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 ORIGIN OF THE ELECTRIC TISSUES OF GYMNARCHUS 
 NILOTICUS. 
 
 By ulric dahlgren 
 
 Professor of Biology, Princeton University. 
 
 Nine plates and nine text-figures. 
 
 [Extracted from Publication No. 183 of the Carnegie Institution of Washington, 
 pages 159-194. 1914. 
 
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Digitized by tine Internet Arcliive 
 
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 http://www.archive.org/details/originofelectricOOdahl 
 
ORIGIN OF THE ELECTRIC TISSUES OF GYMNARCHUS 
 NILOTICUS. 
 
 By ulric dahlgren 
 
 Professor of Biology, Princeton University. 
 
 Nine plates and nine text-figures. 
 
7>I3 
 
 5f2 
 
ORIGIN OF THE ELECTRIC TISSUES OF GYMNARCHUS NILOTICUS. 
 
 By Ulric Dahlgren. 
 
 In the seven types of electricity-producing fishes the exact development 
 by which the electric organs and tissues are produced during the creature's 
 life is known in only two, leaving the other five unknown. It also happens 
 that the two forms which have been studied as to the histogenesis of their 
 electric organs are the only two elasmobranch fishes among the seven, so that 
 we have not as yet seen how the remarkable electric tissues in Malopterurus, 
 Gymnotus, Astroscopiis, the mormyrids, and Gymnarchus are developed. Also, 
 of the five teleost types we know the structure of the full-grown electric organs 
 in all of them pretty well, except in Gymnarchus. This fish is found in Africa 
 and has been rather rare, so that but two workers have published observa- 
 tions on it, both of them a long time ago and from poorly preserved material. 
 
 It was, therefore, with great pleasure that the writer came into possession 
 of some embryos of this rare form through the kindness of Dr. J. Graham 
 Kerr, Dr. Arthur Shipley, and Dr. Richard Assheton, to whom he wishes 
 to express his most sincere thanks. This material was collected in Africa 
 by Dr. Samuel Budgett some years ago and was in most excellent condition, 
 owing to the great care and skill with which Dr. Budgett put it up and cared 
 for it. The collecting was done unflinchingly and faithfully, under con- 
 ditions of hardship and sickness that few white men could stand, and Dr, 
 Budgett lost his life from exposure and illness incurred in part by this work. 
 A full account of his trip and of the scientific results should be read in the 
 Budgett memorial volume issued by Dr. Shipley, Dr. Kerr, Dr. Assheton, 
 and others in 1907, through the Cambridge University Press (24).' 
 
 It is somewhat unfortunate that the structure of the electric organs in 
 the adult fish could not be worked up at the same time that this paper was 
 written, but the writer has material on the way from Khartoum and hopes 
 to publish a second paper shortly. 
 
 But three papers have been published on the electric organ of this inter- 
 esting fish, one by Erdl (15) in 1847 and another by G. Fritsch (19) in 1885. 
 Riippel's publication on the subject could not be found, but Fritsch states 
 that Riippel mentioned the peculiar structures which we are considering, 
 and so he stands at present in the writer's knowledge as the first one to have 
 seen and reported to science the electric organs of this fish, although he 
 was in doubt as to their significance. Erdl used a specimen which was so 
 
 ^ The figure3 in parenthesis refer to the literature cited, p. 193. 
 
I62 
 
 Papers from the Marine Biological Laboratory at Tortugas. 
 
 poorly preserved and so soft that when he cut the animal across its body, 
 the electroplaxes and connective tissue ran out of the muscle in which they 
 are embedded. Nevertheless Erdl stated that it was probably an electric 
 organ, coming to this conclusion by a comparison, of such features as he 
 could make out, with the structures found in the tails of the other electric 
 fishes, as Raja, Mormyrus, and Gymnotus. 
 
 
 Fig. I. — General view of Gymnarchus nilolicns. (Drawn from a 
 lantern slide made from a figure in Jordan's "Guide to the Study 
 of Fishes," New York, 190s.) 
 
 Fritsch (19) in 1885 worked on better material and gave a more com- 
 plete account of the anatomy of the organ, especially of the histology of the 
 electroplax; but he came to the rather strange conclusion that it was not 
 an electric organ at all, assuming the erroneous position that, on account 
 of the large blood-supply, the organ acted in some way as a storage for 
 oxygen during the period of hibernation made necessary by the drying of 
 waters at certain seasons. Fritsch was also mistaken in calling the fibrous 
 
 Fig. 2. — Scene drawn from descriptions to illustrate the habitat of Gymnarchus and its manner 
 of swimming. (Drawn from lantern slide made from a drawing, by Bruce Horstall.) 
 
 contents of the electroplaxes "connective tissue." He went a great deal 
 farther than Erdl, however, in describing the gross anatomy of the electric 
 organs and surrounding tissues. 
 
 Not having full-grown material, the writer must rely on Fritsch's figures 
 and descriptions for the adult gross anatomy, although the oldest embryos 
 
Origin of Electric Tissues of Gymnarclius Niloticus. 
 
 163 
 
 Fig. 3. — Section through mid-tail region 
 of body of an adult Gyjmiarchus, 
 showing position of the eight electric 
 spindles and their relations to sur- 
 rounding muscular and bony struc- 
 tures. D, dorsal spindles; U.M„ 
 upper median spindles; L.M., lower 
 median spindles; F, ventral spindles. 
 (After Fritsch.) X unknown. 
 
 (about 42 days) used for this paper represent practically adult material 
 so far as the histology of the electroplaxes is concerned. A short resume 
 of the adult anatomy will make a good basis for the embryological descrip- 
 tions to follow. 
 
 The fish (see text-fig. i) is a mormyrid of elongate form, so much so as to 
 make it almost eel-shaped, although not quite so much so as the Gymnotus 
 of South America. It possesses an extensive development of the dorsal 
 fin, which extends from forward on the neck to within a short distance 
 of the tip of the tail, where it suddenly stops, leaving the tip of the tail 
 naked of fin; whence the name of the fish, Gymnarchus niloticus. This 
 heavy fin, well provided with a 
 series of lateral ray-muscles, is 
 used extensively by the fish as a 
 means of propulsion, by holding 
 the body straight and stiff and 
 causing a series of lateral undu- 
 lations to pass from behind for- 
 ward, thus driving the body bsxk- 
 ward (see text-fig. 2) or, from front 
 to rear, which causes the fish to 
 move with its head forward. I 
 have not heard, but I presume 
 that in moments of unusual effort 
 the animal can swim by means of 
 the common sinuous body-move- 
 ments used by other elongate fishes, 
 as is well illustrated in the eel. 
 
 The posterior tip of the creature's body is interesting. As has been 
 mentioned, this end is free from the fin for some distance (see text-fig. 2). 
 Also it is round in section and ends bluntly. When swimming backwards 
 the animal uses it like a finger to feel its way. The peculiar round and 
 blunt end may be explained by the fact that this tip contains the largest and 
 best-developed portion of the electric organ, which fills the lateral parts of 
 the body at this point almost to the exclusion of the ordinary muscle. 
 
 As Erdl and Fritsch have described, the electric organ consists of eight 
 long "tube-like" or cylindrical structures, four on each side, embedded in 
 the muscle tissue as close to the median bony parts as a little connective 
 tissue in between will permit. Four of these are present on each side 
 (see text-figs. 3 and 4), and they may be called in order from above down- 
 ward, the dorsal, the upper middle, the lower middle, and the ventral cylinders, 
 or spindles, of the electric organ. In a section cut through the body at a 
 point midway between tail-point and anus (see text-fig. 3) the dorsal spindles 
 are to be found, just above the union of the neural spines of the vertebrae, 
 and set closely together with only the dorsal spine and some connective tissue 
 between them. The upper median spindles are more widely separated by 
 
1 64 
 
 Papers from the Marine Biological Laboratory at Tortugas. 
 
 the neural canal and its contained spinal cord. This second pair of spindles 
 are at the level of the upper part of the spinal cord. The lower median 
 spindles are found much below, on each side of the ventral processes of the 
 vertebra. They lie at about the level of the caudal artery and almost as 
 widely separated as the upper median pair. Lastly, the ventral pair of 
 spindles are found just below the latter pair and slightly below the level of 
 the caudal vein, which lies between them. They are separated a little less 
 than the lower median pair by the narrower bony structures at this point. 
 
 ,Upper median 
 Dorsal 
 
 Ventral Lower median 
 
 Fig. 4. — Diagram showing longitudinal position and extent of electric spindles in Gymnarchus. 
 Those of one side only can be shown in figure. 
 
 The four spindles are largest in diameter in the tail, especially out in the 
 thick, finger-like, naked extremity. From this part they taper to a smaller 
 size as they go forward in the body and they finally become thin and end at 
 points in the neighborhood of the anal opening (see text-fig. 4). All are not 
 of equal length. As Fritsch has shown, the dorsal organ is the shortest and 
 extends for about 20 cm. in a fish of 89 cm. length. The ventral pair reach 
 for about 5 cm. further, or 25 cm. in length in a fish of the same size. The 
 two median spindles reach for about 40 cm. from the tip of the tail. 
 
 Each spindle is marked off clearly from the neighboring muscle, and other 
 tissues which are found next to it, by a distinct connective-tissue covering. 
 In my largest specimen, which is a young fish of 40 days, this is well shown 
 and is exactly like other dividing connective-tissue sheaths that surround 
 the various muscle divisions. Like them, it often contains pigment cells 
 which show golden-brown pigment granules. 
 
 The important contents of these spindles are alternate, cylindrical 
 segments of a denser, deeper-staining, muscle-like substance, the electro- 
 plaxes; and a connective tissue of jelly-like, grayish transparence which in 
 all ways appears to be similar to the "electric connective-tissue" found 
 between the electroplaxes in the other electric fishes. In this tissue are 
 found the blood-supply, which is largely in contact with the ends of the 
 electroplaxes, principally the anterior end; and the nerve-supply of thick, 
 medullated fibers which run towards, and are attached to, the posterior 
 ends of these organs. In this case, according to both Pacini's law and the 
 fish's relationship to Mormyrus, the direction of the current at time of 
 discharge should be from tail towards the head. I have been unable to 
 learn, from the literature of travelers and scientific collectors and observers, 
 if the electric discharge is strong enough to be felt by the hand. Budgett 
 does not mention it, and no other does, so I conclude that it is not a strong 
 shock and that the organ must be classed with the weak electric organs, 
 
Origin of Electric Tissues of Gymnarchus Niloticus. 165 
 
 as is the case with its relatives, the various mormyrids. The natives of 
 Africa are much afraid of the creature, especially at nesting time, and one 
 of its Arabian names, "Abu rhad" meaning "father of thunders," might 
 seem to indicate perceptible electric powers. 
 
 The embryos and young fishes put at my disposal by Dr. Kerr and Dr. 
 Assheton were five in number, and of these three were the suitable stages 
 from which this paper was worked out. The significant development of 
 the electric tissues in Gymnarchus takes place between the ninth day of 
 embryonic life, at which time the embryo possesses a fully formed and 
 complete musculature in the tail with no sign of an electric organ, and the 
 fortieth day of development, at which time it can be seen that the embryo 
 has developed its electric organs, out of a certain part of the previous muscu- 
 lature in the tail, to a degree that shows the farthest advanced electro- 
 plaxes in a practically adult condition. 
 
 The most interesting and critical stages in this metamorphosis of muscle 
 into electroplax appear to take place within much closer limits, and stages 
 from the eleventh to the fifteenth day would include them. These signifi- 
 cant changes have been studied and drawn principally from an embryo 
 12 days of age, fixed in sublimate-acetic, and showing the changes very 
 much to my satisfaction. A point of interest and importance in this study 
 is that, in earlier embryos, the myotomes and electric spindles are youngest, 
 least developed, and growing fastest in the posterior part of the body or 
 nearest the tip of the tail; while in older embryos and in the adult the 
 greatest, most complete, and most characteristic development of the 
 electroplaxes is to be found in the end of the tail or at the posterior end of 
 the spindle. Thus the adult structures in the anterior part of the spindles 
 represent a somewhat inferior and less complete change of the muscle tissue 
 into electric tissue than the posterior parts of the same organs do. 
 
