Elasmobraiichii. Quart. Jour. Micr. Sci., n. s., vol. 37, no. 145, pp. 87-101. Selenka, E. See Gadow, H., nnd Selenka, B. ' Sewertzoff, A. [N.] '95. Die Entwickelung der Occipitalre^on der niederen Vertebraten im Zusammenhang mit der Frage iiber die Metamerie des Kopfes. Bull. Soc. Imp. Nat. Moseou, ann6e 1895, n. s., torn. 9, no. 2, pp. 186-284, pi. 4, 5. Sewertzoff, A. N. '98-99. Studien zur Entwickelungsgescliiclite des Wirbelthierkopfes. I. Die Metamerie des Kopfes des electrisehen Rochen. Bull. Soc. Imp. Nat. Moseou, ann6e 1898, n. s., torn. 12, no. 2-3, 4, pp. 197-263, 393-445, 5 fig., pi. 1-4. Sheldon, Lilian See John.ion, Alice, and Sheldon, Lilian. Spencer, W. B. See Marshall, A. M., and Spencer, W. B. Stannius, H. '49. Das peripberisclie Nervenystem der Fische, anatomisch und pLysiolo- gisoh untersucht. Rostock, iv + 156 pp., 5 Taf. Stannius, H. '51. Uber den Bau der Muskeln bei Petromyzon fluviatilis. Nachr. Univ. u. Gesellsch. Wissensch. Gottiugen, 1851, No. 17, pp. 225-235. Stefani, U. :01. Se all' atropinizzazione dell' occhio succedano modificazioni nelle cellule del ganglio ciliare. Atti. Beale Istit. Veneto sci. lett. ed arti, tom. 60, pt. 2, pp. 393-408. Review in Neurol. Centralbl., Jahrg. 20, 1901, no. 16, pp. 751, 752. Strong, O. S. '90. The Structure and Homologies of the Cranial Nerves of the Amphibia, as determined by their Peripheral Distribution and Internal Origin. Zool. Anz., Jahrg. 13, No. 348, pp. 598-607. Thane, G. D. '95. The Nerves. Quain's Elements of Anatomy. Ed. 10, vol. 3, part 2. London, pp. 231-403, fig. 140-241. Thompsen, R. '87. Ueber eigenthiimliche aus veranderten GanglienzeUen hervorgegangene Gebilde in den Stammen der Hirnnerven des Menschen. Arch. f. path. Anat. u. Physiol., Bd. 109, Heft 3, pp. 459-465, Taf. 12. CARPENTER : DEVELOPMENT "OF THE OCULOMOTOR NERVE. 227 Timoteew, D. '98. Beobachtungen iiber den Bau der Nervenzellen der Spinalganglien und des Sympathicus beim Vogel. luteniat. Monatssclir. f. Anat. u. Phvsiol., Bd. 15, Heft 9, pp. 273-281, Taf. 15. •\^ignal, W. I '83. M^moire sur le developpement des tubes nerveux chez les embryons des raammiferes. Arcb. de physiol. norm, at path., ser. 3, torn. 1, pp. 513-535, pi. 10, 11. Volkers, C. See Hansen, V., und Volkers, C. Wiedersheim, R. '98. Grundriss der vergleiohenden Anatomie der Wirbelthiere. Aufl. 4, Jena, xxiii -f 559 pp., 361 Kg., 1 Taf. Wijhe, J. W., van. '82. Ueber die Mesodermsegmente und die Entwickelung der Nerveu des Selachierkopfes. Natuurk. Verb, koninkl. Akad., Decl. 22, Amsterdam, 50 pp., 5 Taf. Wijhe, J. W., van. '86. Ueber Somiten und Nerven im Kopfe von Vogel- und Reptilien- embryonen. Zool. Anz., Jahrg. 9, No. 237, pp. 657-660. Zimmermann, [W.] '91. Ueber die Metamerie des Wirbelthierkopfes. Verb. Anat. Gesell., Versamml. 5, in Miinchen, pp. 107-114. 228 bulletin: museum of comparative zoology. EXPLANATION OF PLATES. All figures are from preparations of chick embryos except when otlierwlse stated. All, with the exception of the photomicrographs and diagrams, were drawn with the aid of the camera lucida. The magnifications follow the descriptions of the figures. Abbhbviations. cl. ...... , Indifferent cell. ■cL' Indifferent cell. (See explanation of Fig. 16.) cl." Indifferent cell. (See explanation of Fig. 16.) 'cl. coinit " Accompanying '' cell. ^ lirs. incubation), a, $, y. Ganglion cells in ophthalmic branch of trigeminal nerve ; a, near Gasserian ganglion ; $, opposite ciliary ganglion in position marlsed i8 in Figure 25; 7, more distal, in position marlced y in Figure 25. S, Ganglion cells in ciliary ganglion. Fixed in Zenker's fluid and stained with iron hematoxylin. X 1400. Carp'Enter— OculomotorWerve IB Chick. Plate 3. par.ms e.v. ®0 V^ ..m'6Z." f '. /*\"~ /> : cl.comit. f ' ■ !: '\*J '•■vel.marn. ,T^$^ ;V\ 'ms'ench. ■n'hi: mb.lim.ex. cl.med.miy. A-i\ ■,vel.marg. -d.comil! jjar.nis e.t'. W.^-. ^^ y ,^- .