$7454) 078 tte - Sf5050 7; eo - 0 * 2 bee AO .*e * “+ + +. .* > “ss a ee ¥ Oy 7? - a aad > * * a ’ 7d «4, + * . 7,4 +" a*s e%s * +? $75 . * « * a @ . * « + ee 4 oe ©. 9,4 4% née. 6.8 *¢ 4-9 ee ere SAO wel if Banyo THE JOURNAL OF COMPARATIVE NEUROLOGY EDITORIAL BOARD Henry H. DoNALDSoNn ApoLF MEYER The Wistar Institute Johns Hopking University J. B. JOHNSTON Ourver S. Strona University of Minnesota Columbia University C. JuDsON HERRICK, University of Chicago Managing Editor VOLUME 34 FEBRUARY—OCTOBER 1922 PHILADELPHIA, PA. THE WISTAR INSTITUTE OF ANATOMY AND BIOLOGY tein ; SE (Lift i me yeh SEOs atin Pa coe] Di gia a r - a ; } ee CONTENTS No. 1. FEBRUARY ALBERT Kuntz. Experimental studies on the histogenesis of the sympathetic nervous ERECT pM DETCr TO UTCS. Ame 2.4). coaces tera An iy apinn coy a eee aint By BE Ei Scthe RIGehe ciate Saas ws e,i6 ld e il L.8. Ross. Cytology of the large nerve cells of the crayfish (Cambarus). Five plates Gime Tn —OUC) At OUETES)) 5 ees, oc. fc, 8 chp Releta ore, Mier aye See SEN o/c SR, MS op Ns ES hae ca 37 Marion Hines. Studies in the growth and differentiation of the telencephalon in man. ihe jissurahippacampy, Hifty-one figures. ... 86 (62) 4. cy Mabe ee chow oe one ee 73 No: 2, APRID N.E.McInpoo. The auditory sense of the honey-bee. Twenty-six figures.............. 173 G.W. Bartetmez. The origin of the otic and optic primoridainman. Ten figures...... 201 Davipson Brack. The motor nuclei of the cerebral nerves in phylogeny. A study of the phenomena of neurobiotaxis. IV. Aves. Sixteen figures....................000005 233 No. 3. JUNE H. Saxton Burr. The early development of the cerebral hemispheres in Amblystoma. MRED NES UNH AUIES han icyou ay ek Gc.e dBc on oats othe mtv anal tte meer meneRS avetotine cis sal ed ae le 277 Cart Caskey Spempen. Further comparative studies in other fishes of cells that are homologous to the large irregular glandular cells in the spinal cord of the skates. ipcoyplates toirteen aneres): sc fers: shee KR ne ee eR PAE ie eee CoE ates ee hs aR 303 Roy L. Moopiz. The influence of the lateral-line system on the peripheral osseous elemenis;of fishes and amphibia.» “Five figures.) )...5-...2ic0. dates 5 esis & be de ale es Sele 319 J.M.D.O.tmstep. Taste fibers and the chordatympaninerve. One figure.............. 337 No. 4. AUGUST Roy L. Moopre. On the endocranial anatomy of some Oligocene and Pleistocene mam- LAS ANWONIGY EV OT OMEROE cA) avela cies te ko erter sacah da Septal newark Seles 56 Ja Cri eyGrcel ania a TET Ace ee as aa Nee rn ee a Ev a ais AS as 57 SOUT ETS poht Ye, Ba. bes 1.08 Carat A SE gn AN Net Peete Sy Bt ee 58 MEGS TAL UTE IL CCL pasar. fice Neh os Paterna rere eo ahs SRA Se oe time tiat St tt BU 58 INTRODUCTION Since the discovery of mitotic cell division histologists have given much more attention to the nucleus of the cell than to the cytoplasm. This was due in part, and perhaps principally, to the search after the bearer of heredity resulting in the belief that the chromatin is such a bearer. For some time the cyto- plasm was more or less neglected, but of recent years it has re- ceived more attention, some cytologists insisting that in some degree, at least, it shares with the nuclear material in functioning as a transmitter of heritable characters. Even though this function be denied it, yet its importance is realized because of its relationship to the nuclear material as its immediate environ- ment, and of the interactions between them. ‘The complexity of the structure of the cytoplasm is manifest as a result of studies Norr.—I gratefully acknowledge indebtedness to Prof. R. R. Bensley, Prof. C. Judson Herrick, and Prof. G. W. Bartelmez for kindly criticism and valuable suggestion. 37 THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, No. 1 38 L. S. ROSS of recent years, but problems of its morphology yet remain unsolved. The cytologist is confronted in his work by some obstacles that at present seem almost insurmountable. It is difficult for him to determine in all cases whether he is observing structures that are present as such in the living cell or whether he is observing artefacts. Do Nissl bodies, neurofibrillae, Golgi internal reticular apparatus, etc., have existence in living undisturbed cytoplasm, or are they the results of violent disturbances caused by reagents? Important as the answers to these questions may be as concerns the ultimate structure of protoplasm, yet is it really of vital importance to the cytologist in comparative work, provided he realizes and recognizes limitations? Does this lack of knowledge of necessity vitiate all his conclusions? That artefacts are produced by reagents is not doubted, but the difficult question is, what appearances are those of artefacts and what ones are not. Certainly, some conjectures based upon various experiments in producing artefacts are not warranted. Unquestionably, re- agents produce artefacts in albumen solutions, as reticulations, etc., but comparisons between the effects produced by reagents upon the cell and upon colloidal solutions in a test tube are not wholly justifiable. At best the results from such experiments can give rise to inferences of probability only. As a generaliza- tion, however, similarity of results indicates similarity of materials upon which the technic was used. Likewise, dissimilarity of results indicates dissimilarity of materials. The importance of the study of living and postvital, fresh, unfixed cells cannot be overestimated, as apochromatic objectives and the true vital dyes have enabled us to distinguish various elements of the cytoplasm under these conditions. If the cytologist finds that a particular appearance is pro- duced by the same technic upon cells from the same source, and also by a variation of technic, and if he finds further that cells from different sources respond similarly to the same technic and to variations of technic within limits, then he has a basis for cytological work upon cells of a specific kind and also for com- parative cytology—a basis, it is true, that is not so satisfactory NERVE CELLS OF THE CRAYFISH 39 as that of recognized structural elements in the living cytoplasm, but one, nevertheless, upon which a stable scientific superstruc- ture may be erected. If Nissl bodies, neurofibrillae, etc., are made evident in various cells by the same technic, whether these be artefacts or not, the observer is justified in the opinion that there is probably also a similarity between the cells in the living condition. MATERIAL AND METHODS The nerve cell of the crayfish serves well as a subject for cytological study. Material is readily obtained and but little work has been done upon it. The ventral ganglia with their large cells are so easy of access that no postmortem changes need occur. Only two or three minutes are needed for the killing of the animal and the removal and transfer of the cord to the fixing solution. The large cells lend themselves especially well to the study of the trophospongium and of the origin of the axis cylinder. Most of the material used was from young specimens and adult specimens of Cambarus soon after they were collected in the field. Practically all of the current neurocytological methods were tried, but only the following yielded results at all satis- factory. 1. Intravitam staining After intravitam staining with thionin or methylene blue the neurofibrillae could be clearly seen in the fresh teased material. A .01 per cent aqueous solution of thionin or methylene blue was injected into the pericardial sinus. Upon the death of the animal, twenty to sixty minutes later, the abdominal nerve cord was removed into a thionin solution of the same or double strength and teased. For thirty minutes after removal no postvital changes were usually noted so that there was oppor- tunity to study the cells with some care. 2. Silver methods All of the silver methods proved to be exceedingly fickle. There were striking differences between the reactions obtained in young animals 4 to 53 em. long, and those obtained in adults. 40 L. S. ROSS For neurofibrillae the most satisfactory results were obtained by the Cajal procedure used by Boulé (’07) for Lumbricus which I have modified only by using formol neutralized with magnesium carbonate instead of adding ammonia to the commer- cial solution. The method employed by Cowdry (712), which involves fixation in the 6:3:1 mixture of Carnoy followed by silver impregnation and reduction with pyrogallol, and the Ranson pyridine-silver method were used also. For the internal reticular apparatus of Golgi the original Golgi technic and the Kopsch and Cajal methods were tried, but nothing was obtained in the large ganglion cells which could be interpreted positively as this apparatus. 3. General histological methods In some respects the most striking results were obtained by the use of Bensley’s (’11) ‘A. O. B.’ method. The mitochondria are preserved and stained, the Nissl substance can be dis- tinguished, and the intracellular axone is sharply differentiated with the individual fibrillae distinct. The procedure is as follows: 1. Fix 2 to 4 hours in: Potassium bichromate, 2.5 per cent, 16 cc. Osmic acid, 2 per cent, 4 ce. Acetic acid, two small drops. . Distilled water, few minutes. . Grades of alcohol to absolute, 2 to 4 hours each. . Aleohol 100 per cent and bergamot oil, half and half, 1 hour. . Bergamot oil, 1 to 2 hours. . Bergamot oil and paraffin, half and half, 1 hour. . Paraffin, 60°C., 2 hours. Section 41; fix to slides. . Remove paraffin with toluol or xylol. . Aleohol 100 per cent down through grades to distilled water. . Potassium permanganate 1 per cent, 15 to 30 seconds. . Oxalic acid 5 per cent, 15 to 30 seconds. . Stain 4 to 5 minutes at 60°C. in Altmann’s anilin fuchsin: Anilin water, 100 ce. Acid fuchsin, 20 grams. 13. Rinse in distilled water. 14. Methyl green or toluidin blue 1 per cent solution or Wright’s blood stain, a few seconds to be determined by trial. 15. Drain, dehydrate in alcohol 100 per cent, usual procedure for mounting. For tissue masses larger than crayfish ganglia it may be necessary to extend the time in the process through no. 7. See Nr COON ®D OP | LO NERVE CELLS OF THE CRAYFISH 41 For neurofibrillae a modification of the method of Donaggio (05) proved very serviceable. It is as follows: 1. Fixation, 24 hours in Heidenhain’s corrosive sublimate. Remove excess of sublimate with iodine solution. 2. 2 to 3 hours in distilled water. 3. 48 hours in pyridine, changing pyridine after 24 hours. 4, 24 hours in distilled water, changed frequently. 5. 24 hours in aqueous solution ammonium molybdate, plus 1 minim hydro- chloric acid to 1 gram of molybdate. Solution should be fresh. (I used ammonium picrate in place of molybdate.) . 24 hours in distilled water, changed a few times. . Dehydrate and imbed. Usual procedure. 8. Stain thin sections, 4 to 5u, in 0.01 per cent thionin, (Istained overnight. Stain is removed rather rapidly in dehydration.) For the study of the Nissl substance various additional fixa- tives were used. MHeidenhain’s corrosive sublimate solution, formol bichromate (Regaud-Cowdry), acetic-osmic-bichromate (Bensley) and 2 per cent aqueous osmic acid show distinct Nissl bodies with irregular clear spaces between them. Formol neutralized with magnesium carbonate or the 6:3:1 mixture of Carnoy show bodies, but they are not so clearly defined. Whether this is due to a different behavior of the stain or to the effect of the fixatives was not determined. All material was imbedded in paraffin and cut 3 to 7y in thickness. LITERATURE Relatively little work has been done on the cytology of the crayfish nerve cell. Holmgren’s theory of the ‘trophospongium’ was based primarily on his observations of Astacus as well as Lophius cells. His interpretation of the former I have dis- cussed in a paper (Ross, 715) in which I suggested confining the term ‘trophospongium’ to the capsular septa which extend into the cytoplasm. In the comprehensive paper of 1900 Holmgren figured a large nerve cell from Astacus which shows the typical intracellular axone, but this condition is not referred to in the text. Owsiannikow (’00) reported two types of neurofibrillae in Astacus cells, the finer fibers forming a network about the nucleus, the coarser fibers being peripherally situated and continuous 42 L. S. ROSS with those of the axone. Prentiss (’03) studied the neurofibrillae of the leech and crayfish. ‘‘ My preparations of Astacus showed no trace of fine fibrillae about the nucleus. . . . The fibril- lae appear relatively large and form a few large meshes in the peripheral region of the cell.’’ Dahlgren and Kepner (’08) refer to the ‘‘implantation cone that reaches far into the cell’ in the lobster, and make a general statement with reference to this condition in arthropods, but give no detailed description. Poluszynski (’11) studied the ganglion cells of Astacus, Squilla, and Homarus with the Kopsch and Sjéval methods, using Golgi’s arsenious acid technic as a control. In opposition to Holmgren he differentiated between the intracellular pro- longations of the capsular cells and the ‘Golgi-Kopsch apparatus,’ and concluded that in arthropods generally the latter appears as a series of isolated threads and granules, and not as a net- work. Since his results are based primarily upon the osmic acid preparations, it is by no means clear that his internal reticu- lar apparatus may not include other constituents of the cyto- plasm. R. Monti (’14 and 715) has concluded that Poluszynski did not see the true form of the Golgi apparatus. She says (transla- tion): Poluszynski, using the methods of Golgi and Kopsch, did not find the reticular apparatus; he observed in the nerve cell only short rods sometimes curved or distinct granules scattered throughout the body of the nerve cell. He maintains that this formation of granules or short rods is homologous with the Golgi reticular apparatus. In support of the results of his pupil, Nusbaum concludes that the Golgi appa- ratus presents diverse forms, as nets, filaments, bacteriform granules, circles, etc. In further discussion, she says that— When the reaction is good a figure may be recognized that corresponds to that given by Poluszynski, but upon following the reaction with the greatest care, a figure of extreme fineness is obtained that demonstrates the delicate formation of the structure to be comparable with the Golgi internal reticular apparatus. With a superb technic at her command, Monti studied the mitochondria and Golgi apparatus in ganglion cells of Astacus NERVE CELLS OF THE CRAYFISH 43 and Homarus, comparing them with corresponding elements in insect and mammalian ganglion cells. Monti’s figures are the only ones which give Golgi apparatus pictures of arthropod cells which resemble those of other forms, and we may assume that the preparations of other workers were incomplete or unreliable so far as this cytoplasmic structure is concerned. It seems that in these forms the apparatus is very delicate and that there is less tendency toward reticular formation than in vertebrate nerve cells. In addition to the diffuse Golgi apparatus, Monti discovered in the small ganglion cells of young Crustacea pre- pared by the Golgi method, a minute tightly wound skein of threads at one side of the nucleus which is directly comparable in appearance and position with the ‘centrophormien’ of Ballowitz or the Holmgren ‘canals’ as they appear in non-nervous cells. Monti reaches the conclusion that in the invertebrates a portion of the ‘chondrioma,’ i.e., the mitochondrial granulation as a whole, becomes the Golgi apparatus and the remainder disappears. ‘This conclusion is based merely on the superficial resemblance between the thick Golgi (arsenious acid-silver) preparations and thin sections stained to show mitochondria. Yet the lack of evidence for such a deduction should not detract from the great value of her morphological contributions. Un- fortunately for the present purpose, she did not obtain any satisfactory preparations of the large nerve cells of either Astacus or Homarus, so her figures cannot be compared directly with those presented in this paper. Tello (14) describes a cell in the electric lobe of the brain of the torpedo in which there is in Cajal preparations an intra- cellular axone almost identical in appearance with that of the crayfish. His‘text-figure 19 would serve as a figure of the cray- fish nerve cell were it not for the presence of a dendrite and of a fibrillar capsule about the nucleus. Because of the closeness of this similarity I give the translation of a part of his description: As may be seen in figures 19 and 20, the nerve cells of the lobe men- tioned (bulb of ray and torpedo) possess a very complicated reticula- tion. Immediately beneath the membrane lies a dense layer of rather coarse primary filaments arranged in bundles parallel and plexiform, 44 res. ROSS which to all appearances, without seemingly leaving the region, pass to one dendrite or another. Further, there is a submarginal delicate concentric zone that colors less intensely with silver, in which in a good preparation there may be discovered a complex network of fine filaments, pale, rather thick and circumscribed polygonal meshes. The compari- son of these preparations with those obtained by Nissl’s method shows that the zone is principally of chromatic granules. This has been well studied and described by Studni¢ka. Jmmediately following, a fibrillar zone is evident, consisting of dense bundles of unequal thickness, dense, concentric, extending through a great part of the protoplasm. The fibrillar layer is almost always discontinuous; often its bundles double and pass to a deeper layer of whorled and spiral appearance, previously observed by authors and especially by Studni¢ka. At the level of the zone the granules of Nissl are absent or are limited in number. Yet it is most interesting of all that its fibers unite at an angle of the soma, become very pale and delicate (usually appearing redder and clearer than the remainder of the reticulation) and producing the principal contingent of the axone. A section somewhat tangential reveals the whorls that the bundles of the layer describe, as well as the complica- tions of their deep derivation. For underneath the aforesaid fibrillar mass we find a thick, irregular layer of bundles, disoriented, and con- stituted principally of a reticulation similar to that of the second zone. And finally around the nucleus we observe an obscure capsule composed of compact primary filaments arranged in a dense network which recalls completely the pattern (emplazada) underneath the cellular membrane. Text figure 19 of Tello’s description shows an encircling intra- cellular axone, and figure 20 shows a whorled condition of fibrils similar to the appearance of figures 5 and 6 of my 1915 paper. Heidenhain (’11) describes a similar arrangement ofneurofibrils in certain spinal ganglion cells of the frog. OBSERVATIONS AND DISCUSSION Intracellular axone and neurofibrillae Since Apdthy’s work revived the interest in the fibrillar structure of nerve cells there has been much controversy as to the relations of the axone fibrils within the perikaryon. This has centered naturally about the possible function of these fibrillar structures. Are they the conducting substance par excellence? Are they morphological expressions of the stresses and strains in the ground-substance indicating the direction NERVE CELLS OF THE CRAYFISH 45 in which the nerve impulse travels, or are they simply supporting structures, as Sziits (’14) has suggested? Das Stiitzgeriist der Nervenelemente ist ihre neurofibrillére Struktur. Es ist in mehreren Fallen gelungen, den innigen Zusammemhang zwischen der Gestalt und der neuvofibrilliren Struktur der Nervenel- emente nachzuweisen. Die Neurofibrillen muss man daher fiir den Triiger und die Stiitze der Zellgestalt und nicht fiir specifische leitende Elemente ansehen. We are in no position as yet to decide this question. There are two problems, however, of fundamental interest which can be discussed profitably: 1) Do the neurofibrillae exist in the living protoplasm? 2) What is their relation to the ground-substance of the cytoplasm? The first of these questions has not received the attention it deserves. Most observers have tacitly assumed the preexistence of neurofibrillae in the living protoplasm. The evidence in favor of it is not convincing to say the least. In 1868 Max Schultze, a microscopist of the most exceptional ability, un- hampered by any preconceptions, discovered a fibrillar structure in the freshly teased cells of the electric lobe of Torpedo. This, as we have seen (p. 43), is as favorable a material for the pur- pose as the large crayfish ganglion cells. Dahlgren (’15) seems to be the only one who has attempted to confirm this observa- tion. He reported ‘“‘some trace . . . . of neurofibrils”’ and figures a faint longitudinal striation. This object deserves further study with the use of vital dyes. M. Heidenhain was impressed with the evidence obtained by Cajal (’07) of the individualistic behavior of neurofibrillae in regenerating nerves. The fact that fibrillar differentiations appear after so many diverse fixations surely indicates that, even if they do not exist as such in life, they represent a peculiar type of organization in the neuroplasm so that their presence in a given cell is strong presumptive evidence of its nervous character. The controversy in the literature has centered about their morphology in fixed and stained preparations which are assumed to give true pictures of their forms and relations. Two categories 46 ie vSs) OSs of fibrillae are generally recognized, the coarser fibers or net- work at the periphery and the more delicate plexus or net about the nucleus. The coarse cytoreticulum figured by Donaggio in the dog is uniform throughout the cell. Such a difference in appearance as compared with Cajal, Bielchowsky, and Bethe preparations is probably due to the peculiarities of the technical procedure. On general principles it is improbable that any networks pervade living protoplasm. ‘The point of view developed through the progress of colloid chemistry postulates that protoplasm consists of a microscopically homogeneous colloidal ground- substance in which are microscopic colloidal aggregates of eranulations, fibrillae, and the like. There are various catego- ries of these, each of which has its own physical and chemical peculiarities. This is not merely speculation. The beautiful cytoreticula that may be seen in the protoplasm of many eggs after acid fixation, sublimate, etc., have been proved to be artefacts by the study of the living protoplasm with modern optical equipment, by the micro-dissection studies and vital staining methods of Kite and Chambers, by the use of less violent fixing agents, and by the convincing centrifuging experi- ments of Lillie and others. The control of the technical pro- cedure in other tissues by the study of fresh material is of especial importance for the study of the nerve cell, for here the difficulties of observing the uninjured cell are very great. Bensley (11) found the acinar cell of the pancreas in the guinea-pig particu- larly favorable for postvital study with and without vital stains. After an extensive series of experiments with fixatives con- trolled by this means, he found that osmic acid mixtures produced less change than any other. This was especially true of his modification of the Altmann mixture, which he has termed ‘A. O. B.’ Nerve cells prepared by this method are particu- larly worth study, although it must be remembered that in such large elements as are considered in this paper it is possible that the acetic acid may often reach the center of the cell before the osmic acid and so produce the picture of an acid reticulum. From a consideration of these facts relative to other cells, I NERVE CELLS OF THE CRAYFISH AZT conclude that it is highly improbable that the neuroplasm of the crayfish nerve cell contains either a fine or a coarse network such as is revealed by the majority of neurofibrillar methods. As Heidenhain (’11) has pointed out, the killing fluids used for silver and gold neurofibrillar methods are poor protoplasmic fixatives. The same may be said of ammonium molybdate, ammonium picrate, nitric acid and aqueous mercuric chloride. In the latter case very different pictures are obtained when one does not treat the material first with iodine and then with pyridine as Donaggio did. The least that we can say is that the existence of a cytoreticulum and the supposed relation of neuro- fibrillae to a perinuclear net rest upon insecure foundations. The use of the term reticulum or reticulation in this paper is for convenience and does not commit the author to the opinion that a reticulum exists in living protoplasm. In his work upon the Crustacea, Retzius (90), pone upon insufficient evidence, reaches the conclusion that the large nerve cells, which are considered in this paper, are motor and the small ones are sensory. Dolley (’13) considers the cray- fish nerve cells as divided into two principal groups, the motor and the sensory, the principal differentiating characteristic being the presence of an ‘intracellular axone’ in the former. On the other hand, Allen (’94) considers that the cells within the ganglionic chain are motor and coordinating and that the sensory cells lie outside the chain. The cells without the ‘intracellular axone’ are much more numerous than those possessing it. Let us turn now to my own observations. The general form of the large cell of the crayfish is pear-shaped with the axone leaving the narrow end, the transition into the extracellular axone being gradual rather than abrupt. Not unfrequently the diameter of the axone, about one-fifth to one-fourth the diameter of the cell body, may be slightly greater near the exit from the cell than it is immediately outside. In some instances the axone is almost, straight at its exit, while in other cases it shows distinctly short, sharp undulations (fig. 7). In many nerve cells, especially in vertebrates, the gross origin of the axone is in the axone hillock or implantation cone near the 48 L. S. ROSS periphery of the cell, the cone usually being rather sharply differentiated from the cytoplasm immediately in contact by the absence of Nissl substance. Only as a name, other than descriptive, can the term axone hillock be used in connection with the large nerve cell of the arthropod (Dahlgren and Kepner, ’08), for the reason that the axone has an intracellular origin and a course almost or quite enveloping the nucleus. In the large cells of the abdominal ganglia of the crayfish the evident origin of the axone is not a definite implantation cone, but rather it is a band or tract of some width curving about the nucleus and composed of numerous fibrillae originating in all parts of the cytoplasmic mass (figs. 1 to 8, 10, 12,14 to 21). The main portion of the tract as it curves about the nucleus usually lies at a little distance from it, although in a few cells observed, in a plane of section parallel with the curve of the long axis of the tract, it not only touches the nucleus, but produces a very marked indentation without any break in the nuclear membrane (figs. 2 to 5 and 7). This cannot be an artefact produced by knife pressure, as there is no indication whatever of tearing of the ganglia; and also the indentations do not occur in an isolated section, but rather in several sections of series in different cells. At the place of contact there is no evidence of continuity of the tract with the nucleus. In some of the cells the compact en- circling portion of the intracellular axone is very broad and trough-shaped or cup-like with the nucleus situated in the depression of the trough. Figures 20 and 21 show extreme widening; these sections are consecutive, showing one limb of the cut tract on one side of the nucleus and ‘the other limb on the opposite side. Other sections in the series make it evident that the two limbs as shown are parts of the one greatly broadened tract. Figure 10 indicates a similar condition in another cell, while figures 17 to 19 show a sharp angle between the two limbs. Only three cells of the large numbers observed indicated such an extreme widening of the compact portion of the tract. Possibly sections of some of the cells in planes other than those followed might have made such a widening evident. NERVE CELLS OF THE CRAYFISH 49 The fibrillae of the intracellular axone in its more compact portions are more or less sinuous in their general course, which is spiral. The spiral arrangement may be observed in longi- tudinal sections and also in cross-sections of the tract. In some cross-sections observed, 4, in thickness, this is shown most beautifully by focusing on different planes. As the objec- tive is raised or lowered bringing different planes into view, there is a very striking appearance of a corkscrew movement on the part of the sections of the fibrillae. In the gold-toned Cajal preparations and also in the Kopsch preparations diverging fibrillae may be traced out from the compact intracellular axone into the cytoplasm for a short distance. Cross-sections of the tract, and also longitudinal sections, may show such fibrillae coming from the cytoplasm throughout the entire length of the intracellular axone. The greater abundance of the diffusely distributed fibrillae, however is to be found in the basal part of the cell and in the portion of the cell on the side opposite the compact part of the axone. One cell stained intravitam with 0.01 per cent thionin showed a fibrillar feltwork throughout the entire cytoplasmic mass, having the appearance of radiating from a point near the nucleus. The apparent reticulation was denser, of more numerous and finer meshes, in the portion of the cell distal to the axone exit. Some other cells showed a more equable distribution of the felt- work. Many of the cells stained in this way are very small and the fibrillae extremely delicate. The latest attempts at intravitam staining with 0.02 per cent thionin in normal salt solution indicate the presence of delicate fibrillae in the same position in the cytoplasm and with the same relation to the nucleus as is so distinctly shown in the fixed and stained speci- mens. ‘Two and one half cubic centimeters of the thionin solu- tion were injected into the pericardial sinus, with some loss, this being followed by two injections of a like quantity, thirty minutes intervening between two consecutive injections. Twenty minutes after the third injection the crayfish was decapi- tated and the cord removed into the thionin solution. Some cells were soon found, within twenty minutes, showing faint 50 L. S. ROSS lines in the cytoplasm. Forty-five minutes after decapitation of the crayfish a cell was observed with delicate stained lines in the intracellular axone. The color was retained for only a relatively short time after discovery of the cell, but long enough for corroboration of the observation by one of my colleagues. Evidently the cytoplasm showing the lines was stained some time before the expiration of the forty-five minutes; how long before there are no means of knowing. Only one of two inter- pretations can be placed upon the observation; the appearance is due either to death coagulation or it is due to the presence of neurofibrillae in the living cell. Dahlgren (’15) found ‘‘some trace of chromophilic bodies to be seen; also of neurofibrils”’ in living (?) nerve cells in the electric lobe of Torpedo. Sections stained with 0.01 per cent aqueous solution of thionin following fixation in Heidenhain’s corrosive sublimate solution show not only the network about the nucleus, but also the fibrillae of the axone; but here the possibility of artefacts produced by the technic intrudes itself. The structure as indicated by silver impregnation appears very similar to that shown by the thionin staining. Some of my sections prepared by the method Boulé (’07) used for the earthworm give indication of the endocellular reticulum, although not with a degree of distinctness equal to that obtained by the use of thionin. Possibly a somewhat greater difference in intensity between the perinuclear zone and the region toward the periphery of the cell is shown by Boulé’s method than by Don- aggio’s thionin stain. The intracellular axone in the Boulé preparations may be blackened in a portion of its length, but unaffected in the remainder. Where blackened, the compact portion of the band shows no differentiation whatever into fibrillae, but appears as a dense black mass. In some sections the band is blackened in its course from the periphery of the cell body and in the near vicinity of the nucleus shades off into rows of brownish granulations that encircle the nucleus, but, as indicated, not as sharply defined as shown by the thionin stain. The preparations by Boulé’s method gave some detail, while those made by Ranson’s method were failures as there was only a diffuse browning of the entire cell. NERVE CELLS OF THE CRAYFISH 51 Sections of cells cut in the appropriate plane show the intra- cellular axone and its position in relation to the nucleus. Prep- arations by Bensley’s A. O. B. method show the origin of the axone to be very widespread throughout the cytoplasm as a diffusely spreading basket of neurofibrillae within which the nucleus lies (figs. 1 to 6, 15, 16). The composite picture pro- duced by a study of various sections, stained with thionin, with silver, and with acid fuchsin, respectively, is almost identical with the one seen in the acid fuchsin preparations. One notable difference between the origin of the axone in the crayfish and in most vertebrates and in some invertebrates, as leech and earthworm, is the difference in the locality where the fibrillae unite to form the compact axone. In the vertebrate, the fibrillae usually pass from the region about the nucleus to the point of exit at the periphery of the cell, where they form the axone. In the crayfish, on the other hand, the axone is formed as a tract within the cell body, receiving additional fibrillae along its course even to its exit. Another difference is that the perinu- clear reticulation in the vertebrate cell seems to be closed, that is, fibrillae form a closed zone about the nucleus, while in the crayfish most of the fibrillae of the perinuclear reticulation, possibly all, have a free origin within the cytoplasm. While it is generally known that mitochondria are found among the closely crowded fibrillae of the axone hillock or the intracellular axone whereas the Nissl substance is wanting, yet no explanation of the fact has been given. Nissl substance Much confusion has arisen concerning the Nissl substance _ because of the purely morphologic point of view of many investi- gators (Cowdry, 712, p. 18). This substance is the only constit- uent of the cytoplasm that has been placed upon a firm chenfical basis. Held (95, ’97) was the first to study it from this point of view: and to postulate its nucleoprotein character. In 1897 Mackenzie reported the presence of organically bound iron in many of the Nissl bodies by the means of the Macallum micro- chemical test for iron. This work was greatly extended by Scott 52 L. S. ROSS (99), who also demonstrated the origin of the Nissl substance from the chromatin of the nucleus in the neuroblast of the pig. Because of its demonstration side by side with the other recog- nized elements of the cytoplasm, there is no reason for confusing it with other substances, in the vertebrates at least, and probably also in the arthropods. In no form has its morphology been adequately cleared up, nor can we hope for this until it has been studied by means of a vital dye in a living nerve cell in situ. No such specific dye has yet been found for the Nissl substance or for any other form of chromidial substance. There are no means, therefore, of knowing whether any of the granules seen in fresh cells are ‘cytochromatin,’ as Heidenhain (’11) has termed it. In certain small spinal ganglion cells of verte- brates (Cowdry, 714), and in the acinar cells of the pancreas (Bensley, ’11) the chromidial substance seems homogeneously distributed throughout most of the cytoplasm. The colloidal particles seem to be ultramicroscopic in size in these cells. Their absence from the axone hillock where mitochondria and ground- substance are present between the neurofibrillae is significant in this connection, for it shows there is a definite arrangement of the chromidial substance in the cytoplasm. Its absence from the great axone tract of the large crayfish cells is very striking (cf. figs. 10 to 12). It is possible that in this case, as in other cells where fixing agents produce definite Nissl bodies, the material is present intravitam in the form of ‘granules,’ but of this there is no evidence. Nissl bodies are figured and-described by many authors as they appear in fixed and stained cells, but it is very questionable if they exist as bodies in living nerve cells. Mott (?12), Marinesco (12), and Cowdry (14) could not find the bodies in postvital material. Failure attended by efforts to observe them in the living nerve cell of the crayfish. It is only after fixation that they become evident. I teased out individual cells from the abdominal ganglia in normal (0.75 per cent) salt solution, and examined them under low power, 4-mm. objective and no. 4 ocular, and under a 2-mm. Zeiss apochromatic oil-immersion objective and no. 4 ocular. Some of the cells were in their NERVE CELLS OF THE CRAYFISH 53 normal shape, the cover-glass being supported, while others were flattened by pressure upon the cover. In no instance was there any indication whatever of Nissl bodies. The cytoplasm is quite uniform throughout in its appearance, with numerous granules more or less irregular in shape and size and optical appearance, distributed with a marked degree of regularity. Some of the granules are almost at the limit of vision with the 2-mm. oil-immersion objective and no. 4 ocular; others are much coarser, being several times greater in diameter. In shape they vary from almost spherical to an angular outline. Some of the granules, at least, are mitochondria, as may be demonstrated by staining with Janus green. ‘There is no evidence of a grouping of granules into concrete bodies. Intravitam staining with methylene blue and with pyronin failed to demon- strate any grouping. Nissl bodies are of such size that they should be observable if they are present in the living cytoplasm, unless such observation is prevented by optical characters. In so far as I have been able to observe, they do not exist in the living nerve cell of the crayfish as formed bodies. The question involved is not as to the existence of Nissl substance, but as to the morphology of the substance, whether it exists in the condition of dispersed granules or in the form of masses, or homogeneously dispersed through the cytoplasmic ground-substance. The substance exists in living protoplasm but in all probability only in the dispersed condition. Golgi internal reticular apparatus The Golgi internal reticular apparatus has been demonstrated in nerve cells from many sources, by certain technic taking on stain and by other technic remaining as clear unstained spaces of various sizes and forms. Misch (03) reported some vertebrate nerve cells as not showing the presence of the ‘Binnennetz.’ Perhaps the results obtained by Misch may be attributed to imperfect technic. On the other hand, Cajal (’03) has found the apparatus in every type of nerve cell he has examined, and he believes it to be universally present in all vertebrate cells. THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 1 54 if So ROSS Some workers think that the apparatus is an artefact (cf. Cowdry, *12). It seems to be definite that at least some of the clear spaces appearing in fixed cells from some sources are not artefacts. By the Kopsch osmic-acid method some of the clear canals in a given section may be stained, while others remain unblackened (Cowdry, ’12). In examinations of the living nerve cell of the crayfish I could not distinguish any apparent reticulations of clear spaces in the cytoplasm; the entire mass was seemingly a mixture of granules in a more fluid matrix. Nor did intravitam staining with thionin, methylene blue, or with pyronin show their pres- ence. The fact that failure attended the search for the appara- tus in the living cell does not give much evidence against its presence. The cell is of such a thickness and it contains so many granules of various sizes that it would be difficult to demonstrate spaces devoid of granules unless the spaces were relatively large. Clear spaces or canals appear in the prepara- tions by the Bensley method, as shown in figures | to 6, 8 to 12, 15, 16, and in preparations fixed with osmic acid, Heidenhain’s corrosive sublimate, etc., but not quite so clearly in the Carnoy preparations. These unstained spaces are connected with somewhat wider elongated spaces radiating from the path of the intracellular axone. Numbers of attempts were made to demonstrate the apparatus by the Kopsch osmic method, by Veratti’s method, and by Cajal’s modified uranium nitrate method, these invariably resulting in failure. If Nissl bodies do not exist as concrete masses of cytoplasmic material in the living cell, then it is very evident that an aggluti- nation is effected by the reagents. As a mechanical result of such action granules might be withdrawn from some portions of the cytoplasm, giving those portions a clear appearance. The shape of the spaces would be determined by mechanical forces. Mathematical regularity would not result, but rather irregularity due to a lack of perfect homogeneity of the cyto- plasm. The general appearance of the clear spaces is that of an irregular reticulation. Some of the Nissl bodies tend toward a polygonal outline, while others are somewhat elongated. ‘The NERVE CELLS OF THE CRAYFISH ao shapes of the bodies and of the spaces are correlated. In the earlier part of my work I was of the opinion that these spaces are the Golgi internal reticular apparatus, but now I believe them to be artefacts. Poluszynski’s work (711) on the ganglion cells of Astacus, Homarus, and Squilla, chiefly by the Kopsch method, but also by the original method of Golgi as a control, shows the struc- tures stained by this method very variable in appearance. Instead of an ‘apparato reticulare’ present in most cells, he finds a series of isolated structures darkened by the procedure. He believes this ‘granular’ form of the Golgi-Kopsch apparatus to be characteristic of arthropods, certainly of Crustacea. His figures of the cells of Astacus show bodies of the same order of magnitude as the reticular apparatus of other nerve cells, but he is somewhat doubtful whether they are normal structures. He considers the most typical form of the apparatus to be delicate filaments blackened by the Kopsch method in Homarus cells. By this same method I have demonstrated similar fila- ments in Cambarus cells, but I am convinced that they are not the Golgi apparatus, but are fine ramifications of the tropho- spongium which anastomose, thus differing, from the reticular apparatus in Astacus as described by Poluszynski. The fila- ments are much more delicate than the clear spaces observed in the same cells after A. O. B. fixation. It may be said also that the Kopsch method is not to be considered specific, since under certain conditions the mitochondria also are well stained by it. The figures given in the papers by Monti represent elongated threads of various shapes convoluted and frequently anastomos- ing that in no way can be considered homologous with the clear spaces appearing between the Nissl bodies in mypreparations. The threads are more regular in diameter and do not form such a continuous reticulation. Both of the papers tend to corrobo- rate my conclusion that the clear spaces between the Nissl bodies in my preparations are not the Golgi reticular appara- tus, but are mechanical artefacts. There can be no question but that the structures observed by Monti in the small nerve cells of Astacus and Homarus are the Golgi apparatus. 56 i 8. ROSS No one has succeeded in obtaining preparations of the large crustacean nerve cells we are here considering which could be interpreted as revealing a typical Golgi apparatus, nor have I had any better success. Whether it is absent or whether the osmotic conditions are such as to make it difficult to obtain com- plete impregnations in these giant elements remains for further work to decide. Figure 13 is made from a drawing of a cell in a Golgi arsenious acid preparation and represents the only type of positive stain that has been obtained in large nerve cells. It obviously bears little resemblance to the Golgi apparatus as we find it in other nerve cells. These isolated elongated ele- ments are of a greater order of magnitude than those obtained by Poluszynski in Kopsch preparations and may possibly repre- sent greatly swollen mitochondria. Trophospongium In some cells in ganglia treated by the Kopsch method very fine blackened bodies, either short or elongate, are evident. Some of them are not larger than mitochondria. Some appear as very delicate rods varying from very short to elongate fila- ments. These seem to have no relation in position to the clear spaces between the Nissl bodies, but are distributed promiscu- ously throughout the cytoplasm, and are more numerous in some cells than in others, while in yet other cells none are ob- served. The more delicate of the filaments are usually found in the interior of the cell and the larger toward the periphery. In some cases those toward the periphery are demonstrably por- tions of the trophospongium, a more or less complex framework continuous with sheath cells and penetrating into the cytoplasm. It seems evident that the very small blackened filaments in the interior of the cell are fragments of the trophospongium and that the elements of the trophospongial framework are present throughout the mass of the cytoplasm even to the center of the cell, and that they attain such delicacy as to tax microscopic vision. That these filaments are not of the Golgi reticular apparatus is indicated by their extreme delicacy, and especially by their connection with the framework invaginated from the NERVE CELLS OF THE CRAYFISH o7 sheath cells. They indicate a delicacy and an intricacy of Structure of the trophospongium more marked even than pre- viously reported (Ross, 715). The finely filamentous structures described by Poluszynski (’11) in Homarus may well have been trophospongium stained by the reticular methods, since hedid not appreciate the extreme delicacy of this supporting framework. Mitochondria ‘“Mitochondria may be provisionally defined as substances which occur in the form of granules, rods and filaments in almost all living cells, which react positively to Janus green and which, by their solubilities and staining reactions resemble phospholipins and, to a lesser extent, albumins”’ (Cowdry, 716, p. 425). Altmann added another element to the known complexity of the nerve cell a quarter of a century ago when he observed that in adult nerve cells there are minute bodies now known as mito- chondria. Since that date the literature upon mitochondria has become voluminous, although most of the work has been of recent years. Mitochondria are now known to be present in the cells of nearly all tissues of both animals and plants. ‘‘ They (mitochondria) occur in almost all cells . . . they are as characteristic of cytoplasm as chromatin is of the nucleus”’ (Cowdry, ’16). Mitochondria are readily demonstrable in the crayfish ganglia, in the cell bodies and along the course of the fibers. Such variations in size and shape as are usual in cells from other sources are to be observed here. Some are granular, others short rod-shaped, or of longer rods, straight or slightly bent; granules or rods isolated or grouped or arranged in rows giving a broken line appearance (figs. 9 to 12). As it was not the purpose of the study to investigate the technic of mitochondria staining, only a few methods were used. A few trials of Regaud’s method (’10) were made without satis- factory results, as no mitochondria were made evident. Janus green gave its typical reaction. Bensley’s acetic-osmic-bichro- mate acid-fuchsin method yielded the best results. Although this stain is not permanent, yet it was retained quite distinctly for two and one-half years or longer. 58 ie (S. ROSS SUMMARY 1. The intracellular origin of the axone of the large nerve cell of the crayfish consists of a large number of neurofibrillae widely distributed in the cytoplasm and almost surrounding the nucleus. The fibrillae converge to form an intracellular axone deep within the cell body, whereas in some invertebrates and in most of the vertebrates the fibrillae converge to an axone hillock near the periphery of the cell. 2. By means of intravitam staining some evidence of the probable existence of neurofibrillae in living cytoplasm is obtained. 3. The Golgi internal reticular apparatus was not demon- strated. 4. Nissl bodies were not demonstrable in living cells, but are evident in fixed and stained cells. The bodies are probably artefacts, although there is no reason to doubt the chemical entity of the Nissl (chromidial) substance. 5. Mitochondria are readily demonstrable in the cell bodies and along the course of the fibers. 6. The trophospongium shows connection with the sheath cells, and may be traced as very delicate filaments, in section, penetrating even to the center of some cells. Drake University, Des Moines, lowa. LITERATURE CITED ALLEN, E. J. 1894 Studies on the nervous system of Crustacea. Q. J. Mic. Sc., N.S., no. 36, pp. 461-497. Ascott, G. von 1911 Zur Neurologie der Hirudineen. Zool. Jahrb., Abt. f. Anat. u. Ontog., Bd. 31, S. 473-494. Bayuiss, W.M. 1915 Principles of general physiology. London. Brenstey, R. R. 1911 Studies on the pancreas of the guinea-pig. Am. Jour. Anat., vol. 12, pp. 297-388. Betue, ALBRECHT 1895 Studien itiber das Centralnervensystem von Carcinus maenas nebst Angeben iiber ein neues Verfahren der Methylenblau- fixation. Arch. f.mikr. Anat., Bd. 44, S. 579-622, Tafel 3. 1897 Das Nervensystem von Carcinus maenas. Arch. f. mikr. Anat., Bd. 50, 8. 460-546, Tafel 6. 1900 Ueber die Neurofibrillen in den Ganglionzellen von Wirbel- thieren und ihre Beziehungen zu den Golginetzen. Arch. f. mikr. Anat., Bd. 55, S. 513-544, Tafel 3. NERVE CELLS OF THE CRAYFISH 59 Bratkowska, W., uND Kutrkowska, Z. 1911 Ueber den Golgi-Kopschschen Apparat der Nervenzellen bei den Hirudineen und Lumbricus. Anat. Anz., Bd. 38, S. 193-207. 1912 Ueber den feineren Bau der Nervenzellen bei verschiedenen Insekten. Bull. de l’Akad. Sci. de Cracovie. Bout, L. 1907 L’impregnation des elements nerveux du lombric par le nitrate d’argent. Le Névraxe, T.9, pp. 313-327, 10 figs. CagaL, Ramén y 1903 Un sencillo metodo de coloracion selectiva del reticulo protoplasmico y sus effectos en los diversus organos nerviosos. Trab. Lab. Biol. Univ. de Madrid, T.2, pp. 129-221, 38 grabados. 1903 Neuroglia y neurofibrillas del Lumbricus. Ibid., T. 3, pp. 227- 285, 4 grabados. Cowpry, E. V. 1912 The relations of mitochondria and other cytoplasmic constituents in the spinal ganglion cells of the pigeon. Internat. Monatschr. f. Anat. u. Phys., Bd. 29, 8S. 473-504. 1914 The comparative distribution of mitochondria in spinal ganglion cells of vertebrates. Am. Jour. Anat., vol. 17, no. 1, pp. 1-29. 1916 The general functional significance of mitochondria. Am. Jour. Anat., vol. 19, no. 3, pp. 423-446. DauwuGREN, Untric 1915 Structure and polarity of the electric motor nerve cell in torpedoes. Proc. Carnegie Inst. of Wash., vol. 8, no. 212, pp. 213-256, 6 pl., 6 text figs. DauLGREN, U., AND Kepner, W. A. 1908 A text-book of the principles of — animal histology. New York. Doutey, D. H. 1913 The morphology of functional activity in the ganglion cells of the crayfish, Cambarus viridis. Arch. f. Zellforsch., Bd. 9. Donaaeio, A. 1905 The endocellular fibrillary reticulum and its relations with the fibrils of the axis-cylinder. Rev. Neur. and Psych., vol. 3, pp. 81-100, 5 plates, 1 text fig. HeEIDENHAIN, M. 1911 Plasma und Zelle. Handbuch der Anatomie des Men- schen von Bardeleben, Bd.§8. Hewtp, Hans 1895 Beitriige zur Structur der Nervenzellen und ihere Fortsitze. I Teil. Arch. f. Anat. u. Phys., 1895, S. 396-416. II Teil. Ibid., 1897, S. 204-289. HotmaGren, E. 1900 Studien in der feineren Anatomie der Nervenzellen. Anat. Hefte, I. Abt., Bd. 15, 8. 1-89. Mackenzik, J. J. 1897 Investigations in the micro-chemistry of nerve cells. Proc. British-Soc. Adv. Sc., 1897, p. 822. Marinesco 1912 Reference to Bayliss, 1915, p. 470. Miscu, J. 1903 Das Binnennetz der spinalen Ganglienzellen bei verschiedenen Wirbeltieren. Internat. Monatschr. f. Anat. u. Physiol., Bd. 20, S. 329-414. Mont, Rina 1914 L’apparato reticolare interno di Golgi nelle cellule nervose deicrostacei. R.Accad. dei Lincei, 8.5, Rend. 23, 1 Semestre, pp. 172- teehee dre 1915 I condriosomi e gli apparato de Golgi, etc. Arch. Ital. Anat. Embry., T. 14, pp. 1-45. Mort 1912 Reference to Bayliss, 1915, p. 470. 60 i. SS. ROSS Nussaum, Joser 1913 Ueber den sogenannten Golgischen Netzapparat und sein Verhiltnis zu den Mitochondrien, Chromidien und andern Zell- strukturen im Tierreich. Zusammenfassendes Sammelreferat. Arch. Zellforsch., Bd. 10, 8. 357-367. Osporn, H.F. 1917 The origin and evolution of life, New York, p. 78. OwSIANNIKOW, PH. 1900 Ueber die Nervenelemente und das Nervensystem des Flusskrebses (Astacus fluviatilis). Mem. Acad. Se. St. Peters- bourg, (8), I. 10, no. 32. PouuszynskI, G. 1911 Untersuchungen iiber den Golgi-Kopschschen Apparat und einige andere Strukturen in den Ganglienzellen der Crustaceen. Bull. intern. Acad. Se. Cracovie, Cl. Se. Math. nat., B., pp. 104-145. Prentiss, C. W. 1903 The neurofibrillar structure in the ganglia of the leech . and crayfish with especial reference to the neurone theory. Jour. Comp. Neur., vol. 13, pp. 157-175. 2 plates. Rerzius, G. 1890 Zur Kenntnis des Nervensystems der Crustaceen. Biol. Unters., N. F., Bd. 1, S. 1-50. Ross, L. 8. 