 The same importance attaches to the fact that the rates of development 
 of the several spindles seem to vary. The lower median spindle starts 
 first to differentiate, extends farthest forward in the body, is larger than the 
 others when developed, and during early development is always in advance 
 of the corresponding parts of the other spindles. The upper median spindle 
 closely follows the lower. The ventral spindle is much behind the two 
 median ones, while the dorsal spindle represents the latest and weakest 
 development and is shorter than any of the others. These facts have made 
 it possible in the present study to get many stages of development from 
 very few embryos. 
 
 STUDIES OF AN EMBRYO NINE DAYS OF AGE. 
 
 This little fish was 26 mm. in length and, while the egg-membrane had 
 been ruptured and cast away, the animal was still forced to remain in its 
 nest because of its huge, elongate yolk-sac, still unabsorbed, and because 
 of its otherwise undeveloped organs of alimentation, locomotion, etc. 
 The posterior part of the body was carefully cut into four portions (see 
 
i66 Papers from the Marine Biological Laboratory at Tortugas. 
 
 text-fig. 5) to be sectioned as follows: First, from in front of the anus to a 
 point about 26 vertebral segments posteriorly. The last three segments 
 of this portion were sectioned transversely and serially (region A-^ while 
 the remaining anterior part was sectioned vertically and longitudinally 
 (region A). Another portion of 19 vertebral segments further caudad was 
 removed and its posterior two segments sectioned transversely (regional), 
 while the anterior part was cut as before in vertical, longitudinal sections 
 
 Fig. 5. — Outline of body of an embryo of Gymnarchus nine days old. Transverse 
 lines and letters indicate parts sectioned for study. For explanations see 
 text. (Copied from Assheton in "The Work of John Samuel Budgett.") X S. 
 
 (region B). A third part, of 18 more vertebral segments, was treated in 
 the same way, except that no transverse sections were taken and the entire 
 piece was cut vertically and longitudinally (region C), while the remaining 
 portion or tail-tip was cut serially in transverse sections and forms a series 
 (region D). 
 
 Figure i, plate i, shows a transverse section through the body a rather 
 short distance from the extremity of the tail or at a point where we are sure 
 that the electric organ will be well developed a little later in life. It may 
 be thought that such a section could be taken for study to better advantage 
 in a more anterior position on account of the earlier anterior development 
 just discussed, but it must be taken into account that the tail segments are 
 being added and are still growing rapidly at this age, that they are very 
 short and very crowded, and therefore the location of this section is, in 
 reality, fairly well forward in the future electric spindles. Conditions were 
 much the same in region C. 
 
 This section shows a good development of muscle fibers as indicated 
 by the shaded area. As is usual in vertebrates, the most advanced stages 
 of muscle-cell development are to be seen at the lateral periphery of the 
 myotome. Here, at the point indicated in figure i , plate i , by the dotted line, 
 a layer of the outer muscle cells, two or three deep, has acquired an average 
 of about 26 myofibrils (an average of 14 counts). These are grouped in one 
 (fig. 2, plate I, B and C) or sometimes two bundles in the cell (fig. 2, plate i, 
 A) and their correct spacing and their thickness and staining power indicate 
 muscle cells of normal development and good functional activity. The 
 remaining and inner muscle cells of the myotome, forming its larger bulk, 
 show, as one examines them successively farther inward (toward the median 
 line), a series of earlier stages, until at many points on the inner edge of the 
 
Origin of Electric Tissues of GyinnarcJms Niloticus. 167 
 
 myotome the smallest cells are seen with large nuclei and no myofibrils at all 
 (fig. 2, plate I, C). These youngest cells are particularly abundant at the 
 dorsal and ventral edges of the myotome. 
 
 From what we know of the position of the future electroplaxes and their 
 relation to the muscle-masses, we can be sure that it is from the inner edge 
 of the myotome that the electric tissue is to come, and a close scrutiny of 
 the cells which form this edge shows that at two points only is there any 
 indication of such a development. 
 
 One of these points, marked with a circle ( © ) in figure i, plate i, is v/here 
 the two myotome segments (dorsal and ventral) m^eet and close in against 
 the notochord. Here we see, in some of the sections, several especially 
 large and strongly developed fibers, somewhat detached from the rest of 
 the myotome and resting against the vertebral disk. Several reasons exist, 
 however, why these fibers do not represent the future electric organ. First, 
 they are in the exact median position which remains constant during growth 
 and in which no electric tissue is to appear. Second, they are very short 
 and are not attached to each other longitudinally, as other fibers are, by 
 means of connective tissue, but are attached to the bodies of the future 
 vertebra. They may be called the vertebral fibers. 
 
 A second point can be seen where some muscle-tissue shows unusual 
 development. At this point (marked with a circle (O) in fig. i, plate i) are 
 several muscle cells which show from 10 to 18 myofibrils each, and a degree 
 of development almost equal to the cells in the outer layer. These cells 
 are represented as seen under high magnification in figure 3, plate i. They 
 are, I believe, destined to be the future electric cells, several of which will 
 unite and form one of the electroplaxes of the lower median spindle of the 
 electric organ. I base this assertion only on their position and their some- 
 what advanced development as muscle fibers, for they show at this time no 
 indication, other than their size, of developing into electric tissue. 
 
 Their position is slightly too far dorsal for the lower spindle, in an adult 
 Gymnarclms, but when we examine the 12-day-old and 42-day-old stages, 
 we find that the normal growth of the muscle-mass will carry this point to 
 exactly the proper position for that spindle to lie at, in the grown fish. 
 
 An important point in studying these cells in figure 3, plate i, is to note 
 that they are separated from the rest of the myotome and from one another 
 by other muscle cells of weak or of earlier development and containing only 
 a few myofibrils. These weaker cells, and even some of the more advanced 
 ones, are destined to degenerate during the development and growth of 
 the electroplax. In figure 3, plate i, 7 cells are present that will probably 
 take part in the formation of the electric spindle at this point. Some of 
 them are contiguous and others are widely separated. 
 
 In figure 4, plate i, we have a fortunate longitudinal section from the 
 adjacent region of this same embryo. The section shows in longitudinal 
 view, and under low magnification, the same group of muscle cells marked 
 with the circle (O) at both ends. Also, it shows a portion of the ventral 
 
l58 Papers from, the Marine Biological Laboratory at Tortugas. 
 
 part of the myotome with its strongly developed outer layer {o.f.). The 
 vertebral fibers mentioned above are not visible, but their position in a few 
 following sections is indicated by a line one can imagine to be drawn 
 between the points marked with the figure © . It has been considered 
 unnecessary to figure the myofibrils longitudinally under large magni- 
 fication. The cross-striation is very plainly visible and is the same in all 
 parts. Each of several regions of this embryo was examined and in all its 
 parts were the same conditions found. We may sum up by stating that the 
 embryo of 9 days age shows no electric tissue and but a very weak indication 
 of the development of such a tissue, all of the myotome cells being decidedly 
 of the muscular type. Our only evidence is drawn from future stages, as 
 to the position of such electric tissues and from the otherwise unexplained 
 precocity of the fibers in our 9-day embryo, in that same position. 
 
 STUDIES OF AN EMBRYO TWELVE DAYS OF AGE. 
 
 This animal (text-fig. 6) showed a considerable increase in size, not so 
 much in length as in thickness. The posterior part of the body of this 
 specimen was divided into the following pieces: First, from a short dis- 
 tance behind the anus to 14 
 vertebral segments further 
 caudad. Two-and-one-half 
 segments were cut off of this 
 by serial, transverse sections 
 from the posterior end (re- 
 gion ^ — i) and the remaind- 
 er was cut into vertical, lon- 
 gitudinal sections from right 
 to left (region A). Second, 
 15 segments more were re- 
 moved and 3 segments of 
 the posterior end were again 
 cut as serial, transverse sections (region Bi) and the remaining 12 segments 
 were cut serially in horizontal, longitudinal sections (region B). Third, 14 
 segments were again cut off and the posterior 3 segments were cut trans- 
 versely (region Ci), while the remainder was cut in vertical, longitudinal sec- 
 tions (region C). The next 18 vertebral segments formed a piece that 
 was cut in vertical, longitudinal sections (region D), without any transverse 
 sections being made, and this left a small bit of somewhat curled tail-tip, 
 which was so young in development that its segments could not be easily 
 counted. This is cut transversely in series (region E). 
 
 Study of this embryo may best begin by examining a transverse section 
 through the posterior part of the body (fig. 5, plate 2) to see what has 
 become of the inner parts of the myotomes. Looking first for the point 
 L.M. as seen before in figure i, plate i (marked with O), we can see at once 
 that it is present as a compact mass of muscle-like tissue now clearly 
 
 Fig. 6. — Diagram of body of an embryo of Gymnarchtu 12 days 
 old. Lines and letters indicate the regions studied. For ex- 
 planations see text. (Copied from the same source as fig. 5.) 
 X about 3.2s. 
 
Origin of Electric Tissues of Gyninarchus Nilotictts. 169 
 
 separated from the rest of the myotome. Further, there are to be seen on 
 each side three other similar sections of the same muscle-like tissue, all 
 more or less also separated from the main mass of the myotome. The most 
 important part of our study now consists on the one hand in proving that 
 this mass L.M., in figure 5, plate 2, is derived from the undoubted muscle- 
 fibers (marked with O), as seen in figure i, plate i, and on the other hand in 
 showing that this same structure is to become the finished electric tissue as 
 seen in such advanced development as in figure 21, plate 8, for instance. 
 
 In tracing it back to the muscle, we are much assisted by the fact that 
 the dorsal and ventral spindles are always in an earlier or in a less complete 
 stage of development than the upper middle spindle, or, particularly, the 
 lower middle spindle under consideration. And, since in this embryo of 12 
 days the development has gone to considerable length, and since a slightly 
 younger stage, say a 10- or 11 -day embryo, was not included among the 
 embryos at my disposal, this fact is of much importance, because it will be 
 fair to take the left ventral spindle as an intermediate step in the compari- 
 son. Figure 9, plate 3, is a highly magnified section of the locality of the 
 left ventral spindle from region C in the embryo of 12 days, and in it we can 
 see a mass of muscle-like cells, closely associated and lying at the inner 
 edge of the myotome. Certain changes clearly differentiate them from the 
 rest of the muscle, however. One change is the fact that the myofibrils 
 have shown a large diminution in size or thickness and have also suffered in 
 power to take the stain ; particularly on the periphery of some of the muscle- 
 columns they almost refuse to take it. Their spacing is also irregular and 
 they show a distinct tendency to clumping together and, in some cells, to 
 get close to the nucleus or even to surround it. They can be readily com- 
 pared in figure 9, plate 3, with the well-developed young muscle cells just 
 outside and to the left of them. The dotted line marked XX indicates a 
 separation of the two. All of those to the right of this line show the con- 
 dition of the myofibrils mentioned above; those to the left show the usual 
 condition of muscle cells of this age in fishes. 
 
 A second characteristic of the changing muscle cells under discussion 
 is in their cytoplasm. It appears more abundant, although this may be 
 due to the smaller fibril bundles. But it also stains more heavily with 
 such stains as eosin, erythrosin, and orange G. With the eosin, for example, 
 it also shows a more yellowish tinge than the cytoplasm of the usual muscle 
 cells in the same sections. And lastly, some of the muscle cells seem to 
 have entirely disappeared or to have greatly shrunken. This latter fact 
 causes a loose and separated condition to obtain among the metamorphosing 
 cells which shows in sharp contrast (fig. 9, plate 3) to the compact con- 
 dition seen in the typical muscle cells to the left of or outside of the line XX. 
 