//: |i o ^' V, cl" (¥. /» ^v 1/ " #i f? nUjc'mot. ins'euch. d.comit" <> d.comit. EW.G del RELIOTyPE CO., BOSTON. Caopenteb. — Oculomotor Nerve iu Chick. PLATE 4. Fig. 11. Section transverse to longitudinal axis of mid-brain of an embryo in Stage III (88 hrs. incubation), showing root of oculomotor nerve. Fixed in corrosive-acetic mixture and stained with iron lisematoxylin. X 600. Fio. 12. Keconstruction from seven consecutive parasagittal sections througli an embryo in Stage III (93 hrs. incubation), viewed from left side, and showing longitudinal section through distal portion of ophthalmic branch of trigeminal nerve. Fixed and stained in vom Rath's fluid. X 66. Fig. 13. Diagram of relations between fundament of ciliary ganglion and oph- thalmic branch of trigeminal nerve. Beconstructed from 28 consecu- tive sections transverse to longitudinal axis of mid-brain of an embryo in Stage IV (100 hrs. incubation). In the Figure dorsal is up, median to the left. Ganglion cells a, fl, 7, S, f, correspond to ganglion cells with the same designations in Figure 16 (PI. 5). X 200. Fig. 14. From an embryo pig. A, Proximal end of neuraxon of oculomotor nerve. B, Longitudinal section through oculomotor nerve midway in its course. Fixed in corrosive-acetic mixture and stained with Brazilin. X 733. CA5lPE:NrEH- OCJLCMCTCR IT W^''^ ..nidl.oc'mot. ,-/• v-_ -cr. eC f. ■trt.fbr.v. < .-T .;'r4'' >i (^5^ II n odrnot. ^^ ms'fnrh. rm.opth.trig.^ :k\ pdl.opt. 12 v.crd.a. 14 TOfc.Zim.KK. rm.opth.trig n.oc^mot i — 1^ rn. ^ 19 n.ahd. nidl.oc'mot. ! ti ttO^ooO^-OOo^ 20 ms'ench. n.oc'mot. o tri.fbr.v. F, F, FW.Cdel HELIOTYPe CO., B031 Cabpenteb. — Oculomotor Nerve in Chick. PLATE 7. All the Figures of this Plate are from photomicrographs. Fig. 21. Transverse section of oculomotor nerve of fowl corresponding to A Figure 3 (PI. 2). X 86. Fig. 22. Transverse section of branch of oculomotor nerve of fowl supplying dorsal rectus muscle. X 86. Fig. 23. Section frontal to hind-brain of an embryo in Stage II (70 hrs. incu- bation), showing connection of maxillo-mandibular portion of Gas- serian ganglion with ectoderm, and fundament of posterior rectus muscle. Stained in haemalum. X 266. Fig. 24. Section transverse to longitudinal axis of mid-brain of an embryo in Stage III (88 hrs. incubation), showing longitudinal section through entire length of oculomotor nerve. Fixed in corrosive-acetic mixture and stained with iron hematoxylin. X 44. Fig. 25. Parasagittal section of an embryo in Stage V (119^ hrs. incubation), viewed from right side, and showing longitudinal section through communicating ramus between ophthalmic branch of trigeminal nerve and ciliary ganglion. For significance of /8 and y, see explanation of Figure 10 (PI. 3). Fixed in Zenker's fluid and stained with iron hasmatoxylin. X 173. Fig. 26. Parasagittal section through an embryo in Stage V (119^ hrs. incuba- tion), viewed from right side, and showing oculomotor nerve, ciliary ganglion, abducent nerve, and fundaments of eye muscles. Fixed in Zenker's fluid and stained with iron liEematoxylin. X 40. Note. — By an oversight the /)Ziis sign (+) has been omitted from the Plate in the abbreviation mu. rt. v. + a. 23 ee'ih-m. gn.i,i:r-mil.Gas. Begiiin of smnU iiPvra.rdH m.c HELioTye CO., eosTOK, The following Publications of the Museum of Comparative Zoology are in preparation : — Reports on tlie Kesults of Dredging Operationa in 1877, 1878, 1879, and 1880, in oliarge of Alex- ANDISB AOASBiz, by tlie 0, S. Coast Survey Steamer " Blake," as follows: — H. AUGENElt. The Annelids of the " Blake." C. HARTI/AaB. The Oouiatulae of tlie "Blake," with 15 Plates. H. L0DWIG. The Genus Pentaorinus. A. MILNE EDWARDS and E. h. BOUVIBR. The Crustacea of the " Blake." A. B. VEltltlLU The Aloyonaria of the " Blake." Reports on the Scientific Results of the Expedition to the Tropical Pacific, in charge of Alexander Agassiz, on theU.,S. Fish Commission Steamer '* Albatross," from August, 1899, to March, 1900, Commander Jefferson P. Moser, U. S. N., Commaudiug. LOUIS CABOT. Immature State of the Odonata, Part IV. £, L. MARIC. Studies on tjepidoBteus, continued. ' " ^ On Aradinactis. , R. T. HILL. On the Geology of the Windward Islands. W. MoM. WOODWORTH. Ou the Bololo or Palolo of Fiji and Samoa. AGASSIZ and WHITMAN. Pelagic Fishes. Part II., with 14 Plates. Reports on the Results of the Expedition of 1891 of the U. S. Fish Comnilsslon Steamer "Albatross," Lieutenant Commander Z. L. Tanbek, U. S. N, Commanding, in charge of Alexander Agassiz, aa follows: — A. AGASSIZ. The Pelagic Fauna. ' S. J. HICKSON. The Antipathids. The Pauamicl)eep-Sea Fauna. J. P. McMURRlCH. The Actinarians. H. B BIGELOW. The Siphonophores. , E. L. MARK. Branohioeerianthus. K. BRANDT. The Sagittae. JOHN MURRAY. 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