1915 The trophospongium of the nerve cell of the crayfish (Cam- barus). Jour. Comp. Neur., vol. 25, no. 6, pp. 523-534, 3 plates. SancHEZ, Dominco 1904 Un sistema de finisomos conductos intraproto- plasmicos hallado en las cellulas del intestino de algunos isopodos. Trab. Lab. Univ. de Madrid, T. 3, pp. 101-111, 6 grabados. Scott, F. H. 1899 The structure, microchemistry and development of nerve cells, with special reference to their nuclein compounds. Trans. Canadian Inst., vol. 6, pp. 405-488. SOUKHANOFF, SERGE 1904 Contribution 4 1’étude du réseau endocellulaire dans les éléments nerveux des ganglions spinaux. Le Névraxe, T. 6, pp. 77-80. Sztts, ANDREAS von 1915 Studien iiber die feinere Beschaffenheit des Nerven- systems des Regenwurmes, nebst Bemerkungen iiber die Organisierung des Nervensystems. Arch. f. Zellforsch., Bd. 13, pp. 270-317, Tafel VIII-Ix. Tevio, F. 1904 Las neurofibrillas en los vertebrados inferiores. Trab. Lab. Biol. Univ. de Madrid, T. 38, pp. 113-151, 20 grabados. Van GEHUCHTEN, A 1904 Considérations sur la structure interne des cellules nerveuses et sur les connexions anatomiques des neurones. Le Névraxe, T.6, pp. 82-116, Planch 1. WeicL, R. 1912 Zur Kenntniss des Golgi-k opschschen Apparat in den Nerven- zellen verschiedener Tiergruppen. Vehr.d.8. Internat. Zool. Kongr. z. Graz, Jena, 1912, S. 589-595. EXPLANATION OF FIGURES All figures are of abdominal ganglion cells of Cambarus. Figures 1 to 16 show the distribution of the neurofibrillae and the origin of the intracellular axone. Mitochondria are shown in figures 9 to 12. Photomicrographs and drawings, except figure 14, were made by the author from sections 4y in thickness, except figure 13, from a section 7u in thickness. Figure 14 was drawn by Mr. Streedain, to whom acknowledgment is due for valuable suggestions. ABBREVIATIONS a, intracellular axone n, nucleus c, clear spaces, artefacts (?) o, origin of intracellular axone m, mitochondria tr, trophospongium N, Nissl bodies un, undulations in intracellular axone 61 PLATE 2 EXPLANATION OF FIGURES The sections illustrated in plate 1 are from abdominal ganglion cells prepared by Bensley’s A. O. B. method, and were cut approximately parallel with the long axis of the intracellular axone. Sections are 4u in thickness. Figures 1 to 4 represent photographs of a series of sections from a single cell, showing the intracellular axone curving about the nucleus and in contact with it in figures 2,3, and 4, but without organic continuity. Figure 4 shows a lateral bending of the axone fibrils as the plane of the section passes to one side of them at the right of the nucleus. Figure 5 is of a section from another cell, and it likewise shows contact of axone fibrils with the nucleus. Undulations of the intracellular axone are to be observed near the point of exit from the cell body. Figure 6, from yet another cell, shows better than the others the diffuse character of the intracellular axone not far from its exit. Neurofibrillae are evident in all the sections, but most distinctly in figure 6. A reticulation of clear spaces, artefacts (?), is evident in all the figures. Magnification: figures 1 to 4, X 539: figure 5, X 576; figure 6, X 563. PLATE 1 NERVE CELLS OF THE CRAYFISH L. 8. ROSS PLATE 2 EXPLANATION OF FIGURES Figure 7 is diagrammatic to the extent that it was drawn from two different sections in the series, one cell from one section and the other from one three sections farther along in the series. Several sections show the two cells side by side. The plane of the section lies parallel with the intracellular axones and with the curve around the nuclei. Here also, as in figures 2 to 5, plate 1, the axone fibrils are in contact with the nuclei, forming indentations. One cell shows marked undulations in the axone near its exit from the cell body and shows also the neurofibrillae in a position parallel with the undulating surface. The neuro- fibrillae could not be traced far into the cytoplasm. The clear reticulations show continuity with spaces radiating from the path of the intracellular axone. Figure 8 is a photograph of a section from another specimen. In this the curve of the intracellular axone is in the opposite direction from that in fig- ure 7. The plane of section lies within one axone for a slightly greater distance than it does in the other, in the one being more nearly through the center of the nucleus. Both figures are from A. O. B. preparations. Magnifications: figure 7, X 562; figure 8, < 750. 64 PLATE 2 NERVE CELLS OF THE CRAYFISH L. 8. ROSS PLATE 3 EXPLANATION OF FIGURES Figures 9 to 12 show the clear reticulations, Nissl bodies, mitochondria in the cytoplasm and in the intracellular axones, and small portions of the tropho- spongium. Figure 10 shows the compact portion of the somewhat cup-shaped intracellular axon partially encircling the nucleus, a condition observed in but few cells. Figure 12 illustrates a section whose plane is parallel with the curve of the intracellular axon. Only a relatively small part of the axone appears in figure 11. The mitochondria are distributed irregularly throughout the cell body; they appear in large numbers in the intracellular axone, with their arrange- ment in general parallel with the neurofibrillae. Figure 13 shows bodies that possibly are greatly swollen mitochondria. Figures 9 to 12 were drawn from A. O. B. preparations, and figure 13 from an arsenious acid silver preparation. Magnifications: figures 9 to 12, < 700; fig- ure 13, X 866. 66 NERVE CELLS OF THE CRAYFISH L. S. ROSS PLATE 3 67 PLATE 4 EXPLANATION OF FIGURES Figure 14 is diagrammatic and shows the general relationship of position between the nucleus and the intracellular axone. The drawing shows the widely diffuse arrangement of the neurofibrillae as they originate in the cytoplasm and as they converge to form the compact portion of the axone. The nucleus is represented as being free from the axone fibrils, as this is the usual condition. x 1200. Figures 15 and 16 are photographs from an A. O. B. preparation, both from the same cell. The diffuse origin of the intracellular axone is shown especially well in figure 16. A small portion of the compact portion of the intracellular axone appears in both figures. > 600. 68 PLATE 4 NERVE CELLS OF THE CRAYFISH L. 8. ROSS 69 PLATE 5 EXPLANATION OF FIGURES Sections from two cells are illustrated in this plate; figures 17 to 19 in series from one cell of a Kopsch osmic-acid preparation, and figures 20 and 21 from one cell of an A. O. B. preparation. These are presented to show the wide spreading of the intracellular axone within the cell body that is sometimes found. In figures 17 to 19 the angle between the two limbs appearing in the photograph is acute, while in figures 20 and 21 the limbs join in a wide curve. Magnification: figures 17 to 19, X 700; figures 20 and 21, X 327. NERVE CELLS OF THE CRAYFISH PLATE 5 L. S. ROSS 71 Resumen por la autora, Marion Hines. Estudios sobre el crecimiento y diferenciacién del telencéfalo del hombre. La fisura hipocdmpica. En la pared madia del dehisferio cerebral de los embriones humanos de 16 a 30 mm. de longitud la fisura hipocdmpica (Bogenfurche de His) aparece en forma de un surco poco pro- fundo que se extiende desde el bulbo olfatorio hasta el extremo del l6bulo temporal. El hipocampo primordial puede ya re- conocerse en los embriones de unos 10 mm. de longitud a con- secuencia de su pared mds gruesa, matriz mds estrecha y un velo marginal mds claramente definido que el del drea situada lateralmente en inmediata proximidad. Esta separado del drea epitelial media por el surco limitante del hipocampo. Esta es la primera diferenciacién cortical conocida en el hombre. El telencéfalo medio est dividido en el plano medio en una placa terminal y un techo por el dngulo terminal. El drea epithelial contfigua a la regién del plano medio se diferencia en tres dreas caracteristicas: el septo ependimario, el drea intecalada y la l4mina epitelial. La fascia dentata procede de la matriz del borde ventral del hipocampo. La fisura primaria de His apareco en embriones de 25mm. coincidiendo con la mareada evaginacién del bulbo ol- fatorio. En las fases tempranas el hemisferio cerebral se dilata mediante crecimiento intrinseco de cada sector particular y por la mareada aceleration del tejido neopalial. Se alarga mediante aceleration del crecimiento de la linea media en la l4mina terminal y el pliegue ditelencefalico mediante la proyec- cion de arcos de nuevos tejidos que forman los lébulos frontal, parietal, occipital y temporal. Por consiguiente el crecimiento relativo de las estructuras del plano medio susministra un nuevo método para medir el crecimiento telencefalico. Translation by José F. Nonidez Cornell Medical College, New York AUTHOR’S ABSTRACT OF THIS PAPER ISSUED BY THE BIBLIOGRAPHIC SERVICE, JANUARY 16 STUDIES IN THE GROWTH AND DIFFERENTIATION OF THE TELENCEPHALON IN MAN. THE FISSURA HIPPOCAMPI MARION HINES Hull Laboratory of Anatomy, University of Chicago, and The Carnegie Institution of Washington, Laboratory of Embryology, Baltimore FIFTY-ONE FIGURES CONTENTS JH GTS ROLOTIOLIIO & oc.0.0 So Ce a eee ee BPE CRS IG DOR ce TEER el ds ene oI 73 IST ELOI A bg Sd a dtd Odio HO REIERS oR Oe SCE REE Ree eee eee ee ee 74 IMIsAreisiDl) Grae! MNOS ens Geri ecerct Ses oR His encanta aS A i ns Mlb 80 CeneralemonpnOlo syepyra dlasits oo aes ae bok eeieh SE tong le sak oe aie one ahs 81 (Elia OlOmiCallestnu ChUe tas ei ays Sari toon feeb hy eels weet pual 5 NEM eat ead ger Seen 104 IDRSGUSENOM:. pe aSoeloD oes ab aloe bh eis 8D OG SiGe OLIN Ene can Cote nee pee aro 118 PS LEMCe DO ALOM TMECNURIN cepa ckys 21s 5s erahe oN rkestrmeres 2a lak Sea na x oh De ae alee ee 118 Js SHECN (3 ONT ORL 0S2) J Cia tr ean Rae yc aig tee an eh ed Sreed MeL OR a 126 SEP AHOCL CEI ED pL ast hey ESR Actor eee ares at A et pacts A betas Sar in AE aa ee Ne eS 151 FATT © CENTIN PULSE oe sean Pe aoe Poe cs RS eh ak cle yon RP rare ee oaks there bash res 155 HIER EELE Re NE NIC ATU Sy. ad wer onsher Pcrs rans akc, «on Ae nas See re oe 158 The relation of the hippocampus to the neopallium.................... 162 SUITTIGEN a5 oe Aces re thaate CREOLE GI aE BCA ORR ERROR CLA CLG & CREME Tease a 167 UTE O STO kanya ere aeons a voy cial syalore Aivayenelel oS ckeakoie loves iat Oe OMB ROOD oat ae atte 169 INTRODUCTION No question in the history of biological science has gripped the imagination of the student of living matter as much as that of growth. ‘To understand the processes of life, the movement of growth must be studied. In the work to be presented this problem may be approached by singling out movement arrested by death and calling the morphology of its expression stages of growth. It is the interpretation of the morphological and his- tological changes between these so-called stages out of which the student may build a dynamic conception of growth. The first step in such an analysis is the establishment of landmarks or points from which to measure change. It is the purpose of 73 THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, No. 1 74 MARION HINES this paper to establish landmarks in the growth and differentia- tion of the telencephalon, such that accurate measurements of the varied components of the developing cerebral hemispheres may be taken. Such a purpose is the outcome of a considera- tion of the question, long a mooted one, of early telencephalic fissuration. That history is complicated by a group of uncor- related and seemingly contradictory facts, which bear the names of the most eminent neurologists and embryologists of the latter part of the nineteenth century. The solution of that problem depends upon a more modern technique and a consideration of histological structure. The reality of certain fissures which appeared in the medial wall of the cerebral hemispheres of the human embryo between the second and the fourth months was under debate from 1868 to 1904. These fissures were variously named, the most impor- tant being the arched fissure (or the Bogenfurche, the fissura ammonis, the fissura hippocampi) and the fissura prima. The Bogenfurche was divided into an anterior and a posterior limb and sometimes radial folds and an arched accessory fissure were added. Are they real or are they artefacts? If they are real, why do they disappear after four and a half months and seem to play no part in the future fissuration of the medial wall? Pre- vious investigators answered them each in his own manner. Those answers have a peculiar bearing upon the present discussion. HISTORY Meckel (1815) thought that fissures appeared on the medial wall which later ‘‘grew into each other, so that the surface of the brain both inside and outside again becomes smooth.” ‘Tiede- mann (1816) pictured them, but did not consider them to be transitory; rather, he thought them to represent earlier condi- tions of permanent sulci. The study of the central nervous system remained fifty years where Tiedemann had left it. Bis- choff (’68) reported these fissurations as due entirely to alcoholic fixation. However, in the next year, Ecker reported finding them in the fresh brains of mammalian embryos. Schmidt (92) referred to these fissures as temporary furrows, but failed THE FISSURA HIPPOCAMPI TO to find them in the sheep, ox, and pig. ‘The maceration arte- facts are so great in Marchand’s embryos (’91, p. 312) that his description of the hintere Bogenfurche is of little value. But in treating of the vordere Bogenfurche (45-mm. embryo, figs. 1 and 2) he says that the olfactory ‘‘bulb is separated from the sub- stantia perforata by a transverse sulcus, which is continued on to the medial surface as the ‘vordere Bogenfurche’ (incisura prima).”’ In 1909, although he had recognized the radial folds as arte- facts, Marchand nevertheless described a slight indentation (in the fourth month) which extended from the tip of the temporal pole to the region just anterior to the lamina terminalis. The following year Cunningham (’92) defined the fissures in question as “‘a series of furrows which radiate in a stellate manner from the fissura arcuata (Bogenfurche) toward the free border of the hemisphere.’”’ He believed that ‘‘the influence at work in calling the infolding of the cerebral wall into existence appears to be a purely mechanical one, viz., a restraint placed upon the longitudinal growth of the hemisphere; and this being the case it is easy to understand how the number and depth of the fissures will vary with the degree and kind of restraint which is applied” (p. 14). As to their obliteration, he thought that as the cerebral vesicle thickens and the hemisphere elongates, the stellate fissures become detached one by one from the previous arcuata. ‘‘In all cases, however, the posterior hippocampal portion is preserved in situ’ (p. 16). The anterior part is obliterated at the time of the disappearance of the radial folds. He suggested, as did Anton (’86), that the disappearance of the transitory fissures has some connection with the appearance of the corpus callosum. He cites several cases of radial fissuration in the brains of Macro- pus and Halamaturus in which the corpus callosum is rudi- mentary, and a few instances of congenital absence of the corpus callosum in man. In these brains the radial appearance of fissuration is evident. In 1898 Hochstetter reported that he could produce the fissura- tion under discussion by waiting several hours after the death of young fetuses before fixing them. Six years later he challenged His to meet him in Jena. But His could not come. Hochstetter 76 MARION HINES demonstrated his preparations without a dissenting voice. Those preparations consisted of three embryos, a 13.6 mm., a three months fetus of 50 mm. C. R., and a model of a 19.4 mm. C. R. There was no fissura arcuata in any of these specimens. How- ever, he noted in well-preserved brains, two to four months old, ‘a slight trough-like invagination of the medial hemisphere wall” (p. 31), but could not identify an arched fissure. More- over, in speaking of a sixteen-week embryo (p. 33), he says that he can find no trace of the posterior arched fissure, but that the primordium of the pes hippocampi is visible, not as an infolding of the brain wall, but rather as a thickening at that point. At this meeting Schaper demonstrated the fissureless condi- tion of the medial wall in the two embryos 10.5 cm. and 4.6 em. Further, he pointed out an insignificant invagination in the region of His’ anterior arched fissure in the Hochstetter fetus of 49 mm. ‘The discussion must have attained some warmth or Fick would not have advised that the term fissure be stricken out of embryological terminology. It is difficult to imagine the necessity of calming a morphologist. . Goldstein (’04) described upon the smooth surface of the me- dial wall ‘“‘at the point where the anterior arcuate fissure should be sought, a well defined sulcus which extends approximately vertically from the insertion of the olfactory bulb and which His considers the equivalent of his ‘Bogenfurche’”’ (p. 581). This is the fissura prima, and not the true fissura arcuata. For him, the ventricular aspect of the cortex is little disturbed. The broadening and deepening is not an expression of an infolding, but rather a condition of nervous differentiation of the outer level (p. 582). Comparing the posterior arched fissure or the fissura hippocampi with His’ figure 86, he says that the fissure is not as deep as that of His; the outer cortex describes only an arched line so that a slight indentation can be seen (p. 586). In 1901 J. Symington reported at a meeting of the British Association for the Advancement of Science that the frequency and depth of the temporary fissures had been exaggerated, but that they occurred in well-preserved material. Although the arcuate fissure was not a product of fixation, it could have no THE FISSURA HIPPOCAMPI 77 morphological significance and was in no way related to the hippocampal fissure. In one brain he was able to trace the hippocampal formation from the region of the temporal pole to that of the developing transverse commissures. He further called attention to the fact, formerly supported by comparative anatomy alone, that the gray and white formation above the corpus callosum in the adult human brain is the remains of a hippocampal formation. Mall (03) examined some fifty brains in his collection. He found that those fixed in alcohol or in weak formalin showed fis- sures on the medial wall, while those fixed in strong formalin showed no such structures. He concludes ‘‘according to the experience of Hochstetter, Retzius and myself, the transitory fissures are not found in fresh brains. They are therefore arte- facts and of no morphological significance.” G. Elliot Smith (03, p. 217) says: ‘‘Two kinds of so-called ‘transitory fissures’ have been described in the fetal human brain. There is the group of irregular puckerings of the neo- pallium, which are found in those fetuses of the 3rd and 4th months in which putrefaction changes have begun; and there is a second group which are found in fetuses of the 5th, 6th and 7th months. It is quite unnecessary to discuss the first group, because their true nature as postmortem wrinklings of the neo- pallium has been conclusively demonstrated by Hochstetter, and the results of the examination of all known fresh fetuses of the third and fourth months amply confirm the results obtained by Hochstetter’s researches.” The heat of the discussion centered around His. But in no instance did he describe the radial folds of Cunningham and the earlier workers. His findings may be compared to those of Retzius (01), although they share neither the terminology nor the interpretation. In the young embryos (His, ’04, fig. 40, embryo C. R. 13.6 mm.) he describes the fissura prima, the an- terior and posterior arched fissures and the fissura rhinalis; in later embryos, he adds the fissura arcuata accessoria and the fissura calcarina. The fissura rhinalis and fissura calcarina are permanent: and morphologically significant structures. The 78 MARION HINES fissura prima is the result of the separation of the olfactory lobe, or the anterior smell brain, on the medial side through a medial continuation of the lateral fissura rhinica (p. 76). This fissure does not extend to the lamina terminalis. It remains in the adult as the fissura parolfactoria posterior of the B. N. A. The posterior arched fissura or the fissura hippocampi is a reality because it contains a characteristic structure in no wise connected with postmortem changes. ‘The fissura arcuata accessoria lies above the hippocampal in His’ published models. He fails to state whether or not there is any characteristic histological structure present in its depth. Retzius (01, p. 92) says that, although the lateral hemisphere wall is smooth, there is a broad sagittally placed furrow or in- denture, in the medial wall forming a hillock, which butts into the ventricle. A year later (p. 66) he says the same in regard to the matter. Such is the history of the transitory fissuration in the human telencephalic vesicle. But what of lower animals? Only two workers have identified structures similar to the findings of Goldstein, Retzius, and His; they are Martin and Gronberg. Martin (94) was able to identify the anterior arched fissure in a transverse series of cat embryos, 1.3 cm. in length (p. 224). It appeared later than the fissura chorioidea (0.9 cm.). In the cat, the anterior arched fissure resembles the fissure pictured by His (pl. I, fig. 8,’90). The posterior arched fissure appears as a secondary arched fissure (2.2 em., p. 226). The union (at 5 em.) of the anterior and posterior fissures with the ‘seitlichen Balkenfurche’ (i.e., the sulcus fimbrio-dentatus) he considers to be the future fissure of the corpus callosum (p. 242). This fissure maintains the same relationship to the pallial commissure as the sulcus corporis callosi of the rat (Johnston, ’13, fig. 59). The same sequence of fissuration was followed by Grénberg (01) in the brain of the hedgehog. The choroid fissure appears in the 1l-mm. embryo and the arched fissure in the 15-mm. In figure 54 the inner wall is evidently thickened and forms a slight bulging into the lumen of the ventricle. THE FISSURA HIPPOCAMPI 79 This is the first primordium of the hippocampus. The thickening of the wall is limited exactly to the middle part of the groove while the wall retains its original thickness at the transition in the choroid fis- sure and in the outer mantle, the inner edge forms in cross section a stronger band than the outer . . . . (p. 280). The fissura am- monis is not the result of a gradual infolding, but rather is to be con- sidered a secondary formation in the outer layer of the thickened wall ; (p. 281). If the series is followed through one finds that the fold, if we can give the groove this name, is more accentuated in the posteri ior portion than in “the anterior. A division into two origins, an anterior and a posterior, as His described in the human embryo (89), I have not been able to confirm (p. 282). The history of the fissuration on the medial wall of the tel- encephalon of man, during the second, third, and fourth months, falls naturally into the following subdivisions: 1. Fissuration is an artefact and therefore of no morphological significance. 2. Fissuration is not an artefact. It is accompanied by charac- teristic histological structure: a, without future significance; 6, with future significance. If these contradictory statements are true, there is a pos- sible resolution. The solution found rests almost entirely upon a consideration of histological structure and a correlation of development of the tissue in question. To follow the region which lies in the fissura arcuata of His from its earliest differentiation as a tissue distinct from the remainder of the vesicle to its ulti- mate destiny is the purpose of the present paper. This analysis will give the first point of departure in studying the development of the forebrain as a growing tissue with the hope that at some future time the interrelationship of its various parts may be expressed mathematically. During the progress of this research I have sought constantly the aid and advice of Dr. G. W. Bartelmez, and depended largely upon the manifold suggestion and the critical judgment of Dr. C. Judson Herrick. Without them it would have been impossible to begin or carry to completion this piece of work. Iam happy also to acknowledge the debt I owe Dr. R. R. Bensley, not for aid in this particular problem, but for the scientific training I possess. Further, I wish to acknowledge the use of material 80 MARION HINES belonging to the Carnegie Institution, Laboratory of Embryology, Baltimore; the kindly interest of the late Dr. Franklin P. Mall and that of Dr. George L. Streeter. Also thanks are due Mr. A. B.-Streedain and Miss Marian Manly, of Chicago, for the drawings, to Miss Phelps, of Baltimore, for the microphotographs of the embryos belonging to the Carnegie Collection, and to Mr. Ralph Witherow, for drawings of models of those embryos. And I cannot neglect to acknowledge the debt I owe M. L. Fyffe for an interest, long sustained, in the outcome of this contribution. MATERIAL AND METHODS This contribution is based upon a study of human material belonging to the Embryological Collections of the Department of Anatomy, University of Chicago, and of the Carnegie Institu- tion, Laboratory of Embryology, Baltimore. For the elaboration of the technique used in handling human embryos, the Depart- ment at Chicago is indebted to Dr. G. W. Bartelmez. The details of this technique are given by Bailey (’16). There is no better human material than that belonging to the Carnegie Laboratory. Doctor Mall was able to secure the cooperation of clinicians so that the preservation of the embryos studied was the best our present technique can secure. Wax models of the brains studied at Chicago were made, while those belonging to the Carnegie were plaster casts poured by Mr. Heard. All these models have been checked many times with either the photographs or the outline projection of the brains in question, so that the writer believes them to be as accurate as our present methods allow. The embryos studied may be grouped as set forth in table 1. Besides these embryos the following were examined, although their various olfactory centers were not plotted. In all of them the same areas with the same histological differentiation were found (table 2). THE FISSURA HIPPOCAMPI S81 GENERAL MORPHOLOGY The 11.8-mm. embryo, Mall Collection, 1121 (figs. 7, 8, 9, 11) So far as the nervous system is concerned, this embryo lies between the 6.9-mm. embryo of His (Br.) and his embryo C. R. (13.6 mm. N. L.). It is a thin-walled tube easily divided into TABLE 1 Embryos described in this contribution NUMBER eae COLLECTION CONDITION SOURCE FIXATION ee mm. fu PDT LIES Mall Good Abortion Corrosive sub- | 40 limate 940 | 14.0 Mall Excellent | Abortion Formalin 40 (10.0 in alc.) H173 | 19.1 Chicago | Excellent | Abortion Formalin- 10 Zenker 460 | 21.0 | Mall Excellent | Abortion Sublimate- 40 (20.0 acetic in alc.) H91 | 27.8 Chicago | Fair Abortion 10 per cent 20 in for- formalin malin. H41 | 32:1 Chicago | Fair Abortion 10 per cent 20 in for- formalin malin. H163 | 39.1 Chicago | Excellent | Operation for | Formalin- 20 : fibroids Zenker 886 | 43.0 | Mall Excellent | Operation 10 per cent for- | 100 malin and Bowen’s fluid the five brain vesicles of v. Baer. The roof plate is thin in the telencephalon and myelencephalon, but not noticeably so in the diencephalon and mesencephalon. The floor plate is be- ginning to show its characteristic thickenings. ‘The floor of the fourth ventricle is long, broad, and shallow. The lateral recess is not present. There is no evidence of any increase in thickness of its anterior lip. There is an insignificant groove in the floor 82 MARION HINES of the fourth ventricle, continuous with a deeper groove in the wall of the cord and of the mesencephalon. limitans fig. 8, (Sul. lim.). TABLE 2 Other embryos consulted for this contribution This is the sulcus The part of the mesencephalon which NUMBER caeee COLLECTION PLANE OF SECTION THICKNESS CONDITION mm. bE 163 9.0 Mall Transverse 20 Excellent H566 11.6 Chicago Transverse 15 Excellent I eA: 13.7 Chicago Transverse 10 Poor H398 14.5 Chicago Horizontal 25 Excellent 719 15.0 Mall Transverse 40 Fair iH 5 16.0 Chicago Transverse 15 Poor H465 16.0 Chicago Transverse 15 Good 317 16.0 Mall Coronal 20 Good 406 16.0 Mall Sagittal 20 Good 492 16.0 Mall Coronal 40 Excellent H516 17.0 Chicago Transverse 15 Good 576 lino) Mall Sagittal 15 and 20 Excellent 1390 18.0 Mall Sagittal 2, Good 432 18.5 Mall Sagittal 20 Good 431 19.0 Mall Sagittal 20 Good H202 20.2 Chicago Horizontal 25 Fair H 19 20.6 Chicago Transverse 20 Fair 840 24.8 Mall Transverse 50 Good 455 24.0 Mall Transverse 20 Good 632 24.0 Mall Sagittal 40 Fair H 39 25.0 Chicago Transverse 25 Fair 405 26.0 Mall Sagittal 40 Good 1008 26 .4 Mall Sagittal 40 Excellent H 50 29.2 Chicago Transverse 25 Fair 878 36.0 Mall Sagittal 100 Good H 98 38.4 Chicago Horizontal 25 Fair 448 52.0 Mall Sagittal Good 267 59.0 Mall Sagittal Good H 44 60.0 Chicago Transverse 25 Fair lies dorsal to this sulcus is thin and expanded. The part which lies ventrally foretells the future thickening of the mesencephalic floor. This ventricular groove passes through the diencephalon and seems to lose itself in the vicinity of the cavity of the optic THE FISSURA HIPPOCAMPI 83 evagination (Rec. op.). Immediately dorsal to this groove in the diencephalon lies another, here termed sulcus dorsalis (fig. 8, Sul. dors.), which to all appearances arises from the sulcus limitans rostral to the meso-diencephalic boundary. Further forward the sulcus limitans and the sulcus dorsalis fade out in the thalamic wall, dorsal and anterior to the optic ventricle. These two sulci divide the diencephalon into three regions: a large dorsal region, an insignificant central region, here termed the midthalamic region, and a large ventral region, the hypo- thalamus. ‘The first or epithalamus is bounded rostrally by the velum transversum (fig. 8, Vel. trans.), dorsally by a thin ependy- mal roof (fig. 8, Dien. r. pl.), and the epiphyseal evagination (fig. 8, Hp. ev.). The floor of the hypothalamus contains the shallow mammillary recess (fig. 8, Corp. mam.), the broad in- fundibular area (fig. 8, Znf.), and the bed of the optic chiasma (fig. 8, F. b. op. ch.). The infundibulum can be identified as that area to which the anlage of the anterior lobe of the hypophysis clings (fig. 8, Ané. l. hyp.). The midline of the telencephalic roof is only a little lower than the two lateral vaults of the hemispheres. Consequently the foramen interventriculare (fig. 8, For. it.) is only a little smaller than the entire cavity of the whole evagination. ‘The only point of constriction lies in the most dorsal part of the di- telencephalic groove, the region of the velum transversum (fig. 8, Vel. trans.). Following the midline structure forward from the velum transversum, it remains membranous as far as the massive lam- ina terminalis (fig. 8, Lam. term.), below which a slight con- striction marks the preoptic recess (fig. 8, Rec. preop.). Next comes the chiasma ridge (fig. 8, F. b. op. ch.) at the di-telence- phalic junction. The corpus striatum is barely visible as a slight ventricular eminence in the floor of the interventricular foramen. The 14-mm. embryo, Mall Collection, 940 (figs. 10, 12, 15, 14) A difference of only 2.2 mm. between this embryo and no. 1121 has wrought a marked change in the growth of the central 84 MARION HINES nervous system. The basal plate of the cord has grown much thicker. The pontile flexure is evident, but not as accentuated as His pictured for the 10.6-mm. in his collection; nor is the floor plate of the medulla oblongata as he showed it. The lateral recess has appeared. There is no indication of a cerebellar thickening in the superior lip of that recess. The midbrain is not as prominent a feature of the morphology of this brain, due in part to the acceleration in growth of the diencephalon. The mesencephalon is separated from the rhombencephalon by a marked constriction of the total brain tube, the isthmus. In the 14-mm. embryo the transition from midbrain to the dien- cephalon is marked off distinctly in the midline by a sudden dip in the vault. The diencephalic portion of the invagination is the posterior limb of the epiphyseal evagination. The basal plate of the midbrain has grown toward the lumen of the ventricles. The diencephalon is divided into three parts, comparable to those described for the thalamic region of the 11.8-mm. embryo, by the sulcus limitans below and the dorsal sulcus above. The absolute distance between the dorsal sulcus (fig. 14, Sul. dors.) and the sulcus limitans (fig. 14, Sul. lim.) is greater here than in the younger embryo, but the tissue dorsal to this ridge and that ventral to the sulcus limitans has changed little. Above the dorsal sulcus is a well defined ridge which is still more clearly marked in the 19.1-mm. embryo (fig. 15). The diencephalic roof plate is longer, measuring the distance from the velum trans- versum (fig. 14, Vel. trans.) to the epiphyseal evagination (fig. 14, Ep. ev.). The definitive regions of the floor are already outlined. The wall of the recessus mamillaris is not as shallow as that of the 11.8-mm. embryo. The recessus infundibuli is now a definite well in the floor, dipping down into the solid stalk of the posterior lobe of the hypophysis. The bed of the optic chiasma has increased in breadth and thickness. The preoptic and postoptic recesses are actual cavities in the hypo- thalamic floor. The sulcus limitans ends blindly dorso-caudal to the optic ventricle. THE FISSURA HIPPOCAMPI 85 The ventricular communication between the diencephalon and the telencephalon has suffered a tremendous change in its con- tour. Now, for the first time, the future relationships are deter- mined. The dorsal and terminal boundaries of the foramen interventriculare are no longer an arc of a circle, although they are not as yet roof and terminus meeting each other in an angle as described for H173. Instead of the smooth ventricular di- telencephalic union, that junction is marked by a slight eminence, which is continued forward into the floor of the cerebral hemis- phere, the corpus striatum (fig. 14, Corp. str.). The point of entrance of the fila olfactoria (fig. 14, Fil. olf.) into the cerebral hemisphere enables us to locate the region of the future olfactory bulb evagination. The telencephalic vesicle itself extends far beyond the midline, anteriorly and dorsally. Consequently, the great longitudinal fissure now divides the telencephalon into two parts, the cere- bral hemispheres, with a small residual telencephalon medium between. In the 14-mm. embryo the differentiation of the telencephalon medium and adjacent parts of the cerebral hemisphere has advanced to the point where most of the morphologically sig- nificant regions can be delineated. In the following paragraphs these will be described and defined. The telencephalon medium lying between the velum trans- versum and the preoptic recess (fig. 14) may be divided into dorsal and ventral moieties, the area chorioidea (A. ch.) and the lamina terminalis (Lam. term.), each of which is again sub- divided into two parts. The lamina terminalis ventrally is thick and massive (pars crassa, p. c.) and dorsally is a thin epithelium (pars tenuis, p. t.). In the course of development the massive part enlarges dorsalward at the expense of the thin part. The area chorioidea consists of two morphologically dis- tinct portions, the tela chorioidea telencephali medii (Tel. ch. tel. med.) anteriorly and the paraphyseal arch (Par. ar.) pos- teriorly. In this brain, it is impossible to draw the dividing line between the tela chorioidea telencephali medii and the lamina S86 MARION HINES terminalis by the angulus terminalis, as it can be done in the telencephalic roof plate of H173 (fig. 16, Ang. term). The peculiar shape of the paraphyseal arch -is more clearly delineated in the 19.1-mm. embryo (figs. 15,16). It. presents the form of a sharply elevated longitudinal ridge which here forms the floor of the great longitudinal fissure between the two cerebral hemispheres and the roof of the interventricular foramen. Posteriorly it is abruptly terminated by the telencephalic limb of the velum transversum (Vel. trans). Anteriorly it merges gradually with the tela chorioidea telencephali medii. The lateral border passes over at a sharp angle into the medial wall of the evaginated cerebral hemisphere (see the cross-sections, figs. 24, 25, 27). That portion of the hemisphere wall which lies contiguous to the paraphyseal arch in later stages forms the anterior limb of the lateral choroid plexus (see beyond, p. 87). The tela chorioidea telencephali medii is of variable length at different ages. It is defined as that portion of thetelencepha- lon medium which lies between the angulus terminalis (fig. 16, Ang. term.) and the paraphyseal arch. The choroid plexus is never developed in the contiguous portion of the medial wall of the cerebral hemisphere. The structures which have just been considered all belong in the telencephalon medium. In the adjoining part of the cerebral hemisphere there are two structurally distinct regions, ventrally the massive subcortical olfactory centers of the septum, and dorsally a thin area epithelialis. The area epithelialis in the 14-mm. embryo is structurally uniform throughout its extent, but morphologically it comprises two very distinct regions. The portion ventrally of the angulus terminalis and adjacent to the membranous part of the lamina terminalis is part of the septum 1 This term is a modification of His’ angulus praethalamicus. Judging from figures 44 and 45 (’04), His refers to a sharp change in the direction of the midline. This angle is probably the same as that described in this paper, although it is not the anterior margin of the midthalamus region. He says that the closing plate of the medial hemisphere wall passes over at a sharp angle into the medial thalamic wall. This point may be called the angulus praethalamicus; beside it begins the margin of transition of the thalamic wall into the hemisphere wall, i.e., the margo thalamicus of the latter (p. 66). THE FISSURA HIPPOCAMPI 87 (ventro-medial sector of the hemisphere, p. 107) and resembles in form and morphology the septum ependymale of the amphibian brain (fig. 14, Sept. epen.). Almost the whole of this portion ultimately is thickened by intrinsic differentiation of neuro- blasts. The portion of the area epithelialis lying above the angulus terminalis is permanently membranous. Its ventral border is continuous with the septum ependymale, from which at this age it is not structurally distinguishable. Medially it is bounded by the area chorioidea of the telencephalon medium, and dorsally and laterally by the sulcus limitans hippocampi and primordial TABLE 3 Telencephalic structures in and near the midplane MIDPLANE STRUCTURES OF THE TELENCEPHALON CONTIGUOUS AREAS OF THE CEREBRAL MEDIUM HEMISPHERE Recessus preopticus Septum Lamina terminalis Pars crassa Septum Area epithelialis Pars tenuis Septum ependymale Angulus terminalis Area chorioidea Tela chorioidea telencephali medii Area intercalata Paraphyseal arch Lamina epithelialis Velum transversum Lamina epithelialis hippocampus. It extends backward beyond the velum trans- versum toward the occipital part of the hemisphere (figs. 12 and 14, A. ep.), and in later stages it follows the ventral border of the hippocampal formation as far as the tip of the temporal lobe (figs. 17, 18, 20). In the light of future differentiation, the entire area epithelialis may be further subdivided into: 1) the septum ependymale, already referred to; 2) the area intercalata (fig. 14, A. int.) lying contiguous with the tela chorioidea telencephali medii (fig. 14, Tel. ch. tel. med.) within which choroid plexus is never developed; 3) the lamina epithelialis (Lam. ep.) lying opposite to the para- physeal arch and the di-telencephalic junction and in later stages 88 MARION HINES extended backward beyond this junction accompanying the differentiation of the hippocampal formation in the temporal lobe (figs. 16, 18). The whole of the lamina epithelialis of these embryos becomes the lamina epithelialis of the adult lateral choroid plexus. The relations above described are expressed in the accompanying table 3. The morphological changes noted here in comparison with the 11.8-mm. embryo are as follows: 1. The appearance of the pontile flexure and the lateral recess. 2. The marked medial growth of the basal plates in the mesen- cephalon. 3. Slight advance in the delimitation of hypothalamic struc- tures and more marked increase in the thalamic region lying between the sulcus limitans and the sulcus dorsalis. 4, Change in the angle of the vault above the foramen in- terventriculare. 5. Marked growth in the telencephalon, i.e., increase in size of the cerebral hemispheres, 1) by dorsal, caudal, and rostral growth, and, 2) by appearance of the corpus striatum as a ridge in the floor of the lateral ventricle. The 19.1-mm. embryo, University of Chicago, H 173 (figs. 15 and 16) The whole of this brain was not modeled, but from the sections it is evident that the development of both the basal plate and the ganglia of the cord is precocious. The processes of develop- ment, already described in the medulla oblongata, have proceeded with a slight shifting of relative morphology only. The floor plate has maintained a progressive thickening. The depth of the lateral recess has increased. The primordium of the cere- bellum has appeared in the dorsal lip of the lateral recess. The midbrain has grown medially by a ventricular extension of the basal plate, while the roof plate and the alar plate have increased in thickness. In this embryo, as in the 14-mm., the changes in the contour of the thalamus and telencephalon are most marked. ‘The two ventricular markings, namely, the sulcus limitans and the dorsal THE FISSURA HIPPOCAMPI - 89 sulcus, separate the thalamic wall into three parts. The dis- tance between the pronounced dorsal sulcus and thesulcuslimitans has increased both absolutely and relatively, so that this middle portion of the thalamus is becoming the most extensive of the three divisions. The anterior extension of the sulcus limitans cannot be traced to the neighborhood of the optic evagination. A new ventricular suleus has made its appearance. It lies between the medial limb of the corpus striatum and hypo- thalamus, arising in the recessus preopticus (fig. 16, Rec. preop.) and loosing itself in the floor of the foramen interventriculare (fig. 16, For. int.). Immediately dorso-caudal to this sulcus lies the midregion of the thalamus, which now forms the posterior boundary of the foramen (fig. 15). The dorsal boundary of this midthalamic division can be followed rostrally to the region of the velum transversum. However, in the younger stage, the 14-mm. embryo (fig. 13), the posterior boundary of the foramen is here formed by the velum transversum, whose diencephalic limb is continuous with the epithalamus, and its floor is formed by the corpus striatum. In the epithalamus the epiphyseal evagination is more con- stricted. The roof of the whole is a little thicker. In the hypothalamus the infundibular recess is wide and the posterior lobe is not constricted. The floor and the walls have increased in thickness. In the midline the bed for the optic chiasma has increased in volume by growth, not only antero-posteriorly, but also dorso-ventrally. This region is separated from the massive portion (fig. 16, P. c.) of the lamina terminalis by a deep groove, the recess preopticus (fig. 16, Rec. preop.). In the picture of this model (fig. 16) the most striking aspect of the telencephalon medium lying between this recess and the velum transversum is the sharp angle in the midline. This angle, the angulus terminalis (fig. 16, Ang. term.), was commented upon in the description of the 14-mm. embryo. Here the anterior boundary of the telencephalic midline suddenly changes its direc- tion and becomes the vaulted roof of the foramen interventric- ulare. Is this angle a landmark? Can it be used as a fixed point from which to measure change? Further, can it be used as a THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 1 90 MARION HINES fixed point from which to measure the growth of contiguous structures? . One thing is certain, it breaks the midline into two divisions, a ventral and a dorsal, whose ultimate outcome in development is characteristic. The ventral limb is thicker than the dorsal limb. It is the lamina terminalis. In figure 14 (14 mm.) the dorsal part, or pars tenuis ia of the lamina terminalis is much longer than the ventral or massive part (p.c.); but in figure 16 (19.1 mm.) these two parts are approximately the same in length. The former subdivision has been called by Johnston (13) the lamina supraneuroporica. It is a convenient name. The writer would like to use it, but there seems to be no evidence for as complete a separation be- tween the two portions of the terminal plate as Johnston thinks; ‘and, since the last point of closure of the neural tube may lie anywhere, for aught we know, between the recessus preopticus and the velum transversum, it cannot be used to divide the Jamina into two morphologically distinct regions. At present the writer thinks there is no fundamental difference in the later stages between the ventral and the dorsal portions of this area, either in development or internal structure. The dorsal limb of the angulus terminalis (fig. 16, Ang. term.) is divided into two regions, the anterior that of the tela chorioidea telencephali medii (fig. 16, Tel. ch. tel. med.) and the posterior, that of the paraphyseal arch (fig. 16, Par. ar.). Extending laterally into the two ventricles from this arch are the two lateral choroid plexuses. These plexuses are connected across the midline by the vault (here the paraphyseal arch) of the foramen interventriculare (fig. 16), but are not connected pos- terior to the velum transversum. Here they form a broad shallow fissure in the medial wall, known as the fissura chorioidea. Bailey (16a) has called this the posterior limb of the plexus and that adjoining the paraphyseal arch, the anterior limb. Moreover, dorsal to this fissure in the medial wall of the hemisphere and extending more rostrally is a shallow groove. This groove can be followed as an indentation in the medial wall from a region slightly anterior to the angulus terminalis, caudal- ward to the tip of the temporal pole. This insignificant furrow THE FISSURA HIPPOCAMPI 91 _is the fissura hippocampi (see groove, fig. 15) and coincides in extent with the stippled area in figure 16. The greatest change in the di-telencephalon relationship is the ventricular growth in the medial limb of the corpus striatum (figs. 15, 29, and 32). Looking into the cavity of the telencephalic vesicle, several typical markings may be seen. In the latero- ventral sector lies a depression, which divides the nucleus caudatus into a medial and lateral limb. The lateral hillock is a small and insignificant ventricular ridge; the medial, much larger than the lateral, lies in the ventro-medial part of the ventral sector, just lateral to a deep groove which separates the corpus striatum from the septal region. This medial limb of the corpus striatum is most evident from the ventricular aspect of the di-telence- phalic union. The septal region which composes the ventral portion of the medial wall ventral to the angulus terminalis and anterior to the lamina terminalis is separated from this portion of the corpus striatum by a deep groove, called by Herrick (710) in Amblystoma and other vertebrates the angulus ventralis (fig. 31, Ang. ven.). Further dorsalward in the medial wall of the hemisphere a slight ventricular ridge can be followed, from the base of the beginning olfactory evagination to the caudal pole. This is the hillock made on the ventricular surface by the fissura hippocampi. This ridge is limited ventrally by a sharp turn in the tissue wall, a deep, well-marked sulcus, which will be called the sulcus limitans hippocampi (figs. 16, 29 to 31, Sul. vent.). Ventral to this sulcus and caudal to the anterior limb of the paraphyseal arch lies the massive choroid invagina- tion. Rostral to this portion of the paraphyseal arch, the medial wall ventral to the sulcus limitans hippocampi is very thin epithe- lial tissue, the area epithelialis (figs. 16, 31, A. ep.). The outer contour of the telencephalic vesicle is undergoing a change, marked not only by growth rostral to the lamina termi- nalis region, but also now by a seeming swing of the most dorsal portion of the outer di-telencephalic junction caudo-ventrally. This junction liés ventral to the dorsal thalamic suleus, which divides the midthalamus from its dorsal region. This relation- ship of the diencephalon to the telencephalon upon the outer 92 MARION HINES brain surface is foreshadowed in the structure of embryo no. | 940. The morphological changes found in this embryo as compared with the 14-mm. embryo are as follows: 1. Acceleration of the neopallium and appearance of temporal pole. 2. Appearance of the fissura hippocampi. 3. The invagination of the lamina epithelialis, forming the lateral choroid plexus. 4. Increase in length of the lamina terminalis, and the appear- ance of the angulus terminalis. 5. The acceleration of the medial component of the caudate complex. 6. Great growth of the midthalamic region. The 20-mm. embryo, Mall Collection, 460 (fig. 17) The development of the central nervous system as a whole is practically identical with that attained by H 173, the 19.1-mm. embryo (compare figs. 15 and 16 with 17). In this case also only the telencephalon and part of the diencephalon was modeled. Consequently, the morphological description must be confined especially to the growth of the forebrain vesicle. The relation of structures in the telencephalon medium are the same as those found in H 173. The angulus terminalis is essentially the same. And as noted before, the lamina terminalis is divided into two regions. The diencephalic limb of the velum transversum forms a small elevation which marks the posterior boundary of the foramen interventriculare (fig. 17, For. int.). The anterior extremity of the sulcus limitans coincides with the suleus which divides the corpus striatal complex from the hypo- thalamus. This rostral end of the limitans was called sulcus Monroi by His. Milhalkovics, quoting Richert, however, in- cludes in the suleus Monroi not only the rostral end of the sulcus limitans, but also the sulcus separating the medial limb of the striatal complex from the diencephalon. THE FISSURA HIPPOCAMPI 93 In the telencephalon itself there is little change. The em- bryonic fissura hippocampi (lying in the stippled area, fig. 17) extends from the region dorsal to the olfactory bulb (fig. 17, Olf. bulb) to the tip of the temporal pole. The greatest differ- ence in the growth between this brain and that of the 19.1-mm. embryo is in the appearance of neopallium on the medial wall of the definitive occipital pole. In the floor of the lateral ventricle two hillocks lie side by side, separated by a shallow sulcus. The lateral hillock, much larger in this embryo than in H 178, is the lateral limb of the caudate complex. A sulcus separates the medial hillock (figs. 17, 35, and 36, Cor. str. med.) from the thalamus. This groove runs up and over the floor of the foramen interventriculare and ends in the recessus preopticus (fig. 17, Rec. preop.). Rostral to the medial limb of the caudate complex and making up the ventro-medial portion of the cerebral hemisphere is the septum. This area is limited dorsally by the sulcus limitans hippocampi (fig. 17, Sul. vent.) and ventrally by the angulus ventralis (fig. 30, Ang. vent.). The angulus ventralis is a shallow groove which lies between the septum and the rostral portion of the caudate nucleus. Its rostral end becomes lost in the evaginating olfactory bulb. The septum itself has increased in thickness by a marked ventricular growth. There are, then, three morphological differences to be noted in this brain as compared with that of H 173: 1. Increase in the tissue forming the vault of the hemisphere. 2. Growth of the lateral nucleus of the corpus striatum. 3. Dorsal extension of the ventrally thickened portion of the septum. The 27.8-mm. embryo, University of Chicago, H 91 The foramen interventriculare is no longer elliptical in out- line. It is a dorso-ventral slit lying beneath the paraphyseal arch. It is bounded posteriorly by the midthalamus, ventrally by the medial limb of the caudate complex, dorsally by the area chorioidea, and anteriorly by the lamina terminalis. The sulcus Q4 MARION HINES which separates the medial limb of the caudate nucleus from the middle thalamus ends in the: recessus preopticus and runs along the di-telencephalic ventricular junction into the floor of the lateral ventricle. The rostral end of the sulcus limitans almost reaches it. The dorsal suleus which marks the dorsal border of the midthalamus region is almost obliterated. There is, how- ever, a little evidence of it at the rostral end of the diencephalon, where a small ventricular ridge les on the same level in an an- tero-posterior plane with the lateral limb of the velum transver- sum. The roof of the diencephalon has become membranous just caudal to the velum transversum. The angulus terminalis in the telencephalon medium is more obtuse. The thin roof of the telencephalon anterior to the paraphyseal arch has thickened. The dorsal part of the lamina terminalis, the lamina supraneuroporica of Johnston, here called the pars tenuis, has increased in breadth measured from the midplane to the ventricle and in thickness measured dorso- ventrally. This same process has caused an enlargement of the lamina terminalis in all directions (fig. 20, Lt., Bailey, ’16a). The sculpturing within the telencephalic cavity is so modified that the medial and lateral limbs of the caudate nucleus closely resemble each other in the extent of their ventricular expansion. The lateral ridge of the caudate nucleus is as prominent a hillock in the floor of the ventricle as that formed by the medial limb of this nucleus. But the greatest change within the ventricle is due to the increase in thickness of the medial wall, which les ventral to the sulcus limitans hippocampi and cephalad to the massive portion of the lamina terminalis, the septum. This marked growth of the septal region extends rostrally into the base of the evaginating olfactory bulb. The cavity of the ven- tricle is almost filled by the lateral choroid plexus. Upon the medial wall the developing hippocampus forms a continuous bulging on the ventricular wall, long and low at the rostral end, sharp and high at the temporal pole. Immediately beneath this ventricular ridge is the sulcus limitans hippocampi. The ridge is due to a slight infolding of the medial wall, the groove on the outer surface being the fissura hippocampi. In this brain there THE FISSURA HIPPOCAMPI 95 is no interruption of the fissure. Ventral to this fissure is that of the choroid plexus, whose taeniae he so near each other that there is no apparent opening. The caudal pole of the growing hemisphere attached above the telencephalic limit of the di-telencephalic groove is swinging antero-ventrally and carrying with it new tissue, that of the de- veloping neopallium. Consequently the vault is not only increas- ing in height above the ventral ventricular eminences, but is also increasing in extent simultaneously with the forward and down- ward growth of the temporal pole. The 32.1-mm. embryo, University of Chicago, H 41 (figs. 22 and 24, pp. 116 and 117, Barley, ’16 a) The most striking aspect of the ventricular surface of the thalamus is the deepening of the rostral end of the sulcus limitans and the progressive fading of the sulcus dorsalis. The region which lies between these two grooves occupies the major portion of the ventricular thalamic surface. Dorsal to the sulcus dorsalis lies a ridge in the midline which is accentuated in the model by the irregular trimming of the diencephalic roof plate. This ridge was described by His as containing the stria medullaris and the habenula. It is interesting to note that its anterior end reaches the lateral limb of the velum transversum, while its ventral border (the sulcus dorsalis) ‘gradually fades out anteriorly in the same region. Consequently, the foramen interventriculare is closed posteriorly by the great midthalamus. The hypothala- mus remains almost unchanged. Optic fibers are present in the chiasma ridge. The recesses which lie anterior and posterior to the chiasma ridge are deepened. The infundibulum opens into the cavity of the posterior lobe of the hypophysis. The tuber cinereum is thin. The mammillary recess is very shallow. The rostral end of the sulcus limitans joins the suleus which separates the hypothalamus from the medial limb of the corpus striatum, and runs over the floor of the foramen interventriculare into the basal portion of the lateral ventricular surface of the diencephalon. This portion of the corpus striatum together with the dorsal part of the lamina terminalis forms the anterior 96 MARION HINES wall of the foramen. The vault is sealed by the tela chorioidea telencephali medii and the paraphyseal arch. The angulus ter- minalis has become still more obtuse. The dorsal portion of the lamina terminalis is no longer slender and ependymal; but, rather, growth in all directions so characteristic of this region in its ventral area has involved the whole of the terminal plate from the recessus preopticus to the angulus terminalis. The pars tenuis becomes less in extent as the pars crassa of the lamina terminalis increases in length. Looking into the ventricle, the lateral and medial limbs of the caudate complex are separated from each other by a shallow sulcus. The lateral component of this complex has increased in growth relatively more than the medial. The ventricular eminence formed by the fissura hippocampi upon the medial wall is broken in the region dorsal to paraphyseal arch, so that there is now an anterior and a posterior segment. The sulcus limitans hippocampi, lying ventral to this eminence, is continuous from the tip of the temporal pole to a region rostral of the lamina terminalis. The septal region lying between this sulcus and the angulus ventralis has grown in length and width, epecially in the ventral portion near its point of continuity with the diencephalon. The outer contour of the cerebral hemisphere is such that superficially the various poles of the adult hemisphere are recog- nized. The growth which has given this change in the surface is the result of increase in the tissue which is attached medially to the dorsal border of the hippocampus and laterally to the lateral limb of the caudate complex, namely, the neopallium. The most noticeable result of the growth of this tissue has been the swinging of the primitive caudal extremity, posteriorly, ventrally, and anteriorly. Thus the primordium of the temporal lobe is laid down. The fissura hippocampi is a shallow groove rostral to the lamina terminalis, while in the region above the paraphyseal arch it is barely visible. However, caudal to the velum trans- versum this fissure is deeper. Moreover, it follows the new direction of growth of the temporal lobe ventrally and anteriorly, so that its shape upon the free surface of the medial wall be- comes a semicircle. THE FISSURA HIPPOCAMPI 97 The changes which characterize the two embryos last described are: 1. Closure of the choroid fissure. 