HARVARD UNIVERSITY LIBRARY OF THE Museum of Comparative Zoology BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE, IN CAMBRIDGE. VOL. XX. CAMBRIDGE, MASS., U.S.A. 1890-1891. Reprinted with the permission of the original publisher KRAUS REPRINT CORPORATION New York 1967 g -//A-c Printed in U.S.A. C O N T E N T S. Page No. 1.— Contributions from the Zoological Laboratory. XVII The His- tology and Development of the Eye in the Lobster. By G. H. Parker. (4 Plates.) May, 1890 1 No. 2. — On the Rate of Growth of Corals. By A. Agassiz. (4 Plates.) August, 1890 61 No. 3. — Preliminary Account of the Fossil Mammals from the White River and Loup Fork Formations. Part II. Carnivora and Artiodactyla, by W. B. Scott. Perissodactyla, by H. F. Osborn. (3 Plates.) ... 65 No. 4. — Contributions from the Zoological Laboratory. XIX. Cristatella : the Origin and Development of the Individual in tlie Colony. By C. B. Davenport. (11 Plates.) 101 No. 5. — Contributions from the Zoological Laboratory. XX. The Eyes in Blind Crayfishes. By G. H. Parker. (1 Plate.) November, 1890 . 153 No. 6. — Notice of Calamocrinus Diomedae, a New Stalked Crinoid from the Galapagos, dredged by the U. S. Fish Commission Steamer "Alba- tross." By A. Agassiz. December, 1890 165 No. 7. — Contributions from the Zoological Laboratory. XXI. The Origin and Development of the Central Nervous System in Limax niaximus. By Annik p. Henchman. (10 Plates.) December, 1890 169 No. 8. — Contributions from the Zoological Laboratory. XXII. The Parietal Eye in some Lizards from the Western United States. By W. E. Ritter. (4 Plates.) January, 1891 209 No. 1. — The Histology and Development of the Eye in the Lobster. By G. H. Paekee.i Table of Contents. I. Introduction Methods II. Histology 1. Corneal Hypoderrais . . . 2. Cone-cells 3. Distal Ketinulae . . . . 4. Intercellular Spaces of the Retina 6. Proximal Retinulae . . . . Page 1 3 4 6 10 15 19 20 Page 6. Accessory Pigment-cells . . 25 7. Innervation of Retina ... 26 III. Development 31 1. Plan of the Eye ..... 31 2. Optic Nerve 4-3 3. Differentiation of Ommatidia 45 4. Types of Ommatidia ... 56 IV. Bibliography 69 V. Explanation of Figures ... 60 IXTRODUCTIOX. Through the kindness of Mr. Alexander Agassiz it was my privilege to spend the greater part of the summer of 1S87 at the Newport Marine Laboratory. During the preceding winter I had been interested in the structure of the eyes in Arthropods, especially in the inversion of the retina in Arachnoids and my instructoi-, Dr. E. L. ]\Iark, had called my attention to the importance of ascertaining whether the retina in the compound eyes of Crustaceans was inverted or not. At about this time Kingsley ('86") published his preliminary account of the development of the compound eye of Crangon, and claimed that in this crustacean, as in spiders, the retina was inverted. For reasons which I shall mention in the course of this paper, Kingsley's account did not seem fully sat- isfactory to me, and consequently I decided to study for myself the development of the eye in a crustacean. My visit to the Newport Laboratory offered an excellent opportunity to collect embryological material for such a study. During August and September spawning lobsters were easily obtained, and I therefoi'e determined to study the eye in tlie lobster, Hoviarus americantis, Edwards. A series of lobsters' eggs were collected, and before leaving Newport my observa- ^ Contributions from the Zoological Laboratory of the Museum of Comparative Zoology, under the direction of E, L. Mark, No. XVII. VOL. XX. — NO. 1. 1 2 BULLETIN OF THE tious had been carried far enough to satisfy me that the retina in the lobster was a simple ectodermic thickening. On returning to Cam- bridge fi'om Newport, the study of the lobster's eye was continued in the Embryological Laboratory at Harvard College, under the direction of Dr. Mark. Here I completed the observations on the development of the eye, and studied its histology. In the fall of 1888 a brief pre- liminaiy account of the results which are now presented in full was published in " The Proceedings of the American Academy of Arts and Sciences," Vol. XXIV. pp. 24, 25. In procuring at Newport the necessary stages in the development of the lobster I proceeded as follows. Female lobsters with eggs were obtained from the fishermen, and kept in floating latticed boxes which were anchored in the small cove beside the Laboratory. A few eggs were taken daily from each lobster. The reagents which I employed in killing the eggs were Kleinenberg's picro-sulpluu'ic acid, J'erenyi's fluid, a saturated aqueous solution of corrosive sublimate, and hot water. The eggs which were prepared with corrosive sublimate were rendered almost useless by the subsequent formation of a fine precipitate. Those which were killed in Kleinen- berg's pici'o-sulphuric acid and in Perenyi's fluid gave fair results ; the latter reagent left the yolk in good condition for cutting. The best results, however, were obtained by the use of hot water. Eggs which had been prepared in this way could be easily shelled, and the embryos could be readily dissected from the yolk. The separation of the embryo from the yolk proved to be a great advantage, and obviated the necessity of cutting the yolk, a tedious process in an egg as large as the lobster's. In the following account of the development of the lobster's eye, the stages which it is necessary to describe are taken from different sets of eggs. These sets were froin different lobsters, consequently I cannot state with exactness their relative ages. I shall therefore characterize them by their most evident structural peculiarities. Beginning with the earliest stage and proceeding to the later ones, I have lettered them A, B, C, D, E, and F. Set A is in the stage of the " egg-nauplius " ; in this set the characto.'stic three pairs of appendages are easily distin- guishable. In set B the thoracic appendages have begun to form. This stage corresponds very closely to what Pieichenbach ('86, Plate III. Fig. 11) has designated in the crayfish as stage H. In stage C the first trace of pigment in the retina is visible. Stage D is from the same series of eggs as stage C, but is seven days older than C. In both MUSEUM OF COMPARATIVE ZOOLOGY. 6 Stages C and D, the abdomen of the embryo is recurved, and reaches forward covering tlie space between the optic lobes. Stage E corre- sponds to the time of hatching. Stage F is represented by a young lobster one inch in length. The younger stages which follow^ the hatching of the lobster are obtained with considerable difficulty, and I am under obligations to several of my friends for material which covers this period. For some lobsters in the " Schizopod " stage I am indebted to Mr. Sho AVatase. Mr. H. H. Field and Mr. Carl H. Eigeumann kindly collected for me some young lobsters one inch in length. From Mr. F. L. Washburn I received the eyes of several half-grown lobsters, six to eight inches in length. The material which I used in studying the histology of the eye in the adult was very kindly supplied to me by A. T. Xicker- son and Company, of Charlestown, Mass. If et hods. The methods of staining, embedding, etc., which I have employed, are those known to all students of modern histology. In one case, the staining of nerve-fibres, I have used a method which I accidentally discovered while experimenting with Weigert's hsematoxylin. In employing this method it is necessary to stain the sections on the slide. The way in which I have stained sections on the slide has already been described ('87, p. 175). Further experience has shown, however, that the successful employment of this method necessitates a careful observance of certain precautions. These I have not sufficiently em- phasized in my former account, and I therefore redescribe the method, calling especial attention to the precautions. The method consists in a cautious use of Sehallibaum's fixative. The fixative which I have em- ployed is composed of clove oil three parts and Squibb's flexible collo- dion one part. The mixture before being used should be allowed to stand for about a week. After several months it may become ineffective. When working, I usually employ the fixative frequently enough to fol- low its changes, and at the first signs of fiiilm-e I make a new mixture. If for any reason I have not used the fixative for some time, I test it with a few waste sections before employing it with valuable material. In using it a moderate amount is applied to the slide, and the sections in paraffine are placed on it. The slide and its sections are now sub- jected to a temperature of 58° C. for fifteen mini;tcs. It is important to observe carefully both the length of time during which the slide is heated and the temperature to which it is raised. At the end of fifteen 4 BULLETIN OF THE minutes, the slide, wliile warm, is thoroughly washed with flowing tur- pentine. This can be applied conveniently from a small wash-bottle. All of the paraffine should be removed from the slide before it becomes cool, otherwise on cooling some paraffine may solidify. This is liable to loosen the film of collodion. The wash of turpentine should be contin- ued not only till the paraffine is thoroughly removed, but till the slide is cool. Then, and not till then, can the turpentine be safely replaced by alcohol, first 95%, then 70%, 50%, and 35%, and finally it can be im- mersed in water. After once having got the slide with its sections into water, the subsequent treatment with alcohol and water seems to have no effect in loosening the sections, although the film of collodion will dissolve easily in ether. I have veiy generally employed this method of staining for two years, and as it obviates the dilliculties which arise from maceration or partial penetration of dyes, I use it in jDrcference to staining in toto. I have lost very few sections by it, and such accidents as I have had were due, I believe, to a neglect of some of the precau- tions which have been mentioned. The method of staining nerve-fibres which I have employed consists of a modified use of Weigert's haematoxylin. The tissue which was stained by this method was for the most part killed in hot water, although I have also successfully stained nerve-fibres which were killed in chromic acid and Kleinenberg's picro-sulphuric acid. Sections of the optic nerve which had been mounted on the slide and carried into water were treated for about half a minute with an aqueous solution of potassic hydrate 1^0%. They were then thoroughly rinsed in distilled water and transferred to Weigert's hsematoxylin. Here they remained for about three hours at a temperature of 50° C. They were then rinsed again in distilled water, carried through the grades of alcohol, and after being dehydrated with alcohol of about 99%, they were cleared in turpentine and mounted in benzole balsam. Each nerve-fibre when so treated had a distinct blue-gray outline. The sections do not over- stain even when they are kept in the dye for a prolonged period, and there is of course no subsequent decoloring. This method yields fair results when applied to nerves from any part of the lobster's body, but it is especially successful in treating that portion of the optic nerve which intervenes between the retina and the optic ganglion. The Histology. The two movable eye-stalks of the lobster are situated one on either side of the rostrum, at the angle which that structure makes with the MUSEUM OF COMPARATIVE ZOOLOGY. 5 anterior edge of the carapace. The form of the eye-stalk approaches that of a short cylinder terminated by a hemisphere. The cylindrical part of the stalk resembles the general surface of the body in that it is covered with a tirm, calcified cuticula. Excepting a portion of the surface next the rostrum, the whole of the hemispherical part during life is black, and covered with a flexible cuticula. The black area de- fines the position of the retina. That portion of the hemispherical surface which is not black, and which faces the rostrum, is covered with a peninsula-shaped piece of inflexible cuticula. A broad isthmus of the same kind of cuticula connects this with the shell of the cylindrical part. The absence of the retina from the peninsula-shaped portion of the hemisphere is due in all probability to the fact that the field of vision for this part of the hemisphere is cut off" by the rostrum. The remainder of the hemisphere, that part on which the retina is devel- oped, faces away from the lobster's body, and its field of vision is not permanently obstructed by any part of the animal. A section perpendicular to the sm-face, and cutting the eye-stalk in a region where the cylindrical and hemispherical parts unite, is shown in Figure 26. The thick, calcified cuticula of the cylindrical part is indi- cated at eta. On the inner surface of this cuticula is a thin hypoder- mis (Ji d.). The hypodermis is bounded on its inner face by a basement membrane (jnh.). The cuticula of the hemispherical part {cm.) is thin and flexible. It can be designated by the name corneal cuticula. (Compare Patten, '86, p. 544.) Eesting on the deep face of the corneal cuticula is the thick cellular layer, named by Lankester and Bourne the ommateum (omm'.). The proximal face of the ommateum is limited by a basement membrane, which is continuous with tliat bounding the corresponding face of the undifferentiated hypodermis. The onunateum is continuous with the hypodermis, and in fact can be regarded as a thickening of that layer. Can-iere ('85, p. 169) has already pointed out in the eye of Astacus a similar relation between the hypodermis and ommateum, and he believes that this relation holds good for all Decapods. On inspecting the external face of the corneal cuticula, one finds it divided into an immense number of square facets, one of wliich is shown in Figure 2. Although as a rule the outline of the facet is squai^e, it is not invariably so ; for on the margin of the retinal area close to where the ommateum passes over into the undiflerentiated hypodermis, the outline often becomes somewhat irregular, and more frequently presents the form of a hexagon than of a square (Fig. 59). The number of facets in each eye of an adult lobster is about 13,500. 6 BULLETIN OF THE In the ommateum the cells are arranged in specialized groups or ommatidia. There is a single ommatidium under each corneal facet, consequently in any given eye the number of ommatidia equals the number of facets. The cellular composition of each ommatidium is best understood from a comparison of longitudinal and transverse sec- tions. Figure 1 represents a longitudinal section through an ommatid- ium. The thick lamellated layer {c7-n.) at the distal end is the corneal cuticula. Directly below this is a thin layer of cells, the corneal hypo- dermis (cm. hd.). Following on the corneal hypodermis are the cone- cells {d. con.). They are very long, and extend from the corneal hypodermis inward till their proximal ends disappear in the deep part of the retina. In reality they terminate upon the basement membrane. Their distal ends in the region of the crystalline cones are surrounded by pigment- cells, to which I give the name distal retinuhc {rtn'. dst.). These, like the cone-cells, extend to the deeper part of the retina. Here the proxi- mal retinulcC and accessory pigment-cells occur. The proximal retinulaj are elongated cells (i-tn'. px.), and contain black pigment. They sur- round the rhabdoraes (rhb.). The accessor}' pigment-cells are irregular cells, which fill the space between the deep ends of the pi'oximal reti- nulaj. They contain a pigment which is whitish by reflected and yellow- ish by transmitted light. Their nuclei are shown at 7iL pi'j.. Figure 1. The last two kinds of pigment-cells described rest upon the basement membrane {mh.); below this membrane the fibres of the optic nerve can be seen (n.fbr.). From this description it will be seen that the ommateum lies between the corneal cuticula and the basement membrane, and is comjwsed of the following kinds of elements : cells of the corneal hypodermis, cone- cells, distal retinula?, proximal retinula?, and accessory pigment-cells. The numl^ers and positions of these cells are best made out from trans- verse sections. The several kinds of cells will be discussed in the order named. The Corneal Hypodermis. That the corneal cuticula in Decapods is separated from the cone-cells by an intervening layer of cells is a view which has been held only by recent investigators, Grenacher ('79, p. 123), in his account of the eyes in Decapods makes no mention of such a layer, and leaves one to con- clude that the cone-cells abut against the cuticula. Clans ('8G, p. iu) suspected the presence of a corneal hypodermis in Decapods, Schizopods, and Stomatopods, but his seai-ch for it was in vain. MUSEUM OF COMPAKATIVE ZOOLOGY. 