 Another important fact can be seen among the changing cells in figure 
 9. Those near the center of the group show a tendency to touch or 
 coalesce with each other. Already at this early stage this marks a differ- 
 ence in the group. Those cells within the dotted circle are destined to 
 
I/O Papers from the Marine Biological Laboratory at Tortugas. 
 
 unite with one another in a compact bundle to form the future electroplax; 
 while those of the group which lie outside of the dotted ring are to degenerate 
 and atrophy altogether, in order to make room for the growing electroplax. 
 Just mediad of, or to the right of, the dotted ring is seen a cell that has 
 entirely lost its myofibrils and whose cytoplasm is vacuolated and about to 
 be absorbed. At other points, to the left, still other cells can be seen that 
 have dwindled to a smaller size than any of the normal muscle cells. The 
 cells within the ring are judged by the writer to be those which will form 
 the electroplax at this point, because they are starting to unite and, also, 
 because they occupy the position at which the electroplax will lie. Another 
 reason is that their myofibrils are weakening and clumping. Some of those 
 cells lying inside of the ring may also atrophy. This can not be infallibly 
 judged; but certainly all of those outside of it and to the right of the line 
 XX are about to degenerate and are not found in later stages. 
 
 The connective tissue creeps into the neighborhood of and among these 
 metamorphosing muscle cells at this time, and good mitotic figures can be 
 seen, showing that it is increasing the number of its cells. It does not, 
 however, penetrate the groups of future electric cells inside the dotted line. 
 Also blood, pigment, etc., are to be seen in characteristic positions. 
 
 A transverse section of the body at region Bi need not be illustrated at 
 this point by a low magnification figure, because it is so like figure 5, plate 2, 
 in general appearance. But it happens that in such a section several very 
 interesting stages, forming a sequence of which figure 9, plate 3, can be taken 
 as the first member, were noticed, and figure 10, plate 3, is the second in 
 this series. This drawing represents the right ventral spindle in region Bi, 
 and a number, five to be exact, of the transforming cells can be seen here in 
 closer union than the corresponding cells were in figure 9, plate 3. One or 
 possibly two of these may disintegrate a little later. A dotted ring is not 
 necessary, because the connective-tissue cells have partly marked off the 
 electric cells, and outside of this incomplete ring and above it can be seen 
 two muscle cells which are atrophying. A third cell is shown in a final 
 stage of disintegration. Its cytoplasm is almost clear, or all gone, and its 
 myofibrils have united in a single lump, which will soon become a round 
 droplet or cell-inclusion that will afterwards disappear. 
 
 Moving to the right upper middle spindle (fig. 1 1 , plate 3) , we need very 
 little explanation to see how the four or more muscle cells that first com- 
 posed this structure have come into a still closer union. Below and to the 
 right (outer) are still seen some of the degenerating muscle cells — five in 
 this figure. 
 
 ■ Several important points must be discussed in connection with this 
 figure; the component cells, as seen here, are not simple, single muscle 
 fibers. The lower one can easily be seen to have two myofibril bundles as 
 well as two nuclei. The presence of two widely separated fibril bundles, 
 as well as the large size of the cell, makes it nearly certain that the structure 
 was formed by the coalescence of two muscle fibers. In the upper region 
 
Origin of Electric Tissues of Gymnarchus Niloticus. 171 
 
 of the future electroplax, however, a large cell is seen which has only one 
 fibril bundle and four nuclei. It is possible here that this was one cell and 
 that it is growing in size and multiplying its nuclei by amitotic division. 
 As this is the same process which goes on in ordinary muscle cells of this 
 age, it is not surprising to find it going on here, and in older specimens we 
 shall find it the rule. The cytoplasm of all these electric cells is abundant 
 at this stage and is dense staining with the acid dyes. 
 
 As compared with figure 11, plate 3, the next illubtration, figure 7, plate 
 2, is most interesting and is a step of some magnitude in the development 
 of the electroplax. The cytoplasm of such cells as compose this young 
 electroplax is all united into a single mass and the relation of nuclei to fibril 
 bundles is completed. The nuclei are always peripheral and the fibril bun- 
 dles appear to form a single central mass. This is the permanent condition 
 which will obtain throughout the life of the organ, and is also the condition 
 common to some other electroplaxes, as, for instance. Raja and Mormyrus. 
 
 Just how the several fibril bundles become massed as a single bundle is 
 not to be positively stated at this time. The individual bundles can hardly 
 be imagined as moving together through the cytoplasm. It is probable 
 that some of the several bundles as seen in figure 11, plate 3, are lost and 
 absorbed, but it is not probable that all but one are so removed. The central 
 bundle in figure 7, plate 2, looks large enough to be composed of several, 
 such as are seen in figure 11, plate 3. The best explanation is that several 
 remaining bundles are moved toward each other by growth currents in the 
 cytoplasm, or by the absorption of material which lies between them. At 
 the same time, of course, the peripheral cytoplasm is growing in mass and 
 all nuclei tend to remain in this external layer. We shall see later that a 
 very few nuclei are left behind in the central fibrous mass in some electro- 
 plaxes. 
 
 In figure 7, plate 2, we see, plainly and indubitably, the first form of 
 the electroplax as found in older fishes. 
 
 The connective tissues which surround the electroplax are becoming 
 more decided in figure 7, plate 2. Also blood-vessels and pigment-cells are 
 oftener seen as in this drawing. One muscle cell, with its myofibrils clotted 
 into several irregular masses and its nucleus in an advanced stage of dis- 
 integration, is seen just between the lower end of the pigment-cell and the 
 electroplax. 
 
 It will be well at this point to examine some of the longitudinal sections 
 of these early stages, in order to make clear several points which can not 
 be so well studied in the transverse views. 
 
 Figure 12, plate 4, is a low magnification picture (X 140) of 6 segments 
 of the caudal part of the body at region D in this 12-day embryo. Only 
 the dorsal part of the body is shown, where a fortunate slant of the section 
 has permitted the knife to pass through both the dorsal and the upper 
 median spindles at the same time. The drawing is an accurate projection 
 from three different sections, so that all parts of each spindle might be 
 
172 Papers from the Marine Biological Laboratory at Tortugas. 
 
 shown, as well as their relations to the rest of the myotomes. Bracket 
 No. I (zone i) in this figure indicates the inner tips of the ventral halves of 
 the myotomes. Zone 2 shows a mass of connective tissue and nerve which 
 runs between these two halves (compare fig. 5, plate 2, Conn T.). Zone 3 
 embraces the inner tips of the dorsal myotomes where they lie between 
 the upper median spindle and the horizontal connective-tissue septum. 
 It will be noticed that this set of muscle fibers appears to be very short, 
 much shorter than the connective-tissue regions between them (not filled 
 out in this drawing). The reason can be seen at once when these same 
 muscle cells are examined under the high power (fig. 15, plate 5, bracket 3; 
 and fig. 6, plate 2), where it is apparent that they are degenerating fibers. 
 Parts of four of these fibers are shown in figure 15, plate 5, and that one 
 nearest the electric tissue which is represented in figure 15, plate 5 (brackets 
 4 and 4i), is the farthest gone, having lost its myofibrils altogether. The 
 nuclei are also distorted and, besides the single large nucleolus, are noticeably 
 empty of any chromatic matter or even of linin (notice the nuclei of healthy 
 muscle and electric tissue). The next fibers to the left in this figure serve 
 well to show how the degeneration takes place from the ends, where the 
 cytoplasm is gathered in heavy lumps which stain light and yellowish with 
 eosin. In fact, there is a singular similarity between the process of degen- 
 eration of these muscle cells and the transformation of the others into the 
 electric tissue. Above, in zone 3, is a rounded cell which I take to be a 
 muscle cell in a final stage of dissolution. It contains large granules of a 
 chromatic substance, which appear to be the remains of myofibrils. Figure 
 6, plate 2, also represents three fibers from this same region in another 
 myotome and shows three stages of the degeneration of the muscle. Two 
 of them indicate that the degeneration begins in the middle of the fiber. 
 
 Zones 4 and 5 in figure 15, plate 5, as well as in figure 12, plate 4, show the 
 young electric tissue of the upper median spindle. Let us first consider this 
 tissue in figure 12, plate 4, where we can get a larger, low-powered view of 
 it. In the first place, the most noticeable feature of the electric tissue here 
 is that it shows scarcely any signs of a division into the original myotomes. 
 A trace of this is seen in zone 4, but in a careful and systematic search 
 through a number of the spindles of this stage very few instances could be 
 noted. Even the myofibril bundles of successive myotomes seem to have 
 united, and these bundles were most carefully examined under a Zeiss 2 mm. 
 1.40 ap. lens. I have no doubt that each spindle does constitute a con- 
 tinuous reticulum of muscle cells at this period. Laterally, the various 
 muscle cells are not continuously united, but, as is shown in figure 15, plate 5, 
 they are united at a number of points much in the way that some heart- 
 muscle fibers are. 
 
 The myofibrils are best seen, of course, in these longitudinal sections, 
 and it can be seen that they are typical. The transverse stria tion is as 
 perfect as in the functional muscle, but owing to the slighter fibrils they are 
 not quite so dark. In the upper median spindle of figure 15, plate 5, some 
 
Origin of Electric Tissues of Gymnarchiis Niloticus. 173 
 
 points were found where this striation was slightly weakened. Cases of 
 relaxation and semi-contraction uniformly characteristic of the normal 
 muscle prevailed. The distances between the bands, as well as the length 
 of the anisotropic parts, was the same as in ordinary muscle and remains 
 as long as striation can be seen. This same fact does not hold true in Raja 
 or in Astroscopus, where it is much shortened. It does hold, however, 
 in Mormyrus, which is closely related to Gymnarchus. 
 
 The longitudinal sections, shown in figure 12, plate 4, and figure 15, plate 
 5, correspond to the spindles before it has become possible to see that they 
 have segmentally divided into electroplaxes and before a central core of 
 fibrous material has become definitely differentiated from a superficial layer 
 of cytoplasm containing all of, or nearly all of, the nuclei. 
 
 We will now advance toward the head of this same specimen, to find 
 material in a more advanced stage of development, in order to study the 
 segmentation of the embryonic spindle into its individual electroplaxes 
 (this segmentation has just been described as absent in fig. 12, plate 4), 
 and to study further the differentiation of the inner fibrous core from the 
 outer layer, and lastly to see how the myofibrils lose their transverse stri- 
 ation (muscle striation). The reader will remember that at this stage the 
 anterior electroplaxes are in advance, developmentally, of the posterior ones. 
 Figure 13, plate 4, shows four vertebral segments, taken from the region C 
 of this same embryo, under low magnification. Bracket i embraces the 
 zone of the epithelium. No connective tissue is shown. Between the 
 muscle-zones 2 and 4 lies the zone of the long, narrow, embryonic electric 
 spindle, marked by bracket 3. It is the lower median spindle which is 
 shown, one which is always in advance of all the others in development. 
 In this figure it becomes quite plain that, while the muscle tissue is arranged 
 segmentally to correspond to the vertebra, the electric organ is not. In 
 this case the electric spindle is separated transversely by connective tissue 
 into three segments, instead of four. Nor is this proportion always main- 
 tained among the several electric spindles themselves. 
 
 In some cases, as will be seen later, a single electroplax corresponds to 
 as many as five vertebrae. Even the electroplaxes do not correspond with 
 each other. A subsequent figure will also demonstrate this (fig. 23, plate 9). 
 Just what factors do determine the length of the electroplax I am unable 
 to say. A closer study of the nerve distribution and blood-supply may 
 throw some light on the matter. 
 
 The transverse sections of the stages represented in figure 13, plate 4, do 
 not show anything of interest over and above what was discussed in con- 
 nection with the conditions seen in figures 9, 10, and 11, plate 3, and figure 7, 
 plate 2. We will, therefore, advance toward the head one step further in 
 this embryo and examine a longitudinal section from the region marked B 
 in text-figure 6. Here (fig. 17, plate 6) the electric tissue is seen in what may 
 be considered as its maximum development in this 12-day embryo. The 
 electroplaxes are distinct from each other and have grown considerably in 
 
174 Papers from the Marine Biological Laboratoryat Tortugas. 
 
 size. The one selected for illustration is from a lower median spindle and 
 shows an actual length of 0.9 mm. It appears with smooth, well-defined 
 boundaries and good separation of core and outer layer. The myofibrils 
 are shown in the core as several closely associated bundles, only to be 
 distinguished from one another by a slight curving and twisting in their 
 course. This would not be visible in a transverse section at most levels in 
 the electroplax, and it gives strength to the view, expressed before, that the 
 fibril bundles of a number of young muscle cells combine to form the single 
 fibrous core of each electroplax. At the levels examined it would appear 
 that a group of from 4 to 7 cells from each myotomie is concerned in the 
 formation of any electroplax and that these same groups of from 2 or 3 
 myotomes are likewise united end to end, thus making it possible that 
 from 12 to 21 muscle cells are united to form each electroplax. 
 
 The beginning of the loss of transverse striation is quite visible in figure 
 17, plate 6. This striation merely fades or loses its staining power, beginning 
 at several points, but usually at the middle of the electroplax. The ends 
 retain the staining power of the M stripes longest. 
 
 The multiplication of nuclei mmst be spoken of at this point. It is 
 well known that the nuclei of future muscle tissue divide by mitosis with a 
 consequent and subsequent division of the cell-body as long as the cells are 
 in the young myoblast stage. As soon as muscle differentiation begins to 
 take place, the mitotic division of nuclei is changed to an amitotic type of 
 division and the cell-body ceases to divide and begins to lay down myo- 
 fibrils. Some few observations against this view are recorded, but it seems 
 to hold in Gymnarclius. 
 
 In figure 15, plate 5, one can readily see many cases of amitotic nucleus 
 division. The few cases of mitosis visible in the figure are of connective- 
 tissue cells. Such cells, it is known, always divide by mitosis during their 
 whole existence. As the electroplax grows, more nuclei are needed, and they 
 are supplied by the amitotic divisions above mentioned. Just how long this 
 process keeps up is not known, but it is probably mostly done before the 
 first 15 or 20 days of development are passed. It is evidently going on 
 fast in the 12-day embryo, and it is possibly finished in the 42-day stage. 
 The nuclei are typical muscle nuclei in appearance and are not to be dis- 
 tinguished from the nuclei of real and active muscle in the same preparations, 
 except, perhaps, they are slightly larger and have a somewhat heavier chro- 
 matic content (larger nucleolus and chromatic granules). This does not 
 hold, of course, for the degenerating nuclei, whose differences have already 
 been described. Connective-tissue nuclei can at once be distinguished by 
 their delicate outline and small chromatin content, which is distributed as 
 very small granules. 
 
 STUDIES OF A LARVA FORTY-TWO DAYS OLD. 
 In order to continue tracing this history of the development and growth 
 of the electric tissue, we will now be obliged to pass to an embryo of some 
 
Origin of Electric Tissues of Gymnarchus Niloticits. 175 
 
 considerable size and one in which the oldest tissues are almost the same, 
 for an understanding of the adult structure as those of a fully grown fish 
 would be. Text-figure 7 gives an outline of this specimen, which was about 
 63 mm. long and whose tail part was cut off and divided and sectioned as 
 follows: Beginning at about the level of the anus, a portion composed of 6 
 vertebral segments was sectioned in a vertical and longitudinal direction 
 (region A) ; then passing caudad, the next 2 segments were cut transversely 
 (region Ai); then the next 12 were cut longitudinally and horizontally 
 (region C) ; then the next 6 were cut transversely (region Ci) ; then 13 others 
 were cut longitudinally and vertically (region D) ; then 7 more were cut 
 transversely (region Di); then the next 14 were cut longitudinally and 
 approximately vertically (region E), while the tip of the tail, composed of 
 some 12 or more segments, was sectioned transversely (region £1). 
 
 In the embryo of 12 days the youngest developmental stages of the 
 electric tissue were found in the most distal portion of the tail, which at that 
 time had but recently been extended by growth from the body and was still 
 in process of extension. The oldest and consequently the most-developed 
 electroplaxes were to be found in the anterior or cephalic end of the spindles. 
 
 In the present embryo, or larva, of 42 days, the conditions are reversed, 
 and the electroplaxes in the extremity of the tail have passed those farther 
 up in the body in their differentiation, and have reached a much greater 
 and more complete development. Accordingly we will select for study one 
 of the most anterior and least developed examples and compare it with that 
 electroplax last studied, which is represented by figure 17, plate 6. 
 
 Figure 18, plate 6, is drawn from one of the ventral electroplaxes of the 
 region C, as shown in text-figure 7. This position makes it fairly well forward 
 in the spindle, although portion B v/ould have shown slightly younger stages. 
 
 Fig. 7. — Diagram of body of an embryo, or larva, of Gymnarclms, about 42 days old. 
 
 Lines and letters indicate regions studied. For explanations see text. 
 (Copied from the same source as text-fig. 5.) X about i and 1.5. 
 
 The fibril core will first attract our attention. The first noticeable 
 feature is that this core is growing in mass and volume all through the 
 electroplax, but also far faster in its center than at either end. Throughout 
 its course it has assumed a unified appearance which shows no trace of the 
 several fibril bundles which have gone to compose it. In the narrower ends 
 the mass is straightest and its component fibers appear most parallel, being 
 but slightly wavy. As we follow them from either end towards the middle 
 it can be seen that their course becomes more and more wave-like, until, 
 in the middle, they have been thrown into decided folds. Their actual 
 
176 Papers from the Marine Biological Laboratory at Tortugas. 
 
 course can be somewhat better followed with thicker sections and deep 
 eosin staining than with preparations of the usual kind. 
 
 As to the fibrils themselves, they have lost the transverse striation 
 altogether. By this I mean that it is no longer stainable. It seems that, 
 in the straight ends, with a good immersion lens one can see traces of this 
 striation, left as a thickening of the fibrils, at points that may represent 
 the previous position of the anisotropic M spindles. 
 
 Whether the increase in the mass of the fibrillar core, which goes on as 
 the electroplax grows, is due to the thickening of the individual fibrils or to 
 the laying down of more and new fibrils or to the deposit of an interfibrillar 
 substance between them, was not decided. The first two conditions seemed 
 the most probable, because of the apparent absence of much interfibrillar 
 substance in the oldest and largest electroplaxes. 
 
 The cytoplasm of the outer layer begins to be of interest at this stage. 
 Its most particular point of interest lies in the fact that it is decidedly 
 different in structure at its anterior end from its posterior end. This is 
 shown in its staining capacity as well as in its actual structure. In the 
 specimens stained in iron haematoxylin and eosin the cytoplasm at the 
 posterior end stains deeper than that at the anterior end, with both dyes. 
 It can also be seen to be granular in structure — a sort of general granulation 
 with a few larger granules of a substance which stains somewhat like chro- 
 matin. In particular, its cytoplasm is darker than the fibrillar core at this 
 posterior end. 
 
 As the cytoplasmic layer is examined in an anterior direction, it is seen 
 to stain lighter and at the same time to contain an occasional vacuole. This 
 condition increases until, at the anterior end, about one-fifth of the length 
 of the entire structure is covered with a cytoplasm which is so much vacuo- 
 lated that it appears as a delicate reticulum in the meshes of which the 
 nuclei lie. The fibril core extends out to the end or almost to the end, and 
 it can be seen that it is darker in color and denser than the cytoplasm 
 covering it. Some of the finer, granular material is found out as far as the 
 reticulated tip. 
 
 A new element of interest begins to become apparent in this stage, and 
 that is the point of attachment of the nerve. The strong medullated 
 fibers come from the cord, pass through the spinal ganglion, and enter the 
 connective-tissue "tube" of the spindle at the level of the posterior third of 
 each electroplax. The fibers wind and turn considerably, in this vicinity, 
 and are finally applied to the surface of the electroplax over an area which 
 may be described in this specimen as its second sixth part from the posterior 
 end. Thus the extreme posterior end does not receive any nerve-endings 
 at this period. 
 
 The cytoplasm shows a number of indentations where the nerve is ap- 
 plied and the axis-cylinders of the nerve can be traced into these spaces, 
 which they apparently fill with a club-shaped nerve-ending. This ending 
 can not be satisfactorily described until some of the special nerve methods 
 can be used to elucidate it. 
 
Origin of Electric Tissues of Gymnarchus Niloiicus. 177 
 
 Another and more simple step in development is indicated in figure 19, 
 plate 6, which was taken from an electroplax in a dorsal spindle of this 
 embryo at the region E (see text-fig. 7) . 
 
 Two points of interest will be spoken of in connection with this stage. 
 The shape has changed as follows: The middle part has both actually and 
 comparatively widened over the breadth shown by the middle part of the 
 electroplax seen in figure 18, plate 6, and this increased width is due to a 
 broadening of the fibrous core alone, the outer nucleated layer remaining 
 the same. 
 
 Accompanying this widening is also an actual shortening of the structure. 
 Thus, part of the increased bulk of the middle is due to an absorption of the 
 two ends. Apparently the anterior end suffers the greater amount of 
 absorption, for it is usually shorter and thinner than the posterior. In 
 this connection we must remember the previous vacuolization of this 
 anterior end as seen in figL.re 18, plate 6. It would appear that the vacuoli- 
 zation was part of an absorption, or rather of an atrophic process. Con- 
 siderable traces of it remain in figure 19, plate 6, and it is also still noticeable 
 that the anterior end does not stain deeply. The posterior end retains its 
 length and vigor to a greater degree. It seems to become considerably 
 narrowed, however. 
 
 The form of the electroplax also begins to show a marked change through 
 the development of spurs, branches, or papillje which begin to protrude from 
 both its ends, particularly from its posterior end. Some indication of this 
 was visible already in figure 18, plate 6, as small lumps or "shoulders" that 
 marked the organ roughly into three parts — anterior, middle, and posterior 
 thirds. These growing spurs begin to give the middle third a distinct 
 truncate or cylindrical form. The nerve fibers also attach themselves to 
 the sides of the growing papillae, particularly to their bases. The papillae 
 show an inclination to grow out of the sides of the posterior third. 
 
 We will now pass to the last and oldest stage of the electroplax which 
 can be found in the embryos which Budgett collected in Africa. This is 
 found in the E region of the 42-day embryo or larva of Gymnarchus and is 
 represented in longitudinal sections under a low magnification by figure 23, 
 plate 9, which shows parts, or the whole, of about 11 electroplaxes lying in 
 place in the ventral, lower middle, and upper middle electric spindles of 
 this region. Figure 21, plate 8, also shows a transverse section just cephalad 
 of this point, in the D region, where cross-sections of the 8 electric spindles 
 reveal partial or complete transverse sections of 7 electroplaxes, but only 
 the electric connective tissue which fills the tube at this point, in the eighth. 
 Figure 20, plate 7, represents a longitudinal section of one of the electro- 
 plaxes shown in figure 23, plate 9, and will serve as a basis for our last study 
 of this series. 
 
 The form has continued to follow the development which was indicated in 
 figure 19, plate 6. The total length of the structure has shortened somewhat 
 more, while its width in the middle part has increased twofold. These 
 
178 Papers from the Marine Biological Laboratory at Tortugas. 
 
 statements are the result of averages of about 15 measurements. The 
 papillse on the posterior surface have increased in length and some of them 
 have begun to rival the posterior portion of the organ in length. We have 
 no suitable figures of the adult electroplax, but from Fritsch's (3) descrip- 
 tions, and from the tendencies shown by these embryos, it would seem that, 
 as Ewart describes in Raja, the original posterior portion of the electroplax 
 shortens and the new papillae lengthen until they all form approximately 
 similar structures. This can only be completely studied when we have 
 secured suitable sections of the grown fish. 
 
 The development of papillae is noticeably weak on the anterior surface. 
 The figure does not show as many as some of the electroplaxes in figure 23, 
 plate 9, but it is a fair illustration. Neither does it show well the usual 
 condition of the main anterior process of the cell at this time, which can be 
 better seen, in some electroplaxes of figure 23, plate 9, to be still in evidence 
 and of considerable length, but of very weak development. This anterior 
 process shows no trace at this age of the general vacuolization of its cyto- 
 plasm which we saw in an earlier stage, and the fibrils extend as a very thin 
 and uncertain core through its length. 
 
 The fibrillar mass which forms the core of the electroplax has now 
 assumed what appears to be its permament condition. The fibrils are 
 very fine and, after having passed straight down through the posterior 
 process, are thrown into flat waves by the process of packing them into 
 the shortening and widening middle part which now constitutes the principal 
 bulk of the structure. This causes the larger part of the fibrils to lie nearly 
 at right angles to the longitudinal axis of the organ, and this appearance 
 was first taken by the writer, who examined the oldest embiyo first, to be a 
 trace of the transverse striation of muscle from which the organ is formed. 
 We now know it to be the myofibrils, lying at right angles to their original 
 course. No conclusion was arrived at, in this stage, concerning the growth 
 of this mass, as to whether the number of fibrils was increased, or whether 
 the larger size was due to a thickening of the original fibers, or to the deposit 
 of interfibrillar substance. The fibrils seemed to be as fine if not finer than 
 in the earlier stages ; certainly they are much finer than functional myofibrils. 
 Since muscle tissue increases the number of its myofibrils long past this age, 
 I see but little reason why this modified muscle should not also do so. 
 
 A closer study of the cytoplasm and nuclei of the peripheral layer was 
 next undertaken in this oldest stage. Beginning on the posterior surface 
 and on the papillse, we find the layer thickest here and composed of at least 
 three distinguishable materials. One was a dense material which was 
 reticular in structure and stained with chromatic stains deeper than almost 
 any other pure cytoplasm that I know of. This material was one con- 
 stituent, while the other substance composing the general field at this point 
 was a far lighter staining material which was homogeneous and clear. This 
 latter seemed to bear the same relation to the denser material that the 
 "nuclear sap" does to a linin alveolum or reticulum in the nucleus. Like 
 
Origin of Electric Tissues of Gymnarchus Niloticus. 179 
 
 the linin reticulum, this denser material was more refractive and took more 
 of the chromatic as well as more of the acid counterstains than the homo- 
 geneous material did. I shall adopt Schneider's (31) name of "Linom" 
 for the denser substance and his name of " Hyalom " for the clearer and more 
 homogeneous material. It seems probable from the works of Biitscheli (9), 
 Rhumbler (27), Hardy (35), Wilson (34), Andrews (i), Strasburger (32), and 
 others that these structures of cytoplasm, as seen under the microscope in 
 fixed material, do not represent the exact condition as it exists in life. 
 After reading the pages of my recent work on the electric-motor cells of 
 Torpedo, one can more easily see that the cytoplasmic linom is a structure 
 which depends upon the fixative used as one factor and upon the chemical 
 and physical peculiarities and the contents of this plasma at the time that 
 the fixative is applied, as another set of factors. Its reticular or alveolar 
 arrangement can most certainly be immensely varied, and all these artificial 
 conditions must be very much different from that which obtains in life. 
 Since I have only a few fixed specimens to discuss, over whose earlier prepa- 
 ration I had no control, I shall not try to solve the question of what the 
 structure was during life, but will merely describe the present specimen as 
 it appears. 
 
 The linom of the cytoplasmic layer on the posterior end is very fine and 
 can only be seen with the best lenses and under the best conditions. This 
 holds particularly for the outer portion of the layer, for as we examine 
 the inner portion the reticulum grows larger meshed until, at its point of 
 contact with the fibrillar core, the meshes are quite easily seen. 
 
 The same is true as we examine this layer in a more anterior position. 
 Here all the meshes are proportionately larger, until in the layer, as found 
 covering the extreme anterior surface, the meshes of the linom are visible 
 with ordinary high powers. I do not refer to the vacuoles which are found 
 at various points, for there is a great difference between the largest meshes 
 and the vacuoles, although the vacuole may be derived from, or originate 
 in, an overgrown mesh. The meshes always contain the hyalom, while 
 the vacuoles do not. My definition of a vacuole in this case will be a space 
 in the linom into which the h^'alom does not extend. Also, the rounded 
 outline of a vacuole shows a surface tension between its content and the 
 cytoplasm, which does not seem to exist between a mesh of the linom and 
 its contained hyalom. These vacuoles appear to have contained a soluble 
 substance during life, and the hyalom is not soluble in fluids used to prepare 
 the specimen. Such vacuoles are found at various points in the cytoplasmic 
 layer, most often at its outer edge, while the large meshes appear at the 
 inner edge and around the nuclei. The vacuoles become so large and 
 numerous at the anterior end of a stage such as figure 18, plate 6, that the 
 cytoplasm looks like foam containing the nuclei and surrounding the 
 unchanged fibrillar core. 
 
 In addition to this structural basis of linom and hyalom, another very 
 prominent content of the cytoplasm is a series of granules of some material 
 
I So Papers from the Marine Biological Laboratory at Tortugas. 
 
 that is denser and somewhat more stainable than either of the others. 
 These granules are very numerous and fine at the periphery, where they 
 cause the outer part of the layer to stain deepest with eosin, and they lie 
 in the finer-meshed linom, and grow larger and fewer as one examines the 
 inner parts of the layer. They are of a slightly more refractive quality 
 than the linom and stain deeper with chromatic dyes the larger they get. 
 The largest also have some color of their own, a slight golden-brownish color, 
 somewhat like that of pigment granules. While the smallest granules seem 
 to lie in or attached to the strands of linom, the larger seem to lie in its 
 meshes in the hyalom. They remind the observer of the granules found in 
 certain other cells, particularly in the electric-motor cells of Torpedo, as 
 well as in other nerve-cells. In figure i6, plate 5, these finest granules are 
 seen in the outer part of the layer stained with eosin, while the inner part 
 does not show the larger ones that lie there. 
 
 The larger granules are prone to gather about the nuclei and about the 
 inner part of the electric layer, as seen in figure 22, plate 8, where they took 
 the iron haematoxylin stain well. The larger, chromatic-staining granules 
 are also found, in groups or singly, in many parts of the fibrillar core. 
 While the larger ones found in the core, around the nuclei, and at the inner 
 edge of the cytoplasmic layer seem to stand in sharp contrast to the more 
 numerous and finer ones found in the outer part of the cytoplasm, one can 
 trace steps between the two kinds. 
 
 The writer believes these granules to be the secreted or prepared nitro- 
 genous material used by katabolic processes to produce electricity, either 
 directly or indirectly. They must be, physiologically, the same granules 
 described by Ballowitz (5) in the cytoplasmic layer of the electroplax of 
 Raja, and by Schlichter (30) in the cytoplasmic regions on both surfaces of 
 the electroplax of Mormyrus. In this latter they were very large and not 
 easily demonstrated. The writer has seen some indication of them in 
 Astroscopus (12), but they would seem to be noticeably absent in other 
 forms, as Torpedo (although they have been figured here by Fritsch (20)), 
 and in Gymnotus (3) and Malopteriiriis (6) , where such granules as Ballowitz 
 has described or figured would seem to be inadequate in size and number 
 for so heavy a duty. It will be noted in the above list that the weak- 
 electric fish seem to have these cytoplasmic granules best developed, while 
 the strong-electric fish show least of them. It is possible that these latter 
 possess them, or their equivalent, in a more fluid and less visible form. 
 
 The cytoplasmic layer is marked off from the fibrillar core by a very 
 definite and sharp line. It can not be said that a definite membrane 
 exists here, although one may exist. The boundary between the outer 
 layer and the core is not an even or a straight one, but shows a wandering 
 course, especially at the two ends. At some points strands of the linom 
 seem to branch into the core (fig. 16, plate 5). At the anterior end in 
 particular it shows many extensive and complicated invaginations of its 
 granule-containing substance into the fibrillar core. Certain small portions 
 
Origin of Electric Tissues of Gymnarchus Niloticus. i8i 
 
 of it also have been permanently left in the main body of the core, usually 
 near its anterior end. These inclusions (fig. 20, plate 7) may or may not 
 include the nuclei. They always contain some of the largest of the granules. 
 
 The nuclei do not show any internal peculiarities which would dis- 
 tinguish them as electric nuclei from some of the other tissue nuclei, par- 
 ticularly from muscle nuclei, which they much resemble. They have a 
 large chromatic content and a particularly large plasmosome which stains 
 deeply. They can be sharply distinguished from some other nuclei, as 
 connective-tissue nuclei for instance, where the more delicate outline and 
 diiTerent chromatic pattern is discernible at a glance. 
 
 Each nucleus shows some sign of a surrounding differentiated layer of 
 cytoplasm. This consists of larger granules and a zone in which the 
 hyloplasm seems to be in greater proportion than elsewhere. At different 
 places in a preparation one may see more or less of a contraction zone 
 around the nuclei. While this may be a physiological condition, it is more 
 probably an artifact due to the fixing or hardening. 
 
 One interesting condition is to be seen in most of the few nuclei which 
 become detached from the outer layer and included with some small portion 
 of the outer cytoplasm in the fibrillar core. These nuclei probably become 
 so placed during a very early stage, and the further they are separated from 
 the layer to which they rightly belong, the larger they grow and the more 
 diffuse their chromatin becomes. The plasmosome diminishes in size and 
 the whole structure looks more like a connective-tissue nucleus, except 
 that it is very much larger. I have seen this same condition in the electro- 
 plax of Raja. 
 
 It was not possible to find a real electrolemma or cell-membrane covering 
 
 this electroplax. A connective-tissue covering, more or less closely applied 
 
 to the surface, was always present, but the fact that this covering possessed 
 
 its own nuclei seems proof that it was a real connective-tissue covering and 
 
 not any product of the activity of the electroplax tissue. At such points, 
 
 as this connective tissue did not actually touch the electroplax, a careful 
 
 examination was made to see if some actual cell-membrane did exist. 
 
 Beyond the fact that the outer edge of the electric layer was sharply defined 
 
 and that its surface was rounded and even as if some membrane was present, 
 
 no real membrane could be demonstrated, either by its refractive properties 
 
 or by its color. 
 
 INNERVATION. 
 
 A general survey of the innervation is desirable, as too little exact topo- 
 graphical work has been done on those fishes in which the electric-motor 
 centers are thinly distributed in character over large areas of the cord, as, 
 for instance, in Gymnotus, Raja, and the mormyrids. Regions D and E 
 were selected in the 42-day-old embryo as the most favorable parts to study. 
 The work was not as exact as it could be if the investigator had had plenty 
 of live material, especially adult material, on which to use some of the well- 
 known neurological methods. But even in this embryo, which was well 
 
Papers from the Marine Biological Laboratory at Toriugas. 
 
 fixed in sublimate-acetic, much could be accurately made out, and it is 
 hoped, too, that the description will prove suggestive. 
 
 The motor cells were first looked for in the spinal cord, especially that 
 region which was posterior and in the neighborhood of the electric organs. 
 Assuming for the present, as a law, that electric-motor cells are larger than 
 muscle-motor cells which innervate an equal weight of muscle, it was very 
 easy to find groups of large, heavy nerve-cells scattered through the sub- 
 stance of the spinal cord from near the anterior beginning of the spindles 
 to the very last point in the tail to which they extend. 
 
 "iG. 8. — Side view of reconstruction (semi- 
 diagrammatic) of spinal cord and motor 
 electric nerves of a larval Gymnarchns 
 42 days old. Arrow indicates anterior 
 and posterior directions. Ni, N2, Nz, 
 and Ni are the four electric nerves formed 
 by branches from motor roots of spinal 
 nerves. E indicates small branches from 
 electric nerves that innervate posterior 
 surfaces of electroplaxes. g marks spinal 
 ganglion, whose afferent nerves have 
 been cut off. 
 
 E jr~ r 
 
 ^^ir~ rsc 
 
 These cells were situated just above the central canal and from i to 4, 
 or even 5, could be seen in every transverse section in most of this length 
 (fig. 14, plate i[, A, B, and C). They did not appear to be divided into two 
 symmetrical groups, but rather to lie in one median group. On the one 
 hand, their dorsal position might appear to be evidence that they were not 
 motor-cells or that they were not to be considered as modified muscle-motor 
 cells. On the other hand, the presence of real ventral muscle-motor cells 
 (fig. 14, plate 4, D and E) in the whole length of the cord, the correspondence 
 of the cells under discussion with the position of the electric spindles, and 
 the fact that Fritsch describes similarly placed cells in the same position 
 in mormyrids as the motor-nerve cells of the electric organ, all seemed to 
 constitute very strong indirect evidence that these were the nerve cells 
 which innervated the posterior ends of the electroplaxes. 
 
 In addition, the writer was able to trace, in a number of cases, the course 
 of the neuraxons through the cord, out into the ventral roots of the nerves, 
 and finally into special bundles of nerve fibers that undoubtedly innervated 
 the electric tissue. No one nerve process was actually traced for the v/hole 
 distance unbroken, but the various parts and regions were so pieced together 
 that the course was well established and corresponds in many ways with 
 Fritsch 's observations on Mormyrus. Text-figure 9 shows a semidiagram- 
 matic cross-section of these courses as seen from in front, while text-figure 8 
 shows the same thing sketched in relief from the side. This topography 
 will presently be described. 
 
Origin of Electric Tissues of Gymnarchus Niloticus. 
 
 The electric-motor cells at this age must of course be considered as still 
 very young, and descriptions from the adult are desirable. We will begin 
 by examining the 9-day-old embryo once more to see if they have started. 
 In this cord (fig. 8, plate 2) there are but very faint traces of any nerve- 
 
 FiG. 9. — Diagram to show position and relations of 
 nerve elements to electric spindles. S.C., spinal 
 cord, showing central canal and four motor elec- 
 tric nerve-cells. Processes from these cells pass 
 out through ventral roots, and distribution of these 
 roots to electric spindles and muscle-masses is in- 
 dicated. M , some of the muscle-masses. D, U.M., 
 ■ L.M.t and V show electric spindles on one side. 
 Sp.G., spinal ganglion; L.L.N,, lateral line nerve. 
 
 cell development, and some neuroblast mitosis is still going on. In the 
 position to be later occupied by electric cells a little enlargement of nucleus 
 and cell-body is visible. One cell here is of great interest, and that is one 
 of the now well-known " Hinterzellen " or giant cells, first described by 
 J. Beard (37), Rohon (29), Studnitzka (33), and others, in the embryos of 
 Salmo and Raja, and later by Fritsch (21), as a different sort of cell, in the 
 adult LopJiius. Still later, such cells were described by the writer (36) in the 
 embryos and adults of various Pleuronectidae and in Pterophryne, where he 
 showed it to be the same cell in both embryo and adult, and described the 
 relations of anterior and posterior branches of the neuraxon. From its 
 size, shape, and position in the present specimen, it seems that this dorsal 
 cell might be, in some way, related to the electric-motor cells, but that 
 question is easily settled when one examines the 12-day embryo and finds 
 that all of the dorsal cells have effectually disappeared before the electric- 
 motor cells begin to differentiate. Besides, the well-known fiber courses of 
 the dorsal cells as worked out by Fritsch in LopJiius, the writer in Ptero- 
 phryne, Harrison in Salmo, and Johnson in Catostomus are sufficient proof 
 that the two have nothing in common. 
 
1 84 Papers from the Marine Biological Laboratory at Tortugas. 
 
 The 42-day larva shows the best development of these electric-motor- 
 nerve cells until an older fish can be described. Here we find a heavy, 
 rounded, cytoplasmic body of about 40 microns in diameter (fig. 14, plate 4, 
 A, B, and C). Its outline is usually pear-shaped, with the axis-cylinder 
 process given off at the pointed end. Dendrites are not visible in the 
 specimen at hand. While most of these cells are of the same size, a few 
 are noticeably smaller. 
 
 The nucleus is large, round, and placed nearly always in a very eccentric 
 position. This position is most frequently on the side away from the 
 neurite, but sometimes it may be very close to the neurite. Its diameter 
 is about 16 to 19 microns in the largest cells of this age, and its outline is 
 hard and round. The nucleolus consists of a single, or rarely double, plas- 
 mosome, which is a very little less than 5 microns in diameter, but which, 
 when double or multiple, is of proportionally smaller size. As I have shown 
 in the nucleus of the electric cells of several torpedoes, when a plasmosome 
 is multiple its several parts, collectively, are larger in mass than is a single 
 normal or usual plasmosome. 
 
 Of course, the nucleus was carefully examined as to any possible ori- 
 entation of its nuclei, particularly the plasmosome, with reference to gravity 
 or to the electric current or to the axis of the cell. Nothing of this sort 
 could be found, although this does not preclude such a condition in the 
 grown fish. It is known that in Torpedo, where such an orientation does 
 exist, this same orientation is not found in the embryonic or larval stages. 
 
 From these cells thin, delicate axis-cylinder processes were traced down, 
 as has been described, and into the ventral roots of the spinal nerves in a 
 sufficient number of cases to assure the observer that they all ran in this 
 direction. The process was thinnest shortly after leaving the cell, and 
 became thicker as it approached the nerve-root. When it once entered the 
 root it again became very thin, although now invested with a connective 
 tissue and a medullary sheath. A large number of connective- tissue nuclei, 
 nerve-sheath nuclei, and some unknown elements cause the motor-root of 
 the nerve to swell to some size just after leaving the cord. It decreases 
 again in size before it enters the foramen and leaves the vertebral canal in 
 company with the much smaller dorsal root. 
 
 Just outside the canal the dorsal root ends in the spinal ganglion, and 
 the motor-root traverses the inner side of the ganglion, from which it emerges 
 on the lower edge, and at once divides into a dorsal and a ventral branch. 
 
 The ventral branch passes backward and downward (see text-figs. 8 
 and 9) to a point at a level with the lower middle spindle, and here it gives 
 off a considerable group of fibers which pass caudad just inside of the 
 spindle capsule. These fibers are joined by similar groups from others of 
 the spinal nerves and the whole mass forms the lower middle electric nerve 
 (text-fig. 8, TVs). At the posterior level of each electroplax this nerve gives 
 off a few fibers (text-fig. 8, E) which branch out and innervate the posterior 
 surface of this electroplax. 
 
Origin of Electric Tissues of Gynmarchus Niloticus. 185 
 
 The remainder of the ventral branch passes farther down and again gives 
 off a branch which goes to form part of the ventral electric nerve (text-fig. 
 8, N^. This latter sends off little branches to furnish the posterior surfaces 
 of the ventral electroplaxes with motor-fibers (text-fig. 8, £). 
 
 Going back to the anterior root, we find that its dorsal branch leads 
 directly upward and, passing between the spinal ganglion and neural 
 arch, slopes gently backward to give off a large branch which becomes a 
 part of the upper median electric nerve (text-fig. 8, iVj). Its remaining 
 fibers reach upward and furnish the dorsal spindle with a part of the fibers 
 that form its dorsal electric nerve (text-fig. 8, iVi). 
 
 Of course, there are muscle-motor elements in both these branches of 
 the anterior nerve-root, the number depending upon the position, back- 
 ward or forward, at which we examine the arrangement. Two examples of 
 the motor-nerve cells of the muscle-tissue from the cord in region D are 
 shown in figure 14, plate 4, D and E. At the level of the anterior parts of 
 the electric organ, when there are large quantities of muscle and the electro- 
 plaxes are very small, the muscle branches are large, particularly the dorsal 
 branches, which have to supply the muscle-bundles for the large dorsal fin. 
 In the posterior region of the tail, on the other hand, the muscle is almost 
 entirely absent and the muscle-branches of the anterior roots are not even 
 easily seen. 
 
 The eight (four on each side) longitudinal electric nerves are interest- 
 ing in that they form a morphological buffer between the conflicting 
 segmentation of the nervous, skeletal, and muscular systems, and the inde- 
 pendent segmentation of the electric system. Were the electric segmenta- 
 tion to correspond with that of the others, we should not find these nerves 
 in this recognizable form. In fact, in the anterior part of the electric organ 
 we do not find them as continuous nerves, in many places, for more than 
 two or three neuromeres at a time. And even posteriorly where they do 
 form continuous nerves, the fibers that enter them from any given spinal 
 nerve do not pass very far back in them before leaving to innervate one of 
 the electroplaxes. Each nerve cell probably lies but a very short distance 
 in front of the electroplax which it supplies. This was decided upon by 
 plotting the relative positions of all motor-electric cells and all electroplaxes 
 in the larger part of the organ. 
 
 While doing this it was also determined how many nerve-cells sent 
 their nerve-processes to each electroplax. Thus, in region E of the 42- 
 day-old larva there were easily counted 414 of the electric-motor cells, while 
 in the same part there were 81 electroplaxes. This makes it quite sure 
 that, on an average, about 5 of the nerve-cells were used for each electro- 
 plax. It was attempted to count the nerve-fibers as they left the electric 
 nerve to branch out over the electroplax, but the elements were too small 
 and the fixation not just what was needed to do this. It could be easily 
 done in a grown specimen. 
 
 I shall now take up the structure of the nerve-fibers as they leave the 
 
i86 Papers from the Marine Biological Laboratory at Tortugas. 
 
 electric nerves to pass to the electroplaxes, assuming that they have no 
 peculiarities of interest during their course through the nerve-tracts between 
 the cord and the electric chambers. (The electric chambers are not divided 
 as in Mormynis by a transverse septum of heavy, opaque, white connective 
 tissue; this can be explained by the fact of their secondary segmentation.) 
 
 As the few fibers destined for any particular electroplax first leave the 
 electric nerve they are directed caudad and are very small in diameter and 
 invested with a fine connective-tissue sheath. This covering is probably 
 medullated in the grown fish. They quickly turn in a gentle curve whose 
 diameter is about half that of the spindle and pass forward through the 
 electric connective tissue to the posterior surface of the electroplax. At the 
 beginning of this curve, or just after leaving the electric nerve, the axis 
 cylinder or neuraxon enlarges to a considerable diameter and becomes very 
 wavy and irregular in diameter. Its course is no longer straight, and it is 
 not possible to find a section of any considerable part of its length, except 
 in some very few cases. Its sheath of connective tissue is loose fitting and 
 the inner side shows a loose reticulum of fine fibrils and plates that form a 
 weak connection between the axon and sheath. 
 
 It is at once apparent in the maze of fibers which approach the electro- 
 plax that the few neuraxones which first entered the compartment are now 
 many in number. Still, it is difficult to see where they branch in the thin 
 sections, owing to their sinuous and irregular courses and to the large 
 numbers of transverse and oblique sections present. In several places this 
 branching was seen, however, and recognized as the same multiple branching 
 which has often been described in the terminal part of nerve-paths and, in 
 particular, in the same comparative region of the electric tissue of Raja by 
 Ballowitz (5) and Retzius (26) , in Torpedo by several authors, and especially 
 in Malopterurus by Ballowitz (4), who refers in this article to many other 
 cases. The writer has also seen and figured it in the electric tissue of 
 Asiroscopus in a paper soon to appear. In the present case, one section, as 
 can be seen in figure 16, plate 5, exhibits two cases of this branching, one of 
 them showing a single fiber dividing into at least three or four branches; 
 also, figure 25, plate 9, in which there is to be seen a well-defined single fiber 
 dividing into two branches just before they end in two club-shaped nerve- 
 endings in the electric layer of the posterior surface of an electroplax. 
 The abundant nodes of Ranvier, described by Schlichter (30) in the case 
 of Mormyrus, were not observable here, probably on account of the lack of 
 osmic-acid fixation. I have no doubt that they are present and they must 
 be present at the points at which the fibers divide. 
 
 As has already been stated, the fibers now approach and end on the 
 posterior surface of the electroplax, as well as on the lower sides of some of 
 the papillae that arise from it. The mode of ending is not difficult to see, 
 apparently, although this much studied and controverted question should 
 not be approached lightly, especially where the material is merely a subli- 
 mate-acetic fixation stained with iron-heematoxylin and eosin. 
 
Origin of Electric Tissues of Gymnarchtis Niloticus. 187 
 
 The nerve-process, carrying its connective- tissue sheath until it actually 
 reaches the surface of the electroplax, ends in a blunt and somewhat thick- 
 ened knob which is embedded in a hollow or invagination in the substance 
 of the posterior cytoplasmic or electric layer of that structure (see figs. 18 
 and 19, plate 6; also fig. 16, plate 5; also figs. 24 and 25, plate 9). This knob 
 may be quite elongate in form and in some cases appears to be branching. 
 The nerve fiber becomes very narrow and apparently dense just before 
 entering the cavity, but it quickly broadens out, to as much as or more than 
 its previous width, to fill the cytoplasmic cavity of the electroplax. Its 
 substance becomes very light-staining, more so than any other part of its 
 length is, and this light-staining quality is most apparent at its extreme 
 distal end in the cavity. The nerve-fibrils, faintly visible in the outer 
 courses of the axon and rather more so in the denser neck just before entering 
 the cavity, can be seen in the light-colored club-shaped ending to be running 
 in an irregular reticulum instead of in their previously almost straight and 
 parallel manner. They could not be traced into the protoplasmic bridges 
 between the club-shaped nerve-ending and the surface of the protoplasmic 
 cavity in which it lies. In very few instances did the nerve-substance fill 
 the recess in the electric layer tightly, probably owing to some shrinkage in 
 both of the tissues. The small space between the two surfaces is crossed 
 by the numerous fine processes or strands, mentioned above, of some of the 
 nerve-tissue remnants, probably of an original closer contact. 
 
 The question now arises as to whether the cavity or depression which 
 the nerve-ending occupies is to be considered as an invagination of the 
 surface of the electric layer or as a real penetration of the electroplax by 
 the nerve. I shall consider it as the former, because the surface of the 
 electroplax appears not to be interrupted by the opening but to continuously 
 follow the inner edge of the cavity all the way around. 
 
 The presence of a perceptible cell-membrane or electrolemma would 
 assist in the solution of this question, but such an organ could not be 
 demonstrated. The edge of the cytoplasm was sharp and definite, but no 
 membrane was visible, either by its staining properties or by refrangibility. 
 It possibly will be found in the adult organ. Nor was it possible to see the 
 "Stabchen" or rodlets which have been described in other electric organs 
 by Ballowitz (3, 5) and his pupils (30). Some striated arrangement of 
 the granular electrochondria or granules described previously was observ- 
 able, but this constituted a fibrillar secretory striation, such as is seen in 
 the surfaces of most cells that are undergoing exchanges of any kind. Here, 
 again, we must await examinations of the adult electroplax before we can 
 say if such "Stabchen" are present; also if they are homologous with the 
 well-defined rodlets found by Ballowitz in Torpedo and in Raja, and whether 
 they are specific structures of any importance in the production of electricity. 
 
 Returning to the relation of nerve-ending to electroplax, we have 
 practically decided that the nerve-club is embedded in an invagination of 
 the surface rather than in a cavity in the substance of the electric layer. 
 
l88 Papers from the Marine Biological Laboratory at Tortugas. 
 
 In doing this it has formed a much more intimate connection than may 
 be seen in other places where the nerve-fibers touch the electroplax very 
 closely, even being partly embedded in it, or running through the fundus 
 that lies between two papillse, where the fiber is closely pressed on three 
 sides by the surface of the electroplax. In these fiber contacts the con- 
 nective-tissue sheath persists, while in the club-shaped or heavily rounded 
 endings the connective-tissue sheath is lost, lefc at the surface, and the 
 little protoplasmic bridges, shown when slight shrinkage has taken place, 
 testify to the intimacy of the contact. In addition a slight amount of fine, 
 golden-colored granules surrounds the nerve-ending, Ij'ing on its surface, 
 between it and the substance of the electroplax (figs. 24 and 25, plate 9). 
 These granules are not found on any other part of the nerve surface. 
 
 Naturally the paper of Schlichter (30) was carefully examined to see 
 what he had found as to the ending of the electric nerves on the related form, 
 Mormyrus oxyrhynchus. He had adult material, but otherwise was no 
 better off than the writer in the possession of material which had been 
 treated especially for neurological study. He describes the nerve-fibers with 
 their medullary sheaths as coming in contact with the large process of the 
 electroplax and then suddenly ending just as they reach certain large inden- 
 tations of the surface of this electroplax. He found in these indentations 
 only a little coagulated material and some nuclei. 
 
 The writer has no doubt that the slight coagulum represents what 
 remains of a club-shaped nerve-ending similar to that which he finds in 
 Gymnarchus. The nuclei, from their position in Schlichter's picture, are 
 evidently the nuclei of terminal connective- tissue coverings. If this idea 
 be correct we will have a very simple but interesting form of nerve-ending, 
 much larger in size than that found on any muscle or any other electric 
 organ and one in which it will be, apparently, easier to study the intimate 
 contact of nerve-substance with motor-substance from a cytological point 
 of view than in any other form. In particular, we should try to stain these 
 endings with the nitrate of silver and methylene blue methods devised for 
 neuro-cytological studies. This work, however, can be undertaken only 
 on the ground, with good laboratory facilities and with an abundance of 
 fresh material. 
 
 The writer has published observations on some peculiar horizontal, 
 pointed rods, or pointed threads, found imbedded in the electric layer of 
 the electroplax of Aslroscopus. At that time he suggested that they might 
 be in some way homologous with or related to the "Stabchen" because of 
 the absence of any other well-defined "Stabchen" in this fish. Such 
 structures are not found in the present Gymnarchus larva, but they have 
 been seen and described in Raja, in a paper soon to be published. Their 
 presence in Raja, in addition to the "Stabchen," proves them to be entirely 
 different cell organs. 
 
 One word in regard to certain posbibilities for the physiological study 
 of the electric organ in Gymnarchus. A recent paper by Bernstein and 
 
Origin of Electric Tissues of Gymnarchus Niloticus. 189 
 
 Tschermak and another by Bethe go to show that the electric discharge of 
 Torpedo and other fishes is produced by different concentrations of sodium 
 chloride in the electroplaxes and in the intervening electric connective tissue. 
 Since there is a long series, theoretically, of these alternate segments of 
 higher and lower degrees of concentration of the electrolyte, and since all the 
 higher concentrations are presumably equal, and all the lower concentra- 
 tions are equal, we would have a series of equal potentials alternately 
 opposed to each other, and the result would be zero or else only the strength 
 of one concentration current, in case there was one more or one less of 
 either of the concentrations. 
 
 To obviate this difficulty a membrane has been imagined, on one surface 
 of each electroplax, presumably the electric or nerve-ending surface (pos- 
 terior surface in this case), which will be permeable only to one kind of 
 the ions, either negative ions or positive ions, and by which the current is 
 thus rendered integral in one direction. 
 
 Two things remain to be proved in connection with the above theory — 
 the fact of different concentrations and the presence of such a membrane. 
 This can be done in any fish in which it is possible to effectively separate 
 the segments in a fresh state, so as to submit them to delicate chemical 
 tests. In Gymnarchus we have a fish whose elements are larger than those 
 in any other one of the seven electric types — large enough to be cut apart, 
 I believe, and analyzed separately in the chemical laboratory; also large 
 enough to submit anterior and posterior surfaces to physical tests that may 
 show its permeability to either positive or negative ions and its imperme- 
 ability to the other kind in one direction. This experimental work can 
 most certainly be done if the proportion between the bulk of electric con- 
 nective tissue and electroplax remains the same in the adult as in the larva 
 (see fig. 23, plate 9). Fritsch shows much less of the electric connective- 
 tissue segments in his figures of the adult organ. 
 
 Numerous other anatomical features of Gymnarchus have caused it to 
 be classed with the other mormyrid fishes. This fact makes it of interest 
 to compare its electroplax with the very different electroplax in these 
 fishes. 
 
 That found in Mormyrus oxyrhynchus will serve as a type and its general 
 plan has been well shown by Ogneff (25) and Schlichter (30). Here it is 
 evident that a number of consecutive and entire myotomes have been 
 converted into electroplaxes and that the middle layer of each electroplax 
 is composed of unaltered and clearly striated myofibril bundles. The large 
 number of these fibril bundles, and their distribution, indicate that the 
 whole electroplax in Mormyrus is a syncytium composed of all or most of 
 the cells which would otherwise have gone to make up the single myotome. 
 In this we find an agreement with the electroplax of Gymnarchus which is 
 also formed from several cells. In the one case, however, all the cells in 
 the myotome have been used (Mormyrus) ; in the latter only those lying in 
 eight particular localities (Gymnarchus). (See paper by Dahlgren (36).) 
 
igo Papers from iJie Marine Biological Laboratory at Tortugas. 
 
 Further homology is seen in the disposition of the probably superfluous 
 myofibrils. In both forms they are relegated to a middle position in the 
 electroplax, while the apparently more important cytoplasm forms layers 
 on the anterior and posterior surfaces of the structure. Also, in both, the 
 now useless myofibril bundles are packed out of the way at right angles to 
 the axis of the electroplax, which remains the same as the former axis of 
 the muscle-cells that were used to form it. 
 
 The only difierence lies in the fact that the striation of the fibrils is 
 retained in the Mormyrus forms, while it is lost in Gymnarchus, the dark- 
 staining anisotropic substance apparently dissolving away. 
 
 From what little can be predicted concerning the possible origin of the 
 electric tissue in the other teleost forms, it is probable that the Mormyrida; 
 (including Gymnarchus) are the only fish in which the electroplax is formed 
 as a syncj'tium from more than one cell. In Astroscopiis, Electrophorus 
 (formerly Gymnotus), and Maloptenirus the structures show every evidence 
 of having been developed from single myoblasts with the exception of 
 Malopterurus, v/here it is a question as to whether they are not evolved from 
 gland-cells instead. 
 
 SUMMARY. 
 
 In conclusion it may be stated that — 
 
 (i) The electric tissues of Gymnarchus are developed by the differ- 
 entiation of certain portions of its normal, striated muscle-tissues during 
 an embryonic or larval period extending from the ninth day to the forty- 
 second day of embryonic life. The critical period of this change takes place 
 in the neighborhood of the eleventh, twelfth, to fifteenth days. 
 
 (2) The muscle fibers which go to form the electric tissue give up their 
 usual segmentation into myotomes, and first form eight long and con- 
 tinuous spindles which afterward segment, each into a lesser number of 
 masses, the electroplaxes. 
 
 (3) The mj'ofibrils lose their transverse striation and form a large inner 
 core for each electroplax. They lie in a wavy mass, mostly at right angles 
 to their former course. The active cytoplasm forms an outer layer which is 
 denser and stains deeper on the posterior than on the anterior surfaces. It 
 contains granules. 
 
 (4) Each electroplax is made up of several muscle cells, 12 to 20 or more. 
 This is different from the two elasmobranch fishes, in which each muscle 
 cell forms only one electroplax. 
 
 (5) The nerve is distributed on the posterior surface and ends in blunt 
 knobs that lie in cavities formed by the invagination of the surface of the 
 electric layer. The current probably runs from tail toward head, as in 
 Mormyrus. 
 
 (6) The development of the tissue gives a strong clew to the probable 
 development of the electric tissues in the other mormyrid fishes. 
 
LITERATURE. 
 
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 teilung. Arch. Entwick. Mechanik, Bd. ill. 
 
 28. RUPPEL, Ed. (Reported by Fritsch. See. No. 19.) 
 
 29. RoHON, V. Zur Histogenesis des Ruckenmarks der Forelle. Sitz. d. Math. Phys. 
 
 Klass ed. Akad. d. Wiss. in Munchen, Bd. 14, 1884. 
 
 30. ScHLiCHTER, Heinrich. Ueber den feineren Bau des schwach elektrischen Organs von 
 
 Monnyrus oxyrhynchus. Zeit. f. Wiss. ZooL, Bd. 84, 1906. 
 
 31. Schneider, C. Lehrbuch der Histologic. G. Fischer, Jena, 1904. 
 
 32. Strassburger, E. Ueber Cytoplasmastrukturen, Kern, und Zellteilung. Jahr. Wiss. 
 
 Bot., Bd. XX.X. 
 
 33. Studnitzka, F. K. Ein Beitragz.vergl. Histologic und Histogenesis des Ruckenmarks. 
 
 Sitz. d. Kgl. Bohm. Ges. d. Wiss. in Prag, 1895. 
 
 34. Wilson, E. B. Protoplasmic structure in the eggs of echinoderms and some other 
 
 animals. Journ. Morph., Boston, vol. 15, supp., 1900. 
 
 35. Hardy, W. B. On the structure of cell protoplasm. Journ. of Physiol, vol. xxiv, 1899. 
 
 36. Dahlgren, U. Origin of the electricity tissues of fishes. American Naturalist, vol. 
 
 37. Beard, J. Thetransient Ganglion Cells and their Nerves in Raja batis. Anatomischer 
 
 Anzeiger, Bd. 7, 1899. 
 
EXPLANATIONS OF THE PLATES. 
 Plate i. 
 
 Fig. I. Transverse section through body of 9-day-old embryo of Gymnarchus in region C. 
 L.M, position of the large muscle cells that will eventually change into the 
 lower median electric spindle; V.F, location of large vertebral fibers; XXXX, 
 locations of electric spindles in an adult fish with reference to bony structures, 
 blood-vessels, and spinal cord; N, canalis centralis; Bi and B2, caudal 
 blood-vessels. X no. 
 
 Fig. 2. Muscle-tissue in transverse section from 12-day-old embryo of Gymnarchus to 
 show typical muscle cells. A, one of oldest fibers with two strong fibril 
 bundles; B, growing muscle cell with fibrils being laid down in a ring; C, 
 extreme upper edge of myotome with youngest cells at top. X 1150. 
 
 Fig. 3. Transverse section through group of larger muscle cells found at L.M. in fig. I. 
 Under greater magnification. For explanations see text. X 1150. 
 
 Fig. 4. Longitudinal vertical section of C region in 9-day-old embryo, ventral portion. 
 Between marks O O are seen same parts of seven myotomes as are shown 
 at L.M. in preceding figure in transverse section. Note heavy development 
 of these cells. A line drawn between marks © © would indicate location of 
 seven groups of vertebral fibers in next few mediad sections. 
 
 Plate 2. 
 
 Fig. 5. Transverse section of body of 12-day-old Gymnarchus embryo in region CCi. D, 
 dorsal electric spindle; U.M, upper median spindle; L.M, lower median 
 spindle; V, ventral spindle; Conn.T, connective-tissue septum; N, canalis 
 centralis. Bi and Bs, caudal blood-vessels. X no. 
 
 Fig. 6. Three degenerating muscle cells from another (3) zone in fig. 12. X 1440. 
 
 Fig. 7. Electric spindle showing first stage where electric muscle cells can be called an 
 electroplax. Complete coalesence has taken place. Nuclei are in peripheral 
 layer. Myofibrils are concentrated in middle to form central fibrillar core of 
 structure. X 1 150. 
 
 Fig. 8. Transverse section of upper dorsal part of spinal cord in a Gymnarchus, embryo 
 9 days old. Neuroblasts, in region that will later show electric motor-cells, 
 show some little differentiation and growth. Near, and above, this region is 
 one of the transitory giant ganglion cells. X 940. 
 
 Plate 3. 
 
 Fig. 9. Transverse section of left ventral spindle from region C in embryo of 12 days. To 
 left of dotted line are normal muscle cells, to right are degenerating muscle 
 cells, in the midst of which, and surrounded by dotted circle, are a group of 
 muscle cells going through the first steps of transformation into electric 
 tissue. X 1 150. 
 
 Fig. 10. Transverse section of ventral electric spindle in process of formation in region B, 
 of 12-day-old embryo of Gymnarchus. At left of figure two muscle cells 
 begin to degenerate, while just to right of upper one of this pair a muscle cell 
 is seen in advanced stage of atrophy. Connective-tissue capsule beginning 
 to form. Electric muscle cells rather more coalesced than in fig. 9. X 1 1 50. 
 
 Fig. II. Transverse section of upper middle electric spindle in same section as fig. 10. 
 Rapidly degenerating muscle cells to left. Further coalescence of electric 
 muscle cells than in fig. 10. Myofibrils weakening but still in separate 
 groups. X 1150. 
 
 Plate 4. 
 
 Fig. 12. Longitudinal vertical section of dorsal part of region D of 12-day-old embryo of 
 Gymnarchus, showing relations of muscle tissue and electric tissue. Bracket 
 I indicates zone of degenerating muscle fibers on inner median edge of five 
 myotomes, on ventral side of median connective tissue; zone 3 shows atrophy- 
 ing muscle fibers on inner median edge of myotomes, dorsad of median con- 
 nective tissue; zone 4, rudiment of upper median electric spindle; zone 5, 
 same of dorsal electric spindle; zone 6, muscle tissue from ventral spindle to 
 dorsal outer edge; zone 7, connective tissue; zone 8, dorsal epithelium. 
 X 140. 
 
 192 
 
Origin of Electric Tissues of Gymnarchus Niloticus 193 
 
 Fig. 13. Longitudinal vertical section of ventral part of region C of 12-day-old embryo of 
 Gymnarchus. Zone I indicates ventral epithelium; zone 2, ventral outer 
 portions of five myotomes; zone 3, lower median electric spindle which is 
 dividing into three parts in five vertebral segments; zone 4, inner degener- 
 ating muscle fibers of five myotomes; zone 5, projected outline of vertebral 
 segments. X 140. 
 
 Fig. 14. Motor nerve cells from spinal cord of 42-day-old larva of Gymnarchus. A, B, 
 and C, three electric motor nerve cells. D and E, two muscle motor nerve 
 cells. From region E. X 1500. 
 
 Plate 5. 
 
 Fig. 15. More highly magnified part of fig. 12. Brackets indicate same zones. X 760. 
 
 Fig. 16. Highly magnified part of transverse section through an electroplax from C region 
 of 42-day-old larva of Gymnarchus. Curving bundles of modified myofibrils 
 are seen in core and cut at various angles. Outer or electric layer is fixed and 
 stained to show finer outer granules or electrochondria. Vacuoles as well as 
 larger meshes of the linom are visible on inner edge of electric layer. Elec- 
 tric nerves are cut in sections at many angles and show medullary or connec- 
 tive-tissue sheath and its nuclei. At two points branching of these nerve fibers 
 is visible and at four points its contact with electric layer of electroplax is 
 to be seen. X 1500. 
 
 Plate 6. 
 
 Fig. 17. Longitudinal section of a young electroplax from B region of 12-day-old embryo. 
 Segmentation of electric tissue is now complete into unit electroplaxes of 
 which figure is good example. Central fibril bundle beginning to lose its 
 transverse striation. Structure is as long as three muscle segments or bony 
 segments, as near-by spinal processes show. This represents oldest stage in 
 embryo of this age. X 210. 
 
 Fig. 18. Longitudinal section of young electroplax from C region of 42-day-old larva of 
 Gymnarchus. Fibrils have lost all striation and become waved in middle 
 part. Cytoplasm shows strong differentiation at two poles (ends). Anterior 
 end strongly vacuolized. Papillae begin to grow from ends of middle part 
 and nerve-endings have become established at junction of posterior third 
 with middle third. This shows youngest stage in 42-day-old larva and is a 
 successively later step in differentiation than fig. 17. X 210. 
 
 Fig. 19. Longitudinal section of an electroplax from E region of 42-day-old larva of Gym- 
 narchus. Fibral bundle much "waved" in middle third, which now begins 
 to show form of electroplax. Papillae more developed. Nucleus and some 
 cytoplasm inclosed in fibrillar core. X 210. 
 
 Plate 7. 
 
 Fig. 20. Longitudinal section of another electroplax from further caudad in E region of 
 same larva of Gymnarchus. This represents oldest stage accessible and is 
 regarded as almost adult in its histological characteristics. Two nuclei 
 shown in fibrillar core; one of these shows characteristic swelling. 
 
 Plate 8. 
 
 Fig. 21. Transverse section of body of 42-day-old embryo of Gymnarchus from D region. 
 Greater part of muscle tissue is here transformed into electric tissue. Section 
 passes through very last part of dorsal fin. Electroplaxes cut at different 
 levels and in one case section passes through right ventral spindle showing 
 only electric connective tissue. D, dorsal spindles; U.M, upper middle 
 spindles; L.M, lower middle spindles; V, ventral spindles; Bi and Bi, 
 caudal blood-vessels; N, neural canal. Ni, Ni, Ns, and Nt, electric nerves; 
 Ns, lateral line nerve. X no. 
 
 Fig. 22. Part of transverse section from middle part of an electroplax in E region of same 
 larva of Gymnarchus. This figure serves well to show larger granules that 
 lie deeper in electric layer, also connective-tissue sheath or wall of electric 
 spindle. X 1500. 
 
 Plate 9. 
 
 Fig. 23. Longitudinal section from E region, showing parts or whole of 11 electroplaxes 
 lying in dorsal, upper middle, and lower middle spindles. Some remaining 
 muscle fibers are visible at sides. Figure serves to show that there is no 
 heavier, transverse connective-tissue wall lying in electric connective tissue, 
 as is found in Mormyrus. This emphasizes secondary segmentation found in 
 Gymnarchus. X no. 
 13 
 
194 Papers from the Marine Biological Laboratory at Tortugas. 
 
 Fig. 24. Small part from edge of basal part of a papilla on posterior surface of an electroplax 
 from C region of 42-day-old Gymnarchus. Shows a single nerve, ending in a 
 somewhat elongate and branched end-plate in electric layer. Outer crowded 
 mass of granules stained deeply witheosin; larger inner granules not stained, 
 X 1500. 
 
 Fig. 25. Small part of edge of another part of anterior surface of same electroplax. Shows 
 a nerve fiber dividing and each branch ending in typical club-shaped nerve 
 ending of Gymnarchus. X 2200. 
 
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