2. Actual evagination of the olfactory bulb. 3. Plexus formation in the anterior portion of the diencephalic roof plate. 4, Great increase in growth of the midthalamie region. 5. Reversal of relative size of the two limbs of the caudate nucleus. 6: Further thickening of the dorsal portion of the lamina terminalis, i.e., the encroachment of the pars crassa upon the pars tenuis. 7. The incipience of the temporal lobe in the cerebral hemis- phere. 8. The fissura hippocampi is not so deep above the area cho- rloidea in the 27.8 mm. embryo as in the 19.1 mm. or the 20 mm. Anterior to the angulus terminalis and posterior to the velum transversum the fissura resembles that found in the embryos previously described. The formation, however, is continuous throughout. In the 32.1-mm. the fissura hippocampi is very shallow anterior to the velum transversum, posteriorly it is relatively a deep groove. The 39.1-mm. embryo, University of Chicago, H 163 (fig. 18) The changes in the medulla oblongata and the midbrain may be seen at a glance. The cerebellum (Cer.) appears as a medially growing thickening of the dorsal lip of the lateral recess, but as yet no fusion has taken place in the midline. The floor plate in both the medulla oblongata (Myel.) and the midbrain (Mes.) has thickened. The floor plate of the latter does not show any of the subdivisions characteristic of the adult mesencephalon. The sulcus limitans (Sul. lim.) can be easily followed from the cord through the myelencephalon and mesencephalon into the posterior part of the diencephalon. The greatest change as compared with H 41 (32.1 mm.) is found in the development of the prosencephalon, especially in the telencephalic portion. In the diencephalon the sulcus divid- 98 MARION HINES ing the medial limb of the caudate nucleus from the hypothalamus runs over the floor of the foramen and ends in the recessus preop- ticus (Rec. preop.) as described before. This sulcus is joined at its dorsal end by the anterior portion of the sulcus limitans. The hypothalamus is the same as described for H 41. Within its floor are found the recessus mamillaris, the infundibulum, its recessus, the bed of the optic chiasma and the preoptic recess. It is not possible to identify the sulcus upon the medial surface of this thalamus, comparable to the sulcus dorsalis found in the embryos previously described. In the epithalamus the habenula (Hab.), the superior commissure (Hab. com.), and the epiphysis (Ep. ev.) are easily identified. Immediately anterior to the habenula the diencephalic roof plate is non-membranous. Its anterior end, however, extends forward over the paraphysis in the form of membranous pockets, the postvelar tubules of Warren (P. vel. t.). These ependymal tubules (Warren, 717, fig. 18, pl. I, p. 125), invaded by vascular connective tissue, seem com- parable to the dorsal sac of amphibians and reptiles. The telencephalon medium joins the rostral limb of the plexus chorioideus ventriculi tertii at the most anterior attachment of the postvelar tubules (fig. 18, P. vel. ¢.). From this point in the telencephalic midline to the preoptic recess the region which shows the greatest growth in length and breadth is the massive portion of the lamina terminalis (fig. 18, Lam. term.). Only a small region immediately ventral to the angulus terminalis has remained thin and tenuous. The antero-posterior measure- ment of the area chorioidea is almost the same as that of the two embryos, 27.8 mm. and 36.1 mm. in length. Of this, the tela chorioidea telencephali medii (fig. 18, Tel. ch. tel. med.) occupies its anterior half and the paraphyseal arch its posterior (fig. 18, Par. p.). The pouch itself extends over the area chorioidea a real evagination of midline tissue continuous with the velum transversum. In the series this is the only embryo which has the typical circular constriction of the stem of the paraphysis. Although the figure delineates but one sac, there are two small lateral pouches which open directly into the central evagination. These relationships are similar to the one which Warren (717 fig. 14, p. 121) has described for a 25-mm. human embryo. THE FISSURA HIPPOCAMPI 99 In the medial wall contiguous to these structures lie the typical subcortical tissues. The most cephalad of these, the septum (fig. 18, Sept.), bulges into the cavity of the lateral ventricle. Dorsal:to the septum, lying between the sulcus limitans hippo- campi and the pars tenuis of the lamina terminalis, is the thin septum ependymale (fig. 18, Sept. epen.). Morphologically it differs in no respect, except in position, from the area intercalata (fig. 18, A. ant.) adjoining the tela chorioidea telencephali medii. The anterior limbs of the lateral choroid plexuses are continuous with the paraphyseal arch a short distance cephalad to the ante- rior limb of the paraphysis. The foramen interventriculare is barely visible from the medial surface. The growth of the midthalamic region, forward and medialward, has restricted its caudal boundary. Its floor is filled with the medial limb of the caudate nucleus. Its anterior boundary is limited by the dorsal region of the terminal plate. Its vault contains the paraphysis and the tela chorioidea tel- encephali medii. Hence the boundaries of the foramen have not changed, although its diameter is narrower than that found in H 41. The surrounding structures have increased in size, growing toward the center of the foramen in all directions. Looking down toward the ventricular floor in the rostral pole of the hemisphere, at the level of the lamina terminalis, the anterior and the medial limbs of the caudate nucleus are seen. The medial nucleus increases in width gradually as far as the level of the anterior limb of the paraphyseal arch. From that point posteriorly it becomes narrower. The lateral nucleus of this complex springs from the side wall at the level of the root of the olfactory bulb. It attains its greatest width at the level of the dorsal portion of the lamina terminalis. Caudally it terminates in the region of the posterior extremity of the hippocampus. The angulus ventralis lies between the medial limb of the caudate complex and the lateral extension of the septal region. It extends from the base of the olfactory evagination into the floor of the foramen interventriculare. The two ventral sectors of the hemi- sphere wall have grown in length, so that the telencephalic evagination projects beyond the lamina terminalis much farther 100 MARION HINES than in the stage previously described. ‘On the medial wall, the hippocampus does not bulge into the hemisphere except pos- terior to the region of the tela chorioidea telencephali medii. However the sulcus limitans hippocampi lying ventral: to the hippocampus extends from the tip of the temporal pole over the foramen and becomes lost just caudal to the olfactory bulb evagination. The original caudal pocket of the ventricle has swung ventro-anteriorly and carried with it the lateral complex of the nucleus caudatus. The neopallial arch which bridges the lateral division of the corpus striatum and the medial olfactory area is much higher than before. The plexus chorioideus ven- triculi lateralis almost fills the cavity. The chorioidal fissure is filled with mesenchyme and blood vessels. The outer contour of the telencephalon begins to resemble that of the adult. In the center of the lateral surface an area of retarded growth is present, the future island of Reil. Dorsal, caudal, caudo-ventral, and rostral to this point of slow growth, the telencephalon swings out, growing as it were between its points of attachment to the diencephalon. Upon the medial surface the line of separation of the cerebral hemisphere from the evaginating olfactory bulb is carried up on the wall as a small indentation, which terminates rostral to the lamina terminalis. This is the fissura prima of His. Caudal to the velumtransversum the fissura hippocampi can be traced to the tip of the temporal pole. Rostral to this point this fissura no longer can be identified on the medial surface of the hemisphere. The changes which have taken place in growth between the two embryos H 41 (32.1 mm.) and‘H 163 (39.1 mm.) are as follows: 1. Marked evagination of the olfactory bulb, which accentuates the fissura prima of His. 2. Great growth of the lamina terminalis, together with a lateral extension of the septal region into the ventricle. 3. The fissura hippocampi is restricted to the medial wall caudal to the velum transversum. 4. Constriction of the foramen Monroi by the growth of the midthalamic region. THE FISSURA HIPPOCAMPI 101 5. No suleus dorsalis thalami can be identified upon the ven- tricular surface. 6. Appearance of the island of Reil. 7. Great growth of the neopallium. The 43-mm. embryo, Mall Collection, 886 (figs. 19 and 20) This embryo measured 39.9 mm. greatest length in alcohol, only 0.8 mm. longer than H 163. It is not strange that the fore- brains of these two embryos are very similar. The development of the cord and the medulla oblongata is the same. ‘The cere- bellum appears as a medial outgrowth from the dorsal lip of the lateral recess. The floor plate is very thick. The midbrain is divided into alar and basal plates by the sulcus limitans. The basal plate is increasing in depth. There is no hint of a division into colliculi upon the roof of the mesencephalon. ‘The ventric- ular markings in the thalamus are the same. The sulcus limi- tans ends blindly in the hypothalamus. It is joined for a short _ distance by the sulcus which delimits the ventral boundary of the midthalamus, the sulcus Monroi. Dorsal to this sulcus is the great midthalamic mass. Here as in the 39.1 mm. embryo there is no visible separation between this part of the thalamus and the epithalamus. The epithalamus contains the characteristic structures, the epiphyseal evagination, the habenula, the habe- nular commissure, and the choroid plexus of the third ventricle. The hypothalamus contains the corpus mamillare, the thin tuber cinereum, the posterior lobe of the hypophysis, the re- cessus infundibuli. The last recess is deeper in this embryo than in H 163 (39.1 mm.). The recess preopticus lying between the chiasma ridge and the inferior limb of the lamina terminalis is longer and more shallow than that of the 39.1-mm. embryo. The structures of the telencephalon medium are similar in relation and development to those found in the 39.1-mm. embryo. The lamina terminalis is slightly longer, but not broader. The angulus terminalis is not as well delineated. The length of the area chorioidea is practically the same as that of H 163. Its anterior division, the tela chorioidea telencephali medii is like the 39.1-mm. but its posterior, the paraphyseal arch, is not the same. 102 MARION HINES Taking for its anterior limit that part of the midline where the two lateral choroid plexuses are continuous over the telencephalic roof and the velum transversum as its posterior boundary, its length is approximately that of the 39.1 mm. However, it was impossible to identify a dorsal outpouching in the region of the paraphyseal arch. This may be due in part to the fact that the embryo was sectioned at 100y, coronal to the cerebral hemisphere. _A thinner section in the transverse plane is more advantageous for the study of this region. In the 39.1-mm. this structure was present for about 175y. : The septum continues its growth into the venimiclel The area of the septum ependymale measures approximately the same and cannot be distinguished morphologically from the area in- — tercalata. The extent of the lamina epithelialis resembles that of the 39.1-mm. embryo. Within the telencephalon the ventricular ridges have the same relationship to each other as that described for the 39.1-mm. embryo, H 1638. The lateral limb of the caudate nucleus is large | and ends in the ventromedial horn of the lateral ventricle. The medial limb of the same complex terminates at the base of the olfactory bulb. The angulus ventralis separates this medial limb from the septal region. The hippocampus forms a rounded hillock upon the medial wall which tends to flatten out anterior to the angulus terminalis. The sulcus limitans hippocampi lies beneath it, extending from the tip of the temporal pole to a region slightly anterior to the rostral limit of the fascia dentata. The fissure of the choroid plexus is completely closed. The vault of the hemisphere is higher in the region of the paraphysis than in the 39.1-mm. The anterior and ventro-posterior extension is not greater than that of the last brain described. The outer contour is not materially changed. The island of Reil is visible on the lateral surface, so that the hemisphere may be divided into various regions indicative of its future lobulation. The olfactory bulb is large and its cavity opens into the ventricle. Along its medial boundary of constriction, extending dorsally anterior to the lamina terminalis lies the fissura prima of His. Posterior to the terminal plate, the fissura hippocampi can be THE FISSURA HIPPOCAMPI 103 followed from the region of the paraphysis to the end of the hippocampal formation. The most marked difference. between this embryo and H 163 (39.1 mm.) is the relative growth of different portions of the telencephalon medium. ‘They are as follows: 1. Increase in length of the lamina terminalis. 2. No discernible paraphyseal arch. 3. Increase in the height of the neopallial vault, especially in the central region of the hemisphere. In the first two brains described, no. 1121 (11.8 mm.) and no. 940 (14 mm.), no fissure or sulcus on the medial wall was visible. But in the description of nos. H 173 and 460, 19.1.mm. and 29 mm., respectively, a shallow groove extended on the medial wall of the hemisphere from the region of the future olfactory bulb evagination to the most caudal and ventral extremity of the lateral area of di-telencephalic fusion. ‘This fissure was also noted in much the same position in H 91 and H 41 (27.8 and 31.1 mm.), although its depth was not as great rostral to the terminal plate. Immediately dorsal to the paraphysis this groove is very shallow, the wall being almost smooth. But caudal to the para- physis the fissure follows the curved contour of the temporal lobe of the hemisphere, lying in the medial wall, describing an are of a circle. In the oldest brains here considered (39.1 mm. and 43 mm.), the rostral division of the medial wall is very smooth. However, just posterior to the region of the paraphyseal arch lies the rostral end of an indentation, the fissura hippocanpi. This fissure may be traced as a shallow depression in the hippocampus to the caudal end of the primitive temporal pole. Besides this fissure another was described, the fissura prima of His. It is found only in the older embryos of the series (32.1 mm., 39.1 mm, and 48 mm.), or in those embryos in which the olfactory evagination is prominent. It seems to delimit the tissue basal to the attachment of the olfactory bulb from the cortex dorsal to its point of evagination. It is the former groove or fissure hippocampi which has claimed the writer’s considera- tion for a number of years. And you, the reader, are you ques- 104 MARION HINES tioning its reality? You may well do so, when its history is remembered. Certain it is that the question of fixation was adequately controlled. The embryos studied were carefully pre- served, their further handling as good as our present technique allows, and yet such treatment did not banish from the medial wall of the early telencephalon the fissura arcuata of His (the fissura hippocampi of others). Nevertheless the description of this fissure seems to indicate that its preterminal limb is transient. If this groove is real, there must be some explanation, first for its appearance and second for its partial obliteration. Further, if it is real, a histological differentiation would be expected and processes of growth differences may account, in part at least, for its appearance and later its disappearance in the prevelar region. It is not a question of fixation. A very meager ex- perience with this material gives an unerring criterion to the investigator for the determination of artefacts. The explanation is to be found, rather, in the development not only of the tissue involved in the fissure itself, but also in the histological differ- entiation of tissue in more remote parts of the developing hemis- phere wall. HISTOLOGICAL STRUCTURE The 11.8-mm. embryo, Mall Collection, 1121 (figs. 21 to 25) The telencephalic vesicle of the brain of this embryo is little more than a single evagination, which has expanded slightly beyond its initial attachment to the diencephalon. ‘The lateral expansion of this vesicle is greater in the region midway between its anterior and posterior poles. ‘The midline extending from the region of the velum transversum rostrally is interrupted by an infinitesimal elevation, the paraphyseal arch. Figures 21 to 25 are line drawings of sections of the vesicle cut at right angles to the chord joining the velum transversum and the preoptic recess. The levels are indicated in figures 8 and 11. The mid- line, beginning in the region of the bed of the optic chiasma, may be divided morphologically into the following regions (figs. 7 and 8 and p. 85 supra): Region of the optic chiasma (fig. 21, F. b. op. ch.). The lamina terminalis pars crassa (fig. 22, Lam. term.). THE FISSURA HIPPOCAMPI 105 The pars tenuis of the lamina terminalis and the septum ependy- male (fig. 23, Sept. epen.). Region of the paraphyseal arch (figs. 24 and 25h ebsi ne gla en Velum transversum (figs. 7 and 8, Vel. trans.). In the region of the bed of the optic chiasma (fig. 21, F’. b. op. ch.) the evaginating vesicles form two arcs of a circle, their in- tersection being marked by a small depression in the midline. Following this are of the telencephalic brain wall around to its point of union with the diencephalon, it appears fairly uniform in histological structure. In figure 22 the vault of the brain wall is more extensive. Within its outer margin a barely discer- nible clear zone appears, the Randschicht of His, or the marginal velum (Man. 1). This zone is more clearly defined a few milli- meters (measured on the figure itself) lateral of the midline. The wall at this point shows a very slight local thickening. In figure 23 the lateral expanse of the vesicle is greater and the marginal velum more sharply defined. The union of the two vesicles 1s made by a thin epithelial plate of cells. In this figure, approxi- mately 6 mm. on either side of this thin line, no mantle zone can be identified in the brain wall (figs. 9 and 11, Sept. epen.). But beyond this region, for 10 mm. on either side, the marginal velum reaches its maximum definition. Figure 24 is a section through the caudal portion of the tel- encephalic roof plate, at the level of what appears to be the paraphyseal arch (figs. 8 and 24, Tel. r. pl.). This small arch in the midline is one feature of a remarkable histological differen- tiation in the telencephalon. Joining the lateral limbs of this arch is a slender, homogeneous epithelial tissue. Immediately lateral to this tissue is an area in which the marginal velum is clearer and wider than any other portion of the vesicle. Again lateral to this area is a region of the brain wall which forms the telencephalic evagination (Tel. evag.) or incipient cerebral hemis- phere, extending backward to the point of junction with the thalamus at the di-telencephalic groove. Upon the ventricular surface the first and second areas are separated by a shallow sulcus, which will be referred to as the sulcus limitans hippocampi (fig. 24, Sul. vent.). THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, No. 1 106 MARION HINES In figure 25 also on the right side reading from the midline lateralward, the following histologically distinct areas are found: 1) the telencephalic roof plate, i.e., the paraphyseal arch (Tel. r. pl.); 2) region of the future lamina epithelialis (not labeled) ; 3) an area which will contain the fornix and the fascia dentata (compare the older embryo, fig. 14); 4) separated from the lamina epithelialis by a ventricular sulcus (Sul. vent.), the sulcus limitans hippocampi, is the primordium of the future hippocampus (Prim. hip). The sulcus limitans hippocampi can be traced from the caudal level of the paraphyseal arch (fig. 11, Reg. par. ar.) to a re- gion immediately anterior to the most rostral extent of the arch. This ventricular marking delimits areas at this stage of develop- ment already histologically distinct. This sulcus, together with the clear marginal velum lateral to it, enables the writer to bound accurately the anlage of the hippocampus. These slight early differentiations are of great importance, because they can be used as landmarks from which to measure the further growth and differentiation in the telencephalon. The 14-mm. embryo, Mall Collection, 940 (figs. 26 to 28) The counterpart of the distinct histological structures, dorsally placed in the telencephalon of the 11.8-mm. embryo, are found either in the unevaginated midline little changed or in the medial wall of the hemisphere of the 14-mm. The hemispheres of this brain extend for an appreciable distance anterior to the lamina terminalis and posterior to the velum transversum. In order to facilitate the description of the shifting position of histologically distinct areas, especially anterior to the lamina terminalis, within the developing hemisphere, they may be divided into quadrants (following the morphological terminology of Herrick, 710). In that paper Herrick suggested a subdivision of the amphibian cerebral hemisphere into four fundamental quadrants, viz., dorso- lateral (pyriform lobe); 2) dorso-medial (primordium hippo- campi); 3) ventro-lateral (striatum complex); 4) ventro-medial, septal complex. To these in mammals there is added (p. 496) a fifth region, the neopallium. In this embryo (14 mm.) these regions can be recognized save for the confusion of the pyriform, THE FISSURA HIPPOCAMPI 107 or dorso-lateral olfactory areas, with the striatal complex, a condition comparable with that of the adult reptile (Crosby, 717). Because of this confusion and the absence of neopallium in the amphibian, the writer prefers the use of sector stead of quadrant. Accordingly, in this brain each hemisphere can be divided into four sectors: dorso-lateral (neopallium), dorso- medial (primordium hippocampi), ventro-lateral (combined stria- tum, lateral olfactory area and pyriform lobe), and ventro- medial (septum and medial olfactory area). At this particular level the two dorsal sectors are of approximately the same thick- ness; the wall of the ventro-lateral sector, however, 1s much thicker than that of the ventro-medial. The dorso-medial sector may be distinguished from the dorso-lateral by a broad white marginal velum within the medial one (fig. 26, Prim. hip.). _ This same layer of the telencephalic wall within the dorso-lateral quadrant contains many neuroblasts (fig. 26, Neopal.). The matrix or zona ependymalis of the medial sector is not as broad as that of the lateral. The dorso-medial region is separated from the ventro-medial by a sharp ventricular groove, the sulcus limitans hippocampi of Herrick (fig. 26, Sul. vent.). The ventro- medial sector itself is divided into two histologically distinct areas: a dorsal! thin layer of tissue whose cells, packed closely together, show no definite arrangement, the septum ependymale (fig. 26, Sept. epen.), and a ventral whose thick wall presents an inner dense matrix and an outer zone, containing a few neuroblasts, the septum (fig. 26, Sept.). The two ventral sectors are separated from each other by a slight indentation in the ventricular surface the angulus ventralis (compare Herrick, ’10, figs. 13 and 14, A. V. with fig. 26, Ang. vent.). Lateral to the ventral angle the brain wall bulges into the ventricle, the first evidence of the corpus striatum, specifically, the medial nucleus of the caudate complex. Between this nucleus and the dorso-lateral sector must lie the future lateral olfactory complex and the lateral limb of of the caudate nucleus. The four sectors of the hemisphere are then, 1) the dorso-lateral or the neopallium; 2) the dorso-medial or hippocampal primordium; 3) the ventro-medial or the septum and the septum ependymale, and, 4) the ventro-lateral or the 108 MARION HINES corpus striatum and the lateral olfactory areas. The particular area which is of immediate interest is the dorso-medial sector, the future hippocampus (fig. 26, Prim. hip.). The peculiar histological character of this tissue, already noted in the 11.8-mm. embryo but accentuated here, (that is, the clear practically cell- free marginal velum and the slightly thinner matrix) extends from the region just dorsal of the future olfactory evagination, sickle- like, over the foramen interventriculare to the caudal tip of the hemisphere (see the stippled area in figs. 12 and 14). Passing caudally, the future hippocampus occupies a progress- ively more dorsal position in the telencephalic wall (figs. 27 and 28). The amount of neopallial tissue present in the caudal portion of the hemispheric evagination is less than that found at its rostral pole. In figure 27, a section through the paraphyseal arch (Par. ar.), there lies between the arch and the future hippo- campal cortex (Prim. hip.) a curved ependymal wall with. the concavity directed outward. This tissue, which joins the hippo- campus at the suleus limitans hippocampi (Sul. vent.) and the lateral limb of the paraphyseal arch, is the first stage in the development of the sulcus of the lateral choroid plexus (figs. 27 and 28, Sul. fu. pl. ch. ven. lat.). The regions differentiated as histologically distinct areas are as follows: The neopallium, the hippocampus, the corpus stria- tum, the septum ependymale, the septum, the two divisions of the area chorioidea, the tela chorioidea telencephali medii, and the paraphysis, together with their respective contiguous struc- tures, the area intercalata and the lateral choroid plexus. Be- sides these, there is the marked ventro-caudal growth of the hippocampus. The 19.1-mm. embryo, University of Chicago, H 173 (figs. 15, 16, 29 to 32) The cell arrangement within the telencephalon of this embryo bears little semblance: to that found in the 14-mm. embryo, with one exception, the future hippocampus. This region of rela- tively cell-free marginal velum can be identified on the medial wall from the region of the base of the olfactory bulb evagina- THE FISSURA HIPPOCAMPI 109 tion to the tip of the temporal pole (see the stippled area in fig. 16). The other components of the vesicle have shifted their places in the sector formation, but continue to maintain their fundamental relationships. ‘The future hippocampus now oc- cupies the centro-medial area, in levels anterior to the lamina terminalis (figs. 29 and 30, Prim. hip.). This tissue swings gradually upward to those levels further caudalward (figs. 31 and 32, Prim. hip). The ventral margin is limited throughout by the sulcus limitans hippocampi (figs. 29 to 32, Sul. vent.). The groove which outlines the future hippocampus on its medial surface is the fissura hippocampi (figs. 29 to 32, Fis. hip.). Not only is the future hippocampus distinguished by the cell-free outer zone, the marginal velum, but also, by the narrower matrix, and a greater thickness of the brain wall itself. Immediately opposite the sulcus limitans hippocampi lies a small group of cells which appear to be migrating into the marginal velum from the matrix. This group of cells is coincident with the extent of the limiting sulcus, and it is the primordial fascia dentata (figs. 29 to 31, Fas. den.). It lies wholly within the hippocampal formation above the limiting sulcus. The remainder of the dorsal medial wall is histologically the same tissue as that composing the dorso-lateral one. It is the neopallium (figs. 29 to 32, Neopal.) and is characterized at this stage of development by a dense matrix and a marginal velum, filled with migrating neuroblasts. In amount it is pro- portionally and actually greater in the rostral than in the caudal division of the telencephalon. The division of the ventro-medial sector into two areas, similar to those described in no. 940 (the 14-mm.), is clearly seen in the section immediately posterior to the level of the tuberculum olfactorium (fig. 31). The more dorsal slender wall is the septum ependymale (fig. 31, Sept. epen.), and the more ventral, the septum (fig. 31, Sepi.). In the latter region a minute differ- entiation in the form of a row of cells lying in the marginal velum may be identified. This is the nucleus medialis septi, which also appears in levels through the tuberculum olfactorium (figs. 29 and 30, Nuc. med. sept.). This cell grouping is continuous 110 MARION HINES over the angulus terminalis with those cells already identified as the primordial fascia dentata, though these two cell masses originate in different sectors and have very different morphological significance. The lamina epithelialis, above the foramen in- terventriculare, has invaginated now to form the lateral choroid plexus. The last sector, the ventro-lateral, shows plainly the be- einnings of the medial and lateral limbs of the caudate complex. The separation of the caudate nucleus from the neopallium is made evident by a shallow ventricular groove. The angulus ventralis, due in the main to the enlargement of the ventricular eminence of the caudate’s medial complex (figs. 30 and 31, Cor. str. med.), bounds the septum laterally. The following histological changes have taken place between the stage last described, the 14-mm., and the 19.1-mm.: 1. The expansion of a mantle zone in the septal region. 2. The acceleration of growth in the neopallium, accompanied by the complete incorporation of the hippocampal anlage in the medial wall. 3. The appearance of the primordial fascia dentata opposite the sulcus limitans hippocampi. It is continuous with a similar group of cells in the septum, the nucleus medialis septi. 4. The appearance of the plexus chorioideus ventriculi lateralis. 5. The obvious indentation of the hippocampal wall, the fissura hippocampi. The 20-mm. embryo, Mall Collection, 460 (figs. 17, 33 to 37) The morphological and histological differentiation of the tel- encephalon of this embryo resembles in large part that of the 19.1-mm.; the boundary lines of the various areas remain un- changed and they occupy the same relative positions in the four sectors. The fissura hippocampi is a broad bow-shaped bend of the medial wall of the hemisphere involving practically the entire extent of the hippocampal formation and extending from the base of the bulbus olfactorius to the tip of the temporal pole (fig. 17, stippled area). Rostral to the terminal plate, the greatest depth of this fissure lies in the center:of the hippocampal forma- THE FISSURA HIPPOCAMPI Lim tion, but caudal to the lamina the greatest depth, of this fissure shifts toward the sulcus limitans hippocampi (compare Fis. hip. in figs. 33 and 34 with the fissure in figs. 35 to 37). The depth of the fissure, then, is found in the center of the future hippocam- pus at those levels in which its ventral limb is continuous with the septum itself; when, however, its ventral limb is joined to the thin septum ependymale or the taenia fornicis, the depth of the fissure shifts ventrally. Within the septum ependymale is now found a thin marginal velum (fig. 35). There is no change in the area intercalata. However, in the lamina epithelialis, the portion which extends to the paraphyseal arch (fig. 36) approaches the cuboidal epithe- lium of much older stages while that portion contiguous to the outer limb of the velum transversum is as primitive as the whole of the area in the 19.1-mm. embryo. The primordium of the fascia dentata extends from a region anterior and dorsal to the angulus terminalis almost to the tip of the temporal pole, opposite the suleus limitans hippocampi (cross-hatched area, fig. 17). In figure 33 (Fas. den.) it lies as a small well-defined group of cells in the marginal velum below the fissura hippocampi (fig. 33, Fas. hip.). Here, as well as in the next figure (fig. 34, Fas. den.), these cells seem to be continuous with the nucleus of the septum, the nucleus medialis septi (Nuc. med. sept.). This nucleus always appears ventral to the suleus limitans hippocampi and can be identified easily in the next level pictured, through the septum ependymale (fig. 35, Sept. epen.). Here also the fascia dentata is well marked. In the last two levels presented this structure maintains its initial relationship to the sulcus limitans hippocampi and the fissura hippocampi (figs. 36 and 37). The first three figures (figs. 33 to 35), when compared with those of the 14-mm., are excellent illustrations of the shifting of the hippocampal primordium to the midmedial region of the cerebral hemisphere. The whole dorsal lateral wall and almost half of the dorso-medial, at this stage, are composed of neopallium. But within the ventro-lateral sector bounded by the sulcus above the lateral limb of the caudate complex and the angulus 112 MARION HINES ventralis, that area which in mammals is said to represent the whole lateral half of the hemisphere evagination, only two cellu- lar layers on the ventricular contour of the caudate nuclei and a diffuse cellular mass beneath them can be identified. Upon careful examination a thin layer of cells may be seen, swinging around the ventro-medial corner of the hemisphere in the marginal velum, the first evidence of the cortex of the tuberculum ol- factorium. ‘The relationship of tissue within the ventro-medial and ventro-lateral sectors is identical in the 19.1-mm. and 20- mm. embryos. This embryo shows an advance over the last in: 1. The appearance of a clear zone in the septum ependy- male. 2. Growth of neopallial tissue in the caudal and inferior aspect of the hemisphere. 3. The hippocampal anlage begins to assume its definite shape caudally, i.e., the caudal end of the hippocampal forma- tion has advanced forward to define the temporal lobe (figs. 16 and 17). 4. The fascia dentata has differentiated as far as the temporal tip of hippocampal primordium. The 27.8-mm. embryo, University of Chicago, H 91 (figs. 38 and 39) The general relationships of the various components of the telencephalon are the same in this embryo as those described for the 20-mm. However, the intrinsic differentiation within the cortex has become evident. Within the neopallium two cell layers have migrated out of the matrix. The outermost layer is that of the pyramids (fig. 29, Pyr. c. l.), and one next the matrix is the intermediate cell layer (figs. 38, 39, Poly. c. 1.). But in the hippocampus the latter group only has appeared. The cells occupy the dorsal lip of the hippocampal fissure. This differen- tiation is especially marked anterior to the paraphyseal arch. Here also, as before, the primordial fascia dentata (figs: 38, and 39, Fas. den.) lies opposite the sulcus limitans hippocampi (figs. 38 and.39, Sul. vent.). THE FISSURA HIPPOCAMPI 1S The olfactory bulb is in the process of actual detachment, so that its separation from the ventral sectors in the rostral pole has produced the fissura prima in the medial wall. Differentia- tion characteristic of the neopalltum appears in the dorsal lip of this fissure. There is in the other embryos, nos. H 173 (19 mm.) and 460 (20 mm.) a small area dorso-frontal to the region where the fila olfactoris enter, which does not show the differ- entiation characteristic of the hippocampal anlage. The septum ependymale is divided into two layers, an outer clear zone and an inner matrix (figs. 38 and 39, Sept. epen.). This same tectonic arrangement is characteristic also of that tissue which adjoins the midvault of the foramen interventric- ulare. The change from a simple epithelium to the two cell layers described seems to be the first step in the process of differ- entiation in the lamina terminalis, a process which proceeds caudally. The progress of growth has added the following: 1. Differentiation of the neopallium into three cell layers. 2. Appearance of the intermediate cell layer in the dorsal part of the primordium hippocampi, a process whose develop- ment is more marked rostral to the terminal plate. 3. The septum ependymale contains two cell layers through- out, thus differentiating it from the static area intercalata. The 32.1-mm. embryo, University of Chicago, H 41 (figs. 40 and 41) At the level of the root of the olfactory bulb (fig. 40) the cortical differentiation extends ventrally as far as the fissura prima. ‘This groove with its dorsal differentiation continues for a short distance upon the rostral end of the medial wall. Caudal to the root of the olfactory bulb there is an area of unmistakably hippocampal formation. Here the intermediate cell layer has crept out of the matrix below the level of the fissura hippocampi and the pyramidal cell layer shows itself in the dorsal lip of the fissure. This migration is characteristic of the preterminal hip- pocampal formation. That part of the future hippocampus 114 MARION HINES which lies between the levels of the lamina terminalis and the velum transversum is an area transitional to the sickle-shaped postvelar tissue in which little or no cortical differentiation has taken place. It is in the latter that the fissure is best indicated. The fissure is best developed, then, in the least differentiated part of the hippocampus. ‘The fascia dentata (fig. 41, Fas. den.) is now a continuous band of clumped cells opposite the sulcus limitans hippocampi. ‘The fascia dentata is not coextensive with either this sulcus or with the hippocampal formation. However, the ventricular sulcus limitans hippocampi (Sul. vent.) seems to delimit its ventral extent except in that region where the an- lage of the hippocampus gradually fades into the olfactory bulb. The condition of the septum and septum ependymale is the same as that of H 91 (28.7 mm.), with the exception that its most dorsal portion is much wider measured antero-posteriorly. The growth changes may be summed as follows: 1. More ventral differentiation of the preterminal portion of the hippocampal anlage with a coincident decrease in the depth of the fissura hippocampi, ventrally. 2. Further thickening of the outer layer in the septum ependy- male. The 39.1-mm. embryo, University of Chicago, H 163 (figs. 42 to 46) At the level of the tuberculum olfactorium (fig. 42, ef. fig. 18, tub. olf.) the vault of the hemisphere from the corpus striatum laterally to the sulcus limitans hippocampi medially presents the typical cortical differentiation. Following the tissue which lies immediately dorsal to the sulcus limitans hippocampi to the level of the lamina terminalis, the typical cortical layers do not extend as far ventrally as the sulcus. In the marginal velum opposite it lies the fascia dentata (fig. 43, Fas. den.). ‘This tissue is the most rostral limit of the hippocampus. ‘The area immediately ventral to the hippocampus is that of the septum. Its growth is very marked in all directions. That growth is largely the result of differentiation of a new diffuse nucleus. THE FISSURA HIPPOCAMPI BES Already in the marginal velum in embryos H 173 and 460 a thin layer of cells, the nucleus medialis septi, was evident. Now, lying between this nucleus and the matrix is a large diffuse group of cells, the nucleus lateralis septi (fig. 48, Nuc. lat. sept.). Here, also, rostral to the anterior commissure, the separation of the ventro-lateral from the ventro-medial sector is emphasized by a prolongation of the angulus ventralis (fig. 43, Ang. vent.). Its accentuation in this embryo is due to the growth of the medial nucleus of the caudate complex. The ventricular angle mark- ing the boundary between the dorso-lateral and the ventro- lateral sectors has become more acute because of changes in the corpus striatum, namely, a ventricular extension of the lateral root of the caudate complex and the appearance of the anlage of the lentiform nucleus (figs. 43 and 44, Nuc. lent.). Examining the relationships of tissue at a level passing through the more caudal part of the lamina terminalis (fig. 44, Lam. term.) we find the septum is narrow. However, three groups of cells may be seen, the inner matrix, a middle diffuse group, the nucleus lateralis septi (fig. 44, Nuc. lat. sept.), and an outer thin layer, the nucleus medialis septi (fig. 44, Nuc. med. sept.). Their development will be considered in the second study. ‘The hip- pocampus joins this tissue dorsally. Within the hippocampus the pyramidal cell layer extends past the middle of the fissura hippocampi, while the intermediate cell group lies in its ventral lip, just dorsal to the level of the sickle-shaped sulcus limitans hippocampi (fig. 43, Sul. vent.). Along the extreme ventral margin of the hippocampus extending dorsally from the region of the sulcus mentioned lies the fascia dentata (fig. 438, Fas. den.). In the space between the fascia dentata and the matrix lie the fornix fibers. These fibers also occupy the space between the medial and lateral septal nuclei. From this level, caudally, throughout the compass of the hippocampus, the fascia dentata grows dorsally along its most ventral margin (figs. 43 to 46, Fas. den.). In figure 45, a section through the paraphysis, the lateral choroid plexuses join the lateral limbs of the paraphyseal arch (Par. ar.). The small tubules lying in cross-section on either 116 MARION HINES side of the arch are the rostral evagination of the roof of the third ventricle, the postvelar tubules of Warren (P. vel. t.). How- ever, in the midline above the arch there are two small tubules whose continuity with the membranous roof is found anterior to the velum transversum. They form a real pouch which ex- tends forward over the telencephalic roof plate (fig. 18, Par. p.). This condition of the human paraphysis is strikingly like that figured by Warren (’17) in the 25-mm. embryo (figs. 14 and 15). Rostral to the anterior limb of the paraphysis is the tela chorioidea telencephali medii. It is undifferentiated, a true epithelial tissue. The tissue, which joins it dorso-laterally, is separated into two layers, an inner matrix and an outer layer, which contains fibers, emerging from the hippocampus, the fornix (fig. 45, For.) In a section parallel with the length of the fissura hippocampi (fig. 46, Fis. hip.) the fascia dentata (fig. 46, Fas. den.) is a thin band of cells lying in the marginal layer of the hippocampus. This section cuts the sickle-shaped fissura hippocampi tangently so that the typical hippocampal structure may be seen at two ends. The 43-mm. embryo, Mall Collection, 886 (figs. 47 to 50) Orientation is difficult in this embryo, because it was cut eoronally to the long axis of the cerebral evagination. A com- parison of the levels from which the figures were taken with the median sagittal view (fig. 20) is illuminating. Figure 47 passes longitudinally through the rostro-dorsal extent of the fissura hippocampi. ‘The fascia dentata lies as a band in the outer margin (Fas. den.). The intermediate cell layer is well differ- entiated, while that of the pyramids extends only across the mar- gin of the hippocampus. Lying between the fascia dentata and the matrix are the developing fibers of the fornix system. Figure 48 was taken at a level farther caudally, below the dorsal arch of the fissura hippocampi. The architecture of the hippocampus is the same in its anterior and posterior extent with the exception that the number of developing fornix fibers is greater posteriorly. Following the posterior limb to its caudal THE FISSURA HIPPOCAMPI ez: end, there is no change in these relationships. Anteriorly, how- ever, the cortical differentiation is comcident with the sulcus limitans hippocampi (fig. 49, Sul. vent.) and seems to be continu- ous with the same type of differentiation which lies dorsal to the root of the olfactory bulb. At both of these levels the septum contains three groups of cells as outlined in H 163 (39.1 mm.) and is separated from the hippocampus by the sulcus limitans hippocampi. There are no discernible paraphyseal pouch or postvelar tubules. Development in these two embryos may be summarized as follows: 1. Cortical differentiation in the region of the hippocampal formation, anterior to the paraphyseal arch, and the coincident reduction in depth of the fissura hippocampi in this region. 2. The great growth of the lamina terminalis in all directions and an intrinsic differentiation into three cell groups, the septal nuclei. 3. The dorsal extension of the fascia dentata along the margin of the medial wall. 4, The development of the anterior commissure and the fornix system. The eight brains described in the preceding pages have been arranged according to the time interval between the initiation of the growth process and its arrest, measured as the greatest length attained by the individual. The greater the difference in length the greater the growth changes. These embryos may be divided into the following groups: 1. The 11.8-mm. 2. The 14.0-mm. 3. The 19.1-mm. and the 20.0-mm. 4, The 27.8-mm. and the 32.1-mm. 5. The 39.1-mm. and the 43.0-mm. This division is also a logical one, for when these groups are com- pared to the curve of change through which this particular protoplasm has passed, certain similarities are noticed, from which it seems that the recapitulation of phylogenetic processes is 118 MARION HINES woven into the fabric of their development in such a manner that later additions of brain substance have not erased the early history of the long passage. Consequently, there is a possible interpretation of early growth in the human telencephalon based upon a comparison of its growth with that attained by various representatives of the vertebrate phyla. However, it may ap- pear when this study is complete that this so-called biological inertia which has been used to explain the striking similarities of development in the brains of the vertebrate series has its roots not in heredity as such, but in the more fundamental necessity of the mechanics of growth. Although certain recapitulations are complete, there is no stage of development whose rhythms of growth repeat in any way accurately earlier phylogenetic stages. If, however, the development of one particular tissue is watched with care, the sequence of intrinsic differentiation will appear in the order of a phylogenetic ‘recapitulation. It is possible that the disturbing element is the growing neopallium, whose initial acceleration influences the growth rhythms of the other parts of the telencephalon and may therefore have modi- fied the early phylogenetic relationships of this tissue. DISCUSSION Certain factors seem to be implicit in the growth changes of the telencephalon. They are the landmarks or points in the midline which show individual differentiation. ‘These points are delimited by a peculiar histological structure and a characteristic external morphology. With such a series of stages as presented, the early development in the midline, together with those changes in the telencephalic vesicle whose various parts are confluent with them, may be followed. Telencephalon medium The midplane structures of the telencephalon medium have been divided into two main divisions by the angulus terminalis, the lamina terminalis and the area chorioidea. The lamina terminalis grows progressively thicker, rostro-caudally, and the thickening approaches the angulus terminalis progressively from THE FISSURA HIPPOCAMPI 119 younger to older stages so far as studied. The area chorioidea is differentiated early into two regions, one of whose lateral limbs becomes the thin lamina epithelialis of the lateral plexus and the other an area in which changes are insignificant. The former is the paraphyseal arch and the latter is the tela chorioidea telencephali medii. In table 4 the writer has attempted to measure, somewhat crudely to be sure, two dimensions of these midline structures. The total lengths of the telencephalon medium were taken by TABLE 4 Measurements of divisions of the telencephalon medium TELENCE- TELA CHORIOIDEA EMBRYO PHALON | LAMINA TERMINALIS TELENCEPHALI PARAPHYSIS MEDIUM MEDII Number | Length Length Length Paes Length Width Length Width mm mm. mm. mm. mm. mm. mm. mm. 1121 | 11.8 | 2.388 | 0.944 | 0.054] 0.2481} 0.02 1.24 | 0.02 940 | 14.0 | 1.620] 1.048!| 0.088 | 0.168! | 0.028 | 0.404] 0.032 liege tO. Ve | 1:2.52 ed 72 O17 “1-02.29 0.03 0.51 | 0.04 460 | 20.0 | 2.516 | 1.60 0.15 | 0.40 0.036 | 0.516 | 0.036 HOt} 627.8: | 2.645 | 2.27 O51 70h 15 0.02 0.225 | 0.05 H41 | 32.1 | 2.834] 2.40 | 0.644 | 0.184 | 0.041 | 0.25 | 0.064 HA63)))| © 39.1. |, (3:24 -| 2.56 | 0.80 | 0:21 0.72 0.470 | 0.044 886 | 43.0 | 3.384 | 2.94 | O73 O15 0.75 0.2 0.045 i These are estimates, because the angulus terminalis is not present. measuring the distance between the recessus preopticus and the velum transversum at the magnification of the model studied. That length was then divided by the magnification. Conse- quently, the figures are approximately the actual length in millimeters as found in the particular embryo and therefore comparative. The distribution of the increase lies entirely in the lamina terminalis. The telencephalon medium grows in length mainly because the lamina terminalis increases in length. If the ac- companying figures 1 to 6 be examined, especially figures 3 to 6 sketch 1 and 1d, through the ventral and dorsal divisions of the 120 MARION HINES terminal plate, the growth in depth and width is apparent. This change is a continuous one and, as far as the evidence presented in this paper is concerned, it seems to be one which accompanies the intrinsic growth and differentiation of the contiguous struc- tures. Certain it is, that following the initial differentiation of the septum into matrix and marginal velum and the subsequent appearance of the two septal nuclei, the matrix layer becomes noticeably poor in cells. The whole seems to be a growth at the expense of the cells in situ and not a migration of cells from other centers. In the two oldest embryos only, the anterior ABBREVIATIONS A.ch., area chorioidea A.ep., area epithelialis A.int., area intercalata Ant.com., anterior commissure Ang.term., angulus terminalis Ang.vent., angulus ventralis Ant.l.hyp., anterior lobe of hypophysis Bul.olf., bulbus olfactorius Cer., cerebellum Cor.mam., Corp.mam., corpus mamil- lare , Cor.str., Corp.str., corpus striatum complex Cor.str.lat., Corp.str.lat., corpus stri- atum laterale Cor.str.meéd., Corp.str.med., corpus stri- atum mediale Def .fas.den., fascia dentata Dien., diencephalon Dien.r.pl., diencephalic roof plate Di-tel.gr., di-telencephalie groove Ep.ev., epiphyseal evagination Epith., epithalamus Fas.den., fascia dentata F.b.op.ch., bed of the optic chiasma Fil.olf., filasolfactoria Fis.hip., fissura hippocampi Fis.pr., fissura prima For., fornix For.int., foramen interventriculare Hab., habenula Hab.com., habenular commissure Hip., hippocampus Hypoth., hypothalamus Inf., infundibulum TIs.cal., islands of Calleja Lam.ep., lamina epithelialis Lam.ter., Lam.term., lamina terminalis Lat.olf.tr., lateral olfactory tract Lim.men., limitans meningea Man.l., marginal velum Mat., matrix Mes., Mesen., mesencephalon Mes.f.pl., mesencephalic floor plate Mes.r.pl., mesencephalic roof plate Met., Meten., metencephalon Myel., myelencephalon _ Neopal., neopallium Nuc.ac., nucleus accumbens of Kappers Nuc.lat.olf.tr., nucleus lateralis tracti olfactorii Nuc.lat.sept., nucleus lateralis septi Nuc.lent., nucleus lentiformis Nuc.med.sept., nucleus medialis septi Olf. bulb, olfactory bulb Olf.evag., evagination of olfactory bulb Op.c., optic chiasma Op.evag., Opt.evag., optic evagination Op.s., Op.st., optic s talk Par.ar., paraphysea, arch Par.p., paraphyseal pouch p.c., pars crassa of lamina terminalis P.com., Post.com., posterior commissure THE FISSURA HIPPOCAMPI Pl.ch.vent.lat., plexus chorioideus ven- triculi lateralis Pl.ch.vent.quart., Pl.ch.v.quar., plexus chorioideus ventriculi quarti Pl.ch.vent.ter., plexus chorioideus ven- triculi tertil Pely.c.l., intermediate layer of mi- grating neuroblasts Post.hyp., Post.l.hyp., posterior lobe of hypophysis P.Ra., P.Rat., pouch of Rathke Prim.hip., primordium hippocampi p.t., pars tenuis of lamina terminalis P.vel.r., postvelar arches P vel.t., postvelar tubules Pyr.c.l., pyramidal cell layer Rec.inf., reeessus infundibuli Rec.mam., recessus mamillaris Rec.op., optic ventricle Rec.postop., recessus postepticus Rec.preop., recessus preopticus Reg.par.ar., region of the paraphyseal arch Reg.sept., region of the septum epen- dymale Rhom.fos., rhomboid fossa Sept., septum 121 Sept.epen., septum ependymale Sul.dors., suleus dorsalis thalami Sul.fim.den., suleus fimbrio-dentatus Sul.fu.pl.ch.vent.lat., suleus futurus plexus chorioidei ventriculi lateralis Sul.lim., suleus limitans Sul.mon., suleus Monroi Sul.vent., sulcus limitans hippocampi Tel., telencephalon Tel.ch.tel.med., tela chorioidea telen- eephali medii Tel.evag., telencephalic evagination Tel.r.pl., telencephalic roof plate Te.pl.ch.vent.lat., taenia plexus chori- oidei ventriculi lateralis Te.pl.ch.vent.quar., taenia plexus chori- oidei ventriculi quarti Te.pl.ch.vent.ter., taenia plexus chori- oidei ventriculi tertii Thal., thalamus Tub.cin., tuber cinereum Tub.olf., taberculum olfactorium Undif f.den., Undif.fas.den., undiffer- entiated fascia dentata Vel.trans., velum transversum Ven.lat., Vent.lat., ventriculus lateralis Vent.tert., ventriculus tertius EXPLANATION OF SYMBOLS hippocamaus. fascia dentata area epithelialis subcortical olfactory centers septal nuclei sulcus limitans hippocampi THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 1 122 MARION HINES I commissure has grown across the midline. The bed for these fibers is laid down long before the fibers themselves appear. Further, the growth is appositional, taking place ventral to the center of the main body of the terminal plate and progressing both dorsally and ventrally. The most marked growth is found in the former region, and not in the vicinity of the angulus ter- minalis or the recessus preopticus. There is no constant change in the non-plexiform portion of the area chorioidea and if the structures adjoining this region are examined (figs. 1 to 6, sketch 2) they appear to be constant. J+ + we = ; es i v\ i x. VE Figs.1to6 These are pen sketches of the midregion of transverse sections of the telencephalon, showing the position of cell groups in the tissue; they were drawn either from photographs of the sections or from projections made with the Edinger apparatus. Fig.1 The 11.8-mm. embryo, no. 1121, Mall Collection. X 25 Fig.2 The 14.0-mm. embryo, no. 940, Mall Collection. X 25 Fig.3 The 19.1-mm embryo, H 173, University of Chicago. X 163. THE FISSURA HIPPOCAMPI 2s 4 E fon) Fig.4 The 27.8-mm. embryo, H 91, University of Chicago. X 12>. Fig.5 The 32.1-mm. embryo, H 41, University of Chicago. X 123. Fig.6 The 39.1-mm. embryo, H 165, University of Chicago. X 123. The levels from these various embryos are designated as follows: 1) through the lamina terminalis; 1 v) through the ventral part of the lamina terminalis; 1 d) through the dorsal part of the lamina terminalis; 2) through the tela chori- oidea telencephali medii; 3) through the paraphysis. The various regions of the medial wall contiguous to the midline structures are self-explanatory, see the legend below the list of abbreviations. 124 MARION HINES The only deduction which the present series allows is that the growth in this region is immaterial. In sucha case this measure- ment lends another landmark to the study of the midline. Prob- ably the most striking group of figures presented are those of the paraphyseal arch. There is little change in the antero-posterior length; in this particular series, the younger embryos possess the longer arches. The shortening of the distance between the Fig.7 Medial sagittal view of a model of a 11.8-mm. embryo belonging to the Mall Collection in Baltimore, no. 1121. X 16}. anterior and the posterior limb coincides with the decrease in width and the formation of the actual plexus itself. The con- stant feature is not the pouch; it is rather the simple arch itself. If these figures are substantiated by subsequent work, the para- physeal arch will be a more prominent feature of the younger stages than of the older ones. This finding seems to place the human embryo in phylogenetic line with other mammals, the difference being not in its relative extent, but in the complexity of its bizarre outpouchings. This does not disagree with the work THE FISSURA HIPPOCAMPI 125 of Warren and Bailey; it simply calls attention to a fact not recognized before, that the paraphyseal arch is present even in very young embryos in a simple form, extensive when compared to the whole midline, especially in these brains. It of necessity retains its fundamental importance as a midline structure. cE 1 ah : e vent. qua 5 | um pl ch en 4 v esr pl : Epey va - ; Oe Rec. preop. Dien. r-pl. . Kh : Lam.term. Vel. trans. tnt, Tel.r. pl. Tub. olf. fein OT Sat epen 27” _ Prim. hip lan Undif. f.den. SS dul. vent. Fig. 8 This is a pen-and-ink outline of the same model as that shown in figure 7. Histologically distinct areas in the telencephalic medial wall are pro- jected in their relative positions upon its surface. The key to these areas as well as those of the other models will be found below the list of abbreviations (p. 121). The annotations on all of the models refer to numbers of specific sec- tions, whose photographs are reproduced in this paper or will appear in those which follow. If these midline structures prove to be the landmarks which the writer has shown them to be in the embryos so far studied (and certainly all the well-preserved embryos between the ages men- tioned which have been studied in the Department of Anatomy at Chicago and the Mall Collection in Baltimore substantiate that decision), the growth of the telencephalon adjoining them may be followed accurately. Herein lies an approach to data which 126 MARION HINES will enable the growth of the medial hemisphere wall to be in- terpreted more wisely. It may also give new data upon the de- velopment of the new parts of the cortex lying on the medial wall of the hemisphere. Area epithelialis The area epithelialis lies between the telencephalon medium and the sulcus limitans hippocampi. In the 11.8-mm. embryo this tissue is almost neglible in the rostral division of the hemi- sphere, but it becomes markedly greater in the caudal portions of that evagination. This difference is one of fundamental importance (figs. 1 to 6, sketches 2 and 3). Along the dorsal margin of the area epithelialis on the opposite side of the sulcus limitans hippocampi, the fornix is developed. This sulcus is regarded as the ventral limit of the cerebral cortex and the epithe- lial tissue is the derivative of the most dorso-medial portion of the roof of the primitive evaginated telencephalon. In such a primitive condition, the fornix formed a fringe along its lateral border and the anlage of the hippocampus lay laterally of it (i.e., morphologically ventral to it). This condition is almost exactly realized in the brain of Ichthyomyzon concolor (Herrick and Obenchain, 713). In the process of evagination the relations of the area epithelialis to the fornix and the hippocampus are reversed; that is, the two latter areas are turned first outward, then upward, so that in the medial wall of the hemisphere they come to lie dorsally of the area epithelialis. This seems to be a true statement of the case, because in the 11.8-mm. brain the sulcus limitans hippocampi is coextensive with the epithelial Fig.9 This is the anterior view of the model shown in figures 7 and 8, no. 1121 of the Mall Collection. X 163. There is a slight depression in the midline separating the initial evagination into two halves. Fig. 10 This is the anterior view of the same model as that shown in figures 13 and 14. This 14-mm. embryo belongs to the Mall Collection in Baltimore, no.940. X 16%. Fig.11 Thisisapen-and-ink sketch of the same view as that shown in figure 9, no. 1121. X 162. The areas indicated in figure 8 are shown in their dorsal extent. The planes of section of figures 21 to 25 are indicated. Fig.12 Anterior view of no. 940, same embryo as figure 10. X 163. THE FISSURA HIPPOCAMPI 127 We, |_Aep Figs Hz —e21-3 25 Rate ; —2-2-2 24. ndif. fas.den. — a | Regpara Sul.vent. v x. ae 3 : 23, Prim. hip —— <|\Tel.ch.tel med Sept epen. Sept. Opt. evag ny S54) ——O-Gal 22. Opt st. —s-3-2 al. | Met ie Mesen Figs2s. Sul. vent. Prim. hip.— Tel.ch.tel med 128 MARION HINES tissue and approaches the extent of the differentiation in the hemi- sphere wall termed the primordial hippocampus (figs. 8 and 24). The interpretation of the further development of this tissue must be correlated with this finding, as well as with the facts of its phylogenetic development. This region of generalized morphology and fe stalons was divided for descriptive purposes into three areas, entirely upon the Fig.13 Medial sagittal view of the model of brain no. 940,14 mm. X 16}. basis of their respective destinies, namely, the septum ependymale, the area intercalata, and the lamina epithelialis. The relative growth of the septum ependymale may be seen at a glance by comparing the sections marked /d in figures 3 to 5. This region, an undifferentiated epithelium in the brains of the first four embryos described, begins to show a marginal velum in the 20- mm. embryo, which persists and grows in width in the brains of this group. The differentiation proceeds towards the angulus THE FISSURA HIPPOCAMPI 129 terminalis as a limit. There is always some tissue, therefore, which retains the initial two layers of the early conditions. The area intercalata changes little in morphology. Its extent in- creases most noticeably in the dorso-ventral direction. Intrinsic differentiation is practically identical throughout the older brains of the series (19.1 mm. to 43 mm.). Sul.lim. Mes. pl. i Mes f-pl- Soa SS Rec. inf. 3 Bee fie \ Rec. op. Dienrpl. [\ah. Rec. postop. Lam. ep. Ne Fb.op.ch. A.ep. \ 2 Rec. preop. Alene See = = |y eae Lam.term. Undif. fas.den. “Qs. , ee Fil. olf. qaltven 7 \- oe Sept. Taltenteloneda” Sept. epen. Prim. hip. / A.tnt. |4 Fig. 14 Pen-and-ink outline of the same brain taken from the same point of view as that shown in figure 13. The extent of the medial olfactory areas is projected upon the medial wall of the telencephalon. The arrows indicate the plane of section. The lamina epithelialis lying between the lateral limb of the paraphyseal arch and the sulcus limitans hippocampi increases in relative importance in the older embryos. In the 11.8-mm. the amount of epithelial tissue adjoining the lateral limb of the paraphyseal arch is more extensive than that adjoining the unarched portion of the roof of the telencephalon medium. ‘There is no lateral choroid plexus here; neither is the telencephalon more than a slight evagination. However, in the 14-mm. embryo 130 MARION HINES the paraphyseal arch is a definite dorsal protrusion of the roof plate just anterior to the velum transversum. The tissue_con- tiguous to it is convex lateralward, the first indication of the invagination of the lateral choroid fissure. In all the rest of the series, the plexus exists as described for man by Bailey (16a), namely, the choroid plexus of the lateral ventricle con- sists of two divisions, an anterior, whose lateral taenia is that of the fornix and whose medial is the lateral limb of the paraphyseal Fig. 15 Median sagittal view of the forebrain of H 178. a 19.1-mm. human embryo, belonging to the Chicago collection. > 163. arch, and a posterior, whose lateral taenia is the same as that of the anterior division, but whose medial taenia is the telencephalic limb of the di-telencephalic groove, the taenia chorioidea. In these older embryos the hippocampus forms a complete crescent dorsal and lateral to the plexus. In all stages the primordial hippocampus precedes the choroid plexus as the hemisphere grows caudally. Bailey (16a, fig. 15, a. c. p.) has suggested that the growth of the posterior limb of the lateral choroid plexus is a simple THE FISSURA HIPPOCAMPI 1341! continuous invagination of tissue which lay undifferentiated in the sickle-shaped telencephalic wall adjoining the di-telencephalic groove. There is no indication in the 11.8-mm. or in the 14- mm. embryo of an area peculiarly differentiated in that region. The primitive hippocampus arches over the caudal pole of the developing hemisphere. Since the hippocampal tissue precedes iN Undit fas. den. Primhip. Aint 5 ~ fA Op. sf Post coma) WSS / Recpreop Op.c ‘ SSSA Hypolh. Uy Rec postop 7) MED) Ant Lhyp Bees. Rec. inf “Sul Post hyp. Vi PRa resin Gg g) ; | Ys | 7 7) Rhom. fos. Fig. 18 Median sagittal view of the wax model made from the brain of a 39.1-mm. human embryo H 163 of the Chicago collection. > 81. The positions of the sulcus limitans and the thalamic midgroove are indicated by broken lines. The olfactory area is identified on the medial wall and sketched as if the thalamic wall were transparent. The planes of section of figures 42 to 46 are indicated. 134 MARION HINES ment of the lateral plexus as the hemispheres come more and more to dominate the development of the telencephalon (pp. 526 and 527). However, it dominates only because of the progressively in- creasing importance of the portion of the hippocampus which is developing in the ventro-caudal direction concomitant with the growth of the hemisphere in that same direction to form the Fig. 19 Median sagittal view of a model made from the brain of a 43-mm. human embryo belonging to the Mall collection, no. 886. X 62. temporal lobe. It seems more reasonable, therefore, to place the anlage of the telencephalic lateral plexuses, even in man, in the epithelial tissue which lies between the taenia fornicis or the sulcus limitans hippocampi and the paraphyseal arch and velum transversum and limited rostro-caudally by the anterior limb of the paraphyseal arch and the velum transversum. From this dorsally placed anlage it grows ventro-caudally. If this be true, the primordium of both divisions of the two lateral choroid plexuses lies immediately contiguous to the roof plate of the early unevaginated telencephalon. THE FISSURA HIPPOCAMPI Bs) oO | - Dien.r-pl. Pl.ch.vent. lat. Lam.ep. Vel. trans, A. int. oN aoa prt ~ as, : Ae mon. NN hie inf. ae Ao hyp. p: \ Sul vent, Ope. ieee: epen. a Va / 3 \Avtcom. Rae. op. Def. as.den. J iS Hs -1 Lam. ter see hip. Vi Olf. bulb. Tel.ch.telmed! —/ “| “| “| Tub. olf. a] 2} &] 2} Constr. Nuc. med.sept./ Fig. 20 Outline sketch of the same model as that shown in figure 19. The broken lines indicate the position of the sulcus limitans, the most caudal boundary of the telencephalic evagination, and the separation of the thalamus from the hypothalamus. The extent and the boundaries of the olfactory area projected upon the medial wall of the telencephalon can be seen through the main body of the thalamus. The planes of figures 47 to 50 are indicated. Figs. 21 to 25 A series of transverse sections, pen-and-ink outlines, made with the Edinger apparatus, through the various levels of the telencephalon of no. 1121, the 11.8-mm. embryo. The numbers below the sections correspond to those in figures 8 and 11 and each indicates a specific section. X 50. Fig. 21 At the level of the optic evagination. Fig.22 Through the middle of the lamina terminalis. Fig.23 Through the tela chorioidea telencephali medii. Fig. 24 Through the paraphysis. In this and the preceding figure the wall of each cerebral hemisphere shows four distinct zones. Beginning in the mid- plane they may be designated as follows: 1) A thin septal area, telencephalic roof plate; 2) A thicker homogeneous area, the septum ependymale, bounded laterally by a shallow ventricular sulcus, the sulcus limitans hippocampi; 3) A small area showing a narrow mantle layer emerging from the matrix, the future hippocampus; 4) An adjoining and still more lateral area where no clear mantle zone is visible, the neopallium. 136 THE FISSURA HIPPOCAMFI UeiC Le eas-lel. evag. an Rae GE ms a Pt 3 # An a Fig. 25 This is taken slightly cephalad on the left, but passes through the paraphysis on the right. Upon the right the differentiation of the dorsal telence- phalic wall may be divided into four histologically distinct areas as seen in figures 23 and 24, THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, No. 1 Sept. epen. Sept. Cor. str. Prim.hip. veh 7 %, ul. vent, t me ABR Sul.fu pl. ch. Vent. lat, For. int. Cor. str. THE FISSURA HIPPOCAMPI 139 Figs. 26 to 28 Photographs of three transverse sections through the 14-mm. embryo, no. 940. Two views of a model of this brain are shown in figures 10, 12, 13, and 14 and planes of sections are illustrated in figure 12. Fig. 26 Section just anterior to the lamina terminalis and the tela chorioidea telencephali medii. The division of the medial wall into three regions is evi- dent, namely, 1) a dorsal or neopallial; 2) a dorso-medial or hippocampal; 3) a ventral or septal. Fig.27 Through the paraphysis. In comparing this with figure 24 of no. 1121, the same upward curve in the membranous roof and the division of the dorsal half of the hemisphere vesicle into four regions may be noted. X 60. Fig.28 Through the di-telencephalic groove. Note the thin marginal velum bordering the limitans meningea on the dorsal aspect of the evagination in the primordial hippocampus. The massive undifferentiated area joining the dien- cephalon medially is the telencephalic limb of the di-telencephalic groove caudal to the choroid plexus evagination. Neopal. Vent. lat. Fas. den. fo aa os ita SOP Rs gan RE Tub.olf Fig. 29 Through the anterior part of the tuberculum olfactorium, showing the division of the medial wall into four histologically distinct areas, tuberculum olfactorium, septum, primordial hippocampus, and neopallium. Fig. 30 Through the more caudal part of the tuberculum olfactorium. The region of the hippocampus is more accentuated, the groove is less shallow, the sulcus limitans hippocampi a more prominent feature. A group of cells, the fascia dentata, can be distinguished just above the sulcus. 140 ai, 4°94 Zoe Cor str. “4 & fy lar | ‘dl Cortr. med. - a | 4 | ei -iagunnns mien rer eenstenr iment Te. pl. ch. a vent. lat. Fig.31 Through the septum ependymale, just rostral to the lamina terminalis region. Fig.32 Through the di-telencephalic groove, showing the invagination of the plexus chorioideus ventriculi lateralis, with the taenia fornicis. Figs. 29 to 32. This series of reproductions through the forebrain was taken from embryo H 173, 19.1 mm., of the Chicago collection. This brain was cut transversely to the telencephalon. For the exact positions of these sections refer to figure 16. 20. 141 Lim.men. Fas. den. Fis hip. Cor. sfr. Nuc.med. sept. Seyi gear pee a —— Se BRO AO ee +. “aa Si, Figs. 33 to 37 These are photographs of selected sections from a transverse series of the telencephalon of a 20-mm. human embryo, no. 460, at the Carnegie Institution, the Mall Collection. X25. A modelof the forebrain may be studied by turning to figure 17. The levels at which typical sections were photographed are indicated. 142 Hip. Fas.den. co Cor. sir. lat. Me Cor. str.med. ‘ig ig af Re S v Cor. str, lat. Sulvent. ae ~ Cor.sir-med. Pi. ch. Vent. lat, 37 Fig. 33. Through the more rostral portion of the tuberculum olfactorium. This is comparable with the level shown of brain H 173, figure 29. The islands of Calleja appear as denser cell groups in the marginal velum of the ventral region. Fig. 34 This section is slightly caudal to the previous one and illustrates the deepening of the embryonic fissura hippocampi. Fig.35 Through the septum ependymale. The marginal velum is just visible. Fig.36 Through the foramen interventriculare and the lateral choroid plexuses. Fig. 37 Through the thalamus and the telencephalon caudal to the di-telen- cephalic groove. The roof of the third ventricle is non plexiform. The epen- dymal walls of both the lateral plexus and the diencephalic roof plate appear very thick. The tissue adjoining the taenia of the lateral plexus, both dorsal and ventral to it, is the hippocampus. The same type of cellular arrangement as that shown in the depth of the embryonic fissura hippocampi in more rostral levels is seen. 144 , HS Sul. vent, | Figs. 88 and 39 These figures are reproductions of photographs of a 27.8-mm. embryo, belonging to the Chicago Collection. X 28. Fig. 38 Through the septum ependymale and the lamina terminalis. Fig. 389 Through the tela chorioidea telencephali medii and the foramen interventriculare. 145 Oe, oe Sol. 4 Figs. 40 and 41 These photographs were taken from a transverse series of a human embryo 32.1 mm. in length, belonging to the Chicago Collection, H 41. xX 28. 146 x ye eel Ne opal \, Nuc. lat. sepl: é of Nuc. med. sept . ae 0). ee SS > Nuc. méd “sept Fig.40 Through the root of the bulbus olfactorius, showing the curve in the medial wall described by His as the fissura prima. Fig. 41 Through the tela chorioidea telencephali medii. The primordial hippocampus is bounded by the sulcus limitans hippocampi ventrally, and by the neopallium dorsally. The lamination of the neopallial cortex is easily dis- cernible. Ventral to the sulcus limitans hippocampi is the septum ependymale. Emerging from the matrix a narrow marginal velum is seen. 147 44 Ss H \ Nuc. med sept Pur cl. ; Hip. Fas. den. Sea \ Nuc. lat: Se Ty ce lamten Vent. laf. 4 148 \ Poly. eae LS. Lp. Sul. vent. SS : a ea a7, : Nuc.le nl. \ Vent. tert. \ \ 149 46 Figs. 42 to45 Pen drawings of sections through the 39.1-mm. embryo, H 163. The planes of section are indicated on the drawing of the model, figure 18. Fig.42 Through the septum and the postoptic recess. Fig. 43 Through the rostral portion of the lamina terminalis. The cortical lamination in the hippocampus reaches almost to the sulcus limitans hippo- campi. The differentiation of the pyramidal cell layer seems tardier than that of the polymorphous layer. Fig. 44 This section was taken through a more caudal part of the lamina terminalis. The slight groove in the medial wall, dorsal to a line joining both sulci limitantes hippocampi is the remnant of the fissura, hippocampi of the earlier stages. The polymorphous and pyramidal cell layers are not well as differen- tiated in the regio hippocampi as they are in the two previous figures. Fas. den. Hip. Fas.den. Sept. epen. 7 ry SP Lat.vent. VS Olf. evag.—e, 50 Fig. 45 Section through the paraphyseal arch, showing a few postvelar tubules, the lateral choroid plexuses, and the foramen interventriculare. The cortical layers are not visible in the hippocampus, although they are well devel- oped in the neopallium. Fig. 46 Photograph from the same specimen as the last, through the depth of the hippocampal fissure. For plane of section see figure 18. A thin row of cells lies in the most medial part of the mantle zone of the hippocampus, fascia dentata. xX 14. 150 THE FISSURA HIPPOCAMPI Si Fascia dentata In the medial margin of the hemisphere wall in the 19.1-mm. embryo opposite the sulcus imitans hippocampi lies a group of cells, the fascia dentata. This group of cells does not appear in the younger embryos, but persists in the rest of the series as a mass of cells, which seem to have migrated out of the matrix lying opposite the dorsal limit of the suleus. This differentiation begins anteriorly and passes posteriorly, following in develop- ment the initial differentiation of the future hippocampal region into the outer marginal velum and inner matrix layers. These cells slip along the marginal velum of the developing hippocampus. In the 39.1-mm. embryo and the 43-mm. they form a slender band almost coextensive on the medial wall ventro-caudally with the development of the hippocampal cortex. The rostral limit of the fascia dentata lies in close proximity to the rostral limit of the sulcus limitans hippocampi. The four sketches, in figure 51, present comparable levels through the region under discussion from four different embryos. A, a 16-mm. embryo, shows no differentiated fascia dentata, but in B, a 20-mm., a group of differentiated cells opposite the sulcus limitans hippo- campi may be seen. In C, a 39.1-mm., these cells have slipped along the marginal velum of the hippocampus; while in D, an 85-mm.. they show the characteristic crescentic line-up. In the levels delimited only that of the 85-mm. demonstrates the re- lation of the sulcus fimbrio-dentatus to the growing fascia dentata. Here the fornix fibers lie ventral to the dentate band, among undifferentiated cells of the primordium hippocampi, separated from that gyrus by the sulcus fimbrio-dentatus. Figures 47 to 50 These drawings were made from a coronal series of the forebrain of a 43-mm. human embryo, no. 886, of the Mall Collection. Refer to figures 19 and 20 for the medial sagittal section of the model and the planes of these sections. X 62. _ Fig. 47 Through the hippocampus, showing that tissue in both its caudal and rostral aspects. Fig. 48 A section through the upper border of the thalamus, the septum ependymale, and the anterior and the posterior divisions of the hippocampus. Fig.49 Through the septal region and the posterior part of the hippocampus. Note the cellular groups in the septum. Fig. 50 Through the caudal hippocampus, the middle of the thalamus and the olfactory bulb. P52 MARION HINES Marginal Velum Fissura hippocampi Hippocampus ia dentata+ Fissura Hiopocampu Locus of fascia dentata : Sule fenere Hipporomg ppocampus Hippocamp Lamina 7 Fascia dentata epithelialis p Primordium hippocampi Neopallium Lamina epithelialis Matrix ey ‘ie : i Intermediate Mespelvem Pyramid cell layer cell layer Pyramidal cell layer Subiculum Hippocampus Fissura hippocampi Fissura hippocampi Sulcus Cortex hippocampi Fascia dentata fimbrio- 7% dentatus / Alveus 7~ Fascia dentata ?~ Hippocampus Sulcus limifans hippocamp Primordium hippocampi 22) Sulcus limitans hippocampi Lamina epithelialis (thea C. Lamina epithelialis D. Fig.51 These four pen-and-ink sketches were drawn either with the Edinger projection apparatus or were taken as tracings from photographs of known magnification. They show the relative histological development of the neopal- lium, the hippocampal formation, the fascia dentata and the lamina epithelialis, through a level which subtends the lateral choroid plexus itself or its primordium. Sketch A. X 352. No. 465, 16 mm. (section 11-2-6), University of Chicago Collection. The hippocampal region is clearly delineated by the thickened marginal velum, the thin matrix, and the great convexity toward the ventricular surface. Ventral to the hippocampal formation lies the lamina epithelialis, the site of the lateral choroid plexus evagination. Here it is a thick wall made up of many small undifferentiated cells. The matrix of the neopallium interdigitates with the marginal velum. The fissura hippocampi is a shallow groove beneath which lies the locus of the future fascia dentata. THE FISSURA HIPPOCAMPI 1a} This last is essentially the reptilian condition, and, since some of these fornix fibers in both cases enter the alveus in the adult, it follows that the reptilian dorso-medial cortex is comparable with the human hippocampal cortex rather than with the fascia dentata, as supposed by Meyer (’92) and Levi (94). More- over, the embryological evidence presented by these embryos is decisive in favor of the same conclusion. The fascia dentata arises from cells of the matrix immediately dorsal to the sulcus limitans hippocampi, from which position its cells migrate dor- salward along the outer margin of the hippocampal formation. This mode of origin corresponds in all major particulars with Sketch B. x 352. No. 460, 20 mm. (section 15-1-2), Carnegie Institution, Baltimore. The histological characters of the hippocampus are the same as those described for the 16-mm. embryo. Thearea itself is greater dorso-ventrally and the fissura hippocampi, although shallow, inciudes a greater sweep of tissue. The lamina epithelialis is thinner, composed of only a few rowsof epithelial cells. Opposite the sulcus limitans hippocampi within the marginal velum lie a group of cells. the undifferentiated fascia dentata. SketchC. X78. No.H163,39.1 mm. (section 147-14), University of Chicago Collection. The medial wall in the region of the hippocampal primordium bulges prominently into the ventricle, its ventricular convexity being greater than its medial concavity. Within the dorsal lip of the fissura hippocampi three cortical lamina are present: 1) the inner, or matrix; 2) the middle, or the intermediate cell layer, and, 3) the outer or row of young pyramids. The inter- mediate cell layer, like the matrix, is never as wide in the center or ventral lip of the fissure as it is in the dorsal lip or in the neopallium. The lamina epi- thelialis ismore convoluted and presents at this level a concave surface outward, an indication, the writer believes, of the sulcus fimbrio-dentatus. The fascia dentata has migrated dorsally in the marginal velum of the hippocampus. The locus from which these cells come shows no cortical lamination, although between the fascia dentata and the matrix a few undifferentiated cells remain. Sketch D. xX 10%. No. 1400-23, 85 mm. (section 34-14). A brain belonging to the private collection of Dr. George L. Streeter, studied at the Carnegie Laboratory of Embryology, Baltimore. The hippocampal formation lies in the depth of the fissure. The fascia dentata seems caught in a characteristic twirl. Ventral to the fascia dentata lies a small sulcus, the sulcus fimbrio-dentatus. The fornix fibers are intermingled with undifferentiated cells, the persistent primordium hippocampi. It takes no leap of fancy to bridge the growth process from this stage to the adult. Note. In the untouched photographs from which figures 29-31, 33-36, 38, 39 and 41 were reproduced the histological differentiation of the facsia dentata was clearly visible, as indicated in the pen drawings, figures 42-45, 47-51. This detail in some of the photographs is lost in the reproduction. THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 1 154 MARION HINES Levi's (04) description of the development of the fascia dentata in the rat, but his interpretation requires revision. The identification of the reptilian dorso-medial cortex with the fascia dentata of Meyer and Levi has been questioned by Cajal (11) and by Elliot Smith (10), who believes, however, that it is undergoing differentiation toward fascia dentata, a view supported also by Crosby (17) and in a modified form by John- ston (13, p. 391, and 715, p. 419). By what criterion hail the fascia dentata be eae connections, intrinsic chromatin staining, position, or history? To determine nerve connections in this material is impossible. The primordial fascia dentata shows the characteristic intensive nuclear staining even in its earliest stages of development. But its morphological disposition can be so followed from stage to stage up to the adult form that there is no doubt as to its identity. In vertebrates below the lowest mammals there is no represen- tative of this structure. Levi (04) has pictured the dorso- medial cortex of reptiles as containing a cortical lamination of deeply staining cells, whose connections according to Smith and Cajal are those of the hippocampus. Its boundaries have noth- ing in common with those of the mammalian fascia dentata. But knowing the origin of this tissue in human development, we naturally turn to the homologous region in the lower vertebrates. Such an area in both reptiles and mammals is the undifferentiated primordium hippocampi in the region of the developing fornix fibers. If the fascia dentata is a center for cortical reenforce- ment, as Cajal (11) thinks the neurone connections indicate, and not the main receiving station for incoming impulses over the medial olfactory tracts, as Elliot Smith (96) maintains, then it would seem natural for its development to be in abeyance in lower vertebrates. But if the reverse be true, we are at a loss to supply a reason for its undifferentiated condition in lower forms. It seems logical that it may develop from the cells of the primordium hippocampi, opposite the sulcus limitans hippo- campi, and that the development will be delayed in accordance with Cajal’s hypothesis of its function, until cortical associational mechanisms are well elaborated. Be that as it may, its anatomi- THE FISSURA HIPPOCAMPI £55 eal site of development supports the generalization of Elliot Smith (’96) for monotremes, that this tissue is the fringe of the cortex. Hippocampus Coincident with the appearance of the small area characterized by thicker marginal velum in the dorsal wall of the evaginating hemisphere in the 11.8-mm. embryo, there is a slight thickening of the wall itself. These two features furnish the first differentia- tion of the primordial hippocampus. The histological and mor- phological differentiation is more apparent in the 14-mm., and in the 19.1-mm. and the 20-mm. the whole extent of this tissue is involved in a groove, the fissura hippocampi. Here its wall is slightly thicker than the wall of any other part of the cortex. There is little cell migration from the matrix into the marginal velum. In fact, at this stage of development of the cerebral vesicle, this is the only region where a true cell-free margin is found. The tissue which lies immediately dorsal to the hippo- campus is neopallium and that which is ventral is either septum or a derivative of the area epithelialis. This fissure, in all proba- bility, is not formed by an invagination due to more rapid growth of the central part of this tissue, but rather by a buckling of the wall on itself as the result of the appositional growth of the neo- pallium between the endorhinal fissure and the dorsal limit of the hippocampus, plus perhaps the lack of support, except at one point, by the epithelial tissue ventral to it. The fissure is deeper and less broad where the ventral support is narrow. Since this fissure or groove lies above the sulcus limitans hippocampi and the fascia dentata and involves the whole wall containing the primitive hippocampus, there can be little doubt as to its identity. It is the fissura hippocampi or the fissura arcuata of His, the Bogenfurche of other authors. This fissure must not be confused with the sulcus limitans hippocampi or the sulcus fimbrio-dentatus. It corresponds to the fissura arcuata of Herrick (10) in reptiles, Johnston (’13) to the contrary notwithstanding, if the evidence above be accepted, namely, that the definitive hippocampus is derived from the reptilian dorso-medial cortex. 156 MARION HINES In this case the ventral boundary of the hippocampal formation may be drawn by passing a plane through the wall at the level of the sulcus limitans hippocampi. Such a limit would corre- spond to one similarly drawn diagonally through the medial wall of the brain of Phrynosoma cornutum (Herrick, ’10, fig. 61, p. 533) joining the sulcus limitans hippocampi and a groove on the ventricular surface just dorsal to nucleus lateralis septi. In the brain of the turtle (Johnston, 13, p. 391, and fig. 17, p. 4385) a sulcus which lies above this hypothetical plane described by Herrick is called the sulcus fimbrio-dentatus. This is Herrick’s fissura arcuata. If, now, Johnston and others are correct in assuming that the mammalian fascia dentata is derived from the ventral part of the reptilian area of differentiated cortex above this so-called fimbrio-dentate sulcus, then the reptilian primordium hippocampi gives rise only to the mammalian fimbria and fornix bed and the term fimbrio-dentate sulcus is clearly appropriate, for this sulcus is defined by Johnston (713, p. 391) as ‘‘lying between the fimbria and the developing fascia dentata.’”” But, on the other hand, it has been shown in this contribution that the human fascia dentata actually is developed, not from the differentiated hippocampal cortex downward, but upward from the extreme ventral border of the primordium hippocampi. The hippocampal primordia of reptiles and of these human embryos are apparently strictly comparable structures. The fimbrio-dentate suleus as defined by Johnston cannot, therefore, lie dorsal to the primordium hip- pocampi as he describes it. We conclude, then, that there is no fimbrio-dentate sulcus either in reptiles or in the human embryos here described. The mode _ of its appearance in later developmental stages has not been determined in sufficient detail to enable the writer to treat the subject exhaustively, although sketch D in figure 51, taken from an embryo 85 mm. in length, clearly delineates the fact that the sulcus in question develops later than the fascia dentata and appears ventral to both the fascia dentata and the sulcus limitans hippocampi. It would appear to follow from the conclusion that if the reptilian fissura areuata of Herrick’s description (Ambrio- THE FISSURA HIPPOCAMPI nar dentate suleus of Johnston) is represented in the human embryo at all, it must be homologous with the fissura arcuata of His and with the fissura hippocampi of the adult. If, however, this conclusion is adopted, it must be recognized that the fissure is very differently disposed with reference to the chief mass of the differentiated hippocampal cortex in adult reptiles and mam- mals. But the embryology of the region as far as followed in this paper is almost the exact duplication of the situation as described for reptiles by Herrick (10, pp. 464, 465). And the writer is inclined to think that future study will prove those differences discussed by Herrick to be slight indeed, for the fascia dentata arises from cells in the ventral lip of the fissura, immediately opposite or slightly dorsal to the ventricular sulcus limitans hippocampi. The fibers of the fornix lie between the matrix and the fascia dentata in this region. Moreover, the ventral lip of the fissura hippocampi shows no cortical lamination in the stages presented. ‘This differentiation of the early hippo- campus into a dorsal cortical portion and a ventral non-cortical resembles the reptilian condition, with the exception that opposite the sulcus limitans hippocampi lies the undifferentiated fascia dentata. There is nothing, according to Johnston or Crosby which compares to this differentiation of fascia dentata in either the turtle or the alligator. The writer suggests that the regions in adult reptiles called by these authors primordium hippocampi are the source of the fascia dentata. There are some points, essential for the completion of this argument which remain obscure. In the material avialable it is impossible to determine the absolute ventral limit of the primordium hippocampi caudal to the angulus ferminalis. The paraphysis is universally regarded as a differentiation within the roof plate. The lamina epithelialis in all probability should not be so regarded since it takes part in the evagination of the hemisphere. Its subsequent position, however, is not evident until the 14-mm. embryo is studied. The sulcus limitans hippocampi in this contribution is regarded as markng the ventral boundary of the hippocampal formation. In the region rostral to the angulus terminalis (figs. 3 to 6, sketches 1 v) it 158 MARION HINES separates cortical from subcortical regions; but throughout the regions bordering the post-terminal area epithelialis it separates the thin area intercalata and (in the adult) the choroid fissure from the hippocampal formation. In the series presented no cortical areas are found ventral to it, nor are there any cells which can be proved to be neuroblasts ventral to it. But in figure 51, sketch D, it seems impossible to determine just where the bound- ary between the primordium hippocampi and the lamina epi- thelialis should be drawn. In the sketches it has been assumed to lie as indicated, below the area containing undifferentiated cells of the primordium hippocampi. On this interpretation that portion of the fimbria marked Fornix (fig. 51, D) is a segment of the lamina epithelialis which has been secondarily thickened by the invasion of fornix fibers; but possibly, it should be regarded as belonging within the primordium hippocampi, a possibility which cannot be disregarded until the methods of neurological technique have demonstrated otherwise. Doctor Herrick sug- gests to me that a close series of developmental stages of this region in reptiles or lower mammals would probably be favorable material for the solution of this question. Fissura hi ppocampr Having established the homology of the fissura hippocampi with the reptilian fissura arcuata of Herrick, we are desirous of clearing the situation as it exists in the history of the embry- ology of this region. As development proceeds, the primitive hippocampus presents a smooth contour from its earliest defini- tion (9 mm.) to 16 mm. From that length to approximately 24 mm. the fissure extends into the medial wall from the base of the olfactory evagination to the end of the hippocampal primordium as a shallow groove involving the whole of this peculiarly differentiated area. For approximately the next 10 mm. of growth in greatest length, the fissure grows progressively shallow in the region above the area chorioidea, so that from the surface it appears to be divided into two segments. However, there is no interruption of the hippocampal formation itself. THE FISSURA HIPPOCAMPI 159 During this time of development a new groove appears on the medial wall, the result of active olfactory bulb evagination, the fissura prima of His. The 27.8-mm. and the 32.1-mm. belong to this group. In the 39.1-mm. and the 43-mm. the anterior seg- ment of the fissura hippocampi has disappeared, but the posterior and the fissura prima persist. The cortical lamination of the dorsal lip of the fissura hippocampi is coincident with the flatten- ing of the medial wall. If a dorsal commissure were added to the anterior commissure, now lying in the much-thickened lamina terminalis, the rela- tionships of commissure, fissura hippocampi, and fascia dentata would resemble those of the marsupial. Elliot Smith (97, p. 67) wrote of this comparison as follows: In the Marsupial we have a fissura arcuata or hippocampi, extending from the tip of the temporal pole right round the mesial wall of the hemisphere towards the olfactory peduncle; so, in the fetal child or kitten, we find the Bogenfurche (which we might, with Mihalkovics, appropriately call ‘Ammonsfurche’) following a similar course and shading away towards the cephalic pole of the hemisphere. And it is necessary to remark, in passing, that the so-called part of the ‘Vor- dere Bogenfurche,’ which His calls ‘fissura prima’ has nothing whatever to do with the true Bogenfurche or fissura arcuata, if we regard the latter as the primitive fissura hippocampi. Smith refers to the 1891 paper of Marchand. Moreover, Marchand (’09) denied the existence of such a fissure and Smith (03) reports that Hochstetter’s work on fissuration of the medial wall proves beyond a doubt that all the fissures are artefacts. But the conditions of these tissues in the brains of the 39.1-mm. and the 43-mm. are essentially the same as described by Smith in his first paper. His himself (’04) states plainly that the fissura prima has no relation to the fissura arcuata or the hinterer Bogenfurche; he defines it as follows: ‘‘The continuation of the fissura mesorhinica extends for a distance over upon the medial wall of the hemisphere as the fissura prima. By a deepening of the surrounding sulcus the termination of the lobus olfactorius or this bulbous portion becomes separated more and more from the overhanging frontal lobe. The bulbous portion retains its sagittal direction and becomes separated laterally from a 160 MARION HINES transversely directed portion by a deep sulcus” (p. 66). This fissure is considered by His to be the same as the ‘vordere Bogen- furche.’ ‘The first embryo to have such a fissure is Se (16 mm. G. L.) thought by His to be six weeks of age. There is little doubt that this measurement in no way compares with those in this series. Here there is no well-developed olfactory bulb until the 27.8-mm. is examined, although there is a slight olfactory evagination in the 19.1-mm. Then the fissure in all of the series after 27.8 mm. is the same as that of His’ description. It is quite possible that the bulb in His’ embryos was delimited ar- tificially. His also models a small bulb in C. R. (13.6 mm.), in which case the writer believes that some of the olfactory fila may have been included in the drawings of the projection of the brain. In the later stages the anatomy of the fissure is the same in all cases. Further, he thinks it divides the olfactory system into two parts. ‘‘Atno time does it extend posterior to the lamina terminalis. Its remnant is the fissura parolfactoria posterior of the B. N. A.” (His, ’04, p. 76). This fissure is not, then, the an- terior portion of the fissura arcuata; rather it is continuous with the mesorhinic fissure. Besides this fissure in embryos of the second month, His finds a sickle-like fissure extending posteriorly beginning in the region of the terminal plate. He finds also that in many cases the mesenchyme fills the fissure and that there are no evident postmortem artefacts. He finds the same kind of thickening in the medial wall in the cat embryos of 14 mm. G. L., as Zucker- kandl (01) showed in his paper on the development of the corpus callosum. The hintere Bogenfurche lies dorsal to the fissure of the choroid plexus, its anterior limit does not pass beyond the terminal plate. This fissure is undoubtedly the one the writer has identified as the fissura hippocampi. However, besides these, His described another, the ‘accessoriche Bogenfurche,’ in his draw- ings of three- and four-month embryos. ‘This lies on the anterior part of the medial wall, arching over the terminal plate. The writer finds nothing to correspond with this fissure and considers it an artefact. THE FISSURA HIPPOCAMPI 161 Martin (94), on the other hand, uses vordere Bogenfurche as synonymous with fissura prima and reports that it appears dorsal to the choroid fissure. He thinks also that the hintere Bogenfurche, a groove in the medial wall in the midst of the posterior hippocampus, does not join the anterior Bogenfurche until later in development. If his illustrations are carefully studied, it appears that Martin’s vordere Bogenfurche is the an- terior limb of the hippocampal fissure and seems to become con- tinuous with the hintere Bogenfurche when the tip of the temporal pole has grown ventrally and rostrally. This finding, if so inter- preted, agrees in all poimts with mine. There can be no doubt that Martin’s vordere Bogenfurche and His’ fissura prima are not the same. Grdnberg (01), however, found no separation of the Bogenfurche into anterior and posterior limbs in the hedgehog. This finding of Grénberg agrees in all points with that in man, namely, a fissure coextensive with the primordium of the hippo- campus upon the medial wall. Such workers as Hochstetter, Retzius, Goldstein, and Syming- ton report that in the region of the primordium hippocampi there is a slight thickening of the wall, but no definite infolding, although upon careful examination the medial wall at this point is not smooth. In other words, these fundamental findings agree with those presented in this paper. Investigators who concerned themselves with well-fixed brains of the third and fourth month did not find the radial folds of the earlier work, nor could they identify the fissura arcuata of His. The confusion arose out of failure to distinguish between artefacts of fixation and the accentuation of normal findings. There is no doubt but that maceration plays havoc with the normal contour of the medial wall of the hemisphere before the fibers of the corpus callosum have lent their stiffness toward its support. His failed to emphasize the histological structure which he found in the fissura arcuata as peculiarly distinguishing that fissura from the accessory fissure above. My special contribution to this particular phase of the inves- tigation is the discovery that at a certain stage in development, the fissura hippocampi is coextensive with the hippocampal 162 MARION HINES primordium and that, as cortical differentiation proceeds in that portion which lies anterior to the velum transversum, the hippo- campal fissure disappears. But posterior to the velum trans- versum the fissura remains as the adult fissura hippocampi (fig. 51, sketch D). The relation of the hippocampus to the neopallium The tissue which manifests the most marked and most regular acceleration is the neopallium. In the youngest embryo there is no clear line of lateral demarcation of this tissue. But in the 14-mm. the wall of the ventro-lateral sector is noticeably much thickened. This division between the two lateral sectors is more marked in the 19.1l-mm., where a slight ventricular groove appears. The position of the hippocampus in the developing vesicle depends largely upon the amount of neopallium joining the hippocampus and the latero-ventral complex. Further, the in- trinsic differentiation of the neopallium which is first seen in the 27.8-mm. progresses more rapidly than that of the hippocampus, although that tissue was the first to become at all evident in the developing telencephalon. The process of evaginaton is largely one of growth between the hippocampus and the region of the pyriform lobe, uncus and tail of the caudate nucleus. The appended table 5 gives an idea of the relative differen- tiation in the vesicle of these various embryos. - With these data in hand two factors appear to be involved in the position of the developing archipallium. The first of these is the disposition of the old cortex in the wall of evagination, coincident with the noteworthy acceleration of the neopallium. The second factor is the intrinsic differentiation of the hippocam- pus itself. From these relationships it 1s possible to delineate the method of growth in the evaginating telencephalon. Recalling the form of the telencephalon at a stage where no cortical area has been evaginated from the telencephalon medium into the cerebral hemisphere as exemplified by the case of Ichthyomyzon concolor already cited (Herrick and Oben- chain, ’13, figs. 3 and 4), it is evident that in the process of further THE FISSURA HIPPOCAMPI 163 evagination the most dorsal edge of the massive side wall will become the most ventral edge of the complete evagination and help form the roof of the foramen Monroi(p.126). Anterior to the lamina terminalis the most dorsal border will meet the most ventral edge, making a seam or junction along the medial wall of the growing telencephalon. This seam or junctional zone on the medial wall of the cerebral hemisphere in front of the lamina terminalis is always marked in amphibian and reptilian brains by a cell-free limiting zone and often by a ventricular groove, a sulcus limitans hippocampi. This area is necessarily transitional in type because, besides the approximation of the hippocampal formation with the septum (the primitive dorsal column with the primitive ventral column), it marks the union of the thinner dorsal part of the telencephalic roof plate bordering the hippocampal formation with the septum, the major portion of which sooner or later becomes greatly thickened. The sulcus limitans hippocampi marks the border of the hippocampal formation for its entire length, and rostral to the foramen interventriculare it also marks the junction of the hippocampal formation with the septal complex. In this series the cerebral hemispheres of the 11.8-mm. embryo have reached what may be called the first stage in the evagina- tion; the primordial hippocampus is dorsal throughout and there is no medial wall anterior to the lamina terminalis. The hippo- campal formation lies as a crescent over the top of this evagina- tion, never quite reaching either the anterior or the posterior pole of the vesicle, yet lateral and dorsal to the sulcus limitans hippocampi. In the 14-mm. the hippocampus is entirely medial, anterior to the angulus terminalis, and climbs, as it were, over the crest of the hemisphere to the posterior pole. Here again the differentiation only approaches the posterior pole, but does not reach it. The amount of neopallium is greater in this embryo at the anterior pole than at the posterior. This process continues, so that the formation in the 19.1-mm. and the 20-mm. lies en- tirely upon the medial wall. In the latter embryo, however, the neopallial tissue in the posterior pole has increased. In the 27.8-mm. and the 32.1-mm. the neopallium has grown greatly 164 EMBRYO me pues ES | » ee) i= a 4 mm 1121 | 11.8 940 | 14.0 H173 | 19.1 460 | 20.0 H91 | 27.8 H41 | 32.1 H163 | 39.1 886 | 43.0 | Same True olfactory bulb Same + differentia- MARION HINES TUBERCULUM OLFACTORIUM; * OLFACTORY BULB Olfactory fila only Olfactory fila + a slight evagination | | | Same with slightly more evagination; cortex of tubereu- lum olfactorium just visible Same; cortex of tu- berculum more prominent and fissura prima; same tion of layers of bulb Same; pronounced islands of Calleja SEPTUM Thick wall Same Nucleus medialis septi Same plus margi- nal velum Same Beginning of nu- cleus lateralis septi Large nucleus lat- eralis septi + nucleus accum- bens, anterior commissure Same AREA CHORIOIDEA Area Summary of development Septum cvs Lamina ependymale anlern epithelialis Thin wall Thin | Thin wall wall Same Same | Concave out- ward Same Same | Lateral cho- roid plexus Same Same | Same Same -++ mar- | Same | Posterior limb ginal velum of lateral cho- roid plexus greater in extent Increase in Same | Same marginal velum, whole area thicker Wide margi- | Same | Same nal velum Same Same | Same Py t Corpus striatum — Ventro-lateral sector thicker Angulus ventralisap- pears; ventro-later- | al sector thicker Two hillocks and a few fibers in the lateral gray Same Marked increase in cells lying between the caudate nucleus much | and marginal ve- lum Same plus few inter- nal capsule fibers Nucleus lentiformis, internal and exter- — nal capsule fibers Same fa 4 e ‘E5 MBRYO | PYRIFORM LOBE te ~ oO i} ——— S \ o |7 4 mm. 121 11.8 | Ventro-lateral sector 4 thicker 940 | 14.0 | Same H173 } 19.1 ventricular wall be- tween neopallium and corpus striatum 460 | 20.0 | Same 91 | 27.8 contour of these nuclei H41 | 32.1 | Same 1163 | 39.1 | Appearance of lateral nu- cleus and tract; cortical layers; endorhinal and ectorhinal fissure 886 | 43 Same | . wits of the cerebral hemisphere Appearance of sulcus on Two definite cell layers on THE FISSURA HIPPOCAMPI HIPPOCAMPUS Narrow marginal velum; suleus limitans hippo- campi; ventricular ridge Occupies medio-dorsal wall of hemisphere, wider marginal velum Same plus a few cells in the marginal velum. Fissura hippocampi FASCIA DENTATA Few cells opposite the sulcus limi- tans hippo- campi Same Same Two cortical lamina in dorsal lip of fissure Intermediate cell layer in ventral lip Same Same Growth dorsally along marginal velum of hippo- campi 165 NEOPALLLIUM No differentiation Marginal velum contains neuroblasts Neopallial tissue appears on the medial wall Appearance of neopallial tissue posterior to hippo- campus Increase in total tissue; cortical layers More tissue Same Same 166 MARION HINES in the posterior pole and the hippocampus has made the ventro- caudal twist into the temporal lobe so characteristic of it. In the last two embryos of the series, the 39.1-mm. and 43-mm., remarkable growth has taken place in all regions of the neopallium, so that relatively little area, comparatively speaking, contains the hippocampal formation. And it is worth noting that the major portion of the hippocampus in these last two brains lies in the medial wall posterior to the velum transversum. Thus trac- ing the history of its position in the developing hemisphere lends adequate support to a portion, at least, of Herrick’s quadrant theory of telencephalic evagination. But, further, this brief his- tory of hippocampal position points to the conclusion that its extent is inversely proportional to neopallial growth. It also gives some facts concerning the regional acceleration of this neo- pallial growth, namely, that acceleration seems to shift from the frontal to the dorsal and then to the posterior poles of the de- veloping hemisphere. Concomitant with this change there is the intrinsic differen- tiation characteristic of the hippocampus itself. Although set aside as the first cortical area, its subsequent differentiation progresses so slowly that such layers as are characteristic of the cortex appear in the neopallium long before they are completed in the hippocampus. The differentiation does not proceed in any logical sequence, but seems rather to be subject to rhythms of acceleration. These rhythms of acceleration do not correspond absolutely to those expressed in any arrangement of the adult brains of the verte- brate phylum. In other words, given the stage in human de- velopment of the hippocampus, the differentiation of the fascia dentata or the neopallium will not correspond to the phylogenetic development of the first-named tissue. It is possible, however, to take any one tissue and follow it through a complete develop- ment whose changes fit into its phylogeny. The developing neopallium seems to act as a disturbing factor, not, however, as one which obliterates, but rather as one which obscures the phylogenetic history by suddenly leaping into the foreground and by its great increase in amount and complexity of tissue THE FISSURA HIPPOCAMPI 167 demanding immediate and engrossing attention. It is a dis- turber of growth rhythms and an obscurer of elementary phy- logenetic ‘patterns.’ It is the belief of the writer that there is actually some relationship between these two; that is, that the acceleration of the neopallium results in a change of rhythm of growth in different parts, although it has no effect upon the actual differentiation, except that of obscuring it. SUMMARY 1. The medial wall of the cerebral hemisphere of human embryos 16 mm. to 30 mm. in length is not ‘perfectly smooth.’ Its otherwise even contour is broken by a shallow groove. ‘This eroove extends from the region of the olfactory bulb to the tip of the temporal pole. It is the fissura hippocampi, the ‘Bogenfurche’ of His. It is homologous with the fissura arcuata of reptiles as described by Herrick. 2. In embryos as young as 11 mm. the primordial hippo- campus can be recognized along the line of the future fissura hippocampi. This primordium is identified by the following histological peculiarities: 1) a thicker wall; 2) a narrower matrix; 3) a clearly defined marginal velum; 4) a limiting sulcus, the sul- cus limitans hippocampi. This is the first cortical differentiation known in man. 3. The fascia dentata arises in the matrix of the hippocampal formation from cells in the dorsal limb of the sulcus limitans hippocampi. These cells grow dorsalward, slipping along the marginal velum of the hippocampus. In the series studied no other cortical differentiation has occurred in this region. It is comparable to the persistent primordium hippocampi of am- phibians and reptiles. 4. The fissura prima of His first appears in embryos of about 25mm. The appearance is coincident with the marked evagina- tion of the olfactory bulb. It has no connection with either the fissura hippocampi or the hippocampal primordium. 5. The various regions of the telencephalon medium are distinguished by a characteristic morphology and histology in all the embryos of this series except the 11.8-mm. In the 168 MARION HINES remainder of the group described the angulus terminalis separates the midline structures into terminal plate and roof. The former is the lamina terminalis; the latter, the area chorioidea. The lamina terminalis increases in length and width throughout the series. The area chorioidea changes little in total length. Its anterior division, the tela chorioidea telencephali medii, is practi- cally stationary. Its posterior division, the paraphyseal arch, in the younger embryos forms a tent-like evagination in the roof; in older stages it may become a pouch-like paraphysis with two lateral pockets. 6. The portion of the medial wall of the hemisphere con- tiguous with the area chorioidea and the dorsal thin part (pars tenuis) of the lamina terminalis is termed the area epithelialis. It may be divided into the following parts, enumerated from ventral to dorsal borders: 1) The septum ependymale (fig. 14, Sept. epen.) is that portion of the area epithelialis which lies ventrally of the angulus ter- minalis and borders the dorsal thin portion of the lamina ter- minalis. In later stages it thickens, beginning at the ventral border, differentiating first into matrix and marginal velum, with later migration of neuroblasts of the septal nuclei into the latter. The dorsal portion remains thin and undifferentiated. 2) The area intercalata (figs. 14, 16, A. ini.) lies contiguous to the tela chorioidea telencephali medii. It remains membra- nous and increases but slightly in total surface and thickness. 3) The lamina epithelialis (figs. 14, 16, Lam. ep.) borders the paraphyseal arch and becomes transformed into the lamina epithelialis of the lateral choroid plexus of the adult. Its an- terior moiety subtends the paraphyseal arch and the invagina- tion of the choroid fissure begins between the 14-mm. and the 16-mm. stages. Its posterior moiety contiguous to the di- telencephalic fold of the velum transversum thereafter rapidly expands. 7. The neopallium grows more rapidly than any other part of the telencephalon. Its initial differentiation follows that of the hippocampus; its subsequent development surpasses that of the latter. The identification of the future hippocampus in the THE FISSURA HIPPOCAMPI 169 young stages suggests a certain type of relative growth in the telencephalon. In measuring the growth of the histologically distinct regions of the telencephalon medium and the areas contiguous to them, lying in the evaginated portion of the hemi- sphere, we are able not only to measure the relative amount of growth of the telencephalon, but also to determine the manner in which this growth takes place. In the embryos studied the medial wall was observed to grow in the following manner: first, by the intrinsic growth in the midline, especially in the region of the lamina terminalis and in that of the di-telencephalic fold; second, by the out-growth of a series of arcs of new tissue which forms the incipient frontal- parietal, occipital, and temporal poles. BIBLIOGRAPHY Anton, G. 1886 Zur Anatomie des Balken Mangels im Grosshirne. Zeits. fiir Heilkunde, Bd. 7, Prag, 8. 53-64. Bartey, P. 1916a Morphology of the roof plate of the forebrain and lateral choroid plexuses in the human embryo. Jour. Comp. Neur., vol. 26, p. 79. 1916 b The morphology and morphogenesis of the choroid plexuses with special reference to the development of the lateral choroid plexus in Chrysemys marginata. Jour. Comp. Neur., vol. 26, p. 507. Biscuorr, T. L. W. 1868 Die Grosshirnwindungen des Menschen. Adh. d. k. Bayerisch Akad. d. Wiss., Bd. 10, Abth. II, 8. 446. Casa, 8S. R.y 1911 Histologie du Systéme Nerveux de l’homme et des Verté- brés. Paris, T. 2, pp. 752-754. Crossy, E.C. 1917 The forebrain of Alligator mississippiensis. Jour. Comp. Neur., vol. 27, pp. 325-402. CunnineHaM, D. J. 1892 Contribution to the surface anatomy of the cerebral hemispheres. Dublin. Ecxsr, ALEX. 1869 Zur Entwicklungsgeschichte der Furchen und Windungen der Grosshirn Hemisphiren im Foetus des Menschen. Archiv f. Anthrop., Bd. 3, 8. 203-225. GoupsTHIN, K. 1904 Zur Frage Existenzberichtigung, etc. Anat. Avz., Bd. 24, 8. 579-595. Gronsure, G. 1901 Die Ontogenese eines niedern Siugergehirns nach Unter- suchungen an Erinaceus europaeus. Zool. Jahr., Abth. Morph., Bd. 15, S. 261. Herrick, C. J. 1910 The morphology of the forebrain in Amphibia and Rep- tilia. Jour. Comp. Neur., vol. 20, pp. 413-547. His, W. 1890 Die Formentwicklung des menschlichen Vorderhirnes. Abh. Math.-phys. K]. des Kg. Sichs. Ak. d. Wiss. Leipzig. 170 MARION HINES His, W. 1889 Die Formentwicklung des menschlichen Vorderhirns vom ersten bis zum Beginn des dritten Monats. Abh. math.-phys. KI. d. Kel. Sachs Ges. Wiss., Bd. 15, S. 675-736. 1904 Die Entwicklung des menschlichen Gehirns. Leipzig. HocusteTtTer, F. 1898 Beitrige zur Entwicklungsgeschichte des Gehirns. Stuttgart. 1904 Ueber die Nichtexistenz der sogenannten Bogenfurche an den Gehirnen lebenfrisch konservierter Menschlicher Embryonen. Anat. Anz., Bd. 25, Erginzungsheft, S. 27-34. JounsTon, J. B. 1909 On the morphology of the forebrain vesicle in verte- brates. Jour. Comp. Neur., vol. 19, pp. 457-539. 1913 Morphology of the septum, hippocampus, and pallial com- missures in reptiles and mammals. Jour. Comp. Neur., vol. 23, pp. 371-488. 1915 Cell masses in the forebrain of the turtle, Cistudo carolina. Jour. Comp. Neur., vol. 25, pp. 393-468. K6uuiker, A. 1879 Entwicklungsgeschichte des Menschen und der héheren Thiere, Ed.2. Leipzig. Luvi, Giusnrppr 1904a Ueber die Entwicklung und Histogenese der Am- monshornformation. M. Schultze Archiv, Bd. 64, pp. 389-403. 1904 b Sull’origine filogenetica della formazione ammonica. Archivio Anat. Embriol., T. 3, pp. 234-247. Matt, J.P. 1903 On the transitory or artificial fissures of the human cerebrum. Am. Jour. Anat., vol. 2, p. 333. MarcHanpD, F. 1891 Ueber die Entwicklung des Balkens im menschlichen Gehirn. Arch. f. mikr. Anat., Bd. 37, 8. 298-334. 1909 Entwicklung von Mangel des Balkens im menschlichen Gehirn. Abh. der Kénigl. Siichs. Gesellschaft der Wissenschaften, 58, Math. Phys., Klasse, 31, pp. 371-402. Martin, Pavunt 1894 Bogenfurche und Balkenentwicklung bei der Katze. Jenaische Zeitschrift fiir Naturwissenschaft, Bd. 29, Neue Folge 22, Sage MercxsgL, J. F. 1815 Deutsches Arch. f. Physiol. Halle und Berlin, S. 1-108; 334422. Mrnarkovics, V. 1876 Die Entwicklung des Gehirnesbalken und des Gewélbes. Centralbl. f. d. med. Wiss. nr. 19. Mrine@azzini, G. 1888 U.d. Entwklg. Furchen und Windung des menschlichen Gehirns. Moleschott’s Untersuchungen zur Naturlehre, Bd. 13, S. 498-562. Muysr, A. 1892 Ueber das Vorderhirn einiger Reptilien. Zeit. f. wiss. Zool., Bd. 55, S. 63-133. Rerzius,G. 1901 Zur Frage von den sogenannten transitorischen Furchen des Menschengehirns. Anat. Anz., Engiinzungsheft, Bd. 19, S. 91. 1902 Zur Frage der transitorischen Furchen des embryonalen Men- schenhirns. Biol. Untersuchungen, Neue Folge, Bd. 10, Stockholm, S. 65. Ricutsr, A. 1887 Ueber der Windungen des menschlichen Gehirns. Virchow’s Archiv, Berlin, Bd. 108, S. 398-422. THE FISSURA HIPPOCAMPI Mira Scuapgr, A. 1904 Zur Frage der Existenzberichtigung der Bogenfurchen am Gehirne menschlicher Embryonen. Anat. Anz., Erginzungsheft, Bd. 25, S. 35-37. Scumipt, F. 1862 Beitrige zur Entwicklungsgeschichte des Gehirns. Zeitschr. f. wiss. Zool., Bd. 11, S. 43-51; 64-65. Smita, G. Evtiot 1894a A preliminary communication upon cerebral com- missures of the Mammalia. Proceedings Linnean Society of New South Wales, vol. 2, series 2, Oct., p. 655. 1894 b Brain of foetal Ornithorhynchus. The forebrain. Quart. Jour. of Micr. Sc., vol. 39. 1895 The comparative anatomy of the cerebrum of Notoryctes typhlops. Trans. Royal Soc. of South Australia, p. 167. 1896 The fascia dentata. Anat. Anz., Bd. 12. 1897 a The origin of the corpus callosum. Trans. Linnean Soc. of London, 2 series, vol. 7, pp. 47-69. 1897 b Further observations upon the fornix, with special reference to the brain of Nyctophilus. Jour. of Anat. and Phys., vol. 32, pp. 231-246. 1897 c The relation of the fornix to the margin of the cerebral cortex. Jour. Anat. Phys., vol. 32, pp. 23-58. 1897 d The morphology.of the indusium and striae Lancisii. Anat. Anz., vol. 13, pp. 23-27. 1899 Further observations on the anatomy of the brain in the Mono- tremata. Jour. Anat. Phys., vol. 33, p. 309. 1903 Note on the so-called ‘transitory fissures’ of the human brain, with special reference to Bischoff’s ‘Fissura perpendicularis externa.’ Anat. Anz., Bd. 24, S. 216-220. 1910 Some problems relating to the evolution of the brain. The Lancet, Jan. 1, 15, 22. ; SymMiIneTon, J. 1901 Report of the British Ass. for the Advancement of Science. Glasgow, p. 798. TippEMANN, F. 1816 Anatomie und Bildungsgeschichte des Gehirns die Foetus des Menschen. Niirnberg. 1823 Anatomie du cerveau contenant d’histoires de son developpe- ment dans le foetus avec une exposition comparative de la structure dans les animaux. Trans. par. A. L. Jurdain, Paris, 1823. WarREN, J. 1917 The development of the paraphysis and pineal region in Mammalia. Jour. Comp. Neur., vol. 28, pp. 75-186. ZUCKERKANDL, E. 1901 Zur Entwicklung des Balkens und des Gewdlbes. Sitzungsber. d. K. Ak. d. Wissensch. in Wien. Mathem. nat. Klasse, Bd. 110, Abt. III, S. 57. PROMPT PUBLICATION The Author can greatly assist the Publishers of this Journal in attaining prompt publication of his paper by following these four suggestions: 1. Abstract. Send with the manuscript an Abstract containing not more than 250 words, in the precise form of The Bibliographic Service Card, so that the paper when accepted can be scheduled for a definite issue as soon as received by the Publisher from the Iditor. 2. Manuscript. Send the Manuscript to the Editor prepared as described in the Notice to Contributors, to conform to the style of the Journal (see third page of cover). 3. Illustrations. Send the Illustrations in complete and fin- ished form for engraving, drawings and photographs being pro- tected from bending or breaking when shipped by mail or express. 4. Proofs.. Send the Publisher early notice of any change in your address, to obviate delay. Carefully correct and mail proofs to the Editor as soon as possible after their arrival. By assuming and meeting these responsibilities, the author avoids loss of time, correspondence that may be required to get the Abstract, Manuscript and Illustrations in proper form, and does all in his power to obtain prompt publication. THE JOURNAL OF COMPARATIVE NEUROLOGY VOL. 34, NO. 2, APRIL, 1922 Resumen por el autor, N. E. McIndoo. E] sentido del ofdo en la abeja. Organo productor del sonido: Este aparato consiste en mem- branas situadas entre las axilares en las bases de las alas anteriores. Los museculos del t6rax que se insertan sobre estas axilares se contraen y dilatan con rapidez, produciendo de este modo una vibracién de aquellas; a consecuencia de esto las membranas vibran rapidamente, produciendo el ruido semejante a un chil- lido que se percibe al comprimir una abeja. Los experimentos llevado a cabo por el autor no prueban que las abejas pueden ofr. El autor cree que las abejas no oyen, por lo menos del modo que nosotros oimos; pero a juzgar por todas las pruebas experimentales y anatOmicas que poseemos, parece que su sentido del ofdo no puede separarse de su agudo sentido del tacto, del mismo modo que su sentido del gusto no puede separarse del del olfato. Los supuestos organos del oido: Hasta el presente se han hallado cinco Ilamados 6rganos auditivos en la abeja. A juzgar por su anatomia, las placas porosas, los frascos de Forel, las ex- cavaciones provistas de una prolongacién obtusa y el 6érgano de Johnston, todos ellos situados en las antenas, no parecen bien equipados para actuar como receptores del sonido; pero los 6r- ganos cordotonales, situados en las tibias, pueden estar mejor adaptados para esta propésito, si es que tienen una porci6n ex- terna correspondiente al timpano. Las placas porosas pueden ser sensitivas a las corriented débiles de aire y tal vez funcionen como un aparato de presién de aire que sirve para informar a las abejas de un objeto situado inmediatamente delante de ellas. Los 6rganos de Johnston pueden ser 6rganos estdticos que sirven para registrar los movimientos del flagelo. Las funciones de los frascos de Forel, excavaciones con prolongacién obtusa y 6rganos cordotonales son problematicas. Translation by José F. Nonidez Cornell Medical College, New York AUTHOR’S ABSTRACT OF THIS PAPER ISSUED BY THE BIBLIOGRAPHIC SERVICE, MARCH 20 THE AUDITORY SENSE OF THE HONEY-BEE N. E. McINDOO Bureau of Entomology, Washington, D. C. TWENTY-SIX FIGURES CONTENTS Introduction and methodsses. aes snes... <5 SeEM ac cee se olen «lol etteiciern ecm a= 173 So-called vocal organs of insects............ cece cere eee eee ete eee eens 175 1. Sound-producing organ of honey-bee.............+-0+e0 seer eee eee ees 175 a. Experiments to determine how bees make sounds.............-..--. 175 b. Morphology of sound-producing organ. ......-.-.-+.-+ees rere reese 176 2. Sound-producing organs of other insects......... 6.6... see eset neers 179 So-called auditory organs of insectsS............ ee eee eee eee eee teen e eee 180 Supposed auditory organs of honey-bee.........-.-- ++ ++ see eset rere eee: 180 GaSiructurero VOhmstones OLG@alMesr seas: \crramite es (oe clracr scleral) ity 180 Be Gtructureiok pore platesy. saa cv. cs Se wetness + teeta ele meee lee 186 @uctructure of othermantennalvorgansens:... setae. toe eeis eile 189 d. Structure of tibial chordotonal organs...............-+---ee ese eeees 190 e. Structure of tibial ganglion cells...............2-2222ee eee e eee e cece’ 195 SHITTT Gie ie sae aaa Ie DQeacinitig Guin emcr OImRnS: ort Oca Cr aon arc On. 196 iterature ciced sortase thc aces > Ragone es Olek Gls aed teins law Seige = clekaw are 198 INTRODUCTION AND METHODS Much has been written about the auditory sense of insects, but critics still contend that it has*never been demonstrated be- yond a doubt that any insect can really hear. Most students on insect behavior believe that insects can hear, but only Turner and Schwarz (714) and Turner (’14) seem to have produced good experimental evidence; however, they used only moths in their work. Much less is known about the sound perceptors in insects, and still it is not generally known how insects make sounds which are supposed to be heard by them. It is usually believed that insects can hear for the three following reasons: 1) many have special sound-producing organs; 2) some have so-called auditory organs, and, 3) many of the experimental results obtained indicate that insects can hear. 173 174 N. E. McINDOO In regard to the buzzing of insects, there are four old views to explain how the noise is made, as follows: 1) by the rapid vibra- tion of the wings; 2) by the vibration of the thorax; 3) by a special modification of the occlusor apparatus in the stigmata, and, 4) in Diptera, by the vibration of the halteres. Relative to Diptera and Hymenoptera, Pemberton (’11) and Aubin (’14) show that none of the above views hold good. They determined that the buzzing is made by the extreme bases of the wings, as is shown for the honey-bee in the present paper. The humming, or more common noise produced by the wings, is made by the distal portions of the wings. We should not expect insects to respond to sounds which have no significance to them, nor to sounds not in their category, because they may not hear the sounds that we do. The number of vibrations perceptible to the average human ear varies from 32 to 60,000 per second. Now it may be that the insect ear is so poorly developed that it can hear only sounds having vibra- tions below 32 per second. It may also be that the sense of hearing in insects is on no higher plane than that advocated by Forel (08), who believes that insects do not hear, at least as we do, but compares this perception in them to that in deaf-mutes who feel the rolling of a carriage at a distance. Bee-keepers are agreed that bees can hear, yet they cannot prove it. Von Buttel-Reepen (07), a scientist and an ex- perienced bee-keeper, in discussing the behavior of bees has much to say about their auditory perception, but still he produces no experimental evidence to support his strong statements. To obtain material for the structure of the sound-producing organ, adult bees were used; but for a study of the so-called auditory organs, young bees, nineteen and twenty-one days old (counting from the time the eggs were laid) were employed. Fresh material was fixed in the modified Carnoy’s fluid and was embeded in 60° paraffin. Sections were cut 5 and 8 yu in thick- ness, and were usually stained in Ehrlich’s hematoxylin and eosin, but a few of them in eosin alone. All the drawings are original and were made by the writer at the base of the microscope usually with the aid of a camera lucida. AUDITORY SENSE OF HONEY-BEE 175 SO-CALLED VOCAL ORGANS OF INSECTS In the higher animals the vocal organs are located in the throat, but in insects we should not expect to find their vocal organs in the buccal cavity, because this class of animals has a totally different organization. Hence, we must look elsewhere for the vocal organs of insects. 1. Sound-producing organ of honey-bee In 1911, while determining that bees have an olfactory sense, the writer also ascertained that they have a means of making a piping or squealing noise. a. Experiments to determine how bees make sounds. Hundreds of worker bees with wings pulled off or cut.off were tested, and from mere observations it appeared that the squealing noise was made by the thorax. To be sure that the noise was made by the thorax, the abdomens of several bees were cut off. These mu- tilated bees could walk fairly well and when irritated made the squealing noise. Several other bees with both heads and ab- domens cut off were likewise tested; these were so badly mutilated that in only one case did a thorax make the noise. Bees with wings cut off, when observed under a binocular, further confirmed the view that the squealing noise is made solely by the thorax, because the bases of the wings were seen to vibrate rapidly. In 1920 the preceding line of experimentation was continued, and the following results were obtained. The loud buzzing of bees is made by the distal portions of both pairs of wings, while the squealing noise is made only by the bases of the front wings. When worker bees or drones were squeezed, or when their wings were held firmly, or were cut off or pulled off, these insects usually made the squealing noise. When no noise was heard, the insect being tested was placed on a large inverted pan lying on a table. The pan thus serving as a resonator usually intensified the feeble noise so that the human ear could hear it. Bees with the wing stubs glued did not make the noise, but after dissolving off the glue the insects made the noise as usual. When the muscles attached to the roots or axillaries of the wings were cut, the 176 N. E. McINDOO squealing noise ceased for all time, and when the thorax of a live bee was held gently between the fingers the tingling sensation perceived indicated that these muscles vibrate very rapidly, setting in motion the axillaries and membranes in the bases of the wings. A microscopical examination of all the front wings pulled off showed that every bee with wings thus detached was able to squeal so long as one or two intact axillaries remained in the thorax. Besides the buzzing and squealing noises made by bees, the writer often heard a crackling sound while observing these insects flying around an alighting-board. He could not detect how this sound is made, but imagined it produced by the wings striking together accidentally. All attempts, except one, trying to get bees to respond to the squealing of other bees failed. Or at least the bees exhibited no reactions which could be attributed as signs of hearing. Never- theless, one squealing bee was held in a hidden position a few inches from an alighting-board; at once one of the many workers on this board seemed to take notice and flew to the screen behind which the squealing bee was hidden, and then it came immediately to the squealing bee, which it began to examine by running around it and smoothing its hair. A queen bee, resting on a comb with workers surrounding her, when squeezed, squealed and the near-by workers became excited. Such experiments really do not mean much, because too many interfering factors cannot be eliminated. The original plan of the writer was to carry on experiments in which he hoped to ‘be able to classify and to record on phonograph records the various sounds heard in a hive of bees. If this were possible, he intended to reproduce these sounds and then to determine whether or not bees respond to them. When he was transferred from the division of Bee Culture, this line of experimentation was discontinued. b. Morphology of sound-producing organ. Several live worker bees and drones were held under a binocular and the following observations were recorded: When a bee is held by the legs it buzzes continuously. The wings are held straight out at right AUDITORY SENSE OF HONEY-BEE POE angles; their anterior margins move little, but their flexible posterior portions vibrate rapidly; their bases do not vibrate, but move slowly in and out and backward and forward; no squealing was heard, but the muscles in the thorax vibrate more or less slowly. When one-half of each wing was cut off, a faint buzzing and a feeble squealing noise were heard. When the front wings were cut off as closely as possible and the hind wings were pulled out by the roots, no buzzing was heard, but the squealing noise was quite pronounced. While the bases of the front wings Figs. land2 Base of right front wing of a worker honey-bee, showing special sound-producing apparatus, consisting of membranes (Me) lying between axil- laries (1, 2, 3, and 4X), median plate (WP), head of radius (R), subcosta (Sc), costa (C), union of radius and media (RM), cubitus (Cw), and anal veins (1 and 3A). Nos. 1, ta, 2, and 3, groups of olfactory pores. Fig. 1, dorsal view, and fig. 2, ventral view. X 40. vibrated, two weak lines in them were exhibited, as indicated by lines AA and BB in figure 1. The first one, starting between the ends of the costa (C) and subcosta (Sc) and ending between the first (1X) and fourth axillaries (4X), resembles a stiff hinge; while the second one, starting from the same source, passes through the weak point in the union of the radius and media (RM) and ends between the first anal (1A) and third axillary (3X). This line, along which the wing usually breaks when this appendage is carelessly pulled off, is more rigid than the other one. 178 N. E. McINDOO While observing a squealing bee, the saddle-shaped subcosta (fig. 1, Sc) rotated quickly on the head of the radius (R) which also vibrated; the bases of the cubitus (Cw), first and third anal veins (1A and 3A), and the membranes (Me) between them likewise vibrated, and the median plate (MP) and tegula were observed to move slightly. When the tegula which covers the axillaries was pulled off, the first, second, and third axillaries (1X, 2X, and 3X) and the membranes (Me) between them were seen to vibrate. It was not possible to observe the ventral surface (fig. 2) of the base of the wing on the living bee, but a study of its anatomy shows that this surface is better adapted to produce sounds than is the dorsal surface (fig. 1). Reference to figure 2 shows that there is twice as much membrane capable of being vibrated on this surface as on the other surface, due to the fact that the sub- costa (Sc), head of the radius (R), and median plate (MP) are considerably smaller than they are on the other side. In fact, all the membranes, represented by dots in figure 1, were observed to vibrate, and all of those in figure 2, likewise represented, also probably vibrate. Thus it is evident that the extreme bases on these wings make a good sound-producing organ. Figure 1 is partly copied from Snodgrass (’10), but the present writer carefully verified all the sclerites here represented, and then made a careful study of the ventral surface (fig. 2), which the former writer did not illustrate. Relative to the muscles, attached to the axillaries, and to the mechanism producing the wing motion, the reader is referred to Snodgrass’ bulletin, page 65. In this study the group of olfactory pores on the front wings have been more carefully observed than they were formerly by the writer (14a). Instead of three groups, there are four groups of them; the fourth group, now numbered /a in figure 2, was formerly overlooked in superficial observations, but was called no. 2 in figures 19 and 20, page 328. Groups 1, la, and 2 are really located on the head of the radius (fig. 2, R), and not on the subcosta, and group 3 lies on the other side of the same sclerite (fig. 1), and not on the median plate. AUDITORY SENSE OF HONEY-BEE 179 2. Sound-producing organs of other insects Pemberton (’11) experimented with several species of Syr- phidae, house-fly, honey-bee, and bumble-bee. He says that they do not produce audible sounds by the spiracles or tracheae, but that all humming or buzzing sounds made by them are produced solely by the wings, either by their vibration in the air or by the wing bases striking against the body wall. This author did not study the anatomy of the wing bases. Aubin (14), as well as Pemberton, used the syrphid or common drone-fly (Eristalis tenax) in all his detailed experiments. The former author, after experimenting with this fly and after carefully identifying all the parts in the bases of its wings, con- cludes that the buzzing sound is made by a rapid vibration of certain thoracic muscles, attached to a particular sclerite, which strikes the thorax at a given point. The resonant apparatus, consisting of another sclerite and its attached membranes, is thrown into a state of vibration, producing the buzzing sound which is about an octave higher than the humming noise, made by the distal portions of the wings. In the honey-bee, according to the observations of the present. writer, no part of the wing base strikes the thorax during the vibration. Aubin believes that, according to the laws of acoustics, the resonant areas in the wing base of this fly might respond to the buzzing of other flies and thus form one of the elements of an auditory apparatus. If this were true, a nervous connection would be necessary. In all probability, no such connection exists in this fly, and certainly not in the honey-bee. Judging from the known sound-producing apparatus, and so- called auditory organs in crickets, grasshoppers, and katydids, the males are usually neither deaf nor dumb, but the females are always dumb, although not generally deaf. The males of crickets, katydids, and of some grasshoppers make sounds by rubbing their wings together, whereas other grasshoppers make sounds by rubbing the hind legs against the wings. Both sexes possess so-called ears, which in crickets and katydids (Locustidae) are found on the front tibiae, but in grasshoppers (Acrididae) on the abdomens. As far as known, the female cicada is both 180 N. E. McINDOO deaf and dumb, but her mate is only deaf, his sonorous sound- producing organ being found in the abdomen. ‘Happy is the cicada, since its wife has no voice,’ says Xenarchos, could just as well be said about the males of crickets, grasshoppers, and katydids. Graber, after cutting off the front tibiae of crickets and katydids, found that they responded as well to a violin and to their chirping and singing as before the operation. Stridulation, special sound-producing apparatus, and various types of supposed auditory organs have. been described in true bugs, moths and butterflies, flies and mosquitoes, beetles, and ants, and also in a few larvae and pupae, yet we know very little about this subject. SO-CALLED AUDITORY ORGANS OF INSECTS Since insects have special sound-producing organs, it is natural to suppose that they also have auditory organs. The so-called auditory organs of Orthoptera and of certain other insects, mentioned above, need not be further discussed here, because Comstock (’20, pp. 145-154) has recently given a good summary on this subject. Supposed auditory organs of honey-bee In the following pages the descriptions of five supposed sound receptors are given, and Janet (’11) mentions a sixth one in the bee. From his brief description and drawing the details of this one cannot be interpreted. The following is all that Janet says about it: The chordotonal nerve departs from the antennal nerve a short distance from the brain, and runs toward the integument where it is inserted at a point beneath and a short distance from the articular edge of the antenna. From this point of insertion departs one end of a fusiform chordotonal ganglion, whose other end gives rise to a terminal cord which runs toward the articular membrane of the antenna, and is there inserted. The present writer has not studied this organ, but from the above brief de- scription he would eliminate it as a possible auditory organ. a. Structure of Johnston’s organ. Johnston (’55) pointed out a supposed auditory organ in the second antennal segment of the AUDITORY SENSE OF HONEY-BEE 181 culex mosquito. This structure, later called Johnston’s organ, was thoroughly investigated by Child (94 a,b), who saw it in all the insect orders examined, except one. He found it in several genera of Diptera, one genus of Hymenoptera, and in one or more genera each of Coleoptera, Lepidoptera, Neuroptera, Pseudo- neuroptera, and Hemiptera (Homoptera). Apparently he did not examine the honey-bee, but found it in a wasp (Vespa vul- garis) well developed, although the articular membrane to which the sense cells are attached is not complicated as he found it in mosquitoes and as the present writer saw it in the honey-bee. Of the many specimens examined, Child found this organ most highly developed in the male mosquito (Comstock, ’20, pp. 152-154, for a general description). He also saw sense organs in the second antennal segments of Orthoptera, but decided they were not Johnston’s organs. Recently these have been described as olfactory pores by the present writer (’20). The distal end of the second antennal segment (fig. 3, 2) is considerably larger than the proximal end, but the proximal end of the third segment (3) is the narrowest portion of the antenna. When examining the extreme distal end of the second segment under a low-power lens, a cirele of irregular structures (J), somewhat resembling a miniature mountain chain in shape, passes completely around the segment. Observing a crushed segment under a high-power lens, it will be noted that these structures, known as chitinous knobs (fig. 4, A.) from now on, lie in the ar- ticular membrane between the second and third segments. The top line in figure 4 represents the union of this membrane with the second segment, and the bottom line the union of the same membrane with the third segment. As an average for each caste, a worker has 70 of these knobs; a queen, 72, and a drone, 100. Oblique sections through the articular membrane show that it (figs. 5 and 6, ArtM) is very thin, that the ends of the knobs (K) fit into sockets GS) in the chitin (Ch) of the third segment, and that soft, flexible strands of chitin (figs. 6 and 12, Ch) firmly bind the two segments together. In fact, the hard, rigid chitin (represented by solid black) of the articular membrane (figs. 7 and 12, ArtM) is reenforced by a layer of soft, flexible 182 N. E. McINDOO 9 Figs. 3 to 11 Antennal sense organs of honey-bee. Fig. 3, dorsal surface of right antenna of worker, showing following: Two groups of olfactory pores (Por) on condyle (Co) and scape (1); flagellum, consisting of second to twelfth segments (2 to 12), bearing Johnston’s organ (J), pit pegs (PP), pore plates (P) and Forel flasks (F/); the tactile hairs and pegs are not represented; X 20. Fig. 4, superficial appearance of Johnston’s organ on worker antenna, showing knobs (K); X 320. Figs 5 and 6, oblique sections through Johnston’s organ in drone antenna, showing knobs (A) of articular membrane (Art) in sockets (S) of chitin (Ch), and soft, flexible strands of chitin (Ch) which firmly bind second and third segments together; * 500. Fig. 7, from three consecutive longitudinal sections of nineteen-day-old worker antenna, showing sensory part of Johnston’s organ, consisting of sense cell (SC), its nucleus (Nwe) and sense fiber (SF) and probably the latter’s nucleus (Nwc); note distal end of sense fiber attached to knob (K) of articular membrane (ArtM); X 1000. Figs. 8 to 11, internal struc- ture of antennal organs; X 1000. Fig. 8, olfactory pore from worker condyle. Fig. 9, pore plate from drone antenna, showing plate (P), two grooves (d and e), and double hinge-like membrane (m). Figs. 10 and 11, pit peg and Forel flask, respectively, from worker antenna, showing semitransparent hair (Hr), nerve strand (St), cavity (Ca), and aperture (Ap). AUDITORY SENSE OF HONEY-BEE 183 chitin (represented by broken lines). Consequently, instead of this articulation being weak, it is as strong as any other, and when broken by a steady pull, the articular membrane remains fastened to the third segment, showing that the knobs, although having considerable play in their sockets, nevertheless lend considerable strength to the articulation. . Longitudinal sections through the second antennal segment show the following: A large group of sense cells (fig. 12, SC) lies on either side of the section; two large antennal nerves (JN), called internal and external olfactory nerves by Janet (’11), run through the center of the segment and at various places unite with the groups of sense cells, as shown in figure 12, and a large trachea (7'r) runs near the nerves and sends out branches here and there. A thorough study of these sections under an oil-immersion lens shows the following: The elliptical sense cells (fig. 7, SC) have conspicuous nuclei (Nuc), short nerve fibers (fig. 12, NF) ‘which run into the nerves, and long and comparatively large sense fibers (SF) which run in bunches toward the articular mem- brane. About half-way between the sense cell and articular membrane may be seen small slender nuclei (fig. 7, Nuc), some of which seem to lie on the surface of the sense fibers, but it is more likely that these are hypodermal nuclei, although the nuclei in the hypodermis (fig. 12, Hyp) usually are round and much larger. When the bunches of sense fibers reach the flexible strands of chitin (Ch) the individual fibers separate, run between these strands, then unite singly with the inner ends of the knobs (K). Figure 12 is a diagram showing most of the second segment in longitudinal section and in perspective, and a small portion of the third segment in both cross and longitudinal section and in perspective. It is noted that the thin articular membrane (ArtM), bearing the chitinous knobs (K), is unprotected and fully exposed to the outside air. Two of the knobs are cut length- wise, showing the cone-shaped cavity which opens to the exterior. The other knobs are heavily shaded, showing that they are buried in the articular membrane. 184 N. E. McINDOO A glance at figure 12 shows that the articular membrane re- sembles the head of a drum and that the knobs act chiefly as sense-fiber attachments. It is evident, judging merely from the structure of this organ, that gusts of wind and possibly weak air currents would cause the articular membrane to vibrate, there- by irritating the sense cells. This organ might also receive jar Fig. 12 Diagram, representing most of second antennal segment of worker honey-bee in longitudinal section and in perspective and a small portion of third segment in both cross and longitudinal section and in perspective, showing John- ston’s organ which consists of two large groups of sense cells (SC) whose nerve fibers (VF) run into the two antennal nerves (NV) and whose sense fibers (SF) are attached to the knobs (K) in the articular membrane (ArtM). Two of the knobs are cut lengthwise, showing the cone-shaped cavity which opens to the exterior, while the other knobs are heavily shaded, indicating that they are buried in the articular membrane. JT'r, trachea; Hyp, hypodermis; Ch, hard chitin; and Chi, soft, flexible strands of chitin which firmly bind second and third segments together. AUDITORY SENSE OF HONEY-BEE 185 stimuli, but it appears too crude to act as a sound-wave receptor, unless it is able to receive sound vibrations of a very low fre- quency. The most reasonable function that the writer can think of is the one suggested by Demoll (’17), that it may serve as a statical organ to register the movements of the flagellum. Since there can be no muscular sense in the flagellum, because this part of the antenna possesses no muscles, such an organ would seem very useful. The scape or first antennal segment (fig. 3, 7) contains many muscle fibers, most of which run to the articula- tion between the first and second segments. These muscles can only move the flagellum about in all directions, but cannot bend it. Since the antenna is the chief tactile organ of the bee and must be carefully operated, the only way of bending the many- jointed flagellum is by blood pressure. The blood bathes all the internal structures, and consequently any change in its pressure would affect the articular membrane. Even if the Johnston’s organ in the honey-bee receives sound vibrations of a low frequency, or functions in any other way suggested above, we should probably classify it as a tactile organ rather than as an auditory organ. Child (’94 b) says that the function of Johnston’s organ is in general to receive original touch stimuli; it can, however, in a broader sense receive the stimuli of sound vibrations. The auditory stimuli are to be thought of as modified touch stimuli. When the same organ serves both as touch and auditory recep- tors, as is possible in mosquitoes and midges, then the insect will be able to differentiate between the touch response and auditory response. According to Child, this organ is of hypodermal origin, arising from a ringlike fold near the antennal funnels which are in- vaginated in the head. Several years ago the writer discovered two groups of olfactory pores on the base of the antenna, but they are here described for the first time. One group of about twenty-five pores lies among a bunch of tactile hairs on the distal end of the articular knob or condyle (fig. 3, Co), and the other group (Por) of twelve pores lies on the proximal end of the scape. So far as known, these are 186 N. E. McINDOO the only olfactory pores on the antenna of the honey-bee. The external and internal structure (fig. 8) are like those already described by the writer (14 a). b. Structure of pore plates. The pore plates or sensilla placodea, according to the writer’s discussion of the antennal organs (’14 b), were first studied in 1847 by Erichson, who called them olfactory organs. Since this date they have been studied by about three dozen other investigators whose views concerning their function differ widely. In 1851 Vogt suggested that they perform a func- tion combining those of smell and touch. In 1858 Lespés com- pared them to the ears of higher animals, and a year later Hicks called them auditory organs. Practically all of the other authors up to 1888, who have studied the pore plates, regard them as olfactory organs. Ruland (’88), after having boiled antennae in caustic potash, saw that a pore plate is suspended on a membrane, resembling a double hinge, similar to that observed in sections stained in eosin by the present writer. Owing to this arrange- ment, he called them auditory organs. In 1894 Nagel favored the olfactory view, but also thought that the pore plates might have a mechanical function. He suggested that air pressure might affect them. Nine years later Schenk (’03) stated that the thick plates in these organs eliminated the possibility of these structures being olfactory organs, but judging from their anatomy he regarded them as having a mechanical function. He favors the view that they are pressure points to inform the bee of the object immediately in front of it. According to Schenk’s calculations, a male honey-bee has 31,356 pore plates and a female has only 3,648. According to the calculations of the present writer, a drone on an average has 29,718 pore plates; a worker has 4,744, and a queen has 2,776. Those of the drone are much smaller than those of the worker or queen, but supposing that their sensitiveness is in direct pro- portion to the total area of all their plates, then if the sensitive- ness of those on a worker equals 1, that of those on a drone equals 3, and that of those on a queen equals only 0.6. These organs (fig. 3, P) are found only on the fifth to twelfth antennal segments of the worker and queen, and on the fifth to thirteenth segments AUDITORY SENSE OF HONEY-BEE 187 of the drone. They are rather equally distributed over the various segments. Using the average number of pore plates on a worker antenna as an example, the segments and number of these organs are: 5th, 322; 6th, 345; 7th, 332; 8th, 288; 9th, 284; 10th, 283; 11th, 278, and 12th, 240. Twelve per cent of these lie on the ventral surface and 88 per cent on the dorsal surface. Rela- tive to the pore plates on the queen and drone, only 3 per cent of those of the former and 25 per cent of the latter lie on the ventral surface of the antenna. Viewed superficially with transmitted light, a pore plate (fig. 13, P) is seen to consist of an elliptical light spot, which is surrounded by three concentric bands; the first and third ones (a and c) being light in color, and the second or middle one (6) being dark. A section through this organ shows that the hard and thick plate (figs. 9 and 18, P) is suspended on a membrane (m), resembling a double hinge, which viewed by transmitted light causes the above dark band (6), while an inner groove (d) causes the first light band (a) and an outer groove (e) produces the other light band (c). In reality this outer, groove is not a true groove, because its walls or sides lie against each other and allow no cavity, except perhaps when the plate is vibrated. This fact explains why other observers have overlooked it. Ruland saw it in sections made from caustic potash material, which must have been considerably distorted. The present writer has also seen it many times in the same kind of sections, besides in other sections made from material not treated with KOH. Any dark stain obliterates this groove, and consequently the writer was able to see it by using eosin alone. Judging from the structure of a pore plate, the elliptical plate (fig. 13, P) may be moved in and out on the double hinge (m), thereby moving the large nerve strand (St) and consequently affecting the large sense cell group (SCG). These organs, there- fore, might be an air-pressure apparatus, as suggested by Nagel and Schenk. It has been observed by Schenk and the present writer that bees, when flying toward an object, such as a window, light on their feet instead of butting their heads into the object. Now, it may be that the pore plates act as an air-pressure appara- 188 N. E. McINDOO tus, in which capacity they inform the insects of the objects immediately in front of them. In case of the honey-bee, they might also be sensitive to the weak currents of air caused by workers fanning. It is possible that the sense hairs are not affected by these weak currents, and therefore some method is Fig. 13 Diagram, representing a block taken from terminal antennal seg- ment of worker honey-bee, showing tactile hairs (Thr), pegs (Pg), pit pegs (PP), Forel flasks (Fl), and pore plates (P) in both perspective and in section. a and c, light colored bands; b, dark band; d, inner groove; and m, double, hinge-like membrane of pore plate; St, nerve strand; SCG, sense cell group. badly needed to keep the bees constantly informed whether or not the fanners are working properly. If these interpretations are correct, we here have another form of touch. Of course these organs might also be sensitive to wave vibrations of a very low frequency, but if this interpretation is correct, we would yet AUDITORY SENSE OF HONEY-BEE 189 probably be more correct by classifying them as tactile organs rather than as auditory perceptors. c. Structure of other antennal organs. Relative to the other antennal organs, there are four, all of which are really hairs. The tactile hairs (fig. 13, 7 Hr) or sensilla trichodea are scarce on the antennae of the male honey-bees, but numerous on the antennae of the females. They are regarded by all the observers as tactile organs. ‘The pegs (Pg) or sensilla basiconica are absent in the males, but numerous on the antennae of the females. They are generally considered as olfactory organs, because their tips are covered with very thin chitin. The present writer believes that they are very delicate touch organs. The pit pegs (fig. 138,-PP) or sensilla coeloconica and Forel flasks (FJ) or sensilla ampullacea are hairs inside of pits. On the antennae of the males both of these types are somewhat numer- ous, but on the antennae of the females they are comparatively scarce. Viewed superficially, one type cannot be distinguished from the other, but sections show that the pit pegs are usually the larger in diameter. Relative to the antennae of workers, most of these organs (fig. 3, PP and Fl) lie in groups on the sixth to twelfth segments, and counting both types combined there are not more than 100 individual organs on each antenna. In regard to their internal structure, they differ somewhat, as may be seen by referring to figures 10, 11, and 13. In both types the semitransparent hair (Hr) ends in a cavity (Ca), which com- municates with the exterior by a minute aperture (Ap), and each hair is connected with a nerve strand (St), which runs to a sense cell group (SCG). The function of these organs is usually regarded as problematical, but still a few authors have called them auditory organs. The present writer has no conception of what their function is, but for some time he has looked upon them as more or less degenerated structures. At this place the writer wishes to call attention to an erroneous idea which text-book writers still persist in handing down. Be- fore understanding the internal anatomy of antennae, some of the early microscopists imagined that they saw gland cells among the masses of sense-cell groups. This led to the idea that in order 190 N. E. McINDOO for the antennal organs to function as olfactory or gustatory organs the secretion from these glands must pass through the thin chitin and must keep the outer surfaces of the organs moist and thus fitted for the reception of chemical stimuli. This is nice in theory, but there is not one iota of truth in such an assumption, because not one of the later investigators mentions having seen glands connected in any way with the antennal organs. Ruland in 1888 denies the presence of them, and the present writer has never seen anything in the antennae which he could call glands. Berlese (’09, p. 610) maintains that the essential feature of these chemical sense organs is the presence of antennal glands, and Comstock (’20, p. 133) quotes Berlese on this subject and then describes various types of hairs which have been called organs of smell and taste. The present writer (16) does not believe that insects have a true gustatory sense and regards it absurd to consider any form of hair capable of receiving chemical stimuli. d. Structure of tibial chordotonal organs. Schén (711) de- scribed and illustrated the structure and development of the tib- ial chordotonal organs in the honey-bee and ants. The present writer has carefully studied the structure of the same organs in the honey-bee and differs with Schén only in a few details. Sections through the tibiae of all three pairs of legs of workers and drones were made and a chordotonal organ was invariably found in each tibia sectioned. It lies (fig. 14, O) in the proximal end of the tibia, about one-fourth the distance from the femoro- tibial articulation to the tarsotibial articulation. This portion of the tibia is divided into two distinct chambers by the large trachea (figs. 15 and 16, 7'r). The blood chamber (fig. 15, B) contains only blood and the chordotonal organ (©), while the other chamber contains blood, muscles (MM), apodemes (A), nerves (NV), fat-cells (Ff), etc. In longitudinal sections this organ expands fan-like across the blood chamber and usually appears to be attached by its proximal end to the hypodermis (figs. 14 and 19, Hyp), but in other sections where the tibia is considerably compressed both its proximal and distal ends are attached to the hypodermis on the anterior side of the leg (fig. 19). In a series AUDITORY SENSE OF HONEY-BEE 191 of cross-sections it may be seen to arise merely as a nerve attached to the hypodermis; then the nerve suddenly runs to a few large cells (fig. 15); a few sections further on the nerve dis- appears, and the organ assumes a spherical shape (fig. 16, O), and the walls are lined with a thick layer of large cells, leaving a cavity in the center, which is apparently filled with a liquid (probably only blood). Figs. 14 to 18 Sections, showing structure of tibial sense organs. Figs. 14 and 17, each a semidiagrammatic drawing from two consecutive longitudinal sections of hind tibia of a drone, showing relative position of tactile hairs (THr), olfactory pores (Por), chordotonal organ (QO), and ganglion cells (@), all of which are innervated by the same nerve (N, too wide here), although a small branch of this nerve, called the subgenual nerve (SN) runs directly to the chordotonal organ; fig. 14, X 32, and fig. 17, X 58. Figs. 15, 16, and 18, cross-sections through tibia of a worker, showing relative position and shape of chordotonal organ (QO) and ganglion cells (@); X 53; fig. 15, through proximal end of group of ganglion cells. A, apodeme; B, blood chamber; F, fat cell; Hyp, hypodermis; M, muscle; N, two main branches of nerve; 7’r, trachea. Using Schén’s terminology, the detailed description of a single chordotonal organ is as follows. The tibial nerve after im- mediately emerging from the femur is apparently divided into two large branches, because in all of the cross-sections made two comparatively large nerves (figs. 15, 16, and 18, N) were found. A short distance from the femorotibial articulation one 192 N. E. McINDOO of the branches gives off fibers to some tactile hairs (fig. 14, T’H7) and also a smaller branch which innervates the olfactory pores (Por) and the chordotonal organ (O). Schon calls this small branch (figs. 14 and 19, SN) thesubgenual nerve (Subgenualnerv). It runs into the sense cell group and gives off a fiber to each individual sense cell (fig. 19, SC). The spindle- but sometimes diamond-shaped sense cells le in a mass which extends diagonally half-way across the blood chamber. In all of the present writer’s sections, the organ is anchored at the base of the sense cell group, but only occasionally was it also fastened at the other end of the organ. According to Schon, it arises from the distal end and should always be fastened at this end. The distal end of a sense cell is terminated into a long, slender, sac-like enveloping cell (EC, Umbiillungszelle) whose elongated nucleus (Nuc.) some- times nearly fills the entire lumen of the cell. Running the full length of the enveloping cell there is a dark-staining thread, the axial tube (Aw), which ends in a much darker staining body, the cone (Con, Stift), lying in the proximal end of the large oblong or pear-shaped cap cell (CC, Kappenzelle) whose nucleus (Vues) usually lies in the distal end. The walls of the axial tube correspond to the extended walls of Schén’s Stift. Schén also describes two other types of cells which the present writer has not been able to differentiate from those just mentioned. His accessory cells (akzessorische Zellen) lie between the cap cells and his end fibers (Endfasern), which fasten the organ to the hypodermis. A more careful study of the sensory element of the chordotonal organ under a magnification of 1900 diameters shows the follow- ing: Lying around the conspicuous nucleus (figs. 19 and 20, Nuc) of the sense cells (SC) there are large dark-staining particles, the largest one of which seems to be the tail end of the axial tube (Az). In longitudinal sections this particle appears as a dark streak and may or may not reach as far as the nucleus. In cross-sections it appears as a large, dark, solid particle (fig. 21). Cross-sections through the proximal end (fig. 22), middle portion (fig. 23), and distal end (fig. 24) of the enveloping cell (£C) show that instead of the axial tube continuing very far as a rod, Ax§ oO) oY 25 i ri ET Pa BET SaE rea “I 5 = “ TSS OPN EAS fo — Kee ERED Figs. 19 to 26 Sections, showing detailed structure of chordotonal organ and ganglion cells. Fig. 19, semidiagrammatic drawing from several longitudinal sections through anterior portion at proximal end of hind tibia of a drone, twenty- one days old, showing chordotonal organ suspended in blood chamber, but at- tached at both ends to hypodermis (Hyp); X 320. Fig. 20, longitudinal view of one of the sensory elements from fig. 19, showing following parts in detail; sense cell (SC), axial tube (Ax), enveloping cell (EC), axial fiber (AF), cone (Con), cap cell (CC), and nuclei (Nuci, Nuce and Nuc3) of sense cell, enveloping cell ‘and cap cell, respectively. Fig. 21, cross-section of sense cell (SC) through nucleus, showing tail end (large dot) of axial tube; figs. 22, 23, and 24, cross- sections of proximal end, of middle portion through nucleus, and of distal end, respectively, of enveloping cell, showing appearance of axial tube (Az) and its fiber (AF). Fig. 25, longitudinal section, and fig. 26, cross-section of ganglion cellsfrom tibia ofadrone. Figs. 20 to 26, X 1000. 193 194. N. E. McINDOO it becomes a tube with thick walls at first, but the walls gradually become thinner and thinner as one glances at them from the proxi- mal end to the distal end. ‘The outer layer of the walls seems solid, often does not take the stain, but remains a light yellow color. The inner layer does not appear totally solid, but usually is stained more or less. The axial tube seems harder than the surrounding tissue and occasionally the microtome knife fails to cut it, thus leaving it slightly projecting from a cross-section. Hence from all appearances the axial tube is a semichitinous structure. The head end of the axial tube terminates in the cone (fig. 20, Con), from the center of which arises the short axial fiber (AF), scarcely visible under the highest magnification. In position this fiber corresponds to the Sch6n’s axial fiber, which extends the full length of the enveloping cell. Figure 24 shows the rela- tion of the various parts in cross-section just in front of the cone. The dot represents the axial fiber; the inner circle, the walls of the axial tube; the middle circle, the walls of the enveloping cell, and the outer circle, the walls of the cap cell. The cytoplasm in the distal half of the enveloping cell and in the cap cell (fig. 20, CC) appears slightly net-like, although this appearance is exaggerated in figures 20, 23, and 24. In regard to the development of the chordotonal organ, Schén says that eight days after the honey-bee egg is laid a small growth projecting into the blood chamber is seen developing in the hypodermal cells in the tibia from which the organ arises. On the ninth day may be seen the first differentiation of cells, and on the tenth and eleventh days one may distinctly see sense cells, enveloping cells, and cap cells. On the eleventh and twelfth days the cone is formed, and on the thirteenth day the nervous part of the organ is laid down in the blood chamber. On the fifteenth and sixteenth days the end fibers are developed, and on the seventeenth day the organ is fully developed. It is of a purely ectodermal formation. Schon says that since the trachea is so greatly expanded where the chordotonal organ occurs, it may probably have something to do with the function of this organ; but the present writer does AUDITORY SENSE OF HONEY-BEE 195 not think so, because the trachea does not come in contact with the organ and is no more expanded here than in other places in the tibia. So far no external device or apparatus connecting with the internal organ has been found, although Sch6n imagined that he had found the external portion when he thought he saw two rows of sense cones (Sinneskegel) on the proximal end of each tibia. In position and number these cones correspond exactly to the olfactory pores described by the present writer. When observed without the cylindrical tibia being properly rotated, they often externally resemble cones; but when the tibia is rotated slightly, so that they lie on the median line of the tibia, the optical illusion becomes evident. Schén found that-both the chordotonal organ and these imaginary cones are innervated by the subgenual nerve, and consequently he believes that the cones act as the external apparatus of the organ. Schon describes and illustrates the internal anatomy of his sense cones (the present writer’s olfactory pores), but here does not recognize them as cones, for he follows vom Rath by calling them membrane canals (Mem- brankanile). Nothing can be said about the probable function of the chordo- tonal organ, but if it were connected with an external apparatus, similar to that found in some Orthoptera, it might serve as an auditory organ. Schon says that there is a great similarity between this organ in Hymenoptera and that in Orthoptera. These organs in bumble-bees, wasps, and Terebrantidae vary greatly with those in ants and honey-bees. In all the organ is fastened to the hy- podermis and in all he found the sense cones andspindle-shaped sense cells which with the subgenual nerve protrude into the nervous ends of the enveloping cells. e. Structure of tibial ganglion cells. In the first longitudinal sections made, the writer observed a group of supposedly sense cells which he thought was associated with the chordotonal organ, but after studying more sections it was ascertained that these cells are totally independent of the chordotonal organ, because the former (fig. 14, G) are located at the distal end of the tibia 196 N. E. McINDOO and the latter (O) at the proximal end of the tibia, and no con- nection could be found between them, except that the same nerve (NV) sends off a branch to each group of cells. For lack of a more appropriate name, the cells under discussion may be called tibial ganglion cells, although the writer knows of no similar group in insects. Sch6én apparently did not see them and perhaps this is the first time for them to be described. They lie in a mass (fig. 17, @) between two tracheae (Tr) at the extreme distal end of the tibia. The distal end of the group is attached to the hypodermis near the articulation, while the proximal end terminates in a branch of the main nerve. Figure 18 is a cross-section showing this group of cells (G) just departing from the nerve (V), and some of the fibers may be seen between the two tracheae (7'r) running to the hypodermis. This group of cells is shghtly larger than the chordotonal organ, but the individual cells (figs. 25 and 26) in it are practically the same in shape and size as are the sense cells in the chordotonal organ. SUMMARY Bee-keepers are agreed that bees can hear, yet they cannot prove it, and critics still contend that it has never been experimentally proved that any insect can hear; nevertheless, within the last few years some good experimental results have been obtained. The special sound-producing apparatus of the honey-bee con- sists of the membranes lying between the axillaries at the bases of the front wings. Muscles, lying in the thorax and attached to these axillaries, contract and relax very quickly, thereby causing the axillaries to vibrate; consequently, the above mem- branes are caused to vibrate rapidly, thus producing the piping, teeting, or squealing noise commonly heard when a bee is squeezed. Up to date five so-called auditory organs have been found in the honey-bee. Judging from their anatomy, the pore plates, Forel flasks, pit pegs, and Johnston’s organ, all located in the antennae, do not seem to be well fitted to act as sound receptors; but the chordotonal organs, lying in the tibiae might be better AUDITORY SENSE OF HONEY-BEE 197 adapted for this purpose, providing they had an external por tion, corresponding to the tympanum. The Johnston’s organ, lying in the second antennal segment, consists of the peculiarly modified articular membrane between the second and third antennal segments and of many sense cells whose fibers unite with peculiar knobs extending inwardly from the articular membrane. This organ does not seem well adapted to act as an auditory organ unless it is able to receive sound vibrations of a very low frequency. It might also be sensitive to weak air currents and possibly to jars, but the most reasonable function that the writer can think of is that it may serve as a statical organ to register the movements of the flagellum. The pore plates, lying so abundantly on the antennae and called olfactory organs by most of the other authors, were found to have two grooves encircling each elliptical plate, thereby allowing the plate to move in and out on a double hinge. Judging from this mechanism, the pore plates might act as an air-pressure apparatus to inform the bees of an object immediately in front of them, and thus prevent them from striking against objects. They might also be sensitive to the weak air currents made by workers fanning, thereby serving as an apparatus to keep the bees constantly informed whether or not the fanners are working properly. The functions of the Forel flasks and pit pegs are problematical. The chordotonal organs, found in the proximal ends of the tibiae, are very complicated in structure and are similar to those found in the tibiae of crickets and katydids, but the former do not have external membranes, while the latter do. Nothing can be said about the function of the chordotonal organs in honey-bees. A group of ganglion cells was found in the extreme distal end of each tibia, but nothing can be said about its function. In conclusion, it may be that the sense of hearing in insects is on no higher plane than that advocated by Forel (’08), who beHeves that insects do not hear, at least as we do, but compares this perception in them to that in deaf-mutes who feel the rolling of a carriage at a distance. Forel says: 198 N. E. McINDOO Hearing is a physical sense. Sonorous waves, especially those of low sounds, are nearer to large mechanical vibrations than luminous, caloric, or electric waves. Hearing, therefore, must be in its origin connected with touch, but we make a distinct difference between the perception of a very low sound by touch and its perception by hearing. We must not forget that the specialization of the organ of hearing has reached in man a delicacy of detail which is evidently not found again in lower vertebrates. It is, I believe, the sense which removes us most from the lower animals. In animals as high as fish the auditory nerve is con- fused with other nerves, and the portion of the labyrinth most specially affected by our audition, the cochlea, has disappeared. LITERATURE CITED AusBiIn, P. A. 1914 The buzzing of Diptera. Jour. Royal Micr. Soc., pp. 329-334. BERLESE, ANTONIO 1909 Gli Jnsetti, vol. 1, Milano. Cuitp, C. M. 1894a Beitriige zur Kenntnis der antennalen Sinnesorgane der Insecten. Zool. Anz., 17. Jahr., S. 35-38. 1894b Ein bisher wenig beachtetes antennales Sinnesorgan der Insekten, mit besonderer Beriicksichtigung der Culiciden und Chiro- nomiden. Zeitsch. f. wiss. Zool., Bd. 58, S. 475-528. Comstock, J. H. 1920 An introduction to entomology. Part I, 2nd ed., Ithaca, N. Y. DrEMoLL, REINHARD 1917 Die Sinnesorgane der Arthropoden, ihr Bau und ihre Funktion. Braunschweig. Foret, Auguste 1908 The senses of insects. English trans. by Yearsley. London. JANET, CHARLES 1911 Sur 1’existence d’un organe chordotonal et d’une vésicule pulsatile antennaires chez l’abeille et sur la morphologie de la téte de cette espéce. L’Apiculteur, 55e Année, no. 5, May, pp. 181-183. Jounston, Cur. 1855 Auditory apparatus of the Culex mosquito. Jour. Micr. Sci., Old Series, vol. 3, pp. 97-102. McInvoo, N. E. 1914a The olfactory sense of the honey-bee. Jour. Exp. Zool., vol. 16, no. 3, pp. 265-346. 1914b The olfactory sense of insects. Smithsn. Mise. Collect., vol. 63, no. 9, pp. 1-63. é 1916 Thesense organs on the mouth-parts of the honey-bee. Smithsn. Mise. Collect., vol. 65, no. 14, pp. 1-55. 1920 The olfactory sense of Orthoptera. Jour. Comp. Neur., vol. 31, no. 5, pp. 405-427. PEMBERTON, C. E. 1911 The sound-making of Diptera and Hymenoptera. Psyche, vol. 18, pp. 114-118. Rvuuanp, Franz 1888 Beitriige zur Kenntniss der antennalen Sinnesorgane der Insekten. Zeitsch. f. wiss. Zool., Bd. 46, S. 602-628. Scuenx, Orro 1903 Die antennalen Hautsinnesorgane einiger Lepidopteren und Hymenopteren, mit besonderer Beriicksicktigung der sexuellen Unterscheide. Zool. Jahrb., Bd. 17, S. 573-618. AUDITORY SENSE OF HONEY-BEE 199 Scuén, ARNoLtD 1911 Bau und Entwicklung des tibialen Chordotonalorgane bei der Honigbiene und bei Ameisen. Zool. Jahrb. Anat. und Ont., Bd. 31, S. 489-472. Snoperass, R. E. 1910 The anatomy of the honey-bee. U.S. Dept. Agr., Bur. Ent. Tech. Ser. 18. TuRNER, C. H., anp Scowarz, E. 1914 The auditory powers of the Catocala moths; an experimental field study. Biol. Bul., vol. 27, no. 5, pp. © 275-293. | Turner, C. H. 1914 An experimental study of the auditory powers of the giant silkworm moths (Saturniidae). Jbid., no. 6, pp. 325-332. Von Butret-REeeren, H. 1907 Are bees reflex machines? pp. 1-48. English trans. by Mary H. Geisler, A. I. Root Co., Medina, Ohio. Resumen por el autor, G. W. Bartelmez. El origen de los primordios 6tico y 6ptico en el hombre. El presente trabajo se basa en datos obtenidos en dace em- briones humanos normales en los estados de cuatro a dieciseis somitas. En el hombre, a diferencia de lo que sucede en la mayor parte de los vertebrados, la placa 6tica y su ganglio asociade aparecen muy temprano, y hasta preceden al primordio dptico. La placa 6tica aparece enfrente de la segunda divisi6n del cerebro posterior, y el ganglio cerca del borde dorsal del pliegue neural adyacente. En los estados de nueve y diez somi- tas las células del esbozo ganglidnico comienzan a perder su dis- posicién epitelial y la masa entera se separa del pliegue neural. Se deriva de la pared del futuro tubo neural. Durante este periodo el epitelio 6tico se diferencia de una manera caracteristica y después se invagina. El esbozo 6ptico puede reconocerse por vez primera como un engrosamiento de los pliegues neurales, correspondiente a la region del cerebro anterior. En este primordio de la ‘‘cresta 6ptica’”’ se produce lateralmente la vesicula 6ptica; de las por- ciones media y caudal parte una proliferacién de la cresta neural. El] sureo 6ptico puede reconocerse en el estado de ocho somitas. La evaginacién se dirige al principio ventralmente, pero al aproximarse los pliegues neurales se dirige lateralmente hacia el ectodermo. En el estado de dieciseis somitas la vesicula 6ptica esta en contacto con el ectodermo. Se deriva enteramente de las paredes del tubo neural definitivo, porque una parte de la pared del futuro cerebro dorso-lateral interviene desde el principio entre el esbozo 6ptico y el ectodermo. Translation by José F. Nonidez Cornell Medical College, New York AUTHOR’S ABSTRACT OF THIS PAPER ISSUED BY THE BIBLIOGRAPHIC SERVICE, MARCH 20 THE ORIGIN OF THE OTIC AND OPTIC PRIMORDIA IN MAN G. W. BARTELMEZ Department of Anatomy, The University of Chicago, and the Laboratory of Embryology, Carnegie Institution of Washington TEN FIGURES INTRODUCTION The early history of the nervous system in man and in the great majority of other mammals is but imperfectly known. In the whole order there is only a single detailed study of the origin of the cranial ganglia. The data on this phase of human development are for the most part included in descriptions of single specimens; they are few and incomplete and marred by faulty interpretations due to the lack of a human series and neglect of comparative material from other forms. We shall here confine our attention to the otic and optic pri- mordia, although the identification of them in the various em- bryos of the series is based in part upon our interpretation of the primary subdivisions of the nervous system. This evidence will be presented in a subsequent paper. MATERIAL The present observations are based upon the study of com- plete serial sections of twelve normal embryos ranging between stages of three and sixteen somites. In addition, the data from the published descriptions of four others have been available, so that the series is a reasonably complete one, even though some of the specimens leave much to be desired in the matter of histo- logical detail. We have models of eight of the embryos, and in the case of some of them several models were prepared. The specimens studied in this connection are as follows: 201 202 10 11 12 G. W. BARTELMEZ DESIGNATION OF EMBRYO Be ee LENGTH H279 Univ. of Chicago 4 2.5 mm. in formol Coll. ‘Klb.’ Normentafel no. 5 to 6 1.8 mm. in alcohol 3 * 391 Carnegie Coll. 8 2 mm. in formol (Dandy 1910) H87 Univ. of Chicago 8 Cire. 2 mm. in for- Coll. mol Eternod embryo ‘DuGa’|} Probably | 2.12 mm. from num- (1896) 9 ber of sections in the series H637 Univ. of Chicago 11 Distorted. 1.85mm. Coll. from number of sections H 197 Univ. of Califor- 12 1.15 mm. (much nia Coll. flexed) H392 Univ. of Chicago 11 3.6 mm. in formol Coll. *4 New York Univ. 14 2.3 mm. after fixa- Coll. (Wallin 1913) tion HS Univ. of Chicago 14 3.3 mm. in alcohol Coll. ‘Pist. uP Normentafel 14 Cire. 2.6 mm. x6 *470 Carnegie Coll. Probably |3.8 mm. in 95% 16 ‘alcohol ACKNOWLEDGMENTS CONDITION Fair, abun- dant mitoses Excellent Fair Good Excellent Excellent his- tologically Good Fair clumped mitoses Good histolog- ically Poor Excellent Fair clumped mitoses This paper is part of a study of human development during the early part of the period of somite formation, begun in 1915 in the Carnegie Laboratory at Baltimore in conjunction with Dr. H. M. Evans (1917). The complete account is to appear OTIC AND OPTIC PRIMORDIA IN MAN 203 as a joint paper in the Carnegie Institution’s ‘Contributions to Embryology.” Although Dr. Evans’ own material, his ex- tensive studies on embryos in the European collections and in the Mall collection have been used freely, this part appears under my name as I am assuming complete responsibility for the interpretation of the nervous system. Needless to say, I am under great and varied obligations to Doctor Evans which I gladly acknowledge. The work was begun at the suggestion of the late Dr. F. P. Mall, to whom we owe much. Dr. G. L. Streeter has continued to support it and has helped and advised. Most of the drawings are the work of Mr. J. F. Didusch, whose understanding help has been invaluable. We would express our appreciation of the courtesy of Profs. Franz Keibel, A. C. F. Eternod, and H. D. Senior for permission to study the young human embryos in their collections. To the following physicians we are indebted for the embryos of our own series, for their cooperation and for the care which they took to preserve these delicate specimens: Dr. J. P. Spooner, of Peru, Indiana, for H279; Alpheus B. Streedain, of Chicago, for H87; Dr. Edwin Hirsch, of Chicago, for H637; Dr. Robert T. Legge, of Berkeley, California, for H 197; Dr. Ethel Rice, of Chicago, for H392; Dr. J. F. Burkholder, of Chicago, for H8. The exceptional success we have had in obtaining young embryos has been due in large measure to the support and cooperation of Dr. R. R. Bensley. THE OTIC PRIMORDIUM The earliest sensory primordium that can be recognized in man is a thickening of the ectoderm opposite the neural folds of the hindbrain. This is the beginning of the otic plate. Sev- eral statements in the literature indicate that the otic plate appears early in human development. In 1908 Keibel identified ‘die Hérplatte’ in the Unger embryo (Normentafel 4), which had about nine somites. Wilson (14, p. 325) suggested that a pair of diffuse thickenings in his two- to three-somite embryo (H3) might be the ‘auditory areas.’ Tracing the otic plate back through our series leaves little doubt but that he was correct THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 2 204 G. W. BARTELMEZ in this. Ingalls (20, p. 67) likewise identified it in his slightly older specimen (Carnegie Collection no. 1878), and with greater certainty, as he had several of our models of older stages for com- parison. Figure 1 shows the conditions in the first of our present series, a four-somite embryo (H279) with wide open neural folds. The dorsal part of the fold near the beginning of the second subdivision of the hindbrain (cf. p. 207) is enlarged; a swelling protrudes toward the ectoderm, ac. fac. gang.; and there is a Fig. 1 A photomicrograph of section 54 of the four somite embryo H27 (Univ. of Chicago Coll.). xX 100. The plane of section is almost horizontal to the hindbrain. ac. fac. gang., anlage of the acousticofacial ganglion. The over- lying cap of ectodermal cells is more deeply stained than the ganglion. of. pl., otic plate; am., amnion; y. s., yolk sac. corresponding ventricular sulcus which does not show clearly in the photomicrograph. This is very like the condition described by Schulte and Tilney (’15) in the cat. The enlargement is termed acousticofacial ganglion in accordance with the customary mammalian usage (cf. p. 214). The ganglionic anlage is capped by a single layer of ectoderm cells which appears as if it had slipped over the top of the neural fold.. The adjacent ecto- derm is obviously thickened as the otic plate, ot. pl. The section, which is nearly horizontal through this region, shows almost the OTIC AND OPTIC PRIMORDIA IN MAN 205 whole rostrocaudal extent of the otic plate. In most sections it fades off gradually into the surrounding ectoderm so that its limits are difficult to determine. Both the ganglion and the otic plate are present in the six- somite ‘Klb.’ of Keibel, but our tracings are not sufficiently detailed to permit of an accurate description. At the time they were made the presence of the anlagen was not suspected. They can be found in the eight-somite embryo first described by Dandy (10) (Mall, No. 391). Figure 2a is taken from a section through the middle of the otic plate of this embryo and shows the gan- glionic primordium clearly on the right side. This appears as an outpouching of the neural fold with the characteristic cap of overlying ectoderm. The ‘otic sulcus’ is manifest in a model of this region made at 400 diameters. It is a broad shallow pit near the dorsal edge of the fold extending through four of the 10 uw sections. In this case also it belongs to the second hindbrain segment and lies just caudal to the first visceral pouch.! The depression to the left in the figure is not the beginning of the otic pit, but a chance wrinkling of the ectoderm. With the illustrations at hand it will be easier to visualize these relations in the other eight-somite embryo, H87. Figure 3 represents the dorsal aspect of a model and figure 4 is from a projection reconstruction of the embryo cut in the mid-sagittal plane and viewed from the right. The sensory anlagen are plotted in from a detailed study of the sections and indicated by stippling. In the former figure we see the broad expanse of forebrain, still in the neural-plate stage. The deep neural groove has progressed as far forward as the midbrain, behind which are two large hindbrain subdivisions. From the second of these the acousticofacial ganglion is arising and laterally in the ectoderm is the otic plate reaching back as far as the third hindbrain seg- ment. The primary brain segments are better shown in figure 4. 1 Manifestly our interpretation of the nervous system of this embryo does not agree with that of Dandy (’10). Having identified the cranial flexure, the otic plate and its associated ganglion throughout our series, it has been possible to interpret correctly the subdivisions of the neural folds. What he has termed the second brain vesicle is in reality the second division of the hindbrain. e it Fig. 2 Fig. 10 A photomicrograph of the seventeenth section of the twelve-somite embryo H 197 (Univ. of California). % 100. On the right the section passes near the middle of the optic anlage (op.sul.); on the left through its rostral end (op.prim.). Medial to the optic evagination the thickened neural fold is pro- liferating mesectoderm (mesec.). The section passes horizontally through the heart. pe.c., pericardial cavity; am., amnion; ph., pharynx; y.s., yolk sac. dium as mesectoderm. Caudally the primordium continues without a break into the hindbrain where, as figure 8 (mesec.) shows, the mesectoderm is clearly differentiated from the rest of the mesenchyme by its deeper stain. The optic sulcus appears THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 2 220 G. W. BARTELMEZ on the right side of figure 10; on the left the section passes through the rostral end of the optic primordium (op. prim.). The more medial portion of the thickened primordium in this figure shows the indefinite lower boundary which is characteristic of a mesec- todermal proliferation. It would seem, then, that the optic anlage arises only from the lateral part of the area indicated by stippling in figure 3, at levels corresponding to the caudal part of the forebrain, i.e., the future diencephalon. H 197 is the only embrya we have seen which clearly shows mesectoderm formation in the forebrain, nor has it been observed in any other mammal. It can hardly be an abnormal activity in this particular case, for as a whole the embryo fits perfectly into the series. More than that, there are hints of such a process in another undoubtedly normal specimen, as we shall see. It is in better condition histologically than most young human embryos, as the photomicrographs (figs. 8 and 10) demonstrate. The optic anlage of Eternod’s nine-somite embryo ‘Du Ga’ (fig. 9b) is no farther along than in the eight-somite H87. In the eleven-somite H637 it is but little younger than in H 197 and the neural fold medial to it is greatly thickened. ‘There are two spots where cells seem to be preparing to leave the primor- dium, and this evidence. from an absolutely normal specimen affords the best evidence for regarding the extensive prolifera- tion in H 197 as normal. Unfortunately, the mechanical in- juries to the head in H 637 make modeling practically impos- sible. Inthe other embryo of this group, H392 (eleven somites), the optic sulcus is decidedly deeper than in H 197, as may be seen by comparing figures 9d and 10. Here, as in ‘Du Ga,’ the progress of the cranial flexure has bent the rostral end of the neural folds ventrally so that in the transverse series the first sections pass tangentially through the midbrain. Because of the oblique sections of the neural folds, it is impossible to get convincing pictures of neural-crest formation. So far as the more caudal part of the optic-crest primordium is concerned, it is clear that in H637 (eleven somites) the optic anlage is continuous with the neural-crest proliferation of the mid- and hindbrain. In the beautiful 5y sections of this series OTIC AND OPTIC PRIMORDIA IN MAN 221 one can observe every stage in the slipping out of the epithelial cells from the neural fold and Veit’s (’18) excellent description of his ‘craniale Ganglienleiste’ can be verified in detail. In ‘Du Ga’ and H392 and the N. Y. U. No. 4 (fourteen somites) there is a continuous crest anlage in the midbrain and the pre- otic hindbrain. In the first it reaches almost to the acoustico- facial ganglion, while in H392 (eleven somites) it cannot be recognized below the trigeminal level. Early history of optic vesicle We may turn now to the mass movements in the rostral divi- sion of the anlage which produce the optic vesicle. The general features of the process may be gathered from a survey of the sections reproduced in figure 9. The first is taken from H87 (eight somites). On the left we have a slightly more caudal level than on the right as the head end curved somewhat to the left. The level of the section is indicated on figures 3 and 4. The thickening of the neural fold, viz., the optic anlage, is indi- cated by the solid color. The optic sulcus and the bulging of the anlage into the mesenchyme stand out clearly. From the lateral portion of it the optic vesicle will arise, as we have deter- mined by the study of the corresponding region in the older embryos. This is an earlier stage of the optic primordium than has been described for any mammal. Figure 9b is from the twelfth section of Eternod’s nine-somite embryo ‘Du Ga.’ The rostral limb of the cranial flexure appears below; above is the caudal mesencephalic limb. Only the optic anlage is indicated by the solid color; it shows the characteristic thickening and the optic suleus. Above in the figure is the mesectodermal proliferation with its corresponding ventricular sulcus. The optic anlagen in this specimen begin four sections (40) behind the rostral end of the neural folds and can be recog- nized in five or six sections. The next stage we have is in the fourteen-somite embryo of the New York University collection. In this case the forebrain appears to have lagged behind the rest of the embryo in develop- 2), G. W. BARTELMEZ ment. Unlike the preceding specimen (fig. 9b) or the following one (fig. 9d), this has the anlagen cut transversely (fig. 9c). Active evagination has begun and on one side there is a deep pit at the center of the primordium, which is, however, confined to the single 5u section here drawn. Figure 10 (op. sul.) shows the deepening optic sulcus as it appears in the twelve-somite embryo H 197. The optic portion is differentiated from the rest of the anlage as has been said and appears as a long narrow field measuring 60 x 200u with the suleus running through the middle. The optic anlage of the eleven-somite H637 is practi- cally identical in appearance with this one. Figure 9d was made from the ninth section of the eleven-somite embryo H392. The space separating the two neural folds here is the neural groove, the bottom of which is one-tenth of a millimeter caudalward in the series. The sulcus is cut obliquely, but is actually deeper than in any case we have considered. It will be noted that the optic evaginations are still directed ventrally. They correspond closely in position and extent to those of ‘Du Ga,’ in fact these two embryos are very similar in most respects. Our next stage is found in the fourteen-somite embryo ‘Pfst. IIV (Keibel u. Elze, ’08, Taf. 6), which is well known from the work of Low (’08) and others. This has the earliest optic anlage that has been described hitherto in man. It begins about 130u behind the rostral tip of the nervous system and is present in twenty-one of the 10u sections. Throughout this region the neural folds are still open. A section through the middle of the anlagen may be seen in figure 9e and a more caudal level appears in figure 2e. The striking change here is that the optic evagina- tions are now directed laterally toward the overlying ectoderm as a result of the rapid approximation of the neural folds. Ac- cording to Low, there is an area of contact between the young optic vesicle and the overlying ectoderm. Keibel und Elze describe the vesicle as ‘close’ to the ectoderm, Bach und See- felder (’14) intimate that there is no actual contact nor do Doc- tor Evans’ tracings show any. Certainly, there is no mesoderm between the two epithelia and in the immediately following stages the lateral side of the vesicle comes into close contact with OTIC AND OPTIC PRIMORDIA IN MAN 223 the future lens epithelium. ‘This is true of two embryos in the Carnegie Collection, No. 12 (fourteen somites) and No. 470 (sixteen somites). In the latter, as figure 9f shows, we have a fully formed optic vesicle resembling that of the His embryo ‘EB’ (04) and of the twins described by Watts (15). It is younger in the fourteen-somite embryo H8 in which the lumen is narrower and there is no ectodermal contact, although the anterior neuropore is as small as in the sixteen-somite No. 470. Discussion—The optic vesicle If we compare e and f of figure 9, it is clear that the optic anlage has begun to balloon out, by an interstitial growth as well as by a thinning of the wall. The optic sulcus has become V-shaped so that the anlage is a trough about as long at the base as it is deep. This holds not only for the fourteen-somite HS8 and the sixteen-somite <470 (Carnegie coll.), but also for the embryos of Watts and that described by Bremer (’06). It is not until the twenty-three-somite stages that the vesicle has assumed a more or less spherical shape. It is difficult as yet to say how much of the area indicated by the solid color in figure 9e enters into the formation of the definitive optic vesicle. The narrow band of neural epithelium which separates vesicle and head ectoderm appears to grow very rapidly as the optic sulcus widens and deepens, and this growth certainly plays a part in the approxi- mation of the neural folds. After their closure this lateralmost part of the original neural plate constitutes all there is of brain wall separating the two vesicles dorsally. It is of course possible that all of the dorsal and lateral diencephalic wall which we find between the optic stalks in later stages is derived from it. On the other hand, it seems more probable that, as the vesicle gradually pinches off, some of the original evagination is incorporated into the brain wall both dorsal as well as rostral and caudal to the developing optic stalk. Perhaps it would be better to say that a portion of the lateral brain wall is at first dragged out with the optic evagination. This would hold particularly for the zone between vesicle and head ectoderm. Schulte and Tilney (15) 224 G. W. BARTELMEZ have presented strong evidence that this is so for the cat. On the basis of a complete series of models, they have described an absolute decrease in the size of the optic vesicle during the proc- ess of its separation from the brain and the formation of the stalk. It is not unlikely that the same conditions obtain in man. With the rapid enlargement of the optic ventricle the wall of the vesicle becomes thinner, whereas the diencephalic wall dorsal to it remains as thick as before. These conditions emphasize another significant fact. The optic primordium is laterally placed, but not in direct continuity with the future skin ectoderm. The future roof plate and part of the alar plate intervene. This strongly supports the theory that the vertebrate eye originated within the central nervous system. Other evidence for this has been convincingly pre- sented by Parker (’08). If the optic vesicle and its derivatives were lateral ectoderm incorporated into the neural plate, it would be necessary to assume that the hiatus left by the separa- tion of the vesicle was filled by an ingrowth of neural epithelium from either side. Such ingrowth should be manifest first as a notch in the side of the anterior neuropore or later as a suture. There is no evidence of such conditions in the pertinent stages we have examined, and we may confidently say that mammalian ontogeny offers no support for the theory of the peripheral origin of the eye. The anatomical evidence here, as in vertebrates generally, indicates that the optic primordia are lateral in position from the outset. The experimental evidence which has been well sum- marized by Mall (’19) is conflicting. The experiments of Stock- ard on Amblystoma (’14) are the most complete. In order to substantiate his theory of cyclopia, this investigator set out to prove that the earliest optic anlagen are median in position. In most of the embryos which survived the removal of the middle third of the rostral end of the neural plate the optic vesicles were subsequently lacking, whereas they were usually present when lateral moieties were extirpated. Stockard was convinced that the evidence demonstrated the existence of a median origin of the two subsequently lateral vesicles. It is possible that he did OTIC AND OPTIC PRIMORDIA IN MAN 225 not consider all the factors involved and that the results may be explained differently. It may be that his median extirpations had a much more general effect than he assumed and were in fact comparable to the general inhibition experiments with anaesthetics. The growing tip of the nervous system was re- moved, and this, in terms of Child’s gradient hypothesis, is the dominant region of highest metabolism. According to the severity of the injury, the development of one or both eyes was more or less inhibited. The extirpations of lateral areas would be more convincing if there had been any attempt to map out the morphologically differentiated optic areas and remove them. Even then the regulatory restitution of an entire optic vesicle from a fragment of the primordium intrudes itself. It might prove possible to make small definitely localized injuries and trace them through their subsequent migrations, as Patterson (10) did in his gastrulation experiments, and thus obtain more conclusive evidence. COMPARATIVE DATA From the comparative point of view, there are several interest- ing aspects of these observations. Man is the only vertebrate species on record in which the otie primordium appears before the optic. The otic plate can be recognized at an extraordinarily early period—earlier, in fact than in any other form for which we have accurate data. Conversely, the optic anlage is differ- entiated relatively later than in most other mammals, if we take into consideration the fact that the earliest stage described in the literature corresponds to our fourth stage (fig. 9d). The optic sulcus can be identified in man at a slightly earlier stage than that in which the otic plate appears in other mammals. The following résumé includes only the available mammalian literature. Artiodactyls Bonnet’s (01) account of early sheep embryos gives only enough to make it clear that the optic primordium precedes the otic in this form. Neither was identified in his twelve-somite 226 G. W. BARTELMEZ specimen (p. 10), but a fourteen-somite embryo (fig. 13) had well-developed optic vesicles, which indicates that the first anlage will be found at least as early as it isin the human. ‘Das Ohrgriibchen’ is first mentioned in a nineteen-somite stage. The Normentafeln of Sakurai (06) for the deer are more complete. The optic pit is first indicated in table 15 (eleven- somite embryo), while the otic plate appears at fourteen somites (table 17). Keibel’s first stage of the optic vesicle in the pig is a well- defined pit in the forebrain of a nine-somite embryo (’97, table 30). There is also the first hint of the otic plate here, but it must be remembered that the optic anlage doubtless appears earlier than this stage. The ten-somite series from which the Ziegler model was made has early optic vesicles rather than simple ‘foveolae.’ There is a marked evagination with a relatively wide lumen as yet directed ventrally, not laterally. Carnivores Weigner (01) found the ‘first signs’ of an optic vesicle in ferret embryos 1.2 to 1.5 mm. in length in which the neural tube had not yet completely closed and the otic pits were already present. His data are not sufficiently exact to make it clear which primordium is actually the first to appear. In his care- ful study of a three-somite ferret embryo, Yeats (11) refers to an ‘optic prosomere,’ and it would seem from his figures that there is an optic suleus here. No mention is made of otic primordia, There are two excellent papers on the early sensory anlagen in the cat. The earlier work of Martin (90) can be best con- sidered in the light of the more complete and thorough studies of Schulte and Tilney (15). Here (p. 322) it is probable that the optic anlage was indicated in the three-somite embryo. The four-somite specimen had optic sulci which resemble those of man (fig. 9d) rather than the optic ‘foveolae’ of the pig. In both cases the neural folds were still open throughout their extent. In the older one they identified trigeminal and acousticofacial OTIC AND OPTIC PRIMORDIA IN MAN 227 ganglia, but the otic plate is not mentioned. It is possible that the latter appears early in the cat, although it certainly does not precede the optic anlage as in man. The neural crest deserves further study in the carnivores. Weigner found no evidence for mesectoderm formation in the ferret and Schulte and Tilney affirm that all cells which leave the neural folds enter into the cranial ganglia. Martin (p. 342) found ‘neural crest’ beginning ‘dicht hinter’ the optic vesicle and extending through the midbrain giving rise to the sensory components of the third, fourth, and part of the fifth cranial nerves. If the microscopic picture in these forms is really not complicated by a mesectodermal proliferation, they are partic- ularly favorable material for the study of the origin of the cranial ganglia and especially of such problems as the origin and fate of the muscle sense cells of the oculomotor nerves. On the other hand, it is also possible that the particular embryonic stages during which the mesectoderm migrates out have not yet been studied. It would seem that in the cat the nervous system is differentiating more rapidly than the axial mesoderm and that a close series, so far as the number of somites is concerned, may have distinct. gaps in it. In man the period of pro-otic mesecto- derm formation is limited to stages between eight and twelve somites, and it may therefore be that this phenomenon occurs between five- and seven-somite stages in the cat—a period which was not represented in the Columbia University series. For the dog we have the descriptions of Bischoff (45) and Bonnet (’01). Figure 36 of the former shows an embryo of about ten somites with obvious optic vesicles. The latter’s figure 39 is taken from a section of an eight-to-nine-somite specimen and seems to pass through the edge of the optic primordium. Bonnet’s ten-somite embryo has both optic vesicles and otic pits. Here, then, as in the cat the two primordia arise at about the same time, the optic, however, taking precedence. Rodents When one considers the large collections of rodent embryos in many embryological laboratories, it is surprising that so little 228 G. W. BARTELMEZ work has been done on the early development of the nervous system in these forms. The only complete study is the Nor- mentafel of Minot and Taylor (05) for the rabbit. In the first embryo of their series they noted the broad expansion of the neural folds rostrally and suggested that this might be the first indication of the optic anlage. This specimen had five somites. Since the eight-somite embryo (no. 4) had well-developed vesi- cles, it is probable that there were optic anlagen in the cephalic plate of the former. The thickening for the otic plate as well as the ‘acoustico-facial ganglion’ and the trigeminal ganglion were recognized in the nine-somite specimen of table 5. Keibel, in 1889, figured the optic primordia of a guinea-pig embryo 16 days, 7 hours, old in sagittal section (fig. 44 and 45) and Foriep (’05, p. 157) has reproduced a photograph of a trans- verse section from a 3-mm. embryo. In both instances there is a deep broad pit situated laterally in the neural fold. Bischoff’s monograph on the guinea-pig furnishes no data on this subject. I nsectivores For the mole we have concrete data in the classic monograph of Heape (’87). He recognized the optic grooves in an embryo of three somites (stage E) where they are as well developed as in our eleven-somite H392 (fig. 9d). Heape calls attention to the very early appearance and typical form of the optic primordia in this form where the eye is subsequently degenerate. The otic plate is referred to in stage J when the optic vesicles are well developed and the neural folds are closed as far forward as the cranial flexure. His figure 25 is from a section of a fourteen- somite embryo in stage H and shows a slightly thickened otic plate. Primates Our knowledge concerning the pertinent stages of primates other than man is due entirely to Selenka and Huprecht. Selenka (’91, ’00) obtained three embryos from the period we are considering. His figure of the Hylobates embryo ‘Ab’ (’00, OTIC AND OPTIC PRIMORDIA IN MAN 229 ' fig. 24) represents a dorsal view of a three-somite specimen, of which the rostral end of the neural plate resembles that of H87 (eight somites). It shows two well-marked converging grooves which may well be the optic sulci. The thirteen-to-fourteen- somite embryo of Semnopithecus (‘Wa,’ Selenka, ’03, figs. 11 and 12) has large optic vesicles, but no otic plate is indicated. There is a deep otic pit in the twenty-three somite Cercocebus embryo, ‘Cc,’ however. From the Normentafeln of Tarsius (Huprecht and Keibel, 07) it would seem that the eight-somite embryo (figs. 8 and 9) had optic anlagen somewhat further advanced than those of our eight-somite embryo H87 (fig. 9a). The youngest stage in which the otic plate could be recognized is that of twelve somites (table 5), where the neural folds were closed in the region of the optic vesicles. Here, then, as in most other forms, the optic primordia precede the otic in development. The scanty and inaccurate data of this survey emphasize the need of detailed studies of early mammalian embryos both on their own account and for the light they will throw on human development. CONCLUSIONS 1. The earliest sensory anlage in man is the otic plate which can be recognized in an embryo of two to three somites as a diffuse thickening of ectoderm in the hindbrain region. A four-somite embryo shows the beginning of the associated acous- ticofacial ganglion, though its fate is not yet completely known. 2. The ganglion arises near, but not exactly at the dorsal edge of the open neural fold and the outermost part of the appar- ent evagination delaminates from the fold before the process of tube formation is completed. It is clearly derived from the wall of the definitive neural tube. 3. The otic epithelium differentiates by an elongation of the distal ends of the cells and the appearance of a brush border. Between ten- and twelve-somite stages invagination begins and there is a deep otic pit at sixteen somites. 230 G. W. BARTELMEZ 4. The otic plate has been identified earlier in man than in any other vertebrate of which we have accurate data, and in this form only does it precede the optic anlage. 5. Isolated thickenings (growth centers) of the cranial neural folds appear at a stage of seven to eight somites, which promptly fuse to form a continuous ridge, the ‘optic-crest primordium.’ An associated ventricular sulcus in the forebrain levels of the ridge indicates the position of the optic anlage. This is the earliest stage of this anlage which has been recognized in a mammal. 6. The non-optic part of this primordium proliferates mesecto- derm and a large part of the trigeminal ganglion. 7. The optic anlage appears laterally in the neural fold, but between it and the ectoderm there is an intervening zone which gives rise to part of the alar and roof plates of the future dien- cephalon. The optic vesicle is therefore derived entirely from the central nervous system. 8. At a stage of sixteen somites the optic vesicle is in contact with the overlying ectoderm. LITERATURE CITED BatLey, P. 1916 Morphology of the roof plate of the forebrain and the lateral choroid plexuses in the human embryo. Jour. Comp. Neur., v. 26, pp. 79-120. ; Bacu, L., UND SEEFELDER, R. 1914 Atlas zur Entwicklungsgeschichte des men- schlichen Auges. Leipzig und Berlin. Biscuorr, T. L. W. 1842 Entwicklungsgeschichte des Kaninchen-Eies. Braun- schweig. 1845 Entwicklungsgeschichte des Hundeeies. Braunschweig. Bonnet, R. 1889 Beitriige zur Embryologie der Wiederkaiuer, gewonnen am Schafei. II. Ar. f. Anat. u. Physiol., Anat. Abt., S. 1-106. 1901 Beitrige zur Embryologie des Hundes. II. Anat. Hefte, Bd. 16: S. 231-332. Bracuet, A. 1906 Recherches sur l’ontogénesé de la téte chez les amphibiens. Arch. d. Biol., T. 28, pp. 165-257. Bremer, J. L. 1906 Description of a four millimeter human embryo. Am. Jour. Anat., vol. 5, pp. 459-480. CasaL, S. R. 1912 Algunas variaciones fisiologicas y patologicas del aparato reticular de Golgi. Trab. Lab. Inves. Biol., vol. 12, pp. 127-228. Danpy, W. 1910 A human embryo with seven pairs of somites measuring about two millimeters in length. Am. Jour. Anat., vol. 10, pp. 85-100. OTIC AND OPTIC PRIMORDIA IN MAN ail Erernop, C. A. F. 1896 Sur un oeuf humain de 16.3 mm. avec embryon de 2.11mm. Actes Soc. helv. de se. nat., pp. 164-169. 1899 Il y a un canal notochordal dans V’embryon humain. Anat. Anz., Bd. 16, S. 131-148. Evans, H. M., anp Bartetmez, G.W. 1917 A human embryo of seven to eight somites. Anat. Rec., vol. 11, p. 355. Fortep, A. 1905 Die Entwicklung des Auges. Hertwigs Handbuch der Ent- wkges., Bd. II, H. 2, S. 139-240. Giceuio-Tos, E. 1902 Sui primordi dello sviluppo.del nervo acustico-faciale nell’uomo. Anat. Anz., Bd. 21, S. 209-225. His, W. 1893 Ueber das frontale Ende des Gehirnrohres. Ar. f. Anat. u. Physiol., Anat. Abt., S. 157-171. 1904 Die Entwickelung des menschlichen Gehirns wihrend der ersten Monate. Leipzig. Incautis, N. W. 1920 A human embryo at the beginning of segmentation with especial reference to the vascular system. Carnegie Contrib. to Emb., vol. II, pp. 61-90. Jounston, J. B. 1909 The morphology of the forebrain vesicle in vertebrates. Jour. Comp. Neur., vol. 19, pp. 458-537. KEIBEL, F. 1889 Zur Entwickelungsgeschichte der Chorda bei Séiugern. Ar. f. Anat. u. Physiol., Anat. Abt., S. 329-388. 1896 Studien zur Entwickelungsgeschichte des Schweines. II. Morph. Arbt., Bd. 5, S. 17-169. 1897 Normentafel z. Entwickelungsgeschichte des Schweines. Jena. KEIBEL, F., unD Euze, C. 1908 Normentafel zur Entwickelungsgeschichte des Menschen. Jena. Krause, R. 1905 Entwickelungsgeschichte des Gehérorgans. Hertwigs Hand- buch der Entwkges., Bd. II, H. 2, 8S. 82-138. Lanpacre, F. L. 1910 The origin of the cranial ganglia in Ameiurus. Jour. Comp. Neur., vol. 20, pp. 309-412. 1921 The fate of the neural crest in the head of Urodeles. Jour. Comp. Neur., vol. 33, pp. 1-44. Lenuoss&ék, M. 1891 Die Entwickelung der Ganglienanlagen bei dem men- schlichen Embryo. Ar. f. Anat. u. Phys., Anat. Abt., S. 1-25. Low, A. 1908 Description of a human embryo of 13-14 somites. Jour. Anat. and Phys., vol. 42, pp. 287-251. Matz, F. P. 1917 Cyclopia in the human embryo. Carnegie Contrib. fo Emb., vol. 6, pp. 5-33. Martin, P. 1890 Die erste Entwickelung der Kopfnerven der Katze. O6estr. Monats. Tierheilk., Bd. 15. Minot, C. S., anp Taytor, E. 1905 Normal plates of the development of the rabbit. Keibel’s Normentafel no. 5. Jena. ParkeER, G. H. 1908 The origin of the lateral eyes of vertebrates. Am. Naturalist, vol. 42, pp. 601-609. Patterson, J. T. 1909 Gastrulation in the pigeon’s egg. Jour. Morph., vol. 20, pp. 65-124. Sakxural, T. 1906 Normentafel zur Entwickelungsgeschichte des Rehes (Cervus eaprolus). Keibels Normentafel no. 6. Jena. 2ae G. W. BARTELMEZ Scuuute, H. v. W., Ano Titney, F. 1915 The development of the neuraxis in the domestic cat to the stage of twenty-one somites. Ann. N. Y. Acad. Se., vol. 24, pp. 319-346. SELENKA, E. 1892 Studien iiber die Entwicklungsgeschichte der Tiere. Wies- baden. 1900 Menschenaffen. Wiesbaden. Zweite Lieferung. 1903 Ibid., Fiinfte Lieferung. Stockarp, C. R. 1914 An experimental study on the position of the optic anlage in Amblystoma punctatum, etc. Am. Jour. Anat., vol. 15, pp. 253-289. Veit, O. 1918 Kopfganglienleiste bei einem Embryo von acht Somitenpaaren. Anat. Hefte, Bd. 56. Watuin, I. E. 1913 A human embryo of thirteen somites. Am. Jour. Anat., vol. 15, pp. 319-331. Wart, J. C. 1915 Description of two young twin embryos with 17-19 paired somites. Carnegie Contrib. to Embryol., vol. 2, pp. 15-54. Weicner, K. 1901 Bemerkungen zur Entwicklung des Ganglion acustico- faciale und des Ganglion semilunare. Anat. Anz., Bd. 19, 8. 145-155. Witson, J. T. 1914 Observations upon young human embryos. Jour. Anat. and Phys., vol. 48, pp. 315-351. Yeats, THos. 1911 Studies in the embryology of the ferret. Pt. I. Jour. Anat. Phys., vol. 45, pp. 319-335. Resumen por el autor, Davidson Black. Los nticleos motores de los nervios cerebrales en la Filogenia. Un estudio de fenodmeno de la neurobiotaxis. lV. Aves En la parte descriptiva de este trabajo el autor describe con detalle la morfologia y relaciones de los nuicleos motores de los nervios cerebrales de Cacatua roseicapilla y Ciconia alba, com- parandolos con las condiciones observadas en otras aves, asi como con las observadas en los reptiles e ictidpsidos. El plan de los nucleos motores cerebrales de las aves, si bien exhibe algunas variaciones importantes en las diferentes familias, es esencial- mente el mismo en todas las formas observadas y caracteristica- mente diferente del que se observa en los otros grupos de los vertebrados. La asociacion de los nicleos motores del V y VII nervio y la situacién de los grupos celulares faciales sobre, y mas frecuente- mente, delante del nivel de salida de su raiz motriz constituye en las aves un cardcter que se encuentra solamente entre los verte- brados en los cicléstomos. Del mismo modo la asociacién de los nticleos dorsales del glosofaringeo y vago, que es caracteristica de las aves, es un rasgo que solamente se encuentra entre los demas vertebrados en los petromizontes, mientras que en la diferenciacién del complejo intermedio motor X-XIT ocupan una posicién vinica en el tipo de los vertebrados. La posible signifi- cacion de estas particularidades en el plan de los nucleos motores del cerebro de las aves es objeto de discusion, aduciendo el autor nuevas pruebas que tienden a confirmar el concepto neurobio- tactico de la emigracién nuclear dentro del sistema nervioso central. Translation by José ¥. Nonidez Cornell Medical College, New York AUTHOR'S ABSTRACT OF THIS PAPER ISSUED BY THE BIBLIOGRAPHIC SERVICE, MARCH 20 THE MOTOR NUCLEI OF THE CEREBRAL NERVES IN PHYLOGENY. A STUDY OF THE PHENOMENA OF NEUROBIOTAXIS IV. AVES DAVIDSON BLACK Central Institute for Brain Research, Amsterdam, Holland, and the Anatomical Department, Peking Union Medical College, Peking, China SIXTEEN FIGURES CONTENTS HintRO CUE OIA creer eae ae eee ee eos A EE Peds baehedt ba aed 233 Motor roots and nuclei in Cacatua and Ciconia..............-.e.-.-eeece: 234 ING EO ORI UIE. Paine Re eS Oe a gee Te aie ow ae 5”) es 234 ERAS LEAs DOM HN TIGG UO EP ae UP ae aE A 7 i 238 INTHE WD eo aon 6 Sea hte ol Sachi eRe atc me A a ee TO 242 WIG @ WA oS ee Boel 6 Ae be Oe hee ie ce eee ne ne ee Pa 244 INGE Weg Sc ac he BS Sle etek eee ie tie Ain AOE A Sra oo De ae oN a 246 Iiermyag JUL in| UNS 5 eis Sie A, Sea ee ee ais cies ee epee wR 249 DISCUSSION OT Neate cee Se eee scar, Say tea, Mee ha 2 261 Ie eRnEM ING, CONMPLOK eet AAR hs. Sos ace a ee dacahioadrs ask caeres woo at 261 2 eeNSCECLA EM OGOLLTUG Le epee emer et aan 6 cn tn cece See slo boas 264 INGE UStACCESSOLIUSS Mee ee ey seek HRA ee Boodle soem 264 Nerves! ireeande Xx. Me ume eee is le occ da ene Ries echo ene 9265 MGtOr Vee Complexe meme Mime kes | Aue ipl we 8) ko ames 267 SE eITUSE LCR TCRROR MS pie fy he Ree oo aral Riy Tren Mg a ey ot og Saran 269 INET Vee VD SRL teens Ml Blt S SPEER TL SEAS LA eg RE ea TA 269 Nemes Jeane Passes my ee rhc ot, ene Se rere tS 269 ‘CTCLURI OT OD SAtoe re. See iat: MRR any ke a oe eS. i eee 5 ane 270 eT GUe™ CGEM Grae x eectane Wneeet oe ecy Pays ROE ee eee vehiaes oan cot 4 Eee 6. ER ERE SNS 273 INTRODUCTORY The following paper on the avian cerebral motor nuclei con- stitutes part 4 of a study on the phylogeny of these nuclei in vertebrates. Reference to my earlier communications on this subject renders unnecessary a further explanatory statement (8, 9, 10). 233 THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 2 234 DAVIDSON BLACK Cacatua roseicapilla, which is here described in comparison with other avian forms, is a member of the subfamily Cacatuinae of the family Psittacidae, suborder Psittaci (Gadow, 26; Evans, 21). Its distribution is restricted to Australia, where it has a wide range from New South Wales to the north-coast region (Salvadori, 48, p. 132). Among the members of this interesting family the fleshy tongue with its intrinsic musculature is rela- tively highly developed and the intrinsic specialization of the syrinx is also well marked. In these respects Cacatua offers a marked contrast to Ciconia alba, which has been restudied in the present connection and in which the tongue is rudimentary and the syrinx of simple and almost primitive structure (Bed- dard, 4, p. 65, et. seq.). The motor nuclei and roots have already been studied and reconstruction charts have been made in the following avian forms: Columba, Ciconia, and Chrysomitris (Kappers, 32); Ciconia (Kappers, 33 and 35); Casuaris, Spheniscus, and Colym- bus (Kappers, 34). Of these, reconstruction charts of the last three are reproduced in the present paper in figure 16, page 260. MOTOR ROOTS AND NUCLEI IN CACATUA AND CICONIA Nerve XII Among birds two nuclei are usually concerned in the origin of the fibers which combine to form the hypoglossal roots. These nuclei, which have been variously named by different authors, consist essentially of a more specialized dorsal cell group usually intimately associated with cells derived from the dorsal motor X column, and a less specialized ventral nucleus forming the rostral extremity of the cervical somatic motor column.! In the following description Kappers’ term ‘nucleus intermedius’ (33) has been applied to the former complex, in which there is usually to be distinguished a visceral portion, the pars vagi, and a somatic portion, the pars hypoglossi. For the less specialized 1On the other hand, Kosaka and Yagita (38) have concluded from their in- vestigations that no true hypoglossal fibers take their origin from the rostral extremity of the cervical motor column in the birds examined by them (Columba, Gallus, and Anas). This conclusion, however, does not appear to be correct. MOTOR NUCLEI IN PHYLOGENY 235 ventral nucleus Brandis’ term ‘nucleus ventralis hypoglossi’ (13) has been retained.? In Cacatua the nucleus intermedius XII forms a well-marked column of large multipolar cells which rests throughout its ex- tent upon the periphery of the periependymal gray matter and is at all levels quite sharply demarcated from the adjacent nu- cleus motorius dorsalis X (figs. 1 and 3). In the closed portion of the medulla it lies ventro-lateral to the central canal, there being in this form a relatively large amount of periependymal gray matter between the latter structure and the dorsum of the raphé (cf. figs. 1 and 2). In this character Cacatua, in common with other members of the parrot family, differs from Ciconia and most other birds (Brandis, l.c., p. 631). In the open por- tion of the medulla the nucleus intermedius XII occupies an analogous position beneath the floor of the fourth ventricle. Except at its rostral extremity, the area of the nucleus inter- medius XII in cross-section is considerably greater than that of the dorsal motor vagus column, while in total length the former nucleus exceeds the latter and extends caudally for some distance beyond it (fig. 16, D, p. 260). Around the nucleus and within its interstices, numerous very fine oblique and longi- tudinally arranged medullated fibers are to be seen which give to it a very characteristic stippled appearance on section. These strands constitute the fibrae propriae nuclei hypoglossi of Koch (37), to which Brandis has also drawn attention (l.c.). The root fibers arising in the nucleus intermedius XII, which in Cacatua constitutes the chief source of the hypoglossal nerve, converge for the most part towards the ventro-medial margin of this cell group. Here they become combined to form well- marked nerve strands, which then pass obliquely ventral and - lateral to reach the periphery approximately along the lateral border of the inferior olivary nucleus. As in other birds, none ® The term nucleus intermedius, pars hypoglossi (or simply nucleus inter- medius XII) is synonymous with Brandis’ (l.c.) hypoglossal portion of the common vago-hypoglossal nucleus. The nucleus ventralis XII is equivalent to the nucleus XII of Turner (50) and of Kappers (88, fig. 5) as well as to the latter author’s ‘Fortsetzung des Cervicalmarks’ (9). 236 DAVIDSON BLACK of the root fibers arising in this nucleus appear to cross the raphé to emerge with those of the contralateral nerve (cf. Brandis, |.c.). The nucleus ventralis hypoglossi is of relatively slight impor- tance in Cacatua, since in this form comparatively few fibers contributing to the hypoglossal nerve arise here. The nucleus lies within the gray reticulum of the cervical motor column, of which indeed it forms the most rostral portion. Some of the Nu.dinX. Nu.int. XT. Fig. 1 Cacatua roseicapilla. Transverse section through the medulla just caudad of-the calamus. Fig. 2 Ciconia alba. Transverse section through the medulla caudad of the calamus. This drawing is at the same magnification as figure 1. C.i., commissura infima (Bok, 11, p. 510); #.s., fasciculus solitarius; Nw.d.m. X., dorsal motor vagus nucleus; Nw. int., nucleus intermedius X—XII; Nu. int. XII., nucleus intermedius hypoglossi; Nu. XJ. v., ventral hypoglossal nucleus; O.i., inferior olive; R. desc. V., radix descendens trigemini; R. XJJ., hypoglossal rootlets. root fibers arising in this nucleus appear to cross the raphé and emerge by way of the contralateral hypoglossal nerve. Similar relations have been described in other birds by Brandis, who was able to confirm his observations by means of degeneration experiments. In contrast to Cacatua, the majority of the fibers of the hypo- glossal nerve in Ciconia arise from the nucleus ventralis XII. MOTOR NUCLEI IN PHYLOGENY 237 In the latter animal this nucleus forms a slim well marked cell column which extends rostral from the cervical motor column into the medulla for some distance above the exit level of the first hypoglossal rootlet (fig. 16 C, p. 260). Caudally the nucleus ventralis occupies a dorso-medial position within the gray reticu- lum of the cervical motor column, in which its identity becomes gradually lost. Undoubtedly some of the root fibers arising in this nucleus present a crossed relationship. As in Cacatua, the nucleus intermedius in Ciconia may be readily distinguished throughout its course from the ventrally situated cell clusters constituting the nucleus ventralis XII. In Ciconia, however, the nucleus intermedius is not a nucleus of origin for hypoglossal rootlets only, but, on the contrary, it is a cell complex in which arise both vagal and hypoglossal fibers. In this form, therefore, we have to distinguish two component cell groups in the nucleus intermedius, viz., the pars hypoglossi (nucleus intermedius XIT) and the pars vagi (nucleus intermedius X) (fig. 2).3 Though intimately associated, the courses of their root fibers enable the limits of these two nuclear components of the intermedius cell group to be distinguished with considerable accuracy, and for this reason they have been plotted as separate entities on the reconstruction chart (fig. 16 C, p. 260). In Ciconia comparatively few of the fibers composing the hypoglossal nerve have their origin in the cells of the nucleus intermedius XII and the cross-sectional area of the entire inter- medius cell complex is at all levels much less than that of the adjacent dorsal motor vagus column. In these characters Ciconia presents a marked contrast to Cacatua. On the other hand, the hypoglossal origin in Ciconia closely resembles that described by Kappers in Colymbus, Casua- ris, and Spheniscus (34). A third mode of hypoglossal origin has been described by Brandis and other investigators, who have shown that in some birds the XII nerve apparently may arise in its entirety from the rostral prolongation of the cervical motor column (nucleus 3’ The latter term is synonymous with the Brandis vagal portion of the com- mon vago-hypoglossal nucleus. : 238 DAVIDSON BLACK ventralis XII) in a manner very similar to the mode of origin of this nerve in many reptiles (fig. 15, p. 259). On the basis of the current descriptions of the central origin of the hypoglossal nerve in birds, the forms thus far investigated may be arranged in three main groups, as follows: Group I. Birds in which apparently the hypoglossal nerve arises wholly from the rostral prolongation of the cervical motor column: Lophortyx, Phasianus, Numida, Laurus, Anser, Phoenicopterus, Fu- lica (Brandis, 13).4 Group II. Birds in which the hypoglossus arises from both the nucleus intermedius XIJI and the nucleus ventralis XII: Anas, Gallus, Columba (Kosaka and Yagita, 38;> Koch, 37; Brandis, 13); Casuaris, Spheniscus, Colymbus (Kappers, 34); Ciconia; Grus, Machetes, Falco, Struthio, Cairina, Corvus, and Passeres without exception, Cypselus (Brandis, 1.c.); Gallus embryos (Bok, 11); Columba (Koch, 37; Turner, 50).° Group III. Birds in which by far the greater part (or all?) of the hypoglossal nerve arises from the nucleus intermedius XII: The par- rot family, among which two cockatoos (Cacatua roseicapilla and C. galatea) and two parakeets (Melopsittacus and Palaeornis) have been investigated (Brandis, I. c.). Nerves IX, X, and XI In Cacatua, as in all other birds examined, the dorsal motor vagus nucleus is continuous rostrally with that of the glosso- pharyngeus. These two nuclei together form the posterior visceral motor column, which in this form is relatively short. Though it extends rostral slightly above the exit level of the motor IX root, it is not prolonged far as a continuous cell column within the closed portion of the medulla and its caudal end falls some distance short of that of the nucleus intermedius XII (fig. 16 D260). 4In the latter two, the flamingo and the coot, Brandis noted that the hypo- glossal roots were small and the central nuclei poorly developed; and the origin of some of the nerve fibers from scattered cells representing a dorsal XII nucleus could not be excluded. 5 As already noted, these investigators did not consider that any true hypo- glossal fibers had their origin in the cell column described above as the ventral XII nucleus (l.c., p. 167). 6 Turner, however, identified the nucleus intermedius XII as the spinal acces- sory nucleus. MOTOR NUCLEI IN PHYLOGENY 239 From their nucleus of origin in the rostral end of the posterior visceral motor column the motor IX rootlets pass almost directly laterad and reach the periphery for the most part dorsal to the radix descendens trigemini (fig. 4). The dorsal motor vagus nucleus forms a somewhat slim cell column which tapers towards its caudal end. In the latter situa- tion the nuclei of either side lie dorso-lateral to the central canal and dorsal to the nucleus intermedius, being separated from one another by but a slight interval (fig. 1). Above the calamus these nuclei diverge from one another and lie within the gray matter of the ventricular floor as indicated in figure 3. The most caudal motor X rootlets pass out almost directly dorsal, intermediate members of the series course dorso-lateral to reach the periphery, while the rootlets arising towards the rostral end of the nucleus emerge as do those of the motor IX nerve. In Cacatua roseicapilla I have been unable to observe the origin of any undoubtedly motor vagus fibers from the cell column of the nucleus intermedius. In other words, no true nucleus intermedius X has been identified in this bird, though in the closely related form, C. galatea, Brandis (13) has described the origin of a few motor fibers from this source. In connection with this observation, however, the latter author noted that in parrots this cell group (which he termed the common X~—XII nucleus) was almost wholly concerned in the supply of the hypoglossal nerve. A small but evident ventro-lateral motor X nucleus is present in Cacatua. As in other birds this nucleus is not sharply cir- cumscribed, and its loosely arranged groups of multipolar cells occupy a position lateral and somewhat ventral to the large nucleus intermedius XII. Kappers (l.c.) has already drawn attention to the presence of this nucleus in other avian forms (Chrysomitris, Casuaris, Spheniscus, Colymbus), in which he has definitely established its motor vagus character. Brandis also had earlier described this nucleus (12, pp. 182-3) and had figured it in the guinea-fowl (l.c., Taf. XIII, Fig. 5, Numida), but he failed to recognize its significance as a source for motor 240 DAVIDSON BLACK X fibers. The nucleus in question is indicated on the reconstruc- tion chart, but in view of the scattered arrangement of its ele- ments it was not possible to mark definitely its rostral and caudal limits (fig. 16 D, p. 260). In Ciconia the arrangement of the nuclei which go to make up the posterior visceral motor column is apparently more compli- cated than that observed in Cacatua. ‘This is due to the presence R.desc.V. Fig.3 Cacatua roseicapilla. Transverse section through the medulla a short distance rostrad of the calamus. Fig. 4 Cacatua roseicapilla. Transverse section through the medulla near the rostral end of the caudal visceral motor column. Figures 3 and 4 at same magnification as figure 1. F.s., fasciculus solitarius; Nu. d.m. X., dorsal motor vagus nucleus; Nu. int. XJI., nucleus intermedius hypoglossi; Nu. mag. Vill.c., nucleus magnocellularis of the cochlear nerve (Brandis, 14; Holmes, 29); Nw. 1X m., motor glossopharyn- geus nucleus; O.7., inferior olive; R. desc. V., radix descendens trigemini; FR. VIII. c., cochlear root fibers; R. [X., glossopharyngeal root fibers; R. XJJ., hypoglossal rootlets; Vend., fourth ventricle. MOTOR NUCLEI IN PHYLOGENY 241 of the nucleus intermedius X in relations that may be described as typically avian, since with but slight variation they have been found to obtain in the majority of the birds so far examined (fee, 16 pA, ByoG;,and:|D gps i260), As in Cacatua, the most rostral portion of the dorsal part of the posterior visceral motor column in Ciconia is occupied by the motor IX nucleus. The relations of this nucleus and the mode of origin of its emergent roots in the latter form differ but little from those already described in Cacatua and require no further description here. The dorsal motor vagus nucleus in Ciconia is much larger than in Cacatua and forms a conspicuous cell column which in the closed portion of the medulla occupies a position lateral and dorsal to the central canal within the periependymal gray and dorsal to the nucleus intermedius X and XII (fig. 2). At the junction of the medulla and cord and below the caudal end of the nucleus intermedius X, the dorsal visceral motor cell column becomes reduced in bulk and its elements become arranged in irregularly spaced cell clusters. In the same relative position the cell column, though discon- tinuous in places, was traced caudad a considerable distance. The origin of root fibers from the most caudal part of this nucleus was not demonstrated. The lower limit of the nucleus could not be definitely determined (fig. 16 C, p. 260). The more caudal of the fine rootlets which arise from this cell column pass almost directly dorsal to reach the periphery, and in this respect resemble emergent XI nerve roots (ef. Lubosch, 41). The topography of the roots and nucleus of the latter nerve was not satisfactorily determined in Cacatua nor in Cico- nia, but in neither of these forms does it appear that the dorsal motor vagus nucleus becomes continuous with the accessory nucleus in a manner similar to that observed in reptiles (10), amphibians (9), and selachians (8). In Ciconia the nucleus intermedius X begins a short distance caudad of the exit level of the motor IX root as a ventral enlarge- ment of the dorsal motor X column. From the outset, however, the morphology of its cells serves to differentiate it from the latter 242 DAVIDSON BLACK nucleus. Over the greater portion of its extent it is intimately associated with the cells of the nucleus intermedius XII, the two nuclei forming a complex (fig. 2), the limits of whose com- ponent parts may, however, be distinguished by the course and distribution of their respective root fibers. In contrast to Ca- catua, the cross-sectional area of the nucleus intermedius complex (combined X—XII components) in Ciconia is at all levels less than that of the adjacent dorsal motor vagus nucleus. The nucleus intermedius X is somewhat more extensive than the nucleus intermedius XII, so that it overlaps the latter nucleus both rostrally and caudally. In transverse sections of the brain stem at these levels the nucleus intermedius is formed wholly of motor vagus cells. A ventro-lateral motor vagus nucleus may be clearly distin- guished in Ciconia occupying a position analogous to that of the similarly named nucleus in Cacatua. As in the latter form, its rostral and caudal limits could not be accurately defined, though the position of its chief cell mass is indicated in the reconstruc- tion chart, figure 16 C, page 260. It is evident that the same grouping of avian forms will result from an arrangement on the basis of the vagal connections of the nucleus intermedius as on the basis of the central origin of the hypoglossal nerve. Thus, group I, as noted above, contains those forms in which the nucleus intermedius is largely vagal; group II is composed of forms in which the vagal and hypoglossal components of the nucleus intermedius are both quite evident, and group III consists of those forms in which the nucleus inter- medius is to a very large extent a hypoglossal nucleus. Nerve VII The motor VII nerve in Cacatua has its origin within the brain stem from two distinct cell groups, which from their relative positions are termed, respectively, the dorsal and the ventral motor facial nuclei.7 7 These nuclei were first charted and fully described by Kappers (82), though this author noted at the time (l.c., p. 69) that two such nuclei had been observed MOTOR NUCLEI IN PHYLOGENY 243 The dorsal motor VII nucleus occupies a central position within the formatio reticularis, and its entire bulk les rostrad of the exit level of the motor VII root. At its rostral end this nucleus becomes continuous with a cell group which gives ori- gin to a part of the motor V root, so that the cell column may be considered as a motor V—VII nuclear complex. Thus relations obtain here that are essentially similar to those of the V—VII motor nucleus in Chrysomitris which have been described by Kappers (32). At its caudal end this nucleus is somewhat en- larged and is prolonged ventrad to a level below that of the upper margin of the laterally situated nucleus olivaris superior. The ventral motor VII nucleus in Cacatua is much smaller than the dorsal cell complex and lies very close to the ventro- lateral periphery of the medulla just medial to the nucleus oli- varis superior (fig. 6). The sagittal relations of the two motor VII nuclei are indicated in the reconstruction chart, figure 16 D, page 260. From their nuclei of origin the path of the emergent motor VII fibers to the periphery is indirect. From both dorsal and ventral cell groups these radicles pass dorso-medial and become collected in the dorsal tegmentum above and to the lateral side of the abducens nucleus, where their direction changes caudad and finally laterad to emerge on the lateral surface of the medulla for the most part dorsal to the radix descendens V, though numbers of the emergent VII fibers pierce the latter structure before making their exit (figs. 5 and 6). The origin and relations of the motor VII nerve in Ciconia have already been described and figured by Kappers (382, figs. 46 and 69; 34, fig. 69), and in most essentials they agree with those in Cacatua. The dorsal motor VII cell group is, however, smaller in Ciconia than in Cacatua, and its cells do not become intermingled rostrally with motor V elements; while the ventral by Wallenberg and had also been recorded by Kosaka and Hiraiwa (39) in Gallus. Indeed, the latter authors have described three facial nuclei in this bird: a chief or ventral nucleus, a dorsal or digastric nucleus, and a cell group associated with the latter and termed the ‘Nebenkern.’ It is highly probable that this ‘Neben- ‘kern’ is represented in Cacatua by the caudo-ventral extension of the dorsal motor VII nucleus of this form (vide infra). 244. DAVIDSON BLACK VII motor nucleus is larger in Ciconia and occupies a more caudal position than in Cacatua (cf. fig. 16 C and D, p. 260). With the exception of Casuaris, in all the avian forms in which these relations have been carefully established (Colomba, Chry- somitris, Ciconia, Spheniscus, Colymbus, Cacatua) the motor VII nuclei are situated rostral to the exit level of the motor Vil root. This relation, as Kappers has repeatedly noted, serves to distinguish these forms from all other vertebrates. In Casu- aris, on the other hand, the motor nucleus lies on the level of its root exit in a relation somewhat similar to that obtaining in Rana catesbiana and Petromyzon (figs. 13,14, 15, and 16). Brandis (14) has recognized but one motor VII nucleus in each of the forms which he described, though Kappers’ (82) subsequent careful investigations have shown that, at least in the case of Columba, Brandis’ observations were in error. How- ever, this does not serve to discount wholly the latter author’s observations on the motor facial nucleus in other avian forms, since it is quite probable that not all birds possess two motor VII nuclei (e.g., Casuaris). Nerve VI The abducens nucleus in Cacatua forms a well-marked cell column of large multipolar cells which occupy a dorsal position in the formatio reticularis, to the lateral side of the fasciculus longitudinalis medialis (figs. 5 and 6). The nucleus lies almost wholly rostrad of the exit level of the motor VII root, and ex- tends from this level approximately to that of the caudal border of the motor V root (fig. 16 D, p. 260). The abducens roots are five in number and emerge in series on the periphery rostrad of the exit level of the motor VIT root. In Ciconia the abducens nucleus occupies a somewhat more caudal position than in Cacatua and is considerably longer in rostro-caudal extent, though its position within the formatio reticularis in the two forms is similar. Eleven small emergent abducens rootlets were identified in Ciconia, all of which emerge in series rostrad of the exit level of the motor VII root. MOTOR NUCLEI IN PHYLOGENY 245 ae ay S. ey = ZP \ Nu NT. mv \Nu. Vim. d. 6 Fig. 5 Cacatua roseicapilla. Transverse section through the medulla at the exit level of the motor facial root. R. VI, fifth emergent abducens rootlet. Fig. 6 Cacatua roseicapilla. Transverse section through the medulla at the exit level of the third abducens rootlet (R. VJ.). Figures 5 and 6 at the same magnification as figure 1. F.s., fasciculus solitarius; Nu. parv. VIII. v., small-celled vestibular nucleus (Brandis, 14); Nu. VZ., abducens nucleus; Nu. VII. m.d., dorsal motor facial nucleus (= caudal part of motor V-VII nuclear complex); Nu. VII. m.v., ventral motor facial nucleus; NV. VI//. v., vestibular root; O.s., superior olive; R. desc. V., radix descendens trigemini; R. V/T., root fibers of the facial nerve; R. VII. m., intramedullary motor facial fibers; Vent., fourth ventricle. 246 DAVIDSON BLACK In Cacatua, Ciconia, Colymbus, and Columba the rootlets of the abducens nerve make their exit from the brain stem rostrad of the exit level of the motor VII root in a manner similar to that obtaining among reptiles in Boa and Varanus. On the other hand, in Spheniscus, Chrysomitris, and Casuaris, some of the abducens rootlets emerge caudal to the lower border of the emergent motor VII root and resemble in this respect the mode of exit of the abducens rootlets in Alligator (figs. 15 and 16, pp. 259, 260). Nerve V In Cacatua the motor V nerve takes its origin within the brain stem from three quite distinct cell groups which are here distinguished as the dorsal motor V nucleus, the combined V-VII motor nucleus and the ventral or chief motor V nucleus. The dorsal motor V nucleus forms a small circumscribed group of large multipolar cells situated beneath the gray matter of the ventricular floor and lateral to the fasciculus longitudi- nalis medialis. It is rostral of the abducens nucleus and is more dorso-laterally placed than the latter (cf. figs. 6 and 7). The radicular fibers arising from this cell group pass laterad and then ventro-laterad to emerge at the periphery, ventral to the enter- ing sensory trigeminal root. From the rostral end of the cell column whose relations as the dorsal VII motor nucleus have already been described, a num- ber of radicular fibers emerge which take a characteristic indi- rect course dorso-medial through the tegmentum to join those arising from the dorsal motor V nucleus and to reach the periphery in common with the latter. The nucleus from which these fibers arise constitutes the combined V—VII cell group of the trigeminal motor nuclear complex, or the rostral continuation of the dorsal motor VII cell group of the facial motor complex. The major part of the motor root of the trigeminal nerve in Cacatua arises in a large nucleus which is situated in the ven- tro-lateral area of the tegmentum, ventral to the emergent fibers arising in the dorsal motor V nucleus. From the ventral nucleus the emergent V root fibers pass directly ventro-lateral MOTOR NUCLEI IN PHYLOGENY 247 and emerge on the periphery ventral to the entering sensory root of this nerve (fig. 7). At its dorso-medial angle the chief or ventral motor V nucleus comes into contact with the rostro- ventral border of the combined V—VII motor cell column (fig. LGD ps 260): Fig. 7 Cacatua roseicapilla. Transverse section through the brain stem at the exit level of the motor trigeminal root and first abducens rootlet. Same magnification as figure 1. An absence of sharp radicular differentiation rendered obscure the relations of the mesencephalic V root at its exit level in Cacatua, a condition which van Valkenburg also found to obtain in both Ciconia and Chryso- mitris (52 and 54). JL.l., secondary cochlear tract; Lob. op., optic lobe; Nw. U.L., nucleus of ascending cochlear tract; Nu. V. m.d., dorsal motor V nucleus; Nw. V. m.v., ventral or chief motor trigeminal nucleus; Nu. V.s., sensory V nucleus with rostral end of the descending trigeminal root; Nu. V.s.p., chief sensory trigeminal nucleus (cf. van Valkenburg, 53, figs. 7 and 8, Taf. XIX-XX, Ciconia) ; R.V.desc., descending V sensory fibers; R.V.m., motor trigeminal root., R.V.m.d., dorsal motor trigeminal rootlets; R.V.s., entering sensory trigeminal fibers; R,VI., first emergent abducens rootlet; Tr. b.f., bulbo-tectal tract; Vent., ven- tricular cavity. The origin of the motor V root in Ciconia differs from that in Cacatua chiefly in the absence of fibers corresponding to those arising in the combined V—VII motor nucleus of the latter form. Motor trigeminal fibers were identified arising in a small 248 DAVIDSON BLACK dorsal motor V nucleus in Ciconia, though in this animal by far the larger number arise in the chief or ventral motor V nu- cleus. The dorsal motor V nucleus has been observed in Ciconia by Kappers (82, 34), but at the time this investigator could not satisfy himself of its indubitable motor trigeminal character, and for this reason the nucleus in question was plotted in dotted lines on his original reconstruction charts. An intermingling of motor V and VII nuclear elements has been observed in other avian forms by Kappers (Lc.). This author also noted that in some forms (e.g., Spheniscus, Chryso- mitris, Colymbus) the ventral motor V nucleus was in reality a compound of two cell groups, the smaller lying medial to the larger. Further, it is probable that the V motor component of the combined V—VII motor column, when present, corresponds to the more medial of these motor trigeminal cell groups. The origin of the motor V nerve in a number of other avian forms has been described in detail by Brandis (16). This author also distinguished three motor V cell groups which he termed the outer (ventral), the middle (which is continuous with the motor VII nucleus), and the inner (dorsal) motor V nuclei.’ In Cacatua the three motor trigeminal nuclei are situated approximately on the same level as their emergent root. This is true also to a large extent in Columba, Chrysomitris, and Colym- bus. In Ciconia, on the other hand, a considerable portion of the motor V nuclear complex lies caudal of the motor V root exit level, while in Spheniscus the reverse condition obtains and the motor V nucleus is placed largely rostrad the level of its root exit. In all these forms the major portion of the motor trigeminal nucleus les in a relatively ventral position within the tegmentum. In the latter respect the birds thus far examined differ markedly from Casuaris, in which the motor V nucleus occupies for the most part a dorsal position such as characterizes §In Turner’s description (50), however, but two motor trigeminal nuclei were distinguished, viz., the lateral motor V nucleus corresponding to the chief or ventral motor V nucleus of the present description, and the deep motor V nucleus which would appear to be equivalent to the dorsal motor V nucleus described above. MOTOR NUCLEI IN PHYLOGENY 249 this cell group among reptiles in chelonians and in Alligator (figs. 15 and 16, pp. 259, 260). Nerves III and IV The oculomotor nerve in Cacatua arises in a highly developed cell complex which, as in all other birds examined, is in close contiguity with the trochlear nucleus. Among birds the cell groups composing these associated nuclei are demarcated more clearly and precisely than is usual in any of the mammalian or- ders. This is largely due to the relatively slight development of small association elements in this region among birds. The trochlear nucleus occupies a dorsal position, lying within the periependymal gray upon the dorsal surface of the fasciculus longitudinalis medialis (fig. 8). In its rostral portion the trochlear nucleus is accommodated within a distinct groove or excavation upon the dorsal surface of the latter fiber tract and a caudal extension of the pars dorsalis of the median oculomotor cell group comes to lie between the nucleus and the median sulcus of the ventricular floor (fig. 11, h and 7). The trochlear root fibers descend within the nucleus and towards the caudal end of the latter become collected into bundles which then emerge from its dorso-lateral margin. The radicu- lar bundles thus formed pass dorso-lateral within the ventricu- lar gray (fig. 8) to reach the superior medullary velum, in which the trochlear decussation occurs and from the lateral side of which the crossed trochlear roots emerge. Within the oculomotor nuclear complex in Cacatua three well-marked subsidiary nuclei are to be distinguished which, in conformity with Kappers’ descriptions (l.c.), are here termed the median, the dorso-lateral, and the accessory cell group, re- spectively. The general arrangement of these cell groups is illustrated in figure 9, where it will be seen that the median cell group is largest in cross-sectional area and is evidently divisible into dorsal and ventral portions. The largest elements within the oculomotor nuclear complex are found in the dorso-lateral — cell group, while the smallest comprise the collection termed the 250 DAVIDSON BLACK accessory cell group. Further, it will be evident that the tro- chlear nucleus at a more caudal level occupies a position analogous to that of the more rostrally situated dorso-lateral oculomotor cell group (fig. 9, f, g, and h). The practical absence of cells comparable to the numerous reticular elements characterizing this region in many mammals is a notable feature, though a well- 3¢ Ure, - . ry hs, Lene ‘ Bote Fig. 8 Cacatua roseicapilla. Transverse section through the brain stem at the level of the junction of the middle and caudal thirds of the trochlear nucleus. Same magnification as figure 1. Ag. c., aqueductus cerebri; Coll., colliculus (= lateral mesencephalic ganglion of Wallenberg, 56); L.l., secondary cochlear tract (Wallenberg, 58); Lob. op., optic lobe; Nu. IV., trochlear nucleus; R. IV., homolateral trochlear root; R. mes. V., mesencephalic trigeminus root (van Valkenburg, 52 and 54; Miinzer u. Weiner, 438, Taf. VII); Tr. b.t., tractus bulbo- tectalis; Vent. op., optocoele. 9 There can be no doubt that, both in Cacatua and Ciconia, oculomotor fibers take their origin from the cells of this nucleus. The origin of such fibers from this nucleus in birds was first noted by Brandis (16), who termed the cell group in question the Edinger-Westphal nucleus. Subsequently these observations have been independently confirmed by Cajal and Kappers and more recently by Brouwer (17). Kappers’ term accessory nucleus has been retained for con- venience in the present description. MOTOR NUCLEI IN PHYLOGENY 251 marked ‘reticular’ nucleus occurs within the ventricular gray below the level of the trochlear nucleus (fig. 9, k). The sagittal relations of these cell groups are shown in detail in the recon- struction chart (fig. 12 F, p. 254). The oculomotor root fibers in Cacatua become collected on the ventral aspect of the nucleus (fig. 10) and then course wee Nudl 2 ees om Fig. 9 Cacatua roseicapilla. Sketches a to k to illustrate the cellular detail at different representative levels from the rostral to the caudal end of the oculo- motor-trochlear nuclear column (cf. reconstruction chart F, fig. 12, p. 254). Nu. ac., accessory oculomotor cell group; Nu. d.l., dorso-lateral oculomotor cell group; Nu. m.d., dorsal part of the medial oculomotor cell group; Nu., m.v., ventral part of the medial oculomotor cell group; Nu. r., large cells on medial periphery of red nucleus; Nu., ret., spindle-celled reticular cell column beginning caudal to the trochlear nucleus; Nu. IV., trochlear nucleus; R. IV., homolateral trochlear rootlets. obliquely rostrad and ventrad to emerge on the periphery as indi- cated in figure 11 (see also fig. 16 D, p. 260). Some of the fibers arising in the ventral part of the medial cell group undoubtedly decussate, as both Brandis and Kappers have observed to be the case in other avian forms. The arrangement and differentiation of the oculomotor and trochlear nuclear elements in Ciconia is in general similar to that 252 DAVIDSON BLACK obtaining in Cacatua and has already been described by Kap- pers in his earlier communications (Kappers, 34, figs. 73 and 76). The sagittal topography of this nuclear complex in Ciconia js Fig. 10 Cacatua roseicapilla. Transverse section through the brain stem at the level of the junction of the rostral and middle thirds of the oculomotor nucleus. Same magnification as figure 1. Aq. c., aquaeductus cerebri; C. post., posterior commissure; Lob. op., optic lobe; Nw. d.l.,-dorso-lateral oculomotor cell group; Nu. m.d., dorsal part of the medial, oculomotor cell group; Nu. m.v., ventral part of the medial oculomotor cell group; R.JII., obliquely cut radicular fibers of the oculomotor nerve; Vent. op., optocoele. illustrated in the reconstruction chart, figure 12 E, page 254 (see also fig. 16 C, p. 260). For accurate comparison of the details of the oculomotor and trochlear nuclear complex, reconstruction charts of these nuclei in Cacatua and Ciconia have been prepared on the same scale as Kappers’ earlier charts of these structures in Alligator, MOTOR NUCLEI IN PHYLOGENY 253 Varanus, Colymbus, and Spheniscus. The latter are here redrawn in figure 12 as mirror images of Kappers’ earlier figures (34, figs. 60, 61, 77, and 78, respectively). The change from the original orientation has been made so that in these figures as in the larger Fig. 11 Cacatua roseicapilla. Transverse section through the midbrain and thalamus at the exit level of the oculomotor roots. Same magnification as figure 1. G.72., interpeduncular ganglion; Lob. op., optie lobe; R.JIJ., emergent oculomotor root; Tr. b. th.s., tractus bulbo-thalamicus (Wallenberg, 59) et bulbo- striaticus (Wallenberg, 57 and 59); Vent. op., optocoele. charts the rostral end of the reconstruction will be toward the left. To facilitate comparison, the charts in figure 12 are arranged so that the rostral end of the trochlear nucleus in all cases lies in the same vertical plane. pees | em A En SSS ee ———_—_ ——<—<——— REDD ASS SSS SSS S32 S SSeS Sew SESS 27. SSeS SSS ee SSeaSee2eS2eaneo8. = Medial oculomotor cell group. HHH Dorso-lateral oculomotor cell group. ieee Nucleus accessorius II. ecm III Trochlear nucleus 254 MOTOR NUCLEI IN PHYLOGENY 255 It would appear from an examination of figure 12 that birds differ from the reptiles chiefly in the possession by the former of an acessory cell group of the oculomotor nucleus. This, how- ever, is not entirely true, for Kappers (34, pp. 63-64) has found in one specimen of Varanus sp.? a cell group which in the mor- phology of its elements and in its general relations resembles closely the avian accessory oculomotor nucleus. No oculomotor fibers could be distinguished arising from this unique accessory cell group in Varanus sp.? and no similar cell group was ob- served in any other Varanus specimens nor in any other reptile examined. Oculomotor nuclear differentiation in birds presents a marked advance over the condition obtaining in reptiles, especially in the differentiation of the median oculomotor cell group, since in none of the latter forms is a fully developed pars dorsalis of the median nucleus present. From the data at hand it would seem that among birds all three divisions of the oculomotor nucleus are well differentiated and that contiguity of the trochlear nucleus with the dorso- lateral oculomotor cell group is a constant character. On the other hand, there would seem to be a considerable amount of variation characterizing the relations of the associated oculo- motor and trochlear nuclei to the exit levels of the III and motor V roots, respectively. Fig. 12 Reconstruction charts of the oculomotor and trochlear nuclei in sauropsidan forms. A, Alligator sclerops (after Kappers, 34, fig. 60); B, Varanus salvator (after Kappers, 34, fig. 61); C, Spheniscus demersus (after Kappers, 9, fig. 77); D, Colymbus septentrionalis (after Kappers, 34, fig. 78); E, Ciconia alba; F, Cacatua roseicapilla. The reconstructions A, B, C, and D have been redrawn as mirror images of Kappers’ original charts, so that in the present paper both in these and in the other reconstructions the rostral end of the chart is toward the left side of the page. For comparison the drawings in this figure have been arranged so that the rostral ends of the trochlear nuclei are in line one below the other. See diagram above for explanation of signs. It is to be noted that no line of demarcation between the dorsal and ventral portions of the medial oculo- motor nucleus is indicated on these reconstructions. The ventral part of the median nucleus in the charts lies below the lower border of the dorso-lateral column, while the dorsal portion is above this line (cf. fig. 9, p. 25). DAVIDSON BLACK 256 “SUBIS JO UOIZBULIAXO OJ DAOGE WRISVIP 990Q "“SNUIL[VO OY} JO O4IS OY} SOPLOIPUT MOLIV OYJ, °8}9]}001 [eqIdr1900-ourds ‘990 ‘ds puv ‘2 ‘A {49]9001 snsvA Iojow 4ysIy “Y ‘yoo peosuArvydossojs 10JouI fy] + S}OOI SuVON pV "7A = (‘| ‘S10 F] Jap wea) sXky}qorowrvpeg pus snioydAjog ‘suvtsXi10ydosso1o oy} Ul 4VY} YIM [VOTJUEpI ySOUT]w SI SsnpozZe19000N Ul Us1e}yed Ivajonu IOJOW IY} ‘UIIOJ SIYY UI SNojONU Avo]YooI4 BY} Jo UoTyISod [vpned ayy Joy ydooxy “(TG “4SIOFY Jop WA Jo}Jv) 1104810} sNpoy -e1a000N ‘qi (8) Bynyyeds uopAjog ‘OD ‘(g) Bvurxvu oyoReg ‘gq ‘(PE ‘stoddey J0zjv) stpyerany uozAworyog Vy ‘OT pu GL ‘FT SounSy yyIA uostrvduo0d Joy suvprisdoAyyYyo! JOMO] UT TepoNU pUB SJOOI IOJOUI OY} JO SJIvYO UOTJONAJSMOIEY ey ‘SLT Snajonu s4a}ieq Brees snupl pawesed IN eee Snajonu PuP O04 4ofou fPIOe4 Joldadns SiUBAIO “Np a snajonu ueondPY () ae ee SS snajonu pur {00d AO POU je} 1d1990-ou dS Sees Snajonu pure }004 40,0W SNUIWNS ty (eeees Snajanu 40,oW snBe/, PZ SN2jONU PUB }oo4 4e9{ 4904 | maa Shajonu pue joo4 AO }OWO|NIC) es Snajonu 40} 01 sna Sud4eydosso| 9) 257 MOTOR NUCLEI I N PHYLOGENY ueoudi(y plouBy) ©) ueiyse|ag | 4q 3W0}SO|2AD V DAVIDSON BLACK 258 *( P € ‘ stoddeyy 194JB) STAvVS]NA UOJ], ‘ Vv "eT OINSY Ul SB SUOTPVIADIGGe pUv SUBIS 10YIQ ‘syoor [essojsodAy ‘77X “(FE ‘stoddey 10qjv) sepxur ouopeyH ‘q ‘(OT) PSsntiayqns viuowed ‘H :(6) BURIqse}eo ‘ suey ‘gq ‘LOPONU PUB SJOOI LOJFOU JO SJAVYO WOTpONASUOIOY FT “Sy 259 MOTOR NUCLEI IN PHYLOGENY "eT OINSY UI SV SUOIPVIADIQG’ pUv SUSIQ ‘sI[vaySN¥ stIUNseD ‘q {sdorepos 10}VSIITy ‘QO {104RBATeS SnuBIBA “gq ‘1OJOIIJSUOD Bog “VY *(Fg) Sdoddvy 1o}Jv lo[ONU PUB S}OOI IOJOUI Jo SJAvYD UOTJONASUODOY CT “BI DAVIDSON BLACK 260 "eT o1n3y Ul sv SUOT}RIADIQgE pu sUsIg *eTIdvoresor BNywoRD ‘q ‘¥Bq]e BIUODID ‘OH (FE ‘stoddvy J04}e) sI;eUOTIzU0}doS snquiAjoy ‘gq ‘(pe ‘suoddey s10qye) snsiowep snostueydg ‘y ‘Tayonu pu S}JOOI 10JOW JO SyIBYO UOTJONAYSUOIOY QT “SIT ik xX (ff LAA mI ATL EE SOR) | (]LLLAAU) : SITE Te rerrrT TPIT IIITTALTELLA POT EE. RQ ' : MOTOR NUCLEI IN PHYLOGENY 261 DISCUSSION 1. Intermedius complex Shortly after the publication of Brandis’ researches (l.c.), Firbringer (24, p. 504) commented on the probable existence of a correlation between the degree of differentiation of the dorsal XII nucleus in birds and that of their syringeal muscular appara- tus. Kappers subsequently has pointed out on numerous oc- casions that the central association of vagal and hypoglossal elements within the avian nucleus intermedius was highly sig- nificant in view of the fact that the musculature of the syrinx peculiar to birds is innervated by XII fibers, while that of the simple larynx in these forms derives its supply from the vagus nerve. The evidence collected by this author (see especially 34), while not conclusive, is strongly in favor of considering the nucleus intermedius X as the motor laryngeal center, while doubt can no longer remain as to the innervation of the syrinx musculature largely if not exclusively from hypoglossal neurones of the intermedius complex. In the simply organized laryngeal apparatus in birds but two intrinsic muscles, an apertor and a sphincter, are present, while in the syrinx one or more pairs of proper vocal syringeal muscles, in addition to the paired extrinsic m. tracheo-clavicu- laris or m. sterno-trachealis, are present in the vast majority of birds. These facts for the most part accord well with the ob- servations noted above that in the majority of birds examined a definite X—XII intermedius complex is present. | There are, however, certain birds in which the syrinx is devoid of intrinsic musculature. All members of the family Ciconiidae lack true vocal syringeal muscles. They are absent also in a few members of the group Gallinae, as well as in Struthio, Casua- ris, and a number of other forms. It is probable that in these birds the absence of intrinsic syringeal musculature is to be considered due to the loss of muscular elements originally rep- resented rather than to the survival of a primitive character (cf. Newton and Gadow, 44; Beddard, 4). Such a conclusion would receive support from the fact that in both Ciconia and in 262 DAVIDSON BLACK Casuaris (34) a quite well-marked X—XII intermedius complex has been observed, while in Struthio according to Brandis (13, pp. 630 and 644), though both vagal and hypoglossal parts of the intermedius complex are small, yet there is no doubt as to their presence. There remains to be considered in this connection those forms in which the nucleus intermedius appears to be largely (if not entirely) composed of elements of but one type, either vagus or hypoglossal, and which fall within groups I or III as defined above on pages 238 and 242. Unfortunately, the motor nuclei and roots in none of the birds constituting group I have been charted, and the only avail- able information on the central relations of the motor X and XII roots and nuclei is that furnished in Brandis’ description (l.c.). The group in question constitutes a quite heterogeneous collection of seven forms belonging to no less than five different orders (Evans’ classification). Further, with the exception of Phasianus and Numida, on the plan of whose syringeal organiza- tion I have been unable to obtain details, each of the remaining five members of the group is equipped with a well-developed pair of intrinsic syringeal muscles. In this respect, therefore, these birds are as specialized as many of the forms included in group II. It may appear that an origin of XII roots largely if not en- tirely from the slightly differentiated rostral part of the cervi- cal motor column might well be a primitive character in birds. On the other hand, the very heterogeneity of the group in which this character has been described, as well as the close relation- ship evidently existing between its members and those of group II, argues strongly against the fundamental importance of this feature. It may be concluded, therefore, that, even should further investigations confirm Brandis’ observations, the ab- sence of a hypoglossal component in the nucleus interniedius complex of these forms should be considered rather as a speciali- zation away from the type characteristic of group II than as the retention of a primitive character. The few forms comprising group III, however, all belong to a small quite sharply defined and specialized natural group. MOTOR NUCLEI IN PHYLOGENY 263 In Cacatua roseicapilla the rostral extremity of the nucleus intermedius XII is practically on a level with that of the dorsal motor vagus nucleus, a relation similar to that obtaining between the motor vagus and hypoglossal nuclei in many mammals. Unlike the latter forms, the caudal extremity of the nucleus intermedius XII in C. roseicapilla extends relatively far below that of the dorsal vagus column. Cacatua differs from the other birds examined in these respects, as well as in the greater relative length of its nucleus intermedius XII and in the excep- tionally rostral level at which the first hypoglossal rootlet makes its exit (ef. figs. 15 D and 16). So far as can be gathered from Brandis’ descriptions and figures, the relations of the nucleus intermedius XII in C. gala- tea, Melopsitticus, and Palaeornis are very similar to those in C. roseicapilla. It would seem, therefore, that in parrots the specialization of a central hypoglossal intermedius cell group has progressed under influences which differ in some important fashion from those operating to produce what may be termed the typical avian intermedius cell complex. It has already been noted that the cell group here termed the typical avian intermedius complex has probably been evolved largely as the central expression of a peripheral specialization in the sound-producing apparatus peculiar to birds wherein a special- ized somatic syringeal musculature works synergically: and coordinately with a simple visceral laryngeal musculature. If this be so, it will be of interest to inquire if any peripheral special- izations of the syrinx or other organ obtain among parrots which might account for the presence in these forms of an intermedius XII nuclear development unique among birds.!! 10 Bath (2 and 3) has shown that taste buds do not occur on the horny tongue epithelium of birds, though they are found in small numbers in the mouth region in the majority of these forms, chiefly near the entrance to the pharynx and glottis and in the mucosa of the buccal floor and margins of the jaws and to a lesser extent on the palate. In mammals Kappers (9) has pointed out that the distri- bution of taste buds innervated by VII and IX fibers over the surface of the tongue is to be correlated with the rostral and dorsal position of the hypoglossal nucleus close to the taste center of these forms. In parrots, however, though the total number of taste buds (i.e., 300 to 400) is much greater than in any other avian form, it is evident, in view of their location elsewhere than in the lingual mucosa, that incoming impulses from this source can play no such réle in deter- mining the location of the XIT nucleus. 264 DAVIDSON BLACK The intrinsic muscles of the syrinx among parrots, though well developed, are not so numerous (but three pairs) nor so highly differentiated as they are, for example, in Corvus (49) and in most Oscine birds (4), where either five or seven pairs of syrin- geal muscles are usually present." On the other hand, Mudge (42), Kallus (Melopsittacus, 31), and others have shown that in contrast to all other birds the chief bulk of the tongue in parrots consists of its highly differen- tiated intrinsic musculature which extends as well in the anterior third as in the caudal portions of this organ. The possibility of coordinate lingual action during phonation in parrots was noted long ago by Owen (47, p. 225). It would seem probable that this surmise is correct (Denker, 20) and that the tongue in parrots forms an integral part of the sound pro- ducing mechanism in these animals, and by alteration of its shape may be capable of modifying in no unimportant manner the quality of the tones produced by syringeal vibrations. The large size and unique specialization of the nucleus inter- medius XII in parrots is thus evidently to be correlated with the bulk of their lingual musculature and especially with the com- plexity of its arrangement and action, and not with any syringeal peculiarities in these forms. In the development of its intrinsic musculature and in the part it may play in phonation as well as in deglutition the action of the tongue in parrots in some respects resembles that of this organ in many mammals. It is, of interest, therefore, to note the close association of the nucleus intermedius XII with the dorsal part of the caudal visceral motor column in parrots in view of the analogous motor nuclear association in this region in mammals. 2. Visceral motor nuclet Nervus accessorius. In all lower forms so far investigated the accessorius nucleus when present is evidently but the caudal prolongation of the dorsal motor vagus column (8, 9, and 10). "An excellent short summary of the number and arrangement of syringeal muscles in various groups of birds is given by Newton and Gadow (44, p. 939). See also Weiss (61, p. 297) and Beddard (4). MOTOR NUCLEI IN PHYLOGENY 265 According to Brandis’ observations (13, p. 634-5), the avian accessory nucleus becomes continuous at its rostral end with the cell group which has been described above as the nucleus inter- medius. It would appear also that Turner’s observations (50) confirm Brandis’ description, since the latter author’s ‘nucleus dorsalis XII’ evidently corresponds to Turner’s ‘nucleus of the spinal accessory.’ The difference between the descriptions of Brandis and Lubosch (41) would also largely disappear if due allowance be made for the latter’s failure to recognize the mixed X-XITI character of the intermedius cell group. Though some variation, no doubt, obtains in the rostral relations of the nucleus accessorius in birds, it is improbable in view of Bok’s recent findings in Gallus (11) that this nucleus is in continuity with the dorsal motor vagus column in any adult avian form. In his careful ontogenetic study of the roots and nuclei of the vago-accessorius complex in the chick (lec., pp. 511-512) the latter author has shown that the elements of the ventro-lateral vagus nulceus, together with those of the nucleus accessorius, originate from the dorsal motor vagus column and migrate ventro-laterad as one cell complex. Differentiation of these two nuclei becomes effected secondarily by the caudal migration of accessorius neurones. Thus, though the nucleus accessorius in both birds and mammals is primarily derived from the dorsal motor vagus column, yet it would appear that in birds its cells migrate first ventrad and later caudad to reach their final situation in the cord, while in mammals the reverse is true. Nerves IX and X. It has been noted above that no vagus component was identified in the intermedius nucleus of Cacatua roseicapilla, though in C. galatea, Palaeornis, and Melopsit- tacus, Brandis (1.c.) was able to demonstrate its presence. How- ever, in view of the great similarity in the peripheral, lingual, laryngeal, and syringeal equipment of C. roseicapilla and C. galatea, it is probable that my failure to identify the component has not been due to its entire absence, but rather to the very extensive development of the hypoglossal elements of this complex. THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, No. 2 266 DAVIDSON BLACK It is well known that both the crop and gizzard musculature of birds are under vagus control (Biedermann, 5, p. 1206). Ver- meulen (55) has shown that in many forms a definite correspon- dence obtains between the size of the dorsal motor X nucleus and the development of the stomach, and similar observations had earlier been made by Kosaka and Yagita (38) in the case of birds. In all parrots a crop is present and, though in cockatoos this structure is of no great size, yet it is well developed. A small gizzard is also present in these animals (v. Oppel, 45, 46; Owen, 47, p. 161 et seq.), so that the specialization and relative size of the stomach in Cacatua would compare favorably with the development of this organ in Spheniscus and Colymbus (23 and 60). It thus becomes difficult to account for the short and relatively slightly developed dorsal motor vagus nucleus in Cacatua which in the proportions of this cell column differs so markedly from the other birds examined. The ventro-lateral motor X nucleus of birds in some respects resembles that of Varanus and Alligator (cf. fig. 15), but is more rostrally placed than in the latter forms. The possible significance of this condition will be discussed subsequently. It is interesting to note that the motor LX nucleus in all birds examined is characteristically associated with the dorsal motor vagus cell group and forms thereby the most rostral part of the caudal visceral motor column. In discussing the relations of the motor nuclei of this region in reptiles (10), it was pointed out that in these forms the association of glossopharyngeal with facial motor perikaryons rather than with vagal elements would seem to be due to a rearrangement of the visceral motor nuclear pattern largely as a consequence of the loss of the hyobranchial pump mechanism for pulmonary ventilation and, as Kappers has noted, under the direct influence of the caudal VIJ-IX taste center. In birds, also, the motor glossopharyngeal and vagus nerves, apart from the control of the simple laryngeal muscles, have lost entirely their original function of innervating respiratory musculature, the effectors concerned in pulmonary ventilation in these forms being wholly of striate somatic character. MOTOR NUCLEI IN PHYLOGENY 267 The gustatory organs of birds are but poorly developed (vide supra) and the pars intermedius VII in these forms is reduced toa minimum. Kappers (36) has shown that but few gustatory fibers can enter the brain stem by the latter path in these forms, the majority of the special visceral afferent impulses being trans- mitted by the glossopharyngeal and vagus nerves. This author pointed out further that the gustatory IX—X components termi- nate in a nucleus which is evidently the homologue of the mam- malian dorso-lateral and dorso-median nuclei of Staderini. The IX—X pharyngeal and oesophageal musculature in birds acts. coordinately during deglutition or in antiperistaltic move- ments of the foregut in practical independence of V—VII effectors. This reflex action of the dorsal motor IX-X nucleus is chiefly inaugurated by afferent impulses of both general and special visceral nature which enter the brain stem almost wholly through the sensory roots of the two nerves in question. The close asso- ciation of the dorsal motor IX and X nuclei in the immediate neighborhood of the terminal special visceral sensory nucleus of their own roots is thus another illustration of Kappers’ principle of neurobiotaxis.” Motor V-VII complex. In Kappers’ earlier publications (see especially 7 and 12) special attention has been drawn to the remarkable fact that in most birds, in contrast to all other verte- brates, the motor VIT nuclei (two or three in number) are situated wholly rostrad of the exit level of their motor root and in close association with the motor V cell groups. An exception to this rule among birds has been encountered only in the case of Casua- ris, where the motor VII nucleus lies on the. exit level of its motor root. Kappers has further shown that the position of the motor VII nucleus of birds rostrad of its root exit and its close association with the motor V nucleus is largely due to a dominat- ing trigeminal reflex influence in the absence of a well-developed gustatory center.” 2 A similar association of [IX—X motor nuclei due to analogous circumstances has already been observed in Cyclostomes (fig. 13 A, p. 257). 13 For a full discussion of this interesting question reference should be made to Kappers’ original communications (l.ec.). - 268 DAVIDSON BLACK Kosaka and Hiraiwa (39) have identified with considerable accuracy the muscular localization within the VII motor nuclei in Gallus and Anas. The ‘Hauptkern’ of these authors was found to be chiefly concerned with the innervation of the m. subcutaneous colli; the “Nebenkern’ with the m. mylohyoideus posterior (m. hyomandibularis and lateralis of Futamura, 25), and the ‘Digastricuskern’ with the m. digastricus (m. depressor mandibulae of Adams, 1). With these facts in mind, it will be of interest to consider further the V—VII nuclear pattern in Ciconia and Cacatua (fig. 16). In Ciconia the ‘Nebenkern’ is not represented as a separate cell group in correspondence with the fact that the tongue is small and not protractile.!* In this form the elements are proba- bly incorporated within the dorsal motor VII nucleus which represents the ‘Digastricuskern’ of Kosaka and Yagita. The ventral motor VII nucleus in Ciconia is larger than that in Ca- catua in correspondence with the more extensive development of the m. subcutaneus colli in the former animal. In Cacatua an intimate association of VII ‘Digastricuskern’ and motor V elements occurs in what has been termed in the foregoing description the combined V—VII motor nucleus. The VII motor elements of the complex are more numerous than the V and the ‘Nebenkern’ of Kosaka and Yagita is probably repre- sented here by the caudo-ventral prolongation of this nucleus described above. The full action of the m. pterygoideus anterior in elevating the maxilla in birds is not possible except when the mandible is widely opened by reason of the contraction of the m. depressor mandibulae (Biedermann, 6). Adams (1) has drawn attention again to the importance of this action of the m. depressor man- dibulae (m. digastricus) in the Psittaci.“ In view of these facts, 144 When the tongue is very protractile or very thick, the m. mylohyoideus posterior consists of two parts termed by Gadow the m. serpi-hyoideus and m. stylo-hyoideus, both innervated by the facial nerve (Newton and Gadow, 44; Biedermann, 7). 16 The so-called m. digastricus of birds which is wholly innervated by the facial nerve is better termed the m. depressor mandibulae, since it is not the homologue of the mammalian muscle of the same name whose anterior belly is innervated by the trigeminal nerve (Adams, 1). MOTOR NUCLEI IN PHYLOGENY 269 the close association of certain trigeminal and facial elements within the limits of a common V—VII nucleus is highly significant as an indication of the probability that these motor V cells represent elements governing the action of the m. pterygoideus anterior. 3. Eye-muscle nerves Nerve VI. But little remains to be said concerning the abdu- cens nerve, which in most respects differs but little from that of reptiles. An interesting series of gradations in the exit level of its motor rootlets is shown in figures 15 and 16. In Cacatua, Ciconia, and Colymbus’ these emerge rostrad of the exit level of the motor VII root; in Spheniscus their emergence is both rostrad and caudad of this level, but in Casuaris the primitive condition is for the most part retained and most of the abducens rootlets in this form emerge caudad of the exit level of the motor root. The dorsally placed abducens nucleus is located for the most part on, or slightly rostrad of, the motor VII root exit, and is somewhat larger than the reptilian abducens cell group, a fact which, as Kappers pointed out, is to be correlated with the greater complexity of the musculature which it supplies in birds (82, 34). Nerves III andIV. Inall birds examined the trochlear nucleus is in close apposition with and frequently overlapped by the caudal end of the oculomotor cell group. The apposition of these two nuclei is evidently due to the rostral migration of the former, whose cells have been shown by Bok (11) to have their ontogenetic origin a considerable distance caudad of the oculo- motor cell group. Though the position of both oculomotor and trochlear nuclei in relation to the exit levels of the oculomotor and motor V roots is subject to considerable variation, the exit level of the trochlear root in relation to its nucleus is relatively con- stant. In Casuaris, Cacatua, and Ciconia it emerges at the caudal end of the trochlear nucleus in Colymbus at the midlevel of the nucleus, and in Spheniscus at its rostral end (cf. Kappers, 34). Thus in no case among birds does the trochlear decussa- tion and emergence take place below the level of its own nucleus as is commonly observed in lower forms (ef. fig. 14). 270 DAVIDSON BLACK The development of subsidiary cell groups within the oculo- motor nucleus has attained to a high state of complexity among birds in ‘correlation with the perfection of the intrinsic and extrinsic oculomotor effectors. In the descriptive portion of this paper special attention was drawn to the accessorius cell group which in its relations, staining reactions, and in the morphology of its elements closely resembles the Edinger-West- phal nucleus of mammals with which it had been homologized by Brandis. In birds there can be no doubt that certain oculo-. motor root fibers arise in this cell group, and for this reason, especially, Kappers’ term nucleus accessorius III has been retained (vide supra, footnote 9, p. 250). Brouwer (17) has recently reinvestigated the Edinger-West- phal nucleus both from the clinical and from the comparative anatomical and pathological standpoint, and he has concluded that there exists the strongest evidence confirming Jacobsohn’s view that the Edinger-Westphal nucleus is the nucleus sympa- theticus nervi oculomotorii (80). In a consideration of the homology of the nucleus accessorius III of birds Kappers’ earlier description of a well-developed nu- cleus accessorius IIJ in one specimen of Varanus sp.? becomes significant, since it is thereby shown that an oculomotor acces- sory cell group (probably of sympathetic nature) has been evolved among reptiles and is present in certain modern repre- sentatives of these forms in a position exactly analogous to that of the Edinger-Westphal nucleus of mammals and the nucleus accessorius IIT of birds. CONCLUSION The cerebral motor nuclear pattern in birds, while showing some important variations in different avian families, is on the whole fundamentally similar in all the forms examined, and characteristically different from that obtaining in any other vertebrate group. Thus the strikingly specialized nature of the modern class Aves is well exemplified also in the topo- graphical relations of the motor roots and nuclei of their cerebral nerves. MOTOR NUCLEI IN PHYLOGENY Zt One of the most characteristic features of this avian nuclear pattern lies in the association of the V-VII motor nuclei and the situation of the facial motor nuclei on, or more frequently rostral . to the exit level of their motor root. It has been pointed out above that a similar association of V-VII motor nuclei is not found as a group character elsewhere in the vertebrate phylum except in cyclostomes. The association of facial and trigeminal motor nuclei in birds would seem to be due largely to the domi- nant influence of sensory trigeminal impulses upon the reflex action of both facial and trigeminal musculature in those forms. The mutual association of the V-VII motor nuclei in close con- tiguity with the chief sensory center acting reflexly upon them thus affords a striking illustration of Kappers’ neurobiotactic concept. An association of the motor glossopharyngeal and dorsal motor vagal nuclei, such as characterizes birds, is encountered as a group feature elsewhere among vertebrates only in petromy- zonts.'6 In birds, though the IX—X musculature has completely lost its primitive respiratory function, yet the effectors concerned act synergically and coordinately in all movements of the fore- gut and are dominated reflexly by visceral sensory impulses entering by way of the afferent IX and’ X roots. Thus these motor nuclei also are associated in the neighborhood of the chief center acting reflexly upon them. The intermedius X—XII motor complex constitutes a third characteristic feature of the nuclear pattern within the brain stem of birds, and one apparently unique among vertebrates. It has been shown to be highly probable that this peculiar com- plex was evolved as a consequence of the development of the laryngo-syringeal mechanism peculiar to birds. Its possession by certain forms, such, for example, as Struthio, Ciconia, and Casuaris, in which intrinsic syringeal musculature is lacking, constitutes strong evidence in favor of considering the-relatively simple syringeal organization of these animals to be due to a 16 Among teleosts, although a similar nuclear association has been observed sporadically in various forms (e.g., Ameiurus, 8), in no case does this character constitute a group feature. 272 DAVIDSON BLACK reduction in the number of elements originally present and not to the retention of a primitive character. In the great development of the hypoglossal component of their intermedius motor complex, it has been shown that parrots differ from all other birds. This fact would appear to be corre- lated with the exceptional development and differentiation of the intrinsic tongue musculature among members of the parrot family. In its morphology and relations the hypoglossal nu- cleus in parrots resembles in many respects that of mammals, though the peculiar psittacine nucleus cannot well have been evolved as other than a specialization of what has already been termed a typical avian intermedius complex (vide supra)... Intrinsic oculomotor nuclear differentiation has attained to a high state of complexity among birds, and the ground-plan of this avian oculomotor specialization is essentially similar to that obtaining in this nucleus in many mammals. In this con- nection it is significant that Kappers has described an arrange- ment of the intrinsic oculomotor nuclei in one specimen of Vara- nus sp.? which closely resembles that obtaining in birds (vide supra, pp. 255 and 270). The occurrence of this phenomenon in a modern reptile shows definitely that the ground-plan of the nu- clear differentiation characteristic of this region in higher forms has already been determined within the class from whose pro- totypes both avian and mammalian forms were evolved. Finally, it would appear that modern birds and reptiles while presenting minor resemblances, show a fundamentally different plan of organization in the arrangement of their cerebral motor nuclei, though either avian or reptilian motor pattern could well have been evolved from a form whose nuclear organization was of a type similar to that obtaining in some modern anurans (e.g., Rana catesbiana, fig. 14 B)!”7 or urodeles (e.g., Triton, fig. 14 A). 17 It has previously been pointed out (9, p. 423) that though the anuran type has certainly been evolved comparatively recently in vertebrate phylogeny, yet ‘“‘within the brain stem in Rana a motor nuclear pattern obtains which on first examination would seem to be much more primitive than the motor nuclear , pattern in selachians.” : an il 12 13 14 15 16 17 18 19 20 21 22 MOTOR NUCLEI IN PHYLOGENY 273 LITERATURE CITED Apams, L. A. 1919 A memoir on the phylogeny of the jaw muscles in recent and fossil vertebrates. Annals N. Y. Acad. 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Bd. 31, 8. 54-71. Kosaka, K., unp Yaaira, K. 1903 Experimentelle Untersuchungen iiber die Urspriinge des Nervus Hypoglossus und seines absteigenden Astes. Jahrb. f. Psychiat. u. Neurol., Bd. 24, S. 150-189. Kosaka, K., unp Hiratwa, K. 1905 Ueber die Facialiskerne beim Huhn.. Jahrb. f. Psychiat. u. Neurol., Bd. 25, 8S. 57-69. 40 41 49 50 51 52 61 MOTOR NUCLEI IN PHYLOGENY 275 Kosaka, K. 1909 Ueber die Vaguskerne des Hundes. Neurol. Centralbl., Jahrg. 28, S. 406-410. Luposcu, W. 1899 Vergleichend-anatomische Untersuchungen iiber den Ursprung und die Phylogenese der N. accessorius Willisii. Arch. f. mikr. Anat., Bd. 54, S. 514-602. Mupaz, G. P. 1901 On the myology of the tongue of parrots, with a classi- fication of the order based upon the structure of the tongue. Trans. Zool. Soe. London., vol. 16, pp. 211-278. Minzer, E., unp Wiener, H. 1898 Beitrige zur Anatomie und Physiologie des Zentalnervensystem der Taube. Monatschr. f. Psych. u. Neur., Bd. 3, 8. 379406. Newton, A., AnD Gapow, H. 1896 A dictionary of birds. London. Oprret, A. 1896 Lehrbuch der vergleichenden mikroskopischen Anatomie. Teil I. Der Magen. Psittaci, S. 198. 1897 Id. Teil II. Schlund und Darm. Psittaci, S. 116. Owen, R. 1866 On the anatomy of vertebrates. Vol. 2. London. Satvaport, T. 1891 Catalogue of the Psittaci or parrots in the collection of the British Museum. Vol. 20, Catalogue of the Birds in the British Museum. SHuretpt R. W. 1890 The myology of the raven (Corvus corax sinuatus). Maemillan. Turner, C. H. 1891 Morphology of the avian brain. Parts V and VI. Jour. Comp. Neur., vol. 1, pp. 265-286. VAN DER Horst, C. J. 1917 Die motorische kernen en banen in de hersenen der vischen, hare taxonomische waarde en neurobiotactische beteekenis Acad. Proefschr., Amsterdam, pp. 1-102. VAN VALKENBURG, C. T. 1911 On the mesencephalic nucleus and root of the N. trigeminus. Proc. kon. Akad. v. Wetenschapp., Amsterdam, May 27, pp. 25-42. 1911 Zur Kenntnis der Radix spinalis Nervi trigemini. Monatschr. f. Psych. u. Neurol., Bd. 29, 8. 407-437. 1911 Zur vergleichenden Anatomie des mesencephalen Trigeminusan- teils. Folia Neurobiol., Bd. 5, Nr. 4, S. 360-418. VERMEULEN, H. A. 1913 Note on the size of the dorsal motor nucleus of the X nerve in regard to the development of the stomach. Proc. kon. Akad. v. Wetenschapp., Amsterdam, Sept. 27, pp. 305-311. WaALLENBERG, A. 1898 Die secundire Acusticusbahn der Taube. Anat. Anz., Bd. 14, 8. 353-369. 1899 Untersuchungen iiber das Gehirn der Tauben. Anat. Anz., Bd. 15, S. 245-271. 1903 Der Ursprung des Tractus isthmo-striatus (oder bulbo-striatus) der Taube. Neurol. Centralb., Jahrg. 22, S. 98-101. 1904 Neue Untersuchungen iiber den Hirnstamm der Taube. Anat. Anz., Bd. 24, S. 357-369. Watson, M. 1883 Report on the anatomy of the Spheniscidae collected during the voyage of H. M. S. Challenger. Challenger Reports, Zoology, vol. 7, pp. 1-244. Weiss, O. 1912 Die Erzeugung von Geriuschen und Ténen. Handb. d. vergleich. Physiol., herausg. v. H. Winterstein, Bd. 3, Iste Halfte. Die Stimme der Vogel, 8. 294-301 PROMPT PUBLICATION The Author can greatly assist the Publishers of this Journal in attaining prompt publication of his paper by following these four suggestions: 1. Abstract. Send with the manuscript an Abstract containing not more than 250 words, in the precise form of The Bibliographic Service Card, so that the paper when accepted can be scheduled for a definite issue as soon as received by the Publisher from the Editor. 2. Manuscript. Send the Manuscript to the Editor prepared as described in the Notice to Contributors, to conform to the style of the Journal (see third page of cover). 3. Illustrations. Send the Illustrations in complete and fin- ished form for engraving, drawings and photographs being pro- tected from bending or breaking when shipped by mail or express. 4. Proofs. Send the Publisher early notice of any change in your address, to obviate delay. Carefully correct and mail proofs to the Editor as soon as possible after their arrival. By assuming and meeting these responsibilities, the author avoids loss of time, correspondence that may be required to get the Abstract, Manuscript and Illustrations in proper form, and does all in his power to obtain prompt publication. THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 3, JUNE, 1922 Resumen por el autor, H. Saxton Burr. EF] desarrollo temprano de los hemisferios cerebrales de Amblystoma. El] estudio experimental del destino del neuroporo de Am- blystoma y un nuevo examen de los estados tempranos de la formacién de los hemisferios cerebrales de dicho animal han demostrado los siguientes hechos: El labio ventral del neuroporo, después de haberse reunido los labios laterales, se transforma en la cresta terminal del embri6n, la cual es invadida mas tarde por la comisura anterior. La fusién de los labios laterales del neuroporo forma la lamina terminal, la cual, por consiguiente, termina en el borde anterior de la cresta terminal. La placa del piso de His no se extiende anteriormente a la fovea del istmo. E] intervalo en la linea media ventral del tubo neural esté ocupado de delante atrdis por la continuacién de un lado a otro de la placa alar anteriomente, y por la de la placa basilar posterior- mente. Como resultado de esto el material evaginado para formar el hemisferio cerebral se deriva enteramente de la placa alar, y a partir de aqui la laminacién dorso-ventral se impone secundariamente sobre la disposicién de las partes en direccién céfalo-caudal. Translation by José F’. Nonidez Cornell Medical College, New York AUTHOR’S ABSTRACT OF THIS PAPER ISSUED BY THE BIBLIOGRAPHIC SERVICE, APRIL 17 THE EARLY DEVELOPMENT OF THE CEREBRAL 4 HEMISPHERES IN AMBLYSTOMA H. SAXTON BURR Anatomical Laboratory of the Yale University School of Medicine TWENTY-SIX FIGURES Since the researches of His (’88, ’92, ’93) his original concept of the structure of the neural tube has been very generally ac- cepted. The work of subsequent investigators well known in the literature has established the main principles of the work of His beyond a reasonable doubt. In his early papers, however, he did not elaborate very fully the rostral relations of the longitu- dinal zones of the neural tube. It was his belief that these four primary columns meet at the ventral lip of the neuropore. On the basis of this assumption, the roof and the floor plates separate the neural tube into two independent lateral halves. It is evident, therefore, that the fate of the ventral lip of the neuropore becomes a matter of prime importance. This matter has been studied carefully by workers, among whom may be mentioned Johnston (05, 709) and Schulte and Tilney (15). In all the work that has been done, however, it has been tacitly assumed, as originally pointed out by His, that the floor plate terminated at the ventral lip of the neuropore. ‘The exceedingly interesting and suggestive paper of Kingsbury (’20) contains the first suggestion that this assumption is not altogether justified by the facts. During the progress of an investigation of the early development of the cere- bral hemispheres in Amblystoma it became evident that a more careful study of the rostral relations of the longitudinal zones of His was necessary. Several years ago Doctor Herrick suggested that a more detailed study of the early development of the cere- bral hemispheres might yield valuable data. The researches of Bindewald (’14), Crosby ('17), Heuser (’13), and others have shown in many forms the fundamental character of the telen- 277 278 H. SAXTON BURR cephalic evagination. In all the cases studied the outpouching of the wall of the neural tube has been closely associated with the olfactory placode. Beginning first in the cyclostomes as an evagination of the olfactory bulb, the further phylogenetic history has involved progressively more and more olfactory association material (Herrick, ’21). The relation of the evaginated portions to the rest of the tube assumes, therefore, a prime importance. The following communication deals with an experimental study of the fate of the neuropore in Amblystoma and the subsequent history of the relations of the longitudinal columns of the neural tube in this region. THE FATE OF THE NEUROPORE The work of Johnston (’09), based on a comparative study of the forebrain vesicle in vertebrates, showed clearly that the ven- tral lip of the neuropore was incorporated into the brain as the terminal ridge lying between the lamina terminalis anteriorly and the chiasmatic ridge posteriorly through the fusion of the lateral lips of the blastopore. He concluded that the preoptic recess, marking the termination of the sulcus limitans and separa- ting the terminal ridge from the chiasmatic ridge, represented the meeting-point of the roof and floor plate and hence the ante- rior end of the neural tube. Hatschek (’09) confirmed in the main Johnston’s researches, but stated that the terminal ridge or ‘Basilarlippe’ marked the primary anterior wall of the neural canal. In any event, it seems evident that this terminal ridge is subsequently occupied by decussating fibers of the anterior commissure. To determine experimentally the above facts, a number of operations were performed on Amblystoma larvae. At the sug- gestion of Doctor Harrison, fine hairs were inserted into the neuropore for a short distance and watched during subsequent development in the hope that the ultimate position of the hair would indicate the final position of the neuropore in the embryo. Difficulties were at once encountered, since a fine hair, unless inserted for a considerable distance into the embryo, would not remain there, but would be extruded. If, on the other hand, the EARLY DEVELOPMENT OF HEMISPHERES 279 fine hair was inserted far enough to prevent its extrusion it became evident that the imbedding of the hair in the tissue deep in the embryo prevented its following the ventral path of the neuro- pore. However, by making a slight wound in the ventral lip of the neuropore and staining it with Nile-blue sulphate, accord- ing to the method of Detwiler (’17), it was found that the stained area could be followed throughout the subsequent development. In fact, the Nile-blue sulphate remained in the tissue for a period QrEy Co) Fig. 1 Ventral view of Amblystoma embryo, showing area of ventral lip of neuropore stained with Nile-blue sulphate, stippled. X 12. Fig.2 Same embryo 24 hours later. X 12. Fig.3 Lateral view of same embryo 48 hours after operation. 12. Fig.4 Lateral view of same embryo 72 hours after operation. X 12. Fig.5 Lateral view of same embryo 96 hours after operation. X 12. of twenty-one days. A series of drawings showing the position of this stained area is given in figures 1 to 5, representing an elapse of four days. A sagittal graphic reconstruction of the brain of the embryo shown in figure 5 is given in figure 6. The region of the stained area is indicated in the figure, rn. The material which makes up the ventral lip of the neuropore produces the ridge in the floor of the neural tube to which John- ston has given the name of the terminal ridge (fig. 6, tr.). It is evident, therefore, that the closure of the lateral lips of the neuro- pore produces the lamina terminalis which ends not in the pre- 280 H. SAXTON BURR optic recess, as Johnston indicated, but in the terminal ridge. By this fact it may be seen that the terminal ridge marks the rostral limit of the roof plate of His. Fig.6 Graphic reconstruction from sagittal sections of brain of embryo shown in figure 5. X 50. chr., chiasmatic ridge; ep., epiphysis; rn., neuropore; sl., sulcus limitans; tr., terminal ridge; vt., velum transversum. Fig. 7 Transverse section through the head of an 1l-mm. Amblystoma embryo, showing the general relations of the lamina terminalis. X 50. Fig.8 Transverse section 190mu caudad of figure 7, showing the general rela- tions of the median alar plate. > 50. Fig.9 Transverse section 110w caudad of figure 8 passing through the cbias- matic ridge. X 50. EARLY DEVELOPMENT OF HEMISPHERES 281 Now the ventral lip of the neuropore is the region at the rostral end of the neural tube where the material which makes up the lateral lips of the neuropore is continuous from side to side across the midline. This material constitutes the rostral portion of the alar plate, and hence we may conclude that in the terminal ridge the alar plate of one side becomes continuous with that of the other. In other words, the assumption of Kingsbury (720) that the alar plate of the neural tube forms an arch about the anterior end of the neural plate may be considered as correct. The terminal ridge represents the median portion of the alar plate with its caudal boundary marked by the preoptic recess where the sulcus limitans of one side becomes continuous with that of the opposite side. The further suggestion of Kingsbury that the basal plate is likewise continuous from side to side across the median line arching around the fovea isthmi, the anterior limit of the floor plate of His, seems to be substantiated by the following facts. In figures 7 to 12 are shown a series of critical cross- sections through the brain of Amblystoma showing the relations and structures, respectively, of the lamina terminalis, the terminal ridge, and the chiasmatic ridge. The gross relationships are shown in figures 7, 8, and 9 and the microscopical structure in figures 10,11, and 12. It is evident from these figures that from the point where the lamina terminalis merges with the terminal ridge the midventral line of the brain is occupied, not by typical floor-plate material, i.e., non-nervous supporting tissue, but throughout its length until the fovea isthmi is reached, by typical nervous tissue. Only one exception can be made to this general statement and that is that part of the floor plate which is to form eventually the roof of the hypothalamus. In the older stages it is reduced to a non-nervous lamina. This, however, has no significance when it is noted that this region in its early develop- mental stages does not differ from the structure of the terminal ridge and the chiasmatic ridge, so far as can be determined from microscopic sections. The thinning of this area is shown in figures 14, 16, and 18. 282 H. SAXTON BURR chr. EO a & ON RSS BEN ee vey SS STD Fig. 10 Section through lamina terminalis, showing cellular detail. X 175. It., lamina terminalis; vc., ventricular cavity. Fig. 11 Section through median alar plate, showing arrangement of cells. X 175. tr., terminal ridge; vc., ventricular cavity. Fig. 12 Section through chiasmatic ridge, showing the continued cellular increase in the median basal region. 175. chr., chiasmatic ridge; vc., ventric- ular cavity. EARLY DEVELOPMENT OF HEMISPHERES 283 It would seem, therefore, that from the terminal ridge caudally to the fovea isthmi we are dealing not with the non-nervous floor plate of His, but with a nervous tissue continuing the basal plate of one side with that of the other. So far as Amblystoma is concerned, it is probable that the fundamental arrangements of the longitudinal columns of His in the rostral end of the neural tube were as indicated by Kingsbury (720) in his figure 6. These rostral relations may be stated as follows: The floor plate of His terminates at the fovea isthmi, about which arches anteriorly the basal lamina, the continuity of which is maintained by the ventral portion of the neural plate lying between the fovea and preoptic recess. Arching about the basal lamina still more ros- trally and separated from it by the sulcus limitans, is the alar lamina. The continuity of the two lateral alar laminae is repre- sented in the midline by the terminal ridge. The roof plate whose rostral portion is the lamina terminalis, therefore, does not meet the floor plate, but is separated from it by the structures above mentioned. EVAGINATION OF HEMISPHERES The general form and arrangement of the constituent elements of the forebrain of Amblystoma are well known through the researches of Herrick, Johnston, and others. As a result of their work, and more particularly that of Herrick (’10, ’21 a), we know that the forebrain of Amblystoma has a fairly well-defined organi- zation. ‘Through his study of Amblystoma larvae of varying stages and of the adult, Herrick has made fairly definite the fol- lowing parts of the hemisphere. It can be divided into a number of relatively distinct nuclei. The most important of these is the olfactory bulb lying at the rostral end of the hemisphere. In addition, four regions are described called, respectively, the ven- tro-medial, ventro-lateral, dorso-medial, and dorso-lateral parts. These converge forward into a relatively undifferentiated nucleus olfactorius anterior at the base of the olfactory bulb. The ventro-medial part is the septum (in the broad sense), characterized by the ventro-medial olfactory tract, the medial forebrain bundle and their connecting tracts. The ventro-lat- 284. H. SAXTON BURR eral part in Amblystoma is an undifferentiated strio-amygdaloid body, characterized by the ventro-lateral olfactory tract, the lateral forebrain bundle and their connecting tracts. The dorso- medial part is the primordium hippocampi, receiving a small dorso-medial olfactory tract, but chiefly characterized by associa- tional fibers relating it with contiguous parts of the hemisphere and by strong tracts directly to the epithalamus and hypothala- mus. The dorso-lateral part is the relatively undifferentiated precursor of the lateral olfactory nucleus and pyriform lobe of higher brains, receiving (among others) the strong dorso-lateral olfactory tract, terminals of the lateral forebrain bundle, and associational fibers from the dorso-medial part. The analysis of the early phases in the development of these structures has been carried out on a series of larvae of Amblys- toma punctatum. Wax-plate models of the brains of five larval Amblystoma were made and checked by microscopic dissection of the entire brain. Drawings of three of these are shown in figures 13 to 18. In addition careful study was made of trans- verse sections stained in haematoxylin and erythrosin to deter- mine the microscopic relations. see OLE) Lee POTS eye. STAINS. Ad Pe 365 Oreodon (Merycoidodon) culbertsoni Leidy...................0005: 365 CreGaGnrAlaCnis LEIGy-nake Mere sige we PUNE t gole S eile ss be oe oer eg os 365 IMeryCOCHOETUSH tt Savarese etre GREE ee owe oer ote we. 5 Che ea areas 367 Mesahippiis!... os Pou? LRP S. SRA SO. MORI P GT OE a 367 SUN Saye yy oe yok Rica tte RF eS SEIS CAR FSB E Pe POSE RU ors « REA CIEE BG 369 LESH RED EO UAEN y ES(0 11 UE A og PS Ce des SRE 9 Wn Beer 370 PISt Ol ab OReWAM EONS en. et er ete HI rok © PSR fee hh Ges 0 ado at Hele ae a 371 Description of foumesr os. 9 BE ona Mae. OM a 372 INTRODUCTION It has been generally assumed by students of comparative neurology that ancient mammalian brains are primitive in nature and that throughout geologic time there has been an evolution of the cerebral cortex. Such an evolution is indicated in the aeluroid and cynoid casts described herewith, this being the first direct comparison of ancient and modern forms to bring out this point. 343 344 ROY L. MOODIE There have been in my possession for several years a number of endocranial casts of fossil mammals, but I have deferred their description for various reasons. Now that Davidson Black has described (’20) the endocranial casts of Oreodon so fully, I feel that there is a basis for further work by those who have not been especially trained in the study of such objects. The field of endocranial anatomy is a very special one and considerable advance has been made since Scott’s (98) results were published. The advancement of knowledge in this field is due largely to the work of G. Elliot Smith, Palmer (713), and Davidson Black (15, ’20). The change in the conception of the nature of endo- cranial casts and their faithful reproduction of the surface of the mammalian brain is dated from the actual comparison of natural endocranial casts with the brains and artificial casts by Davidson Black (’15). The impress which the brain makes upon the inner table of the skull varies greatly with the group, and in the casts here described it is found to be most strongly mani- fested in the Carnivora, reaching its maximum in Smilodon from the Pleistocene of California. In a few cases the entire configu- ration of the cranium is modified by the contained brain, noted especially in Putorius by Schwalbe (’04), where the gyri are evident as external elevations on the skull. It is manifestly impossible for one who has not been especially trained in comparative mammalian neurology to do a great deal in the interpretation of these endocranial casts, but I have been encouraged to place my results on record for other workers in the field. The figures are all accurately done by trained artists and all have been carefully checked with the specimens. LITERATURE An important contribution to the study of endocranial casts is that of Tilly Edinger (’21), who has described the natural cast of the brain cavity of Nothosaurus, a long-headed reptile of the middle Triassic near Heidelberg. This reptile is interest- ing in having a huge anteriorly placed pineal opening. Edinger compares the ancient cast with that of the modern alligator and its endocranial cast, calling attention to the fact that the ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS 345 endocranial cast of Nothosaurus only suggests the form of the brain. Similar results have been obtained by other writers on ancient reptilian endocranial casts. A review of the literature on ancient mammalian endocranial casts will not be made here. All additional literature which has appeared since my annotated bibliography (’15) was printed has been given by Black (15, ’20). MATERIALS The nineteen endocranial casts are all from the White River Beds of South Dakota, ranging from Lower to Middle Oligocene. The casts represent the following forms: 1 cast representing a primitive rodent, genus and species uncertain, possible a sciuromorph (figs. 1 and 2). 3 casts repre- senting primitive insectivores, Ictops acutidens (figs. 5 to 8). 1 cast representing a primitive saber-toothed cat, Dinictis felina Leidy (figs. 3 and 4). 1 cast representing an undetermined felid, possibly Hop- lophoneus (figs. 9 to 11). 1 cast representing a primitive bear-dog, Daphaenus felinus Leidy (fig. 12). 8 casts representing the common oreodont, Oreodon (Mery- coidodon) culbertsoni Leidy (figs. 22 to 28). 1 cast representing a species of Merycochoerus, a large semi- aquatic oreodont, associated with casts of the paranasal sinuses Gage 17); 1 cast of Oreodon gracilis Leidy associated with paranasal sinus casts (fig. 21). 1 cast doubtfully representing Oreodon gracilis Leidy (figs. 18 to 20). 1 partial cerebral cast representing the three-toed horse, Mesohippus (fig. 24). I am obliged to Mr. Paul C. Miller, of the University of Chicago, Mr. E. S. Riggs, of Field Museum, and Mr. H. T. Martin, of the University of Kansas, for the loan of material. For aid in confirming the identification of the casts I am endebted to Mr. E. S. Riggs, Dr. W. D. Matthew, and Dr. E. L. Troxell. 346 ROY L. MOODIE The Pleistocene mammals are represented by a partial brain case of Smilodon from the Pleistocene deposits of the Rancho la Brea of southern California, and a complete brain case of Aeno- cyon dirus, the giant wolf of the same beds. In order that accurate casts might be made, the task of removing the bitumen and taking correct impressions of the rugose endocranial cavities was entrusted to Mr. Adolph Hammer, director of the plastic studios of the department of anatomy. His success will be evi- dent from the figures of these casts (figs. 13 to 16) made by Miss Genevieve Meakin, artist in the department. Comparisons have been made not only with figures of other casts, but with artificial casts of modern mammals. All the material used in this study will be deposited with the Walker Museum at the Uni- versity of Chicago. DESCRIPTION OF ENDOCRANIAL CASTS The casts are all in fairly good state of preservation, though they have suffered the accidents to which all fossils are subject, being cracked, broken, and weathered. While these accidents have obscured certain of the anatomical features, especially of the olfactory bulb and cerebellum, yet sufficient remains to justify description. Photographs of the casts have not been made, since half-tone reproduction usually destroys the greater part of the detail. Measurements of each cast have been made on the basis out- lined by Black (’20) and his terminology has been adopted. The displacement of the objects had been obtained by immersing them in a large graduate partially filled with water. While this method does not give an accurate conception of the endocranial capacity of each mammal, since the casts are often incomplete, yet the capacity is so closely indicated that it is worth recording. The fine work of Bolk (06) has been depended upon for identification of parts of the cerebellum where such was possible and the extremely useful catalogue compiled by G. Elliot Smith (02) has been the guide in the interpretation of parts of the cerebrum. Particular reference is not made to the many general comparative anatomies consulted, such as Gegenbaur, Sisson, ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS 347 Reighard and Jennings, Jayne, Wiedersheim, Johnston, and many others. These have proved useful in certain identifica- tions, such as the endocranial blood vessels, nerves, foramina, etc. Rodentia A single cast represents this primitive mammalian group. It has not been positively identified, but it has been regarded as ‘‘nossibly Paleolagus haydeni Leidy,” which Matthew (’01) refers to as the most abundant hare in the White River beds to which it is limited. A careful comparison of this cast with Paleolagus remains in Yale University Museum by Dr. E. L. Troxell has led to the conclusion that it is not an ancient rabbit brain. The specimen is shown in figures 1 and 2. The cerebrum measures 20 mm. in length; 12 mm. in greatest cerebral width, and the olfactory bulbs, which are beautifully preserved, have a length of 6 mm. and a combined width of 5 mm. The surface of the brain, as in modern rabbits and in the Sciuromorpha, is perfectly smooth. The rugosities which appear in the form of mineral incrustations may represent the meningeal vessels (fig. 1), though I am inclined to think they do not. The smooth projection (fig. 1, cn) just anterior to the olfactory bulb is evidently a portion of the cast of the nasal chamber. The olfactory bulbs themselves resemble in great measure those of the modern rabbit. They are smooth, but are shorter and broader than in Lepus and in the fossil they are more widely separated. The inferior surface is imperfect, so I am not able to determine the number of fila olfactoria. The only surface marking on the cerebrum is a small sulcus (fig. 2), evidently representing the rhinal fissure. This is placed well down on the lateral surface, so that the neopallium is exten- sive. The cerebrum is more slender than in the modern rabbit with the pyriform region more protuberant. The lobes are rounded as in the squirrels, not flat as in Paleolagus and modern lagomorphs. The cerebellum is imperfectly preserved. The base had been partially destroyed by being mounted on a wire pedestal. 348 ROY L. MOODIE So far as I can learn, the brain cast of no ancient rodent: has been previously described. Troxell (’21), in a recent study of material in the Yale Museum representing Paleolagus, with which this cast was compared, remarks that the brain of Paleolagus is relatively small and flat, differing from the present cast in length and breadth and in the small size of the foramen magnum. The small size of the brain is merely a substantiation of the law announced by Professor Marsh (’74), which he indicates in these words: ‘In other groups of mammals, likewise, so far as observed, the size of the brain shows a corresponding increase in the successive subdivisions of the Tertiary.” G. Elliot Smith (’02, p. 195) has called attention to an interpre- tation of the smoothness of the cortex of the brain of certain rodents which is pertinent here. Contrary to the usual assump- tion, he believes that the smoothness of the cortex, such as is exhibited by the endocranial cast of the ancient rodent, does not indicate a primitive condition of the cortex for the group. Some of the mammals, such as the hystricomorphine Rodentia, with cerebral hemispheres the same size as the beaver, possess numer- ous deep sulci. He states: ‘“This is one of the enigmas of cerebral morphology which we are utterly unable to satisfactorily explain at the present time.”’ Insectivora Ictops. The three cranial casts discussed under this heading are identified as Ictops by Doctor Matthew and Dr. E. L. Troxell and for the sake of convenience I have referred to them under the specific name Ictops acutidens, an insectivore of the White River, Oligocene, beds. There is no assurance that the casts represent this species, or even that the casts, found isolated as they were, belong to the same species. There is, however, a wide range of variability exhibited by the skeletal parts of Ictops. The family Leptictidae, representing the primitive hedgehogs, is an ancient group of this curious order of mammals, which Doctor Matthew regards as a group “defeated and disappearing in the struggle for existence.’’ In past time the Insectivora were of ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS - 349 more importance than now; in fact, they have been considered as representing more nearly than any other living order the primitive central group from which all other mammals have descended. Through the Tertiary the group progressed less than most other orders and several families of them became extinct during that period, while the moles and shrews diverged from nearly similar habits to their present peculiarities, and the hedgehogs, prob- ably, acquired their coat of spines. All the casts are imperfect, two of them lacking the olfactory region and with imperfect basis cerebri. They are of unequal size with the following measurements: No. 662 Maximum length...... 29 mm.; the bulbus olfactorius is wanting Missa Winn Gy cts td cadre sian SSS ES eel RE ghered «ety ae 22 mm, No. 628 Maximum length.......... 30 mm., including the bulbus olfactorius. Vicemini UM wyadGlae. 4: c.ce es AIAG AO EE ct SU at 20 mm. drength of oliactory bullae. js 2s). NAS bs EO ae 3mm. - pyWirrcl ts ses feos aye UE ay aks oe acca Reeves SL 5 crn veg oe ve Seeger i oie 14mm. A third cast is somewhat longer with the same cerebral width. These natural endocranial casts (figs. 5, 6, 7, 8) compare very favorably with the figures of the brain of Erinaceus europaeus, the modern hedgehog. They exhibit the same large olfactory bulb, short and broad; the same smooth cerebral surfaces. The cerebellum is partially obscured in all specimens. The only definite indication of a sulcus is a slight groove on the lateral surface, doubtless representing the primitive rhinal fissure (F.rhin.) and a slight depression which runs transversely across the anterior end of the cerebrum and doubtless represents an orbital sulcus (S.orb.).. The position of a lateral sulcus is suggested in one specimen, but it is too imperfect to be definitely identified as such. It is thus evident that the hedgehog brain has not advanced at all in cortical complexity since the Oligocene, and that it had by that time attained all the surface features which the group represents to-day. A comparison of the present casts with the figure of the brain of the European hedgehog given by G. Elliot Smith (’02, p. 189) reveals a number of important differences in the cerebral surfaces 350 ROY L. MOODIE of these two otherwise closely comparable brains. Judging from the portion of an olfactory bulb remaining on one of the endo- cranial casts (figs. 7 and 8), the Oligocene hedgehogs were fos- sorial, since to this underground habit is ascribed the huge olfactory development in the recent form. The olfactory tubercle is not so prominent in the ancient as in the modern form, though its position is well shown in figure 6, where the olfactory trigone may also be discerned. ‘The rhinal fissure on the ancient form is much lower in position, and this rather strikingly increases the neopallial surface in the ancient brain which is so greatly reduced in the modernform. A slight anterior depression, not so strongly marked as in the modern hedgehog, marks the impression known as the sulcus orbitalis. The pyriform lobe in the ancient form is less developed, but its condition is well shown in figure 8. The basis cerebri is well displayed in one specimen, no. 2762, and reveals some things of importance. The optic chiasma is placed well anteriorly, being only 4 mm. posterior to the base of the olfactory bulb. The olfactory trigone and tubercle are both evident as smooth but not very prominent areas. The bases of the trigeminal nerves are quite large and placed well together. Posterior to these arise the cerebral peduncles which are evident as narrow indistinct bands. ‘The pyriform lobes are small and not protuberant. As previously stated, the cerebellum in all three specimens of the endocranial casts of Ictops is imperfectly preserved. On the posterior end of two of them portions of the cerebellar structures can be discerned. While a description of them is justified, a drawing of them in their imperfect state would be needless. The cerebellum of Ictops is simple and possibly primitive, though no more so than the cerebella of some modern mammals, notably Vespertillio. At first glance, this part of the brain seems divided into three vertical areas or lobes, but this is an appearance due to the manner of arrangement of the parts of the lobes. The cerebellum, as a whole, is more advanced than the cerebrum, since there are represented three major folds. The lobus anterior is evident in two specimens, but its limits are indistinct, save in one where the sulcus primarius is a prominent depression ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS ao separating the lobus anterior from the lobulus simplex. The lobus anterior is wholly without lamellae or other complexities. The lobulus simplex is merely a narrow band running across the posterior surface of the endocranial cast. The lobulus petrosus, the lower part of the pars floccularis, is represented in both speci- mens. It stands out so sharply as scarcely to be recognizable as a part of the cerebellum. It is a smooth rounded object some 3 mm. in diameter attached by a narrow pedestal to the side of the cerebellum. Carnivora (Aeluroidea) Dinictis felina Leidy. The most primitive cat represented in the present assemblage of endocranial casts is Dinictis felina Leidy from the Oligocene of South Dakota. The cast (figs. 3 and 4) is an unusually good one, as such objects go, though the olfactory portion is imperfect and the cerebellar portion obscured. The form of the cerebrum is, however, almost perfect and one may see at a glance its typically feline character. When Scott (’89) discussed the systematic position of Dinictis he referred to it as the most primitive cat then known. Later (13) he maintained this position and referred to the opinion of others that Dinictis was in a direct ancestral line with the modern cats. The genus is usually referred to the machairodonts or saber-toothed cats, a line of forms which culminated in the huge Pleistocene species (fig. 16), but was also doubtless close to the stem form from which the modern cats were derived. The cranium of this light-limbed, cursorial machairodont is somewhat elongated, the upper contour of which slopes sharply downwards and backwards from the highest point of the cranium, just back of the orbits, thus leaving but little room for the brain- case. The face in advance of the orbits is much contracted. On account of these cranial formations, the space allotted the cere- brum is comparatively short, while the cavities of the hind-brain and olfactory lobes are long (Scott, ’89). At the time Scott wrote, nothing was known of the form of the brain, save in a general way, and, so far as I am aware, no one has since studied the cerebral form of Dinictis. aoe ROY L. MOODIE The brain cast of a related and contemporary form Hoplo- phoneus, a large, heavier-limbed cat, has been briefly and inadequately described and figured by Bruce (’83). The dis- cussion of an endocranial cast which may represent Hoplophoneus is given below. The cerebrum, almost all neopallial in nature, of this primitive cat, Dinictis, is quite short and thick (fig. 4), and the simple cerebral pattern is evident and clearly impressed on the indurated material preserved. The lateral sulcus (S.lat.) runs almost parallel to the sinus sagittalis, ending gradually and without complexity. At its anterior end this sulcus appears to converge with a slight depression which may represent the coronal suleus and with the termination of the suprasylvian sulcus (S.ss.).. The prominence of the endocranial venous sinuses may be correlated with the well-developed sagittal crest which the skull exhibits. The crest is, however, slight compared to its development in Hoplophoneus. The suprasylvian sulcus (S.ss.) runs approxi- mately parallel to the lateral sulcus, these two being the only clearly marked evidences of cerebral complexity seen on the dorsum of the cerebrum. ‘This gives the cerebrum a remarkably simple aspect, as would be expected from the primitive organi- zation evidenced by the skeleton. Meningeal vessels are not evident and the rhinal fissure is not preserved. The olfactory region was lost as shown in figure 4, X. The broad broken surface of the olfactory bulb, however, indicates a highly developed sense of smell, for the cats have apparently always been predatory. A comparison of the cerebral features of this cat (fig. 4) with the Pleistocene cat (fig. 16) reveals in a striking manner its evolution. The form of the cerebellum, as seen from above, is only indi- cated by the material preserved, since the cast had been badly broken in this place before coming into my hands. ‘The relative size of the cerebellum to the cerebrum and its relation to the broad medulla are worthy of note. There is no indication from the specimen of the cerebrum’s bulging over the cerebellum such as is commonly seen in modern carnivores. ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS a00 The lateral aspect of the natural endocranial cast is especially instructive and is noteworthy on account of the slight develop- ment of the cerebral sulci and gyri. A future complexity of pattern is suggested for the descendants of this cat, and is to be seen in Smilodon (fig. 16). The sylvian fossa is represented by a widely opened depression, the borders of which merge gradually with the adjacent sulci. The floor of the fossa is perfectly smooth. Casts of three cranial foramina indicate the positions of only three of the cranial nerves, i.e., the optic, the trigeminal, and the auditory. The optic nerve is placed well forward, the part represented doubtless being subsequent to the chiasma. The trigeminal, as indicated by the canal, was a large nerve, but presents no unusual characters. The acoustic is represented by a partial cast of the internal acoustic foramen and its short canal. Unfortunately, the basis cerebri is too imperfect for description. mm. MenetnoLen tire cash .of Wimictise.. Sov oc. onc eck ec ee el eee eee cen 80 Gercoraliglenciare pets teret ate 5 fee eS Ye Le od i Pay hey 8 44 ATG EDL WEL ULL tab sc pero cke ts «bart: Aes abe h desk Mend a abecnc eto ehe ie cee 2 40 CeyeUS MELWECI SO NLA. ANG<9. GSNIB. 6 .c.< 1 ofan! abt oo etc bGicte Gaddaar ese 10 Undetermined carnivore, possibly Hoplophoneus. It is singu- larly unfortunate that I am unable to positively identify the best-preserved brain cast in my possession, but it was not accompanied by skeletal parts. It is by far the most beautifully preserved cast I have ever seen. I submitted it to Doctor Matthew for an opinion, and he thinks it may be a felid, related to Hoplophoneus or Dinictis. Since I have already described the brain of Dinictis (figs. 3 and 4), the identity of which was ren- dered certain by accompanying skeletal parts, I conclude that the present cast may be some species of Hoplophoneus. This is rendered more probable by comparing the cast with the figures of the brain cast of Hoplophoneus given by Bruce (’83), where a close resemblance is seen. Then another interesting fact is that Hoplophoneus has a high sagittal crest which in other forms tends to an exaggerated development of the superior sagittal sinus. The present cast (figs. 9 and 10) has such an exaggerated 354 ROY L. MOODIE superior sinus. The skull of Hoplophoneus is longer than in Dinictis and the brain more dog-like. The arrangement of sulci in the present cast (fig. 10) is almost precisely that figured by Bruce (’83) for Hoplophoneus. Bruce’s figures, however, show a somewhat more robust brain than the present specimen. In all other respects, save only the absence of a high sagittal sinus in Bruce’s figure, the two specimens agree. I am not, however, fully satisfied that the present cast represents Hoplophoneus. Its beautiful preservation justifies its description, and we must await future associations of skeletal parts with a good brain cast for a positive determination. That the cast is not cynoid, and hence does not represent Daphaenus, the bear-dog of the Lower Oligocene, is evident in the absence of the sulcus ectolateralis in the present cast (fig. 10). Its felid characters are evident in its robust cerebrum, in the arrangement of the sulci (figs. 9 and 10), and in the base of the brain (fig. 11), especially in the relations of the remnants of the trigeminal nerve. The endocranial cast of this undetermined felid (figs. 9, 10, and 11) differs markedly from that of Dinictis (figs. 3 and 4), especially in its more sharply marked sulci and its more advanced cerebral pattern. It is not so cat-like as Dinictis. The cerebrum does not overhang the cerebellum. Dorsally, the lateral sulcus (S.lat.) runs without interruption from posterior to anterior end of cerebrum, thus differing considerably from Dinictis (fig. 3). The ansate sulcus is not indicated, though it is probably proper to speak of the anterior end of this long lateral sulcus as the coronal sulcus (S.cor.). There is no postero-lateral sulcus and the lateral sulcus ends abruptly against a gyrus separating the sulcus from the tentorial depression. The suprasylvian sulcus of Hoplophoneus, as figured by Bruce (’83), differs from the present specimen in that Bruce’s specimen shows an interruption on the postero-dorsal part of the sulcus. There is no such interruption in the present specimen. The sulcus runs without interruption, clearly and deeply marked (fig. 10), being the more deeply impressed posteriorly. This may be merely a specific difference. ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS 355 The ectosylvian sulcus (S.ectosyl.), absent or only faintly indicated in Dinictis (fig. 4), is represented in the present cast by a shallow semicircular groove. It is more clearly indicated on the left than on the right side, although its course is clear in either case. It begins and ends without joining either of the adjacent sulci. The depression labeled sylvian fossa (F’.syl.) is much narrower and more clearly impressed than in Dinictis. It is not, however, so advanced as in Smilodon (fig. 16), and Smilo- don is not so far advanced in this respect as the modern lion. So that we have four stages of development of the sylvian fissure represented. Dinictis represents the most primitive (fig. 4). Here the fossa is wide and its floor smooth. The undetermined felid (Hoplophoneus?) (fig. 10) represents the next stage of advance followed in sequence by the Pleistocene machairodont, Smilodon (fig. 16), and culminating in the lion. The first three forms are doubtless in direct ancestral sequence, but I do not mean to say that the modern lion is derived from the Pleistocene machairodonts. Anteriorly and posteriorly the sylvian depres- sion, in the undetermined felid (fig. 10), is joined by shallow depressions which may represent sulci, the anterior one repre- senting the rhinal fissure, and a faint fissura rhinalis posterior indicating an extensive expansion of the neopallium. The base of the brain is clearly preserved (fig. 11) and indi- cates a variety of structures. The olfactory region is broken away (fig. 11, X). The optic nerves (N.opt., fig. 11) are indi- cated in the bases of the casts of the optic foramina. The optic chiasma is represented as a narrow structure with evidences of a median sulcus. The chiasma is rather longer than in the modern cat. The optic tracts are visible as smooth elevations on either side of the anterior portion of the pituitary body (fos. pit.), which is, of course, represented by the casts of the cavity of the sella turcica. The pituitary is longer and more oval in this ancient cat than in the modern form. Representing as they do diverse lines of the Aeleuroidea, we should not expect a close conformity of endocranial structure. Lying lateral and somewhat posterior to the hypophyseal elevation are three pairs of structures which I have interpreted 356 ROY L. MOODIE as the casts of the foramina orbitale, rotundum and ovale, or those passageways of the three main divisions of the trigeminal nerve, namely, the ophthalmic, the maxillary, and the man- dibular. Thisisstrikingly similar to the arrangement in the modern domestic cat. The rounded triangular elevation pos- terior to the bases of these nerves may be interpreted as the cast of the cavity in which lay the semilunar ganglion, embedded within the dura mater. Separated as the ganglion thus was from the floor of the brain case, it is difficult to interpret the slight ridges running fanwise posteriorly over the surface of the ganglia. They might be fasciculi if seen on the surface of the ganglia themselves. These elevations extend over on to the surfaces of the cerebral peduncles and may be interpreted as indications of small meningeal nerves. The cerebral peduncles are clearly preserved as a V, between the prongs of which is to be seen the well-marked interpeduncular fossa. The auditory region (Mos.pet.) is not readily interpreted. The internal acustic meatus (M.a.7.) is indicated in both sides as a broken oval base. The characters of the medulla are not clearly shown, since the topographic features of this organ are not clearly impressed upon the skull. The entire cast measures 73 mm. in length and displaces 50 cee. of water. The cerebrum, as preserved, measures 52 mm. in length and 45 mm. in width. The cerebellum extends 36 mm. transversely. The cerebellum (cer., figs. 9 and 10) is fairly well preserved. This is indeed fortunate, since the region was obscured in Dinictis (fig. 3), and we are thus allowed an insight into the nature of the cerebellum in Oligocene cats. The parts are identified according to the terminology of Bolk (’06). As in other primitive mam- malian brains, this organ is divided into three main lobules. The lobulus simplex is very prominent, at least I judge this prominence to be that lobule, though I am unable to identify the situation of the sulcus primarius, due to injury to the specimen. A less prominent division lying to either side of the prominent lobulus simplex is the lobulus paramedianus. It lies somewhat further laterally than in the modern lion, but otherwise has ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS 357 similar relations. Its prominence is due partly to the rather unusual extent anteriorly. The surface of the lobus anterior was a broad, uniformly arched dome similar to the condition found in other cats. On all these subdivisions traces of lamellae are evident, but nowhere are they sharply marked. ‘They are most in evidence on the lobulus paramedianus.