7 The view that the cuticula and cone-cells are in contact, is strongly contrasted with that maintained by Patten ('8G, pp. 62G, 642). Ac- cording to this writer, the corneal cuticula is due to the activity of a layer of cells, the corneal hypodermis, which intervenes between the cuticula and the cone-cells. Patten has identitied the corneal hypo- dermis in the following genera of Decapods : PenaBus, Palsemon, Pagurus, and Galathea. It has also been described by Kingsley ('8G, p. 863) in the eyes of Crangon, and by Herrick ('86, p. 43) in the eyes of Alpheus. Carriere ('89, p. 225) has recorded it iu the eye of Astacus, and there is now good reason for believing that a corneal hypodermis exists in the eyes of all Decapods. Patten's statement ('86, jjp. 665, 666) that the corneal hypodermis "has been invariably overlooked by Grenacher," and Kingsley 's asser- tion ('86, p. 863) that the existence of the corneal hypodermis " was utterly ignored by Grenacher," are perhaps a trifle too strong. It seems much more probable that Grenacher confused the nuclei of the cone- cells and corneal hypodermis. He evidently never saw both kinds of nuclei in the eye of the same Decapod. In some cases he may have described the nuclei of the cone-cells, in other cases those of the corneal hypodermis. In both instances what he described he took to be the nuclei of the cone-cells. In the eye of Mysis, I believe that he ('79, p. 118) described the nuclei of both the cone-cells and corneal hypoder- mis, although iu this case he was of the opinion that both sets of nuclei belonged to the cone-cells. "Where only one set is figured, it is difficult to decide whether he has given the nuclei of the cone-cells or of the corneal hypodermis. So far as I am aware, there are always in each ommatid*- ium of a Decapod two hypodermal nuclei, and four nuclei in the cone- cells. This numerical relation is sufficient to distinguish the groups of nuclei, but it can only be employed satisfoctorily where transverse sec- tions at the proper niveau are given. Unfortunately, in the Decapods, Grenacher did not figure any such sections, and it is therefore difficult to decide in particular cases which kind of nuclei he has described. In the lobster a well differentiated corneal hypodermis has already been pointed out (Fig. 1, cm. hd.). In transverse sections this presents the appearance of squares of granular protoplasm (Fig. 3). Each square contains two nuclei, and is bounded by a membrane. A nan-ow space filled with granular substance separates the membranes of adjacent squares. From the longitudinal section (Fig. 1) it will be seen that these squares are relatively thin, so that their proportions are somewhat like those of square tiles. The outer face of the tile is flat ; its inner 8 BULLETIN OF THE face is hollowed, however, so that its centre is the thinnest part. In a few cases the corneal hypodermis has appeared as cubical blocks, rather than as tiles. This thickened condition probably indicates an increased functional activity, and the more frequently occurring tile-like condition may correspond to a quiescent stage. The two nuclei contained in each square are placed some distance apart, and on one of the diagonals of the square (Fig. 3). Their long axes are approximately parallel to the other diagonal. In a given eye all the squares agree in having the nuclei on parallel diagonals. The pres- ence of two nuclei in a square indicates that the square consists of two cells. Any membrane separating the two cells must necessarily pass between the two nuclei, but all attempts to discover such a membrane have failed. However, for a reason winch will be given shortly, I be- lieve that the protoplasm of the hypodermal square is divided by the diagonal which lies between the nuclei. In the centre of each square several oval or round outlines are usually visible (Fig. 3). These are vesicular bodies which occur in the distal ends of the cone-cells, and which can be seen through the very thin corneal hypodermis. The corneal cuticula is the result of the activity of the corneal hypo- dermis. Viewed from the surface, the cuticula is divided by narrow bands into square facets (Fig. 2). Each facet is external to a hypoder- mal sqiiare. The proximal and distal faces of each facet, as can be seen in the transverse section (Fig. 1), are very nearly flat, the proximal face only being a trifle convex. This convexity, however, is so slight that one cannot attribute to the facet the character of a lens. When a piece of corneal cuticula is cleaned by treating it with potas- sic hydrate, and is then examined in water, the markings which are visible with difficulty in preparations mounted in balsam are easily seen. Each facet in addition to its narrow limiting bands has a faintly marked diagonal band which divides the square into two equal triangles (Fig. 2). In the different facets of a given eye the diagonal bands are pai-allel. Newton (73, p. 327, Plate XYL Fig. 3), in describing the structure of the eye in the lobster, states that each facet is crossed by two diagonals at rio-ht an2:les to each other. This statement I cannot confirm, for, although I have searched with care, I have never succeeded in finding moi'e than a single diagonal in each facet. In the middle of the diagonal there is an in-egular hazy patch. This at times has a distinctly marked cross in it. When the cross is present, one of its axes lies in the diago- nal band, the other extends at right angles to the band (Fig. 2). Whether all of these markings extend through the substance of the MUSEUM OF COMPAEATIVE ZOOLOGY. y cuticula, or whether they are confined to its surface, is difficult to say. The production of the cuticula is such a uniform process that one would naturally expect to find that the marking extended through it, for the successive layers would be similarly marked, and thus bands would be established extending from its deep to its superficial face. Concerning the vertical extension of the bands between the facets there is no ques- tion, for in transverse sections of the cuticula (Fig. 1, x) they reappear in their proper positions, and extend from one surface to the other. Owing to the roughness of the cut face, they are much less readily de- tected in sections than when viewed from the outer surface of the cutic- ula (compare Figs. 1 and 2). The diagonal band and its central spot have not been observed in transverse sections, even when the plane of section is in the most advantageous position for demonstrating these structures. Notwithstanding their apparent absence, both may be pres- ent, although indiscernible. For even in the superficial view, when the outline of the facet was so readily visible, the diagonal band was only faintly seen. In transverse sections, where the distinct boundary of the facet is visible with difficulty, one should not expect to see the much fainter diagonal. On comparing the diagonal band and the boundary of the fiicet by focusing through the corneal cuticula, I was unable to distinguish a greater vertical extension in the one than in the other. Since it has been shown that the boundary^ of the facet extends through the cuticula, this observation supports the conclusion that the diagonal band also extends through it. Patten ('86, pp. 626, 627) has described in the facet of Penseus a band which has many resemblances to the diagonal band in the lobster. It is not diagonal, however, but transverse, and divides the square facet into two equal rectangles, in which the sides are in the proportion of one to two. I have already given my reason for believing that the diagonal band in the coniea of the lobster extends through the sub- stance of the cuticula. Patten states that the transverse band in Penaeus is only a superficial structure, and says ('86, p. 627) that in cleaning the cuticula " when the treatment with caustic potash has been carried to excess, all markings disappear except the contours of the facets." I have subjected the corneal cuticula of a lobster to a boiling solution of potassic hydrate (75%) for a quarter of an hour, and, al- though the potash completely cleaned the cuticula, the outlines of the facets, the diagonal band, and- its spot were as readily visible after this treatment as before. A second and third trial with the same piece of cuticula did not noticeably eff"ect the markings. In this respect, then, 10 BULLETIN OF THE the diagonal band in the lobster is materially different from the trans- verse baud in Penseus, and I conclude that in the cornea of the lobster the limiting and diagonal bands are essentially similar in that they both extend through the cuticula. In all probability the bands between the facets were produced during the secretion of the cuticula by the interference of the partitions which separate the hypodernial squares. If this be true, it is probable that the diagonal bands represent a lil;e interference. It is important to notice that the diagonal band in the cuticula corresponds to the imagi- nary diagonal which lies between the nuclei of each hypodermal square, never to the diagonal which crosses the nuclei (compare Figs. 2 and 3). This diagonal then corresponds to the position in which one woidd look for a membrane between the pair of hypodermal cells ; and although such a structure has not been observed, the diagonal band in the cornea is a strong indication of its presence. Admitting this to be the significance of the diagonal band, it is but natural to expect tliat, if deeper cells touch the cuticula, they would pass outward between the hypodermal cells. The fiict that the hazy patch which lies in the middle of the facet is always on the diagonal band, and directly external to the distal tips of the cone-cells, leads to the belief that this patch marks the place where the cone-cells pass between the cells of the hypodermis and touch the cuticula. I am not of opin- ion that the patch is produced by the secretion of the cone-cells, al- though I have no evidence that the cone-cells cannot produce cuticula at their distal tips. It seems to me more probable that they have given rise to the patch by a series of interrupted interferences with the ac- tivity of the corneal hypodermis. If such be the case, a distinct cross might be produced when the area of interference was definitely circum- scribed. AVhen the area was not so sharply bounded, a hazy patch with indistinct outlines might be the result. From the facts which have been presented, I conclude that each hypo- dermal square consists of two flattened cells, triangular in outline, and very intimately applied on their longest sides. The Cone-cells. One of the most important questions in the anatomy of the cells of the crystalline cones (retinophorse) concerns the relation which these cells bear to the rhabdome. ]\Iax Schultze was the first to maintain ('67, p. 407) that the cone-cells and rhabdomes were separate struc- tures, f^renacher's researches lead to the same conclusion. As an oppo- MUSEUM OF COMPARATIVE ZOOLOGY. 11 nent of this view, Patten ('86, p. 670) has claimed that the cone-cells and rhabdome were continuous, and in fact that the rhabdome of the compound eye was only an enlargement of the proximal end of the cone- cell. Kingsley ('86, p. 863) in his description of the eye in Crangon supported Patten's view. Of those authors who maintain the separateness of the cone-cells and rhabdome, no one, I believe, has given a fully satisfactory account of the ■way in which the proximal ends of the cone-cells terminate. Grenacher, in describing the eye in Paleemon said ('79, p. 123) : " Die fein ausge- zogene Spitze dieser Pyramide [the cone-cells] durchsetzt, bevor sie in contact niit der Retinula tritt, zuerst eine in Form eines Hohlcylinders sie umhlillende Pigmentmasse um sich dann in das Vorderende der Retinula eine Strecke weit einzusenken." A more detailed account was given by Schultze, who, after stating ('68, p. 10) that in some crusta- ceans the cone-cells appeared to terminate a little in front of the distal end of the rhabdome, said that in the crayfish "geht der Kxystallkegel nach unten in vier Spitzen aus, welche sich aus den vier Kanten der Oberflache entwickeln und das obere Ende des nervosen ebenfalls vier- kantigen Sehstabes umschliessen. Die vier Spitzen legen sich dabei an die Kanten des letzteren an und laufen als lange feine Fiiden auf der Oberflache des Sehstabes herab, diesen umklammernd und mit ihm ober- flachlich verbunden aber durch INIaceration isolirbar. Gegen das Ende spitzen sie sich fein zu und verlieren sich auf der Oberflache des Korpers, den sie umfassen." This account is the most complete of any that I have seen, and yet that Schultze w^as not fully satisfied that he had seen the proximal termination of the cone-cells is probable from the fact that he says the fibres are lost on the surface of the rhabdome. The relation of the rhabdome to the cone-cells, and the way in which these cells terminate in the lobster, is as follows. As in other Deca- pods, each ommatidium in the eye of the lobster contains four crystalline cone-cells. Together these cells form an elongated pyramid, with its base next the corneal hypodermis aud its apox on the basement mem- brane (Fig. 1, cl. con.). At the distal end of the ommatidium, in the region which corresponds to the base of the pyramid, tlie four cells are closely applied to each other. This condition is maintained till the deeper part of the ommatidium is reached. Here the four cells, reduced to fibres, separate and end independently on the basement membi'ane. A transvei'se section of the distal ends of the cone-cells is shown in Figure 4. On the external faces of each group of four cells there is a 12 BULLETIN OF THE distinct bounding membrane {mh. pi ph.). This can be called the periph- eral membrane. The four cells in each group are separated one from another by delicate membranes {mh. i cL), which often show undoubted continuity with the peripheral membrane. These membranes, since, they lie between the cone-cells, can be called the intercellular membranes. The distal end of each cell contains coarsely granular protoplasm and a nucleus (Fig. 4, nl. con.). The nuclei usually lie in the external angles of the cells, and do not readily take up coloring matter. The terminal granular protoplasm of the four cells forms a distal cap (Fig. 1, cap^. This cap fills the concavity on the proximal face of the corneal hypo- dermis, and its central distal tip probably passes between the pair of hypodermal cells and touclies the cuticula. The spot or cross which is thus probably produced in the centre of each fticet has already been described. Below the cap of granular protoplasm is the crystalline cone. The firm pcriphei'al membrane of the cap is continued over the cone and pi'oximal part of the cone-cell. Tlie distal end of the cone in cross- section is a s(piare with rounded angles. At this end there is no indi- cation of a division of the cone into four segments corresponding to the four cells. A transverse section midway the length of the cone (Fig. 5) shows no featux'es essontially different from those of the section across the distal end. The proximal end of the cone in cross-section is nearly circular (Fig. 6). On the sides of this end of the cone one often notices small re-entrant angles (Fig. 6, x). The peripheral membrane dips into these. The angles are usually foiu- in number, never more, and occupy positions which indicate the planes of separation between the four cone- cells. In some cases delicate membranes originating from the angles di- vide the substance of the cone into its fuur constituents (Fig. 6, vib. i cL). These membranes correspond in position to the intercellular membranes at the distal end of the cone-cells. The substance of the cone is very finely granular. The four constituents of each cone terminate very nearly at one level. In passing in a proximal direction through a series of sections, the substance of the cone is last seen as a thickening which flanks the cell membranes, especially the intercellular membranes. (Compare Figs. 1 and 6.) Below the cones the outlines of the four cone-cells are well marked by both peripheral and intercellular membranes (Fig. 7). The inter- cellular membranes arc continuous with those seen in the proximal ends of some cones. In this region the cells contain coarsely granular proto- plasm. In passing from the deep ends of the cones to the proximal MUSEUM OF COMPAEATIVE ZOOLOGY. 13 retinulse, the most striking difference noticed in the cone-cells is a dimi- nution in their diameter. (Compare Figs, 1, 6, and 8.) On a level with the distal ends of the proximal retinulse, the groups of cone-cells still retain their four-parted character (Fig. 9, cl. con.). Each group is easily traced between the retinuUe till it approaches the distal end of the rhabdome. The change which here takes place is represented in Fig- ure 12. Of the four ommatidia which are shown in this section, the one indicated at a is cut slightly above the rhabdome. In the case of om- matidia h, c, and d, the plane of section passes through the end of the rhabdome. In each of these three, it will be noticed that the rhabdome is surrounded by four bodies, which correspond in position to the four cone-cells. The four ommatidia which are drawn in Figure 12 are in no way exceptional, but represent a very usual condition. Many such cases have been examined, and whenever the tip of the rhabdome was in the section, it was invariably surrounded by the four bodies previously mentioned. When, on the other hand, the plane of section did not pass through the rhabdome, only the four cone-cells were present. The round bodies at the sides of the rhabdome can be traced from section to section, and I therefore believe them to be fibres. Moreover, there is no break observable in their continuity with the cone-cells, and I therefore further believe that they are the fibrous prolongations of the cone-cells. They have one peculiarity which is worthy of comment. As the cone-cell passes over into' the fibre, a considerable diminution in its diameter takes place. This is accomplished at the distal end of the rhabdome, and within a space equal to the thickness of one or at most two sections (7.5-15 /x). Occasionally there is to be seen a group, in which one or two cells have been reduced to fibres, and the remaining ones are as large in transverse section as an individual in ommatid- ium a (Fig. 12). The conclusions which are arrived at from the study of sections are confirmed by isolation-preparations. Figure 28 represents a portion of an isolated group of four cone-cells from a single omma- tidium. In the distal part of the specimen the four cells are intimately bound together, but at the proximal end they appear as four separate fibres. As in the transverse section (Fig. 12), the continuity of the fibrous and thicker portion of the cone-cell, and the rapid reduction of the cone-cell to form the fibre, are plainly seen. The cone-cells are usually somewhat separated before they reach the rhabdome. It can scarcely be said that they touch the rhabdome, although this is the region in which they are nearest to it. As the fibres pass into the deeper part of the retina they are found to lie nearer the periphery of the ommatid- 14 BULLETIN OF THE ium. They lie between the proximal retinulse, but at some distance from the rhabdome (Fig. 14, cl. con.). They still retain, however, the same relative positions in the ommatidium. Their peripheral location is maintained (Fig. 17) till tliey are very close to the basement membrane. The changes which the fibres undergo as they approach the basement membrane is shown in Figure 19. In this section the plane of cutting was slightly oblique to the basement membrane. Of the three omma- tidia which are here represented, those indicated at c and d are cut at g.bout the same level, and very close to the basement membrane. Om- matidium h is cut farther from the membrane than either c or d. The fibres of the cone-cells are closer to each other in c and (/ than in b. As this condition is generally true in other sections, it follows that, as we approach the basement membrane, the fibres of the Cone-cells con- verge. The convergence is also shown in Figure 21. Of the omma- tidia here figured, b is cut farthest from the basement membrane. In it the four cone-cells (c^. con.) can be seen, and between them a dot which represents the proximal end of the rhabdome {rhb.). Nearer the basement membrane is ommatidium a, in which the four cone-cells can be recognized, and to one side a fibrous area. The rhabdome does not extend as deep as this. The fibrous area represents a region in which the plane of section passes through an elevation on the distal face of the basement membrane. Ommatidium (/ is still nearer the membrane. The cone-cells are here brought more closely together, and are sur- rounded by the fibrous substance .of the elevation. The form of the elevation is now seen to be that of a cross. At x is shown a basal section of the cross-shaped elevation surrounded by four large openings through the basement membrane. The cone-cells are no longer visible at this level, and I therefore believe thnt without penetrating the basement membrane they terminate in these elevations. This belief is further supported by the fact that in transverse sections of the basement mem- brane the cone-cells distinctly end in the substance of these elevations (Fig. 29, cl. con.). The facts obtained from a study of the lobster's eye support the claim made by Schultze and Grenacher, that the cone-cells and rhabdomes are separate structures. In the case of the crayfish, Schultze, moreover, saw the prolongations of the cone-cells, and traced them into the deeper part of the retina. It is probable that in the crayfish, as in the lobster, the fibrous ends of the cone-cells terminate in the basement membrane, but this Schultze did not see. Such an omission is by no means sur- MUSEUM OF COMPARATIVE ZOOLOGY. 15 prising; for when we reflect upon the methods at his command, it is remai'kable what success he had in tracing the course of the fibres, and in demonstrating tiie relation of the cone-cells and rhabdome. Patten has advanced the view that the cone-cells are provided w'ith an axial nerve-fibre, and that the cone itself is the true perceptive ele- ment. I shall defer the consideration of this topic till I describe the innervation of the retina. The Distal Hetinulce. Surrounding each crystalline cone are two pigment-cells, the distal retinulse. These cells not only surround the cone, but extend as fibres into the proximal part of tlie retina (Fig. 1, rt7i'. cht.). The relation which the distal retinulse sustain to the cone can be studied most readily in transverse sections. In a section passing through the distal end of the cone (Fig. 4, ommatidium a), it will be observed that of the four lateral faces which the cone presents, two, the lower and left-hand ones, are covered by a single retinula (rtn'. dst.). The retinula is thickest at the lower left-hand angle of the cone, and becomes thinner the farther it extends on the two adjacent faces. At the more distant edges of these two faces the retinula terminates. Thus the retinula is composed of a central portion and two blade-like extensions. Each blade covers one face of the cone. The second retinula is essentially like the one just described, but lies at the upper right-hand angle of the cone and covers its upper and right-hand faces. In this way the four faces of the cone are sheathed by a pair of retinulcG. On inspecting the arrangement of the retinulse in adjoining omma- tidia (Fig. 4), it is evident that they are so placed that the thick end of each blade-like portion is opposite the thin end of the blade of a neighboring retinula, and that, in passing along the space between the cones, as one retinula becomes thicker the other becomes thinner. The delicate membranes which separate the blades consequently extend obliquely across the spaces between the cones (Fig. 4). In the space w'hich is left between the angles of four adjoining cones the mem- branes of the retinulse are very much thickened. That this is a thickening in the membrane of the retinula, and not due to substance produced by the cone-cells, seems probable for two reasons. First, the membrane is often somewhat thickened in regions between two retinulce, and where the cone-cells could not well touch it. Such thickenings are directly continuous with the larger ones already mentioned (Fig. 4). 16 BULLETIN OF THE Secondly, in isolation-preparations the thickenings always remain at- tached to the retinulaj ; the cones, on the other hand, are covered with membranes of uniform thickness. The thickening in the membrane is not characteristic of the whole length of the retinula, but is pecu- liar to the region corresponding in level to the distal end of the cone (Fig. 1). The foregoing description applies to the structure of the distal retinulse as seen in the plane of Figure 4. This plane passes through the outer ends of the cones. In other regions the retinuloe present somewhat different conditions. The relation of the retinulaj to the hypodermal squares is shown in a section (Fig. 3) which is slightly more superficial than that just described. The two retinuke which were located at the two angles of the cones here occupy the corre- sponding angles of the hypodermal squares. They do not, however, entirely cover the four lateral faces of the square, as they did those of the cone, but from the angles at which they are located they extend over half of each of the adjoining faces. It follows from this that together they flank only one half of the lateral exposure of the square. The blades are now no longer wedge-shaped in transverse section, nor do they overlap neighboring blades, but each one stretches completely across the space in which it lies. Of the lateral surface of the square. that half which is not sheathed by its own pair of retinulai is covered by the arms of four adjacent retinidie. Consequently, six retinula3 in all touch each hypodermal square. Two of these belong to the omma- tidium which is represented by the square ; four belong to adjoining ommatidia. The relation of these will be readily seen by refeiTing to Figure 3. In passing from the plane in which each cone iS surroimded by its own pair of retinulas to the one in which the corresponding hypodermal square is surrounded by six retinulse, the blades of the two retinulte proper to the cone undergo a gradual narrowing ; so that, instead of each blade covering the whole of one face of the cone, it covers less and less, and eventually sheathes only one half of the corresponding face of the hypodermal square. As the blades of the retinulse become narrower, they expose the surface of the cone, but this is still kept covered by the retinuloe of adjoining ommatidia. In any ommatidium there are four blades which become narrow, consequently there are four regions in which adjacent retinulse touch the cones; and as there is a separate retinula for each region, it follows that four additional retinuke here come in contact with the cone. MUSEUM OF COMPARATIVE ZOOLOGY. 17 The distal retinulpe touch the cuticula along the band which marks the boundaries of the facets. That the retiuulas contribute to the formation of the cuticula is very improbable, although I believe that it is largely through their interference that the outlines of the facets are produced. The lateral surfaces of each cone are completely enclosed by retinuliB ; the pair of retinulsE belonging to the cone play the principal part. That portion of each retinula which encloses the proximal two thirds of the cone is densely pigmented (Fig. 1). In transverse sections through the pigmented region one can see that each cone is completely sur- rounded by a pigment band. On a level with the middle of the cone each retinula contains a nucleus (Figs. 1 and 5, nl. dst.). This is im- bedded in pigment. The membranes of the retinulae are less distinct in the pigmented region than near the distal end of the cone. The only membrane which was observed (Fig. 5) was one which corresponds to the thickened membrane shown in Figure 4. At the proximal end of the cone, the retinulje rapidly contract till they are reduced to fibres (Figs. 6 and 7, rtn' . dst.). The pigment is present for only a slight distance below this level. The fibres of the retinula3 are grouped in pairs, and in this relation extend to the proximal part of the retina. It is noticeable that the two fibres which constitute a pair are derived, not from a single ommatidium, but from two adjacent ommatidia. These fibres when seen in longitudinal sections were probably mistaken by Newton ('73, pp. 328, 329) for an investing membrane. At least, in all attempts to demonstrate the existence of such a membrane I have failed, and there is so strong a resemblance between the fibres of the distal retinula^ and the structure which Newton figured ('73, Plate XVII, Fig. 15) as the cut edge of an investing membrane, that I am inclined to think them identical. Transverse sections from the proper region would have settled the question whether these bodies were fibres or membranes, but unfortunately Newton has not figured any such sections. The pair of fibres in passing ft-om the basal ends of the cones to the proximal retinulte retain the same relative position, and are only slightly reduced in diameter. (Compare Figs. 7 and 8, rtn'. dst.) Deeper than this, they are still identifi.able, and can be distinguished from the fibres of the cone-cells by their slightly greater diameter, and by the fact that they are always in pairs. They lie in the space between four ommatidia. (Compare Figs. 12, 15, and 18.) Till within a very short distance of the basement membrane they maintain the condition shown VOL. XX. — NO. 1. 2 18 BULLETIN OF THE iu Figure 18, /•<«,'. dst. Beyond this I have not been able to trace them with certainty. The groups are no longer observable, and it is probable that the fibres have separated. I know that in this region the other cells suffer a very considerable rearrangement ; and such being the case, it would be a very difficult matter to identify single fibres, especially fibres as small as these are. I have not found any satisfactory method of staining the fibres so as to distinguish them, as in the case of the fibrous ends of the cone-cells. I can therefore claim to have traced the fibres only to within about 20 p. from the basement membrane. As I have already mentioned, the distal face of the basement mem- brane has cross-shaped thickenings on it. In the angles which the arms of the cross make with each other, the basement membrane is perforated. There are consequently around each cross four openings through the membrane. Each opening, however, lies between two crosses, so that in reality only one half of each opening belongs to a given cross, or, if one counts whole openings only, half of the four openings, i. e. two openings, belong to each cross. The crosses correspond in number and position to ommatidia, hence there are also two openings for each ommatidium. In each opening, beside three or four large fibres which will be described later, one finds a single small fibre (Fig. 21, rtJi'. dst.). That this fibre represents the continuation of the fibrous end of a distal retinula seems probable, for two reasons. First, the diameters of this fibre and of the fibrous part of the distal retinulse are so nearly the same as to be undis- tinguishable. Secondly, the number of fibres which pass through the basement membrane, two for each ommatidium, agrees with the number of distal retinulae in each ommatidium. I therefore believe that the small fibres which are seen in the openings through the basement mem- brane are the proximal continuations of the fibres of the distal retinulae. If this explanation be true, then it is only natural to expect that, as a pair of fibres approaches the basement, the individual fibres should separate, one passing through each opening. As I have already ex- plained, the fibres, while separated, could be identified only with great difficulty. If the fibres pass through the basement membrane, as I believe they do, they terminate only a short distance below it. For at about 15 /a be- low the membrane all of the fibres are of nearly the same size, i. e. some- what larger than the large fibres which pass through the membrane (Fig. 22). In this region, then, the smaller fibres have either increased to the size of the larger ones, or diminished till no trace of them is left. The fibres here are in groups, however, and these are directly continu- MUSEUM OF COMPARATIVE ZOOLOGY. 19 0U8 with the groups which pass through the membrane. Each group of large fibres as it passes through the membrane consists of either three or four individual fibres. If the smaller fibres disappear, the groups below the membrane should consist of three or four fibres also ; if, on the other hand, the smaller fibres increase in size, the deeper groups should consist of four or five fibres. By either method of change there would be groups of four fibres, so that it is the groups of three or five fibres which will be decisive. As a matter of fact, the fibres are very commonly in groups of three, and not in groups of five ; consequently I conclude that the smaller fibres dwindle out a short distance below the basement membrane. The distal retinulse have not been identified in many Decapods. Carriere ('85, p. 169) has described them in the eye of Astacus. In Peuciius, Patten ('86, p. 634) has observed four cells which belong to the pigmented collar of the retinophora. Two of these, the inner ones, evidently correspond to the distal retinulae of the lobster. They sur- round the cones. The other two, the outer ones, appear to have no homologue in the lobster's eye. The Intercdlidar Spaces of the Retina. In the region of the retina which lies between the proximal ends of the cones and the distal border of the deeper band of pigment, the groups of cone-cells and the pairs of distal retinulse are separated by considerable intervening space (Fig. 1, spa. i cl.). This space is filled with a fluid which contains a very small amount of albuminoid sub- stance. Patten ('86, Plate 31, Fig. 73, x) has figured a similar fluid- filled space in Penseus. On the application of heat this albuminoid substance in the lobster coagulates and forms larger or smaller vesicular bodies, wliich vary much in size. They are usually loosely attached to the cone-cells and the fibres of the distal retinulse. They readily take up coloring matter. They have never been observed in fresh retinas when teased in normal salt solution, nor in maceration-preparations. It was probably these bodies which Newton ('73, p. 329, Fig. 15, c') described as the nuclei on the investing membrane. In addition to the albuminoid substance which I have described, one occasionally meets with a thin layer of homogeneous material which lies slightly in front of the rounded ends of the proximal retinulaj. This forms a dividing membrane which separates the retina into a prox- imal and distal portion. Tlic membrane is of course pierced in many places. There is an opening in it for each pair of distal retinulcC, and 20 BULLETIN OF THE each group of cone-cells. In many cases the membrane has not been observed. It was noticed by Newton ('73, p. 328), and as it is non- cellular it is probably a feeble representative of what Herrick ('8G, p. 44) has described as a " chitinous " framework in the deeper part of the retina in Alplaeus. The Proximal Retinulce. The proximal retinulso are pigment-cells which closely invest the rhabdome. AVith the brownish accessory pigment-cells they constitute the proximal band of brownish black pigment on the distal side of the basement-membrane (Fig. 2G, pig. px.). In some cases they appear to terminate distally in rounded knobs, each of which contains a nucleus (Fig. 1, rtn'. 2>x.). In other instances, and these are of frequent occur- rence, their distal ends, in addition to having a swollen nucleated part, are prolonged into delicate fibres (Fig. 30). These fibres when present extend toward the outer surface of the retina, and are applied, not to the cone-cells, but to the fibi-ous portion of the distal retinulai. The fibres havo been traced only a short distance beyond the rounded ends of the cells from which they originate. As the region into which they extend is one readily studied in both sections and maceration prepa- rations, and as these methods of study have given no evidence of fibres other than that of the very short ones already mentioned, it seems fair to conclude that the distal retinula) terminate as fine fibres a short dis- tance in front of their nucleated portions. In transverse sections the distal retinulee first clearly appear in the plane represented in Figure 9. Here each group of four cone-cells is surrounded by a circle of seven retinula). The section from which this figure was drawn is in a slightly oblique plane. In moving from right to left, one passes into deeper and deeper regions. In the more super- ficial part of the section, the right-hand half, each retinula contains a nucleus, which is surrounded by a small amount of pigmented cell-sub- stance. In the deeper part of the section, the left-hand half, the plane is below the region of most of the nuclei, and one sees the seven reti- nula) densely filled with pigment. In the next section (Fig. 10), the retinulre are broader in transverse section. In their expansion they have so far encroached on the space whicli they surround that it is only large enough to allow the passage of the four cone-cells. The con- traction of the space within the circle of retinulse takes place almost in one plane, as can be seen in the longitudinal section (Fig. 1). In the plane of Figure 10, the retinulae show a tendency to gi'oup themselves. MUSEUM OF COMPARATIVE ZOOLOGY. 21 One can recognize an odd, usually larger retinula, which occupies the lower right-hand corner of each group. The remaining six retinulse are disposed in pairs. In Figure 11, which represents a plane of section deeper than tliat shown in Figure 10, the retinulae, although somewhat reduced in thickness, present nevertheless the same method of group- ing as was pointed out in Figure 10. In this plane one also notices next the odd retinula, a nucleus. This is remarkably constant in its occurrence, both as to position and as to the fact that there is always a single nucleus. When compared with the nuclei of the surrounding retin- ulie, it is found to resemble them very closely. The nuclei of the seven retinulce are characterized by their sharply marked oval outlines, and by the possession of one or two very distinct nucleoli (Fig. 30, nl. px.). In both of these respects the single nucleus agrees so closely with the nuclei of the retinulte, that, were it not for its somewhat smaller size and deeper position, it could not be distinguished from them. The regularity with which it occurs, and its structural peculiarities, incline me to believe that it represents a reduced retinula in which pigment has never been developed. This belief is further supported by the fact, that the additional nucleus is always found next the larger retinula, which from its great size seems to have replaced a second cell. It is therefore probable that each ommatidium of the lobster's eye possesses eight proximal retinula3 rather than seven, and that one of these is rudimentary. Below this additional nucleus, the proximal retinul ■'M. as^ ".f'.'^y^, - lie. •^'^'t^:'" ^t'. i,h y L- rintlst nkhl CHP dp] ^h- Hn/}M .spill cl vl con lU iilpitj lln'lltl clem '^ m drc, ^ 0 9 f"^- 1.1. . /f> M r'r ^5^i^^ f.i ft clem « "4'V >:a> it ffl lin/bl f~ ^° rhi V '/. ■'y*^4-''^^ C^- .'.^ V 'Ak Parker. — Lobster Eye. EXPLANATION OF FIGURES. All the figures were drawn with the aid of an Abbe camera. Unless otherwise stated, the figures are from specimens stained with Grenacher's alcoholic borax- carmine and mounted in benzol-balsam. Where sections have been depigmented the reagent employed was an aqueous solution of potassic hydrate i% (see Par- ker, '87, p. 175). Figs. 1 to 36 refer to the histology of the adult lobster's eye. Figs. 37 to 59 inclusive deal with the development of the eye. PLATE I. All figures on this plate illustrate the histology of the lobster's eye. Fig. 1. A longitudinal section of an ommatidium. This is a composite figure, its parts having been drawn to one scale from various sections, and after- wards combined. The distal retinuhi on the right side of tlie cone contains its natural pigment ; that on the left side has been depig- mented. Tiie letter x indicates the position of the band which limits the corneal facet. The numbers to the right of the figure correspond to the numbers of the foUowing figures of transverse sections, and mark the level at which the latter were taken. X 105. TJie remaining Jigures on this plate (Nos. 2-25) ai'e transverse sections either of ommatidia or bundles of nerve-fibres. In each case the sections were studied and drawn from their distal faces. Figs. 2 to 20 are magnified 375 diameters ; Figs. 2\ to 25, 575 diameters. " 2. A corneal facet. This specimen was cleaned in a boiling solution of potassic hydrate and studied in water. " 3. Four pairs of cells from the corneal hypodermis. Around the lower right-* hand pair of cells the outlines of the six surrounding distal retinulae are indicated in part by dotted lines. " 4. Four groups of cone-cells. The ommatidia to which the cone-cells be- long are indicated by the letters a, b, c, and d. These letters are used to indicate corresponding ommatidia in deeper sections. " 5 to 20 represent transverse sections through ommatidia in the various regions indicated as follows : — " 5. The middle region of four cones. Completely depigmented. " 6. The proximal ends of four cones. The re-entrant angle on the surface is indicated at x. Completely depigmented. " 7. Slightly below the proximal ends of the cones. " 8. Beyond the distal ends of the proximal retinulae. " 9. The distal ends of the proximal retinulas. " 10. The thick distal portion of the proximal retinula\ " 11. The proximal retinulae immediately above the distal termination of the rhabdome. " 12. At the distal ends of tlie rliabdonies. Completely depigmented. Parker. — Lobster E^e. Fig. 13. The rhabdome between its distal end and middle. In tiiis section the ac- cessory pigment-cells of each ommatidiuni arc apparently separated by an intervening space from those of neighboring ommatidia. Tiiis space is probably the result of shrinkage and subsequent rupture. In the same plane as that shown in Fig. 13. Partially depigmented. The middle of the rhabdome. The distal retinula; of ommatidium c have been numbered. Completely depigmented. The proximal end of the rhabdome. The accessory pigment-cells in this section have probably been ruptured, as in Fig. 13. In the same plane as that shown in Fig. 16. Tartially depigmented. The pro.ximal tip of the rhabdome. Completely depigmented. The proximal retinulaj close to the basement membrane. Partially de- pigmented. A diagram of Fig. 10. The proximal retinula' of the different ommatidia have been numbered to correspond with those of ommatidium c in Fig. 15. Tlie retmula; of ommatidium '• are tinted pink. Transverse section of nerve-fibres as tiiey pass through the basement membrane. Tlie Hue n z shows tlie plane of section for Fig. 2it; x indicates the cross-shaped thickening in tlie basement membrane. Completely depigmented and stained in Weigert's hajmatoxylin. (See page 4.) x 575. 22, 23, 24, and 25 represent transverse sections of tlie fibres in the optic nerve. Weigert's hajmatoxylin. X 575. The planes at which these sections were taken are as foHows : — 22. Directly below the basement membrane. Tlie fibres are in groups of threes and fours. 23. At one fourtli the distance from the basement membrane to the optic ganglion. 24. At half the distance between the membrane and ganglion. 25. At the surface of the ganglion. 14. 15. 10. 17. 18. 19. 20. 21. ABBREVIATIONS. ax. n. Nervous axis of retinula. cap. Protoplasmic cap of cone- cl. con. Cone-cell. [cell. cl. ms d. Mesodermic ceil. con. Cone. cm. Corneal cuticula. cm. h d. Corneal hypodermis. eta. Cuticula. enc. Brain. fbr'. Fibrillaj. (jn. opt. Optic ganglion. lid. Hypodermis. ml). Basement membrane. mh. i cpt. Intercepting membrane. 7nb. i cl. Intercellular membrane. luh. pi ph. Peripheral membrane. n.fhr. Nerve-fibre. Nucleus of cone-cell. " corneal hypodermis. " distal retinula. " accessory pigment-cell. " proximal retinula. Ommateum. Distal band of pigment. Proximal band of pigment. Retina. Rhabdome. Distal retinula. Proximal retinula. 111. con. ill. cm. ill. dst. id. pi'j. nl. px. oiiim'. pi(j. dst. pig. px. r. r/ib. rill', dst. rtn'. px. spa. I cl. Intercellular space of retina. The other abbreviations which occur on the plates are explained lu the descrip- tion of the figures with which they are found. Pakker — Lobster Eye. PLATE II. Figs. 2G to 36 deal with the histology of the lobster's eye ; Figs. 37 to 39, with its development . Fig. 26. Vertical section through that portion of the eye-stalk wliere tiie transi- tion from the unditlerentiated hypoderiiiis to tlie oniinateuin is accom- plished. Tlie open space x is due to sliriiikage. X 31. " 27. A group of the four cone-cells of an oniniatidiuin and tlie attaclied cor- neal hypodermis. Isolated and studied in Mailer's tluid. X 865. " 28. Proximal end of a group of four cone-cells where they sei)arate as fihres to pass around the rhabdome. Isolated and studied ni MuUer's tluid, X 365. " 29. Transverse section of the basement membrane. The distal face is upper- most; the proximal face below. The section is taken in a plane which would be represented in Fig. lil by the line z ij. x 575. " 30. A rhabdome and its seven surrounding proximal retinula;. At the distal end the free tips of the seven retinulaj can be seen. At the i)roximal end the rhabdome and the four groups of retinuhe which pass through the basement membrane are visible. Isolated and studied in chromic acid 3^5%. X 200. " 31. An individual proximal retinula. Isolated and studied in chromic acid iA X 200. " 32. Transverse section of a rliabdome and its surrounding cells. The plane of section is about half-way between the middle and distal end of the rliabdome. Completely depigmented with potassic hydrate. X 400. " 33. Oblique section of a rhabdome from the same series of sections as Fig. 32. The upper end of the figure is distal; the lower, proximal, x 4G0. " 34. Transverse section of a rhabdome and its surrounding retinuhr. The nervous axis of each retinula is distinctly stained. Completely de- pigmented ; stained with Kleinenberg's alum-luematoxylin. x 460. " 35. Longitudinal section of a bundle of nerve-fibres extending through the basement membrane toward the optic ganglion. Depigmented ; Weigert's hematoxylin, x 575. " 3G. Optic nerve-fibre. Isolated and studied in Midler's fluid. X 575. " 07. Superficial view of a left optic lobe. Enough of the right lobe is drawn to indicate the position of the median plane (.r y). x is anterior; y is posterior. Stage A (see page 2). x 280. " 38. Posterior face of a section from a right optic disk cut transversely to the longitudinal axis of the embryo (see page .34). At r is an angle formed by the growth of the retinal cells over the undifferentiated ectoderm. Stage A. X 280. " 39. A section from the optic disk of an embryo in stage B. The plane of cutting corresponds to that in Fig. 38. In this figure x indicates, as in the preceding one, the angle between the undifferentiated ectoderm and the growing retina, x 280. Pabker - Lobster eye. Pl.i1. 27. t ■>z cm nib. nlc Hn'pr fhr d.cm. lin'fu- /■ da hd. rub <• >i dcon. y mb iiibicpf. i < \- 59. I '«». WK puipx. 28 Jj ti !^l| V 5/^ ry'i ;■) M rii fn m ti> .■)/ rinpr rir n .'V Hib 31 rAA G.H.P del r< Jfeisel. hth Parker — Lobster Eye. PLATE IIL All figures on this plate illustrate the development of tlie lobster's eye. Fig. 40. A section through tiie left optic lobe and left half of the supra-cesophageal ganglion. The plane of section is tangential to that part of the sur- face of the egg on wliicli the embryo rests. The petition of the median plane is uidicaled at xy. Tlie surfaces tinted with deeper pink in the figure represent areas containing nuclei in the specimen ; those in hgluer pmk, areas in which no nuclei were present. The optic lobe is divided into two parts by a band of large, faintly colored nuclei, which, willi the smaller surrounding nuclei, are sliown in the figure. To the right of the nuclei the broad tinted marginal area represents the retina, r. The remainder of the optic lobe gives rise to the optic ganglion. Stage C (see page 2). X 280. " 41. Posterior aspect of a transverse section of a right optic lobe. The plane of section corresponds to that in Fig. 38 ; x is the angle whicli itnli- cates the separation of the retinal and ganglionic constituents of the intercepting membrane. Stage C. X 280. " 42. This figure is taken from a region wliich corresponds to the left-hand por- tion of Fig. 41j Altliough from the same set of eggs the embryo from which Fig. 42 was drawn was sumewiiat more advanced than that from which Fig. 41 was taken. At x the proximal band of retinal nuclei can be seen ; at y the distal band is shown. Stage C. x 460. " 43. The superficial layer from the distal band of retinal nuclei ; seen from the external surface of the retina. Stage C. X 4G0. " 44. The deep layer of the distal band of nuclei. These are seen in opti- cal section somewhat within the outer face of the retina. Stage C. X 460. " 45. A transverse section of an optic lobe from a lobster at stage D. The plane of section corresponds to that of Fig. 38. As in Fig. 40, the deeply tinted areas were nucleated ; tlie lighter areas were without nuclei. X 145. Parker - Lobster eye. 40 ( gn.opt. b%» ^•^*' 45 enr. ' \ \ mbtcpt 4/. ..-a^%' nQi^cAyi C*^". 3^ :.:;^ ^\ J^ . iJ'i.opt. c^ X-?J-.- '^ __\ij^2»:ia.i_. "I^^r 4" n Jtr. AO V- nidxt. nf.ati. ni.con.. r'^ 45. (SS> nkicpf ■^f^-i^^^s^'- cjnopf. ;7r. ,pl CftC. c^.V-'eisfl. liiK Farker. — Lobster Eye. PLATE IV. AH figures on this plate, except Fig. 59, illustrate the development of the lobster's eye. Fig. 46. A transverse section of an optic lobe at stage E (see page 2). The plane of section and the metliod of coloring the figure are the same as in Fig. 45. X 145. " 47. An enlarged drawing of that portion of the retina which is in brackets in Fig 46. Stage E. X 460. " 48. A view of the external surface of the retina. The distal ends of four ommatidia are seen. Stage E. x 400. " 49. A transverse section of four ommatidia in tiie region of the hypodermal nuclei. (Compare Fig. 47.) Stage E. x 460. " 50. A transverse section of four ommatidia in the plane winch tlio nuclei of the cone-cells occupy. Stage E. x 460. " 51. Longitudinal section of a single ommatidium. Stage F. x 460. " 52. Four corneal facets seen from the external surface. Stage F. X 460. " 53 to 58 represent transverse sections of four ommatidia at Stage F. The numbers on the left side of Fig. 51 indicate the heights at which these sections were taken, and correspond to the numbers of the following figures. In Figs. 53 to 58 tiie magnification is 460. " 53. A transverse section in die region of the corneal liypodermis. " 54. A transverse section through tlie region in which the nuclei of the cone- cells occur. " 55. A transverse section in tlie same plane as tlie nuclei of the distal retinulas. " 56. A transverse section of the proximal ends of two cones. " 57. A transverse section through tlie rhabdomes and proximal retinulge. " 58. A transverse section of a rhabdome from Fig. 57. P'ig. 58 was drawn with a higher magnification than Fig. 57 in order to show the relation of the proximal retinute to the segments of the rhabdome. X 640. " 59. A corneal facet from near the periphery of the retina in an adult lobster. Tiie hexagonal outline is noteworthy. This specimen was cleaned in boiling potassic hydrate and examined in water. X 280. Parker -LoBST^:^ ^■ Jul. ■3 '*> ••> mil III dst III cm nl.cori con ^ r^,.^ & /^'--.y .^1' .J.) r.^ rm / ///. cm '^^jy cue Pl. I\'. nic 51. nlrni 'ilron .)+ /A ;r /?/ if>ri itib I //I. opt r"! .>; 1 cl (Vir -jM jr „ .5.S No. 2. — On the 'Rate of Growth of Corals. By Alexander Agassiz. We know as yet comparatively little regarding the rate of growth of corals under different conditions. Dana has given, iu his "Corals and Coral Islands," * a resume of our knowledge on the subject, so that it is only necessary for me here to refer the reader to his account of the statements of Darwin, Stuchbury, Duchassaing, Verrill, and others, re- lating to this subject. The specimens figured in this communication have been kindly sent me by Lieut. J. F. Moser, commanding the U. S. Coast and Geodetic Survey steamer " Bache.'* They were all taken (as stated by Mr. Hellings, the cable manager) off the cable laid between Havana and Key West, in June, 1888, from a portion of the cable repaired in the summer of 1881 ; so that the growth is about seven years. Lieutenant Moser writes : " Taken from the shore end of the International Cable ; the specimens were taken between the triangular buoys and the outer reef, the shore end being that portion between Key West and the outer reef" The Coast Survey maps indicate a depth of from six to seven fathoms, and this portion of the cable is most favorably situated as re- gards food supply, being directly in the track of the main flow of the tide as it sweeps in and out from the outer reef into Key West Harbor, and over the flats to the northward. Some of the specimens belong to different species from those of which the rate of growth was already known. Orbicella annularis (Plates I. and II.) shows a much greater increase in the thickness of coral formed than the case mentioned by Verrill, where the thickness formed in sixty-four years was not more than about eight inches. The specimens sent by Lieutenant Moser grew to a thickness of two and a half inches in about seven years. * Coral and Coral Islands, by James D. Dana. Third edition. New York, 1890. (Pp 123, 253, 418 ) VOL. XX. — NO. 2. G2 bull?:tin of the Tlie Manicina areolata (Plate III.) shows also a very rapid rate of increase. This coiTesponds to the rate of growth of allied genera. [M idly in size from the first posteriorly, and that of the last is exceedingly depressed and thin. The whole sacrum is quite strongly arched from before backwards. The lumbar region is quite long, and consists of six vertebrae, which are slen- derly constructed ; the centra are anteriorly comparatively narrow and trihe- dral in section, posteriorly they are broader and more depressed. The spines are low and comparatively broad, and are inclined well forward, with concave anterior borders. The transverse processes on the first lumbar are short, de- pressed, but comparatively broad ; these processes lengthen as we pass backwards, but are very slender as compared with those of Antilocapra, and the neural spines are lower than in that genus. The zygapophyses are of the interlocking cylindrical type usual among artiodactyles, and there are no metapophyses. We may infer with considerable confidence that the number of dorsal vertebraa was thirteen ; on this assumption, the most anterior dorsal of this specimen is tlie ninth. In this the centrum is short and trihedral in section, with the infe- rior border sharp and arched from before backwards ; the spine is rather short, and directed very obliquely backwards ; the transverse processes are short and slender, and have well marked facets for the tubercles of the ribs ; the prezyga- pophyses are flat and placed on the pedicels of the neural arch, and, separated MUSEUM OF COMPARATIVE ZOOLOGY. 85 from them by a short interval, arises a pair of small metapophyses. The tenth is the anticlinal vertebra ; the spine is at first very oblique, but curves, and in its upper portion is vertical. In other respects this vertebra is like its predecessor. On the eleventh the spine is directed slightly forwards, but the end is rounded like that of the anterior dorsals; the metapophyses have ap- proached the median line so as to touch the post-zygapophyses of the tenth, while the post-zygapophyses of the eleventh have assumed the cylindrical shape found in the lumbar region. The twelfth and thirteenth vertebrae are much like lumbars in their construction, and are distinctly longer than the three an- tecedent vertebrae ; the spines have the nearly straight thickened free ends seen in the lumbars, and the metapophyses have disappeared. The transverse pro- cesses, however, are very short, though they still retain the rib-facets, even on the thirteenth. The ribs, so far as can be judged from the fragments, are narrow and very slender. Of course this may be true only of the posterior part of the series. The scapula is characteristically ruminant. The glenoid cavit}' is nearly round and quite shallow, the coracoid process is prominent, recurved and thickened at the end ; the neck is very long and much contracted, the borders sloping away from it very gradually ; the coracoid border is thin and rounded at the edge, it curves gently forwards and upwards from the neck ; the glenoid border is very much thickened and somewhat overhanging, from the neck it is nearly straight, and forms a right angle with the very thin suprascapular border. The spine rises abruptly from the neck into the high acromion ; the latter overhangs very slightly, in sharp contrast to the condition found in Aiitilocapra. The spine divides the blade into unequal fossae, the prescapular being much the smaller, as is ordinarily the case among the ruminants. Except for the nearly straight inferior edge of the spine, and the consequent lack of an overhanging acromion^ this scapula very closely resembles that of the prong-buck. Thehumerus has a broad and flattened head, which projects but little beyond the shaft. The external tuberosity is laige, and curves over tlie deep bicipital groove ; the internal tuberosity very small ; both are much less develo]ied than the corresponding processes in Antilocapra. Proximally the shaft is broad and compressed, below it is rounded and slender. No ridges for muscular attach- ment are more than very faintly indicated. The distal end is broken away, but in all probability it was like that of Blastonieryx described above. The pelvis is also entirely ruminant in character. The ilium has a short, deep, and much compressed neck, expanding into a curved and stronglj' everted plate, which projects a consideral)le distance in front of the sacral attachment. The ilium is somewhat tiihedral in section, the median rounded ridge of the plate being more prominent, and the expansion itself smaller than in the prong-buck. The ischium is very long ; above the acetabulum its superior border shows the convexity so usual in the recent ruminants, though in a less marked degree. The tuberosity of the ischium is very long and prominent, and directed straight outwards ; behind the tuberosity the ischium is prolonged further than in the prong-buck. The cannon-bone belonging to this specimen is broken, and its 80 BULLETIN OF THE pruximal end obviously diseased, so that it does not merit description ; the only fact of importance which it shows is the comparative slenderness of the bone. So far as the material will enaT)le us to judge, the feet of C'osoryx differ in no important respect from those of lllastomeryx, and the same statement applies to the long bones of the limbs. Eestoration of Cosoryx furcatus. (See riate I.) This drawing is made from the specimen already described, completed by fragments of others, wliile the feet are drawn from Elastomer ijx; the cervical vertebraj are re[iresented only by the axis, the others being conjectural, as are also the anterior dorsals. The skull is taken chiefly fiom that of the closely allied Eurojiean genus, Fa'ce^menjx, and from specimens of the large Cosoryx ttrca, Cope, belonging to the Smithsonian Institution. The fortunate associa- tion of the mandible in the same specimen with the vertebi'jE, pelvis, scapula, etc., gives a very u.seful standard as to the length and character of the skull, position of the molars, etc. It may be assumed with some confidence that the drawing gives a fairlj' accurate representation of the animal. Marsh's account of the feet uf Cosoryx shows that they were constructed much like those of Blas- tomeryx. In general appearance Cosoryx seems to have had the same light, gracel'ul build as Antilocajira, but with a very dift'erent skull and deer-like antlers. The proportions of the limbs also differ somewhat, the hinder cannon- bone being considerably longer than the fore, while in the prong-buck they are of nearly the same length. Cosoryx was a much smaller animal, the bones are all more slender than in Aiitilocapra, and the carpal and tarsal boues are much higher and narrower proportionately. The view held by Cope that Cosoryx is the ancestor of Antilocapra is very probalily the true one. So far as the dentition, the vertebrae, and the lindjs are concerned, the differences between the two genera are only such as might be expected to occur between a Miocene and a recent ruminant. A distinction of some importance, however, consists in the character of the horns. In Cosoryx they are branched, but probablj* not deciduous antlers; in Antilocapra, a core with a horny sheath, which, however, differs strikingly from the horn of the tyi)ical Cavicornia. But the unique branched horn of Antilocapra not improb- ably indicates, as has been suggested by Cope, a remnant of a former l)ranc.hing of the bony core itself, and so this difference does not preclude a genetic con- nection between the two forms. In Cosoryx the antler was almost certainly covered with skin ; its smooth surfiice, as Schlosser points out, shows that it could not have been naked, as in the true deer. Both Blastomcryx and Cosorijx are probably to be derived from the species re- ferred to the former genus which occur in the John Day beds, but there is no form yet known in the White River which could have given rise to these John Day ruminants. The latter are most probably descemled from some Palceo- racryx of the Old "World, which migrated to this continent. The very close con- MUSEUM OF COMPARATIVE ZOOLOGY. 87 nection between these American genera and the A mphitragulus, Dremotherium, etc. of St. Gerand le Puy is obvious from the most superficial comparison. The collection contains specimens probably indicative of other species of Cosoryx, some of them much larger than C. furcatus ; but in the absence of asso- ciated teeth, it is not possible to refer them to their proper categories. PERISSODACTYLA. ANCHITHERIIDuE. MESOHIPPUS, Marsh. The Brain. Mesohippus had a large and well convijluted brain. The length and breadth indicate that it weighed about one third as much as the brain of the recent horse, while if we estimate the body weights of the fossil and recent animals by the relative size of the humeri, the brain of the Miocene species was proportion- ally heavier. The cerebrum of the horse is, however, much more highly convo- luted, and the frontal lobes are relatively broader. The MesoJiijypics brain is distin- guished in a mai'ked manner by the longitudinal direction of the parietal and occipital sulci, and by the deep trans- verse frontal sulci, as con- trasted with the oblique sulci of all recent unsculates. In fact, in this respect it bears a marked general resemblance to the brain type of recent Carnivora, and conforms with the higher Ungulata of the Figure 10. — Brain of Mesnhippns Bairdiix f. above, and from side. From Eocene. On either side of the lon- gitudinal fissure is a long deep fissure forking anteriorly and marking off the median gyrus, m, of the parieto-occipital region. Parallel with this is a short fissure, which separates the two medilateral gyri, ml, ml'. The third fissure extends to the posterior transverse, and thus entirely separates the supersylvian gyrus, ss, from the medilateral. The fourth fissure is shallower. There are three transverse frontal fissures (Fr. 1, 2, 3) which divide this lobe into three gyri ; the median fissure extends almost to the longitudinal fissure, and sug- 88 BULLETIN OF THE gests the crucial sulcus of the Carnivora. The sylvian fissure is very shallow. The teiuporo-sphenoidal lobe is very pronunent, ami is divided into three gyri (s, m, i) by two sulci. Beneath the third frontal gyrus is a vertical sulcus, par- allel with the sylvian. The cerebellum has a large central lobe with transverse sunple furrows. The Dentition. There are a few new points to be noted in regard to the teeth of Mcsohippus, which bear upon the dentition of the horses in general, and ai-e clearly shown in "^^ f^ >? 7^Q^ ifi, nve Figure 11. — Superior and inferior molars of Mesoluppus Batrdiix \- a series of unworn crowns of the upper and lower jaws. Scott has already pointed out that the incisors in this genus are simple, there being no indication of the infolding of the enamel, such as is seen in Anchitherium aurelianense. In some of the John Day species oi Anchitheruim the enamel is not in- folded, as observed in the lower jaw of a specimen referred to J. equiceps, Cope. The upper molars of Mesohippus clearly show the first step in the formation of the posterior pillar, pp, which is so conspicuous a feature in Anchitherium, in the posterior valley. This can also be observed in a still simpler stage in a specimen of Anc/iilophus from the French Phosphorites. Step by step with the development of this cusp appears the posterior pillar, p, in the lower molars, behind the entoconid ; til is accessory cusp can be traced back to the teeth of Epihippus. When it finally unites with the en- toconid, in Hipparion, it forms the posterior twin cusp (6, h, Riitimeyer), which is analogous to the anterior pair formed by the union of the metaconid and anterior pillar, a (a, a, Riitimeyer). Thus the tran-sition from the Mesohippus to the Anchitherium molars is very gradual, as shown in the accompanying figures. By tracing back the rise of /ict. me Figure 12. — Superior mo- lar of Anchillii'viiim Inni/i- crifte X -}. Superior and external view. Cope col- lection. MUSEUM OF COMPARATIVE ZOOLOGY. 89 the eleven elements which compose the upper Equus molar, we find that six belong to the primitive sextubercular bunodont crown. Two elements of the ectoloph, the anterior pillar and median pillar, rise I'njm the simple primitive basal cingulum of tlie Hijracothcriain molar ; the same mode of devL'lo}imiint, we have just seen, is true of the pjosterior 'pillar. Tlie eleventh el'Uinit, the fold of the postero-external angle of the crown,]), is not prominent until we reach Equus. Tlie term "posterior pillar" is taiven from Lydekker ; the other terms, " median " and " anterior," are applied to parts which have an analogous origin from the basal cingtilum. The remaining coronal cusps are readily iden- tified with their honiologues in the primitive tritubercuhir mular. ? Anchitherium parvulus, Marsh. (Syn. Equus parvulus, Marsh.) Among the Loup Fork specimens collected by Clifford are found two lower molars, m-^ and m^, which are almost identical in size with those of Mesohippus Bairdii. The crown of m^ measures: antero-posterior, .011 m. ; transverse, .009 m. Unlike the Mesohippus molars, there is no external cingulum. The " posterior pillar " has the same degree of development as in Ancliilherium. The fangs are separate. There is no trace of cement. iMarsh has described a di- minutive horse (Equus parvulus), estimated at two feet in heiglit, from the same beds, and it is highly probal)le that these teeth belong to this species. The generic reference is of course very tmcertain. The brachydont crowns jjoint either to Menjchippvs or Anchitherium, Ijut the stage of development of the coronal pattern approximates most closely that in the latter genus, being a little more advanced than in Mesohippus. RHINOCERID^. ACERATHERIUM, The Manus and Pes. The characteristics of the pes of Hyracodon from the lower White River beds have been fully enumerated by us.^ They are ])rincipallv as follows: cnboid not supporting astragalus anteriorly ; lateral digits reduced and not spreading ; ectocuneiform not articulating laterally with mts. II. We may subsequently find that the feet of the later species of Hyracodon varied in some of these re- spects, although this is not probable, owing to the fixity of foot-types once established. We have, however, no ])resent means of distinguishing between the Metamynndnn and Accratherium foot-bones. On page 1(59 of the first Bulletin a high, rather slender tarsus was descrilied, 1 See Scott, E M. Museum Bulletin, No. 3, May, 188.3, p. 10. Also, Oshorn, Mam- malia of the Uinta Formation, May, 1889, Part IV. "Evolution of tlie Ungulate Foot," p. 549. 90 BULLETIN OF THE which probably belongs to the A ceratherium of the lower beds. It differs widely in its proportions from other specimens found in this collection, which belong either to the Aceratherium of the higher beds, or to Metamynodon. The best preserved specimen of this second type (marked a^) is comparatively short and broad, with spreading digits and rugose surfaces for muscular attachment (Figure 13). The pro- portions of the metapodials to the tarsals are similar to those in Ceratorhinus. The calcaneum has a powerful tuber ; the ectal astragalar facet is very convex ; the sustentaculum is narrow, and its oval facet is continuous with the inferior; the cuboidal facet is nearly horizontal. About one fifth of the astragalus rests upon the cuboid. The relations of the cuboid, navicular, and ecto- cuneiform repeat those observed in Rhinocerus. The mesocuneiform is very short, giving mts. II. a wide articulation with the ectocuneiform. The metatarsals are powerful, the lateral pair having approximately the same length as in li. indicus. This type of foot is related directly to that of Aphehps. Tiie maims and pes of a third specimen (marked a^) show several interesting differences. In the pes, the metatarsals are of the same proportions, but the calcaneo-cuboidal facet is oblique and narrow, resembling that in Hyracodon, and the sustentaculum is very small. The remains of the carpus show that the species to which this specimen belonged had a greatly reduced fifth digit, constituting a functionally tridactyl manus. The evideiKie for this is in the greatly reduced lunar-magnum facet, which is invariably characteristic of tridactylism.^ It may be noted here that among the carpals of Titanotherium there is a well preserved lunar, which has its magnum facet much reduced anteriorly, so there is little question that we shall yet discover a tridactyle species of the genus. Figure 13 — Ri^ht pes of Ace- ratherium X i- The Rhinoceros Molars. The peculiarities of the molars of Aphelops will be made more clear by a few observations upon the molars of the rhinoceroses in general. The three main crests of the lophodont crown may now be distinguished in part by terms which express their homologies with the elements of the sextubercular superior and quadritubercular inferior molars of the primitive ungulate, Phenacodus. In the upper molars, the outer crest is formed by the union of the primitive paracone ^ See Osborn, Mammalia of the Uinta Formation, p. 567. feet belong to Metamynodon. It is possible that these MUSEUM OF COMPARATIVE ZOOLOGY. 91 jite .3 ■ cr --cr ^a.cr and metacone, to which is joined the anterior pillar (see Mesohippus, p. 88) ; it may be called the ectoloph. As the anterior crest is formed by the union of the protocone, protoconule, and paracone, it may be termed the protoloph. The posterior crest, which unites the primitive metacone, the metaconule, and the hypocone, may be termed the metaloph. The outer surface of the ectoloph in the primitive molar of the Rhinoceros is marked by three vertical ridges corresponding to its three prim- itive component elements, me, pa, ap; one or all of these disappear in the flattening of the surface. It will be observed that nothing cor- responding to the 'median pillar' of the su- perior molar of the horse is developed. In the lower molars (the paraconid disappearing), the union of the metaconid and protoconid forms the anterior crest, or metalophid, while the hypoconid and entoconid unite to form the hypolophid. The secondary enamel folds, which are de- veloped from the three crests, bear a most in- teresting analogy to those observed in the horse series, beginning with Proto- hippus ; they are outgrowths of the same regions of the crown and subserve the same purpose. They are moreover of like value in phylogeny. The useful descriptive terms introduced by Busk, Flower, and Lj'dekker, should be adopted in part.^ These secondary elements consist, first, of three folds projecting into the median valley, one from the ectolojjh, the crista ; one from the protoloph, the crochet; one from the metaloph, the anticrochet. Secondly, the ecto- loph unites with the posterior cingulura and metaloph. Thus the anterior and posterior valleys may be cut off by the union of these folds into from one to three ' fossettes,' precisely analogous to the ' lakes ' in the horse molar, except that they are not filled with cement. The accompanying diagram is taken from a fossil molar figured by De Blain- ville. (Osteogr. Gen. Rhin, Plate XIII.) It is remarkable in exhibiting all the primary and secondary elements, for they are very rarely combined in a single tooth. Similar accessory folds are frequently developed in the lower molars. FiGUKE 14. — Superior molar of Khinoceios (i-p. iudet.) X 5. After De Blaiiiville. 1 The terms 'protoloph' and 'metaloph' are, however, substituted for 'anterior colHs ' and 'posterior collis ' of Lydekker. The term 'anterior pillar' = 'first costa,' and ' paracone ' = ' second costa.' The mode of evolution of the ' pillar must have been similar to that in tlie horses, where Lydekker has proposed this term for tlie 'posterior pillar.' It is very appropriate, because the pillars in their earliest development can be shown to rise independently from the cingiiltini (see Mesohippus, p. 88), and not as folds of the main elements of the crown, as we should infer from their fully developed stage. 92 BULLETIN OF THE APHELOPS, Cope. The generic characters of Ajihi-lojin have been given by Cope as follows. Den- tition, 1. -J-, C. T, P. -3-, M. §; post-glenoid aiul post-t\ niiianic processes in contact but not co-ossilied ; digits, 3-13 ; nasals hornless. To tliese cliarac- ters may be added : niagiiuiu nut supjiorling Ulnar anteriorly; absence of the 'crista' and invariable presence of tlie more or less strongly developed ' crochet ' and ' antici'ochet ' in the superior molars. The specific nomenclature of Aphdops is in confusion. The type oi A. (Rlii- noccrun) cruasus, Leidy,^ is a last u]>per molar, which is closely siniilui- to that of A. majaloduii ; the characters of the milk molar associated with this type cannot be used in definition.'^ The penultimate upper molar, the type of A. mcridl- anus, Lei-ly,* corresponds in the developmenl of the two 'crochets' to the same tooth in A.fossigcr, Cope, but the posterior ' fossette ' is not ench)sed by the strong cingulum as in the latter species. A. (^Aceralhcriuvi) acutum, Marsh, is identical with A. fossigcr. A. 'innluorhinn!;. Cope, resumbles A. mi:ridianus in tlie open posteri(;r fossette and llie dcNclojimi'iit of the ' ci-Drhcts.' It is inijK.issililc, how- ever, to ch'ar up tiiis synnn^in}' without bringing the original types togetlier for conipari.son. General characteristics ot all the.-e types are the invaiialde development of the 'crochet,' absence of the 'cri>ta,' usual development of the 'anticrocliet.' The specific names propo.sed by Cope are here ado]>ted because they are established upon a very complete linov.iedge of the skull as well as of the teeth. Aphelops fossiger, CorE. Dentition : I. ^, C. 5, P- f > ^I- §• First premolar siin[)le, conical, sometimes absent ; nasals not overhanging premaxillaries ; foramen lacerum medium confluent with foramen ovale ; occiput broad and L)W ; limbs short and bulky ; molars with well developed 'crochet' and 'anticrochet.' Ill the figure given by Marsh (Am. Jonrn. Sci., Oct., 1887, p. 3) and by Cope (Am. Nat., Dec, 1879, p. 771 f)> ^''^ third and Iburth premolars have both the 'crochet ' and 'anticrochet.' There is some ground for the supposition that the sknll here described belongs to a different species, since the 'anticrochet' is not developed in the premolars. This reference is therefore provisional. This is apparently the only species Avhich is represented in this ci^Ilectiou. All the specimens are from Kansas, and include several skulls and well pre- served bones from all jiarts of the skeleton, enabling us to give a complete description and restoration of the animal. 1 See Ext. Man^m. Fauna, Dak , p. 228. 2 Cope has nevertlieless employed the ' cristae ' developed in this milk molar in his definition ni A. crnssiis. "On the Extinct Species of llhinoceriid£e of North America," etc., liulL U. S. Geol. Survey, Vol. V. No. 2, p. 237. MUSEUM OF COMPARATIVE ZOOLOGY. 93 The Brain. One of the most interesting features of Aphelops is the very large size of the brain. The walls of the cranium are solitl. There are no vacuities or air-cells in the diploe of the mid-region of the brain- case, such as attain from 1 to 1^ inches in thickness in Ceratorhinus. Thus the brain is relatively much larger Figure 15 — Brain of Aphilnpsfussirjer X \. Lateral view of intracranial cast. than that of the recent rhinoceros, and presents a marked advance upon that of Aceratherium occidenialc. The bulk of the fore- and mid-brain, or the divisions in front of the cerebellum, is approximately as follows : — Aceratherium, 420 c.c. Aphelops, 1240 c.c. Ceratorhinus, 720 c.c. The bulk of the entire brain is : Aphelops, 1470 c c. Ceratorhinus, 850 c.c. The relative body weight of the two animals can be roughly estimated from a comparison of the femora as Aphelops 4, Ceratorhinus 3. It thus appears that the steady brain growth of the ungulates during the Eocene and early Miocene periods reached its highest point in some fami- lies of the later Miocene, and was followed by a de- generation. The cerebellum in Aphe- lops is small and partly over- hung by the hemispheres. The lateral view of the hemispheres shows a very marked predominance of transverse sulci, which ra- diate from the vertical syl- vian fissure, S, so that in the basal view of the frontal lobes the fissures are antero-posterior. The dorsal surface of the cast is somewhat imperfect, giving an incomplete reproduction of the parietal and^occipitul regions. The .superior Figure 16 •Brain of Ceratorhinus Sumatrensis X i- Lateral view of cast. 94 BULLETIN OF THE sulci of the frontal lobe are directed obliquely backwards to the longitudinal fissure, thus reversing the direction observed in the recent ungulates. The Skull and Dentition. The skull (Plate III.) is broad in relation to its length, owing to the shorten- ing of the ant-orbital region and the recession of the nasals. The niaxillaries spread very widely for the powerful series of molars, while the jvcmaxillaries are slender. The orbit is placed above the first molar. The nasals are com- pressed anteriorly, and extend only so far as to overhang the preniaxillary suture. A .marked feature of the skull is that the upper surface is in a nearly straight line linin the supra-occipital ridge to the tip of tlie nasals, M-hile in A. tncyalodus it is concave. The orbit is very slightly' overhung by the supra-orbital jirocess. The zygomatic arch is deep vertically, but compressed laterally. Tlie post- glenoid process is deep and narrow ; it has contact with the post-tyinj)anic of variable length. 'J'he remarkable feature of the post-tympanic is its extension into a broad flat plate behind the auditory meatus. The occiput is liroad and low, and does not overhang the condyles ; it is deeply cleft in tlie meilian line. On the base of the skull, the foramina rotund um and spheno-orlatale arc con- fluent, as ob.served by Cope. The foramen ovale is either confluent with or separated h\ a slender vidge of bone from the foramen lacerum medium. The molars and premolars are remarkable for the extreme flattening of the outer surface of the ectoloph, all trace of the three vertical ridges having dis- appeared. The first prenndar is a simple conical tooth implanted l)y a single fang ; it is apparently inconstantly developed, lor Marsh makes no mention of it in his de- scription of A. (cicutuni) fossigcr. The inner angles of the protoloph and metaloph unite by the 'crochet' in ^m^ and jnn^ to enclo.se the median valley, as in Aceratherium. The fourth premolar resenddes the molars except in the non-development of the 'anticrochet.' The true molars are characterized as follows : Figure 17. -First superior molar of ,,^. ^j,^ constriction of the inner portion of ApheiopsJ('S.n-! s o O O OQ S es C c O PLATE III. Skull op Aphelops fossigkr. One sixth natural size. Xo. 4. — Crififatdla: the Orujiii and Jkvclopmnif of Ihr valued in, the Colony. By C. 15. ])AVENi'Oia'.' In,!i- Contents. Page I. Tntroduction 101 II. Ar(-liiti.'cture of tlie C(jl()ny . lUo III. Ori-in of tlic Inilivi.luul . . lOG A. (Jb.servatioiis. — Origin of 1. Tlie Uud .... 107 2. The Alimentary Tract 111 3. The Central Nervous System 11-) 4. Tiie Kamptoilerm . 115 5. The Fimienhis ami jMuscles . . . . ll'i 6. Tlie IJotlywall . .117 7. The Radial i'artitions llO B. Comparative ami Theo- retical IJevicw of the Observations on the Origin of the Indi- vidual 120 Origin of the Poiypide 121 Interrehition of the Individuals in tiic Colony 121 Origin of tiie Layers 123 Origin of tiie Alimen- tary Tract . . . .127 1. 2. 3. 4. I';i<:.. 5. Origin of the Central Xervous System . 1"_'7 G. Origin of the runicu- his ami Muscles . li'^ IV. Organogen\'. — Development of 1. The Ring Canal . . 12'.i 2. The Lophophore . . IH" 3. Tiie Tentacles . . I'i-'i 4. The i>oi)hophoric Nerves 13(! T). The Kjjistome . . . 13S (]. The Alimentary Tract l;!'.t 7. The Funiculus .and Muscles . . . . Ul 8. Origin and Develop- ment of the I'arieto- vaginal Muscles . .14:! 9. Disintegration of the Neck of the Poiyp- ide 144 10. Development of the Body-wall .... 144 Summary 14f) Bibliography 148 K.xplanation of Figures .... 152 I. Introduction, At the suggestion of Dr. 1-",. I,. Maik, I liogiui, in tlie sjn-ing of 1889, the stiiily of iVesli-watcr Ilnozoa. While :it the [.ahoi-aturv of the riiite(l States Fish ( 'liimnissidii, at Woods Holl, iMass., where, tlirougl. tlic kindness of ]Mr. A. Agassiz, 1 h;i(I the opjiort unity of speiidiii^' tl: • Contributions from the Zoological Laboratory of the Museum of Comparative Zoology, under the direction of Iv L. Mark, No. XI.X. VOL. .\.\. — NO. 4. 102 BULLETIN OF THE following summer, I gathered most of the material for this study. I found an excellent place for collection in Fresh Pond, Falmouth, where Fredericella and Plumatella were also gathered. Upon my first visit to this pond (July 5th), I found at its outlet Cristatella exceedingly abun- dant on the leaves of the pond-lilies. A month later, the same locality yielded very few specimens ; but about September 5th I found them plentiful again^ and at the same time noticed the phenomenon described by Kraepelin and by Braem, — that some of the statoblasts of Pluma- tella had already hatched. Colonies of from five to twenty individuals were observed with the two halves of the statoblast still adhering to their bases. A few colonies of Cristatella were also gathered in the latter part of August from Trinity Lake, New York. The material collected was killed with a variety of reagents. Cold corrosive sublimate gave the best results. In staining, I always found Czoker's cochineal the most satisfactory dye for the study of the embryonic cells of the bud. As Haddon ('83, pp. 539-54G) has reviewed the most important part of the bibliography of budding in Phylactolsemata which had been published at the time of his writing, I shall be relieved from giving here any extended historical account of the earlier researches. The contributions of Nitsche ('75) and Hatschek ('77) are well known. Reinhard has published a preliminary article ('80', 'SO**) on this subject in the Zoologischer Anzeiger ; but his two more important papers ('82 and '88) I have unfortunately not seen. Braeni's ('88, '80% and '89'') three preliminary papers concerning budding in fresh-water Bry- ozoa correct some erroneous statements of Nitsche, and support Hat- schek's view of the origin of the polypide. The results at which I have arrived concerning this last problem are similar to those of Braem, but his work has apparently been done chiefly on Alcyonella. mine on Cristatella. Finally, T believe there will be found in this paper something new on the organogeny, which Braem does not seem to have especially studied, and which may be of general morphological nnpor- tance. For these reasons, it has seemed to me desirable that I should publish my observations and conclusions, and I am the more inclined to do so because our views are not in all points the same. In the matter of nomenclature, my studies have not led me to a final conclusion as to the homologies of the axes of the individual, and there- fore I fall back by preference on non-committal terms. The individual is bilaterally symmetrical. Parts nearer the mouth end of a line joining mouth and anus (i. e. nearer the margin of the colony) will be desig- MUSEUM OF COMPARATIVE ZOOLOGY. 103 nated "anterior" or "oral"; parts nearer the anal end, "posterior" or "anal." To parts nearer the roof of the colony will be applied the terra "superior," or "tectal"; to those nearer the sole, "inferior." Parts situated at either side of the sagittal plane of the individual are "lateral," and either right or left, — the individual facing the margin of the colony. In naming organs, I have preferably used the terms employed by Kraepelin ('87). I adopt the term polypide simply be- cause it is a convenient name for a number of organs closely united anatomically, and arising from a common source embryologically. II. Architecture of the Colony. The colony of Cristatella, as is well known, consists of a closed sac, which is greatly elongated in old specimens, and has a flattened base or "sole," and a convex roof. The wall of this sac is known as the wall of the colony or cystiderm (Kraepelin). Suspended from the dorsal wall, and hanging in the common cavity of the colony, which may be called the coenocoel, are to be seen numerous polypides in diflferent stages of development. A more careful ol)servation shows that the polypides lying nearest the hiedian plane of the colony are the largest and oldest, those nearest the margin, conversely, smallest and youngest (Plate I. Fig. 1). All young colonies of Cristatella have been derived from one of two sources, eggs or statoblfists. According to Nitsche ('72, p. 469, Fig. 1), there are two polypides of the same age first developed in the cystid, which is a product of a fertilized ovum, and regarding these he fully agrees with MetschnikofF's ('71, p. 508) statement, "Die beiden Zooiden entwickeln sich wie gewdhnliche Knospen.'' Nitsche ('75, pp. 351, 352) observed that in Alcyonella the primary polypides are placed with their oral sides turned from each other, and that the younger buds arise in the prolongation of the sagittal plane of the older polypides, and from that part of the cystid lying between the oesophagus of the older buds and the margin of the colony. As Braem ('89'', pp. 676-678) has shown, there is but one primary bud in the statoblast embryo. The younger buds formed in the stato- blast arise on the oral side of the primary bud. In Cristatella, says Braem ('88, p. 508), the newly hatened stato- blast embryo already exhibits to the right and left of the adult primary polypide two nearly complete daughter individuals of unlike age, which are generally followed by two other sisters in the same relative posi- tions, and a fifth in the median plane, — oral with respect to the 104 BULLETIN OF THE mother bud. These buds may produce new ones until the whole colony has attained the size of a pea ; then young buds arise anal- wards o: the primary polypide, and as the margir» of the colony is pro- truded on each side of this point, the colony becomes heart-shaped. The two upper lobes of the heart are regions of great reproductive activity ; they separate from each other, and thus transform the heart- shaped colony into an elongated one. Through the heaping together of buds effected by this process, a misproportion between the area (Flachenraum) and the circumference of the colony results, and tlie buds, which lie in longitudinal rows, soon come to be crowded. After this, they each give rise to only two daughter buds, a lateral and a younger median one. To these observations of Braem I have little to add. I have figured (Plate X. Fig. 88) a young colony of Cristatella, containing about thirty polypides. This was taken in the latter part of July, and is probably an egg colony. My reasons for thinking so are, that the statoblasts of the preceding year form colonies in the early spring ; that statoblasts of any year have never been seen, like those of Alcyonella, to hatch in the fall ; and that there are, occupying the centre, two polypides of very nearly equal size and development, and probably therefore of nearly equal age. Surrounding these are eight younger individuals, nearly equal to each other in size, and these are in turn followed by two generations, of thirteen and seven individuals respectively, — the last generation evidently being as yet incomplete. As Kraepelin ('87, pp. 38, 139, 167) clearly states, the Cristatella colony is comparable with those of Pectinatella, Plumatella, etc., and may be derived from them by imagining a condensation of those branching colonies. The radial partitions seen in Figure 88, di sejy. r., Plate X., are thus homologous with the lateral walls of the branches of a Plumatella colony; and just as in the latter, so here young individuals arise near the tips of the branches, and the older individuals degenerate. As in Plumatella, young individuals are produced not only distad of older, but also laterad, thus founding new branches, so in Cristatella we find young buds having the same positions. These facts will be better appreciated by a reference to Figure 1, which shows a portion of the margin of a mature colony. It is here clearly seen, (1) that, as has long been known, the youngest individuals are placed nearest to the margin, and that therefore, as one passes towards the centre, one encounters successively older and older individuals; and (2) that, as Kraepelin ('87, Fig. 134) has already figured, the older individuals are arranged in a quincunx fashion. MUSEUM OF COMPAliATIVE ZOOLOGY. 105 The bit of the margin figured may be regarded as typical, not only on account of its symmetry, but also because of the fact that the youngest individuals are placed at the normal distance from the margin. Al- though I have seen these conditions repeated in enough instances to assure rae of their normal nature, yet, owing to a crowding of polypides, both among themselves and to the margin of the colony, and also to the consequent dis- placement of polypides, the appear- ances which I am about to describe are often obscured. First, the interrelations of the indi- viduals included within compartments 1-8 are exactly repeated in compart- ments 9-16. The same repetition holds true for the remainder of this side of the colony. On the opposite side, the number varies from «ix to eight. At the ends of the colony, owing to crowding of individuals, it is difficult to count with accuracy. Since all individuals are derived from pre- ceding ones, the conclusion seems rea- sonable that the inhabitants of these eight branches were derived from a common ancestor. It is interesting that from each of these ancestors the Siime number of branches and an almost equal number of individuals are produced, and that the correspond- ing individuals in each of these fam- ilies, e. g. Figure A, 4, 5 anc* 12, 13, and 7, 8 and 15, 16, are similar in position, and of the same stage of development. Secondly, most individuals figured have given rise to two individuals ; some, on the contrary, to but one. Of the two individuals produced, one (the older) passes into a second (new) compartment, and so forms a new branch. The younger, however, remains in the ancestral com- partment, and thus continues the ancestral branch. See, e. g., individual FlOOKB A. 106 BULLETIN OF THE 4, 5, of Figure A. The buds* which give rise to new compartments may be called lateral buds, iu accordance with Braem's terminology; those which prolong the ancestral branch, median buds. Where only one individual arises, it is a median bud. These conclusions regarding the relationship of buds are based solely upon the length of the radial par- titions, the inner extremities of which correspond to the angle formed by two branches iu branching genera like Plumatella. Thirdly, while the lateral buds. Figure A, 4, 5, and 12, 13, give rise directly to new buds, median buds of the same or younger age, 6, 14, have moved to a considerable distance from their mother buds before giving rise to new individuals. The eflfect of this is, that the median bud comes to lie, not alongside of the lateral bud, but in a quincunx position relatively to it. Fourthly, lateral buds (branches) may arise from either side of the budding individual. Although most of the branching in the part of the colony figured in the out is to the right, yet the youngest lateral buds are being given off to the left. So in compartments 4, G, 7, 12, the funiculus indicating the point where the median bud will arise. To recapitulate : The descendants of common ancestors are arranged similarly in the same region of the colony ; a lateral and a median bud may arise from a single individual, the first forming a new branch, the latter continuing the ancestral one ; median buds migrate towards the margin before producing new buds ; and new branches are formed on either side of the ancestral branches. III. Origin of the Individual. Two essentially different views of the origin of the polypide in the adult colony of Phylactolaemata have been maintained within recent years. The first is that advanced by Nitsche (75, pp. 349, 352, 353), and adopted by Reinhard * ('80*, p. 211, '80^ p. 235). According to these aiithors, the outer of the two layers of the colony-wall gives rise, either by a typical or a potential invagination, to the inner cell layer of the bud, — the layer from which the lining of the alimentary tract and the nervous system both arise, — and pushes before it the inner layer ^ Reinhard says in his preliminary article, " Meiner Meinung nach entwickelt eich die Knospe in Folge einer Verdickung des Ectoderms, in welclie dann die Zellen des Entoderms eindringen," but Brandt's abstract of the paper read by Rein- hard before the Zoological Section of the Russian Association, places entoderm for ectoderm, and vice versa, — a rendering more in accordance with Reinhard's statements in the context. MUSEUM OF COMPAKATIYE ZOOLOGY. 107 of the colony-wall, which thus becomes the outer layer of the bud Hence the buds arise independently of each other. The second view is that advauced by Hatschek ('77, pp. 538, 539, Fig. 3). He asserted that iu Cristatella "Die Schichten der jiingeren Knospe stammen vou denen der nachst alteren direct ab." Finally, Braeni ('88, p. 505) agrees essentially with Hatschek, and believes that a typical double bud, although it does not always appear, is the funda- mental condition. His preliminary account clearly shows that precisely the same condition of affairs, except in so far as modified by the less metamorphosed condition of the ectoderm, exists in Alcyonella as iu Cristatella. A. Observations. 1. Origin of the Bud. — The result of my own work has been to lead me to a conclusion differing from both of these two views, but more like the second than the first. By my view, as well as by Braem's, Nitsche's two types of single and "double" buds are united into one. I would not say, with Hatschek, that tlie two layers of the younger bud arise directly from those of the next older, but that each of the corresponding layers of the younger and next older buds arises from the same mass of indiffer- ent embryonic tissue. In some cases, each of the layere of the daughter polypide does arise from the corresponding layers of the very young mother bud. In other cases each of the two layers out of which the two layers of the older bud were constructed contributes cells to form the correspondiug layers of the younger bud, but the cells thus contributed have never formed any essential part of the older bud. All gradations between these two types occur. For convenience* sake, we may always call the older polypide the mother ; the younger polypide, the daughter. Figure 3 (Plate I.) shows a well advanced bud (Stage VIII.) which con- sists of two layers of cells, an inner, i., composed of a high columnar epithelium arranged about a narrow lumen ; and an outer, ex., of more cubical cells. In a region (I) on the bud which is near the attachment of its oral face to the body-wall there is a marked evaginatiou of the contour, caused in part by a thickening of the outer layer, and in part by a slight increase in the diameter of the inner. This thickening o the wall is the first indication of the formation of a younger bud, which is to arise at this place. Figures 22, II., 16, VL (Plate III.), and II, VI. (Plate II.) show later stages of buds originating in the same manner as that of Figure 3. The mother bud has grown larger, as has also its lumen. The outline in its upper oral region has become much folded as 108 BULLETIN OF THE a result of coll proliferation, and a deep pocket has liecu fornicd Imud by a l.iycr of cells which are still a part of the inner layer of the mother 1)11(1. 'J'he outer layer of tlie latter has also been protruded by the ac- tivity of the inner layer, and its cells go to form the outer layer of the young bud. Still another point is to be observed. The centre of the young bud has moved away from the centre of the neck of the luothci- bud, and thus the fornicr lies nearer to the mar!! account of the mere i-apid elongation of the anal than of the oral side, the axis of the alimcntarv tracts comes to take a horizontal position, as shown in Fig- ures 17 and 18, Plate HI. (Compare also Figs. 27-20, Plate lA^) The blind end of the digestive sac comes very close to the blind end of another pocket formed on the oral side, the osdphagus, and soon the two communicate directlv. At the same time, the inner cell-layer of the MUSEUM OF COMPArvATIVE ZOOLOGY. 113 middle portion of the alimentary tract has been quite cut off from that of tlie atrium by a constriction, the beginning of which is seen at ex., Figure '1\ (Plate IV.) and in a later stage at ex.. Figure 28. The cells of the outer layer are next pushed into the place of constriction and re- move tlie alimentary tract at this point still further from the atrium, as is shown in Figures 18 (Plate III.) and 28, ex. (Plate IV.). Tije error of Nitsche is explainable on the ground that he believed the stage of Figure 18 to be the earliest in the development of tlie alimentary tract. 3. Origin of tlie Central Xervons Sydern. — Metschnikoff ('71, p. 508) first clearly recognized that the supra-oesuphageal ganglion of Phylacto- laemata is derived from the inner cell-layer of the bud, — the same layer which gives rise to the inner lining of the alimentary tract. Nitsche ('75, pp. 359, 3G0) described and figured in an insufficient and not wholly accurate way the process of the formation of this organ. According to my observations, the central nervous system ttriscs directly over the middle of the horizontally placed alimentajy tract in the posi- tion marked f7». in Figure 18, Plate III. (compare also Figs. 17 and 28, pavi. ffn.). The jiroccss by which the ganglion with its internal cavity (Plate VIII. Fig. 73, I/i. gn.) is formed will be more easily understood if the reading of the text be accompanied by reference to the following sec- tions. Figures 17, IS, 19, Plate 111., and Figure 73, Plate VIII., show successive stages in sagittal section. Figures 27-29, Plate IV., from a single individual, are vertical right-and-left sections, the positions of which are indicated by the lines 27, 28, 29 of P'igure 17. Figures 30-32 are similar sections from an oLler individual (see lines 30, 31, and 32, luunliered at the lower border of Fig. 18), and Figures 33-38 are from a still older polypide (compare lines 33-38, Fig. 19). P)V a studv of these sections, it is seen that the cells forming the fluor of the brain, ixim. gn., are derived from the inner layer of the l)ud, and indeed from the very region of the layer which furnished cells to line the alimentary tract (Plate II. Fig. 13, Plate IV. Figs. 25 and 21, gn.), and therefore that the layer of cells forming the floor of the ganglion is dircctlv con- tinuous posterii>rly through the anal opening (Plate II. Fig. 13, an.) with the wall of the rectum, and anteriorlv with the lining of the (esophagus. The first marked differentiation of this region is effected by the sinking of the centre of the floor of the neural tract (Fig. 18, gn.), thus forming a shallow pit, which opens directly into the atrium above. The closure of the walls of the ganglion above must now be considered. (Vmcerning this process, Nitsche says : "Die Pander dieser Einstiilpung [my ' shallow pit '] ^vachsen nun wie die Eiinder dcr Medullarrinue VOL. XX. — NO. 4. 8 114 BULLETIN OF THE eincs Wirbelthierembiyos gegcn cinander, und wic in letzterem Falle cine holile liijhre von der dorsalen Leibeswand des Thieres abgeschniirt wii-d, so wird hier einc hohle Blase von dcr Wand des Polypids abge- schniirt. In unserem Falle ist aber die Wandung an der diese Ah- schnurinig vorsich geht, zweischichtig." The two layers referred to were those of the median walls of a pair of invaginations of the latero-anal sides of the wall of the atrinm, — the beginnings of the lopho])lioric arms (6r. /oyjA., Figs. 37, 38, Plate IV., and Figs. Gl, G2, Plate Vll.). The process of closure is in reality somewhat different from Nitsche's conception of it. The axes of the pockets which go to form the IojjIio- phoric arms are, at first, directed inward, upward, and sliglitly oral- ward (Plate I. Fig. 7, br. loph.). By means of these invaginations the cell layers lining the atrium on opposite sides are brought into contact at a point between the rectum and the gangli(mic pit (Plate V. Fig. 43, Injih.'). This approximation of the walls niay, perhaps, bolter be said to be a continuation upward of the process by which the alimentary tract was cut off from the atrium (after the lumen of the former was formed), and by which cells of the outer layer of the bud came to intrude themselves bctw'ecn tiiese two regions (Plate IV. Fig. 35, ex.); fur the lateral furrows, by the formation of which this act is performed, are, on each side, continuous with the lophophoric pockets, and above end blindly in them. P)y the approximation and fusion of the inner layers of the atrium several things are accomplished. The posterior wall of the brain is formed (Plate IV. Fig. 39, Inph.'), the anus is car- ried farther up (compare Plate III. Fig. 19, and Plate \'ll 1. Fig. 73, na.), and by a continuation of the constricting process the cavities of the loijhophore on opposite sides of the polypide are brought into communi- cation between the ganglion and t!ie rectum at a ptnnt opposite the letters In. gin. in Figure 63 (Plate VII.), whereas they formerly com- municated only outside the alimentary tract. Oralward from the lo]»hophoric pockets there is a thickening of the inner layer above the floor [jxan. gii.) of the ganglion on each side (Plate IV. Figs. 28 and 31). Later, each of these thickenings becomes a fold involving the inner layer of the bud only (Plate IV. Fig. 35). The upper and lower halves of this pair of folds respectively fuse in the sagittal plane, the last point at which the union occurs being near the a'sophagus (Plate III. Fig. 19). Anteriorly the rim of the shallow brain- pit rises up as a third fold, and the ganglion becomes a sac whose mouth is boinided by the edges of the folds, the advance of which causes it to become more and more constricted. These folds are the j)air of folds MUSEUM OF COMPARATIVE ZOOLOGY. 115 above the cavity of the ganglion, and the one between the cavity of the ganglion and the cesophagus. The outer layers of these three folds respectively fuse immediately behind the oesophagus ; the inner layers are constricted off, but without closing the neck of the sac. Conse- quently the neck of the ganglionic sac, instead of opening into the atrium, now abuts upon the inner cell-layer at the angle between the floor of the atrium and the oesophagus. The lower layers of the hori- zontal folds thus become the upper wall of the ganglion (Fig. 35, tct, gn.) ; the upper layers form the new floor of the atrium (Fig. 73, pam. atr.), which lies between the lophophore arms, is continuous with its median walls, and passes over into the walls of the alimentary tract both in front and behind. The outer layer of the young bud only secondarily makes its way in between the upper and lower layers of these folds. It ultimately takes the form of a double layer embracing a space, which is the epistomic canal. (Plate VIII. Fig. 73, lu. gn., Plate V. Fig. 52, la. gn., can. e stm.) 4. Origin of the Kamptoderm. — While the alimentary tract, lopho- phore arms, and nervous system are being marked out in the lower por- tion of the bud, these organs become farther removed from the wall of the colony by an enlargement of the atrium to meet the demands of the augmenting volume of the lophophore. Pari passu with this en- largement of the atrium, its walls diminish in thickness (compare kmp. drm., Fig. 73, Plate VIII., with Fig. 18, Plate III.). This is rather the result of a failure of the cells to multiplj' in proportion as the area of the wall increases, thau of a decrease in the number of cells already formed. Both the inner and outer cell-layers of the bud take part in the formation of this wall, as is evident from the figures. The wall of the atrium was called "tentacular sheath" by Allman ('56, p. 12) and Nitsche, but Kraepelin ('87, p. 19) employs the name "kamptoderm" for this structure. I prefer this term to "tentacular sheath," and have employed it both on account of the reasons given by him and because it may be easily inflected, whereas " tentacular sheath " may not. The kamptoderm, then, is formed of the upper portion of the bud, and both of its cell-layers are concerned in its formation and persist in the adult. 5. Origin of the Funiculus and Muscles. — Nitsche ('75, pp. 353, 354) did not see the origin of the funiculus, but states that it suddenly occurs lying close along the oral side of the bud, to which one end is at- tached. Its proximal end is fastened, he says, to the inner layer of the colony-wall, and by the growth of the latter between the funiculus and the neck of the bud this end retreats from the young polypiile. Bnieni 116 BULLETIN OF THE ('88, p. 533) asserts that the funiculus arises as a lopgitudiual ridge ou the outer layer of the oral wall of the young polypide at the time of the formation of the alimentary tract, and that the cells of this ridge are cut off from the bud to form the funicular cord. Soon after this, embi-yonic cells from the inner layer of the young polypide penetrate into the midst of the cord through its proximal end, and thus lav the foundation of the statoblast. Concerning the origin of the muscles, Nitsche ('75, p. 354) states that they are simple elements of the outer cell-layer of the bud, which were originally situated in the angle of attachment of the bud to the inner layer of the colony-wall, and that by the growth of this wall they be- come drawn out into spindle-shaped cells. I have decided to treat of these two organs together, since their ori- gin and development are curiously similar. According to my belief, both arise, in part at least, from the inner cell-layer of the colony- wall. At a stage slightly earlier than that of the first appearance of the fully formed funiculus (Plate II. Fig. 11, cl. fun.), I have always found a disturbed condition of the coelomic epithelium. This is particularly noticeable on that side of the young lateral bud upon which the median bud is about to arise. In some cases I have seen the cells of this layer taking on all the characters of wandering cells, as seen at cl. fun., Figure 22, Plate III., where some have already begun to group themselves into a funiculus-lilve cord. At Figure 57, cl.fun., Plate VI., the funiculus is seen lying close to the oral wall of the polypide. That it has not arisen in precisely the manner described by Braem is probable from this figure alone, for the proximal end of the funiculus is not yet connected with the wall of the colony. If my view is correct, this connection arises only secondarily (Fig. 2, fun.). I am, however, inclined to be- lieve that the distal end of the funiculus arises in a difi'erent way from the proximal, and in the manner described by Braem. My evidence for this is, tliat I have twice seen at this point cells in the act of dividing so as to contribute daughter cells to the funiculus. Figure 53, Plate VI., shows the condition of the distal end of the funiculus, /mw., which passes, without any line of demarcation, into the outer layer of the bud ; this layer is normally one cell thick, but in the region of funicular formation it is two cells thick. The proximal end of the funiculus is, at this stage, attached to the coelomic epithelium of the roof of the colony, tct. That an attachment should occur in this man- ner, and become quite intimate, is not strange, considering the origin of the funiculus from amceboid cells, and the fact that, even at a late MUSEUM OF COMPARATIVE ZOOLOGY. 117 stage of development, this character is still retained by much of its tissue. (See, for example, Fig. 11, fun., Plate IX.) The great retractor and rotator muscles have, I believe, like the funic- ulus, a double origin. They arise from the outer layer of the bud, on the one hand, and from the ccelomic epithelium on the other. The first indication of the differentiation of the muscle cells consists in a disturbance in the upper lateral edge of the outer layer of the bud at about the stage of Figure 17, Plate III. This is shown in dorso-ventral sections through this region (cl. mu., Figs. 24, 26, Plate IV.). Later, tlie disturbance becomes more marked, and cells having a semi-amoeboid character appear to be proliferated (cl. mu., Fig. 33, Plate IV.), and to migrate from the bud towards the ccelomic epithelium. During this process the cells of the latter layer also are active, and some of them, elongating, reach towards the young polypide, as seems to be clearly shown at cl. mus., Figure 54, PUite VI. It is significant that, since each of the two upper lateral edges of the bud lies near a radial partition, the muscles also are always formed in close proximity to one (disep, r., Fig. 54, Plate VI. ; Fig. 30, Plate IV.). It will thus be observed that, both in the case of the funiculus and of the muscles, the end which is attached to the wall of the colony arises at a point which is remote from that of its attachment to the adult. The migratiim to the later positions will be treated of farther on. (See page 141.) 6. Origin of the Body-wall. — As already shown (page 104), the body- wall of the individual of a Cristatella colony includes not only the endocyst of authors, — the roof and the sole, — but also the radial partitions. Braem ('88, pp. 506, 507) concludes " dass die polypoide Knospen- anlage . . . nicht allein das Polypid nebst den Tochterknospen liefert, sondern dass auch die zugeliorigen Cystide ans ihr und zwar aus ihrem Halstheil entwickelt werden." I believe that a portion of the " cystid," or body-wall, is thus formed in Cristatella, but not the whole. If one compares the relations of the polypide to its daughter bud in Figures 3 (Plate I.) and 17 (Plate III.), and reflects that later the daugh- ter bud is to be found still farther from the mother bud, he is forced to one or the other of two conclusions : either the young bud is pushed from the mother by a proliferation of cells from the neck of the polyp- ide without causing an increase in the length of the body-wall itself, or there is an actual increase in the length of the body- wall, produced either by the proliferation of cells already existing in it, or by the addi- tion and subsequent proliferation of cells from the neck of the mother 118 BULLETIN OF THE polypide ; and this increase in length, occurring between the polypide and bud, carries the two apart. Unfortunately, I am unable to state definitely how this migration of the young bud away from the mother is eftected. If the ectoderm increases in length between the two buds by the proliferation of cells already existing in it, that fact ought to be evinced by a distorted condition of the old cell-walls of the highly meta- morphosed cells of the ectoderm. For, since most of the active proto- plasm is at the base of the ectoderm, its area will increase faster than will the area of the surface of the ectoderm ; and the latter will either rupture or stretch, or else the ectoderm will become concave on its outer side. An application of these criteria to sections of the body-wall in the budding region leads to the conclusion that the ectoderm of Cristatell^ increases here very slightly, if at all, by a proliferation of cells already existing in it. A search for cell division in this region has yielded the same negative results. There can be no doubt that cells are added to the ectoderm from the neck of the polypide. The process takes placo, however, after the daughter bud is well established at some distance from the mother bud. The proliferation of these cells ruptures the old cell-walls of the ectoderm, and increases the area of the body-wall. I shall have occasion to speak of this process more fully in treating of the later period to which it belongs. There remains, then, the conclusion, that the cells which go to form the inner layer of the young bud are pushed from the neck of the next older bud by a proliferation of cells in the stolon-like mass, without causing an increase in the area of the body-wall itself. Moreover, I have seen cell proliferation in the stolon-like mass. Another series of facts will lead us to this same conclusion. Though the body-wall does not increase by cell proliferation between buds, it does so, I believe, at the margin of the colony. This, it is true, cannot be directly observed with ease, since the multiplication of cells, which tends to increase the breadth of. the colony, must also occur at the margin, and one cannot be certain what dimension of the colony wall will be augmented by any given case of nuclear division. My belief rests on the following evidence. (1) In the same adult colony the distance of the youngest bud from the margin is not the same in all regions. This is not what we should expect if the distance of the youngest polypides from the margin remained unchanged during the growth of the colony. (2) There is a gradual increase in the amount of metamorphosis exhibited by the cells as one passes from the margin towards the middle of the roof. Figure 60 (Plate VI.) shows a rather MUSEUM OF COMPARATIVE ZOOLOGY. 119 marked example of a very common, although not universal, condition of the lateral margin of tlie colony. The epithelium of the margin is composed of columnar cells, which are higher (54 jx) than those of the roof (48 yu,), and also of a less average diameter (8.4 /v.) than the latter (18.2 /x). ^loreover, the cells are very little metamorphosed. In pass- ing towards the roof (td.), the cells are seen to become more and more metamorphosed, the secreted bodies (cp. sec.) becoming relatively larger. Fi^nu'e do represents the margin in a more metamorphosed state than Fignre 60. Although this condition of things is not incompatible with the idea of a passive margin, it strongly suggests that this region is one of proliferation, by which cells are added to the roof, and tlius the distance from the youngest polypide to the margin is virtually increased. This conclusion receives a very important confirmation from the study of the origin of the radial partitions, the treatment of which must be deferred for the moment. Although new cells are being added to the roof at the margin, yet the distance from the youngest polypide to the margin is not greater in old tlian in young colonies. How, then, is the approximate constancy of this distance maintained ? Evidently it can only be by the process (which I have already shown must take place) of migration of some of the young buds at the base of the ectoderm, particularly in the case of median buds. The ten- dency of the migration of young buds towards the margin is to diminish the distance between the front of the budding region and the margin of the colony. The tendency of cell proliferation at the margin is to increase that distance. The actual distance is the resultant of these two opposing factors, and may be less or greater in different parts of the same colony, according as the one or the other is the more active. If we assume, further, that the cells added to tlie roof and sole from the mai-gin plus those derived from the necks of tlie polypides are equal in amount to those lost by the degeneration of individuals in the middle of the colony, we have a sufficient explanation of the fact, observed long ago, that the adult colon v of Cristatella maintains a nearly constant width. 7. Origin of the Radial Partitions. — I know of nothing on this sub- ject by any previous author. The radial partitions consist of a muscu- laris covered on both faces by a very thin epithelial layer (Plate X. Fig. 95, 1). The muscle fibres of tlie muscularis arise from the already formed longitudinal muscles of the wall of the colony at the region of transition from the sole to the roof (Plate VI. Fig. 55, mu.). As the muscle fibres move into the coenocoel, they carry before them the ccelo- 120 BULLETIN OF THE niic epithelium of the region from which they ai'ise. It is owing to thiis method of origin that the epitiieUum comes to ch>the both faces of the jtartition. 'I'iie process by which the muscle fibres move int.) the ccenocoel appears to be this. The end of a fibre nearest the roof becomes fixed to a certain part of the muscularis of the roof, and is left behind with it when the margin is carried outward (potentially) as the result of cell proliferation. Thus from a nearly horizontal position the fibres attain a direction at first obliciue, and then perpendicular to the sole. In some instances the u[)per ends of the fil)res move tlu'ough an arc of more than ninety degrees, so that they are ultimately directed upward and inwaixl, i. e. towards tlie centre of tlie cokmy. (('ompare mu, via , mit", Fig. 55, Plate VI.). This pi-ocess is also indicated in two horizontal sections (Plate X. Figs. 95 and 90), the foi-mei' being nearer the sole than the latter. This is a region of active budding, and in consequence new compartments or branches are being riipidly formed. The numbers 3, 4, 5, and G (Figs. 95, 90) sliow the positions of young partitions, which are shorter al)ove tlian l)elow, owing to the oblique position assumed by tlie innermost muscle fil)res of tlie partition. The oblique position is duo to the fact already demonstrated (Fig. 55, Plate VI.), that tlie tectal end of tlie nmscle of the partition first appears at the margin nearer the sole than the roof. At 2 (Fig. 9G) there is apparently an interesting case of the formation of a new par- tition by the detachment of certain fibres from the nmscularis of an old one. The fibres, moving away laterally, take with them a covering of ca^lomic epitheliuui. Near the sole this process has progressed far- ther than it has nearer the roof, so that in iMg. 95 the detachment appears complete, whereas in Fig. 96 the union is still visible. This method of formation is intelligilde when one considers that the muscu- laris of the partition often contains more than a single layer of muscle fibres. Thus, in Figure 87, vui., there are two or three layers of fibres in the section. Figure 86 represents a section cut vertically and at right angles to a partition near its union with the marginal wall of the colony, and shows three fibres of the longitudinal or inner layer of the muscularis lying side by side in the partition. ]}. — Comparative and Theoretic.\l Review of the Observations on THE Origin of the Individual. What bearing have tiie facts here adduced on those given for other groups of Bryozoa, and what is their probable significance in relation to the general problem of non-sexual reproduction'? MUSEUM OF COMPARATIVE ZOOLOGY, 121 1. Orifjin of the Polyp'ule. — Lateral budding (as distinguished from linear budding, such as occurs in Turbellaria, Chretopods, d'c.) may be roughly clas.sitied under two types, in one of which the young indi- vidual arises directly from the body-wall of the parent, as in Hydra. In the other, the young arise, one after the otlier, from a mass of embryonic material derived from a parent individual, — from a stolon, as in Salpa. In the group of Bryozoa both of these methods seem to be present. In such a form as Paludicella (Allman, '5G, pp. 35, 36, Koi'otneff, '75, p. 369) we have an example of the direct type; in Pedi- cellina we have a stoloniferous genus. Also in the marine Ectoprocta examples of both types appear to occur (e. g. Flustra, Hypophorella). To which of these classes does budding in Cristatella. belong'? It seems to me that we have here an instructive example of a transitional condi- tiun. The young polypide of Figure 3 arises directly from the mother polypide, and may represent a case of the first class. Is the type of Figure 2 a representative of the stoloniferous class? It seems to me that it partakes of the essentials of that class, although, as I have shown, it may be united l)y intermediate stages with the first class. I understand a stolon, in its morphological sense, to signify a mass of embryonic cells derived from a parent individual, and capable of repro- ducing non-sexually one or more daughter individuals at some distance from that parent. The condition shown in Figure 15, Plate II., in which the embryonic cells of the two layers represent the stolon, may fairly be said to answer to this definition. The mass of cells (III.) represents, then, the distal end of the stolon. But the stolon does not end here, although its further pi'ogress towards the mai'gin is delayed. Not all of its cells go to form the polypide which arises at this place. On the conti'ary, some of them remain in the " neck " of the new polypide, in an indifi'erent histological condition, and later give rise, either directly, or by the intervention of a typical stolon, or by both, to one or two new buds. Tlmse cells of the neck which do not thus pass over into new buds for the most part degenerate (page 141). According to this view, the neck of the polypide is to be regarded as at first essentially a portion of the stolon. 2. Interrelation of the IndlvlJucds in the Colony. — The interrelation of individunls in the colony in C'heilostomata has been most carefidly investigated from a morphological standi)oint by Nitsche ('71, pages 35, 3(5), who showed that, in opposition to Smitt's theory, each new indi- vidual arose from a single preceding one, and that the latter, in order to increase the breadth of the colony, might give rise to two individuals 122 BULLETIN OF THE instead of one. Reicherfc ('G9, p. 311, Fig. 28, Plate VI.) has shown that in Zooboti'yon (one of the Ctenostomata) " an der Mantelfliiche, und zvvar einseitig, inseriren die Bryozoenkupfe mit Alternation in par- allelen, wie es scheint, langgezogenen spiralig verlaufcnden Keihen an- geordnet." Nitsche ('75, p. 370) states that the buds in Loxosoma arise from the mother alternately on opposite sides, and that the younger the bud, the nearer it is to the foot of the parent individual. Both Hatscheck (77, pp. 517, 518, Fig. 33, Plate XXIX.) and Seeli- ger ('89, p. 176) show that in Pedicellina young individuals are devel- oped in the plane of the older ones, and are successively formed at tlie growing tip of the stolon, towards which the oesophageal side of all individuals is turned. This relation is the same as that wliich wc have found in Cristatella. In Cheilostomata, however, it is apparently the anus which is turned towards the buddiuir maririn. Thus, throughout the group of Bryozoa, we find that the position which young buds assume in relation to older individuals is very definite. I am inclined to believe that the radial partitions of Cristatella sep- arate the morphological equivalents of the isolated branches of such a form as Plumatella punctata (see Kraepelin, '87, Taf V. Figs. 124, 125). The type of budding which gives rise to the series of median buds may, then, be represented, as seen from the side, by Figure B. The margin (*) will then represent that portion of the body-wall of the youngest individual, which will give rise to a part of FiGCBE B. ' *= • T 1 , the body-wall of the next younger mdividual. The process by which the body-wall of the individual of Cristatella is formed is therefore, in my opinion, different from that which Braem describes in the case of Alcyonella, for he maintains that in Alcyonella the proper body- wall of an individual arises later than its polypide. In fact, the tip of the branch of Alcyonella is somewhat different from that of Cristatella. In the former, it is occupied by the polypide of a budding individual ; in the latter, a part of the body-wall of the bud- ding individual is pushed out beyond the polypide. In the former, the foundations of the daughter polypide are pushed out upon the body- wall of the mother, and begin to form their own proper body- wall; in the latter, the young bud migrates away into the modified part of the body -wall of the mother, which forms the extremity of the branch, and which now becomes a part of the body-wall of the daughter polypide. This distal part of the body -wall grows independently of the polypide by interstitial growth, and thus differs from any part of the body-wall MUSEUM OF COMPARATIVE ZOOLOGY. 123 of the individual of Alcyonella, for all of it, according to Braera, is de- rived from the neck of ite own polypide. This last method of origin of the body-wall I believe to be also present in Cristatella, as well as in Alcyonella, as I shall have occasion to show later (page 144). In Paludicella, according to both AUman ('56, pp. 35, 36) and Korot- nefF ('75, p. 369), the formation of the body-wall of the new individual is begun before the appearance of the polypide. In Cheilostomata, as both Nitsche ('71, p. 22) and Vigelius ('84, p. 75) have shown, and in Ctenostomata, as demonstrated by Ehlers ('76, pp. 91, 92), the " zooe- cium " arises before the polypide takes on its deBnite form. 3. Origin of the Layers. — Although later researches have only con- firaied the conclusion arrived at long ago, that in Tunicates cells from all three germinal layers of the parent pass over into the bud, the facts in Bryozoa have seemed not to favor the view of the fundamen- tal nature of this process. To be sure, Hatschek ('77, pp. 517-524) believed that he had found evidence of a condition in the budding of Pedicelliua exactly comparable with that in the budding of Tuni- cates ; but the more recent studies of Harmer ('86, p. 255) and Seeliger ('89) have failed to confirm his results, if they have not satisfactorily explained the source of his error. What is the relation of the condition I have described in Cristatella to the question of the transmission of a part of each germinal layer to the bud, and in how far do the conditions here agree with our present knowledge of the budding process in other groups of Bryozoa] Al- though my results accord with Hatschek's in this, that the youngest and next older buds are intimately related, that the corresponding layers in each are derived from the same cell layers, and that the inner layer of the bud is not derived directly from the overlying ectoderm, they do not strengthen the idea of the fundamental importance of his doctrine, "Die Schichten der jiingeren Knospe stammen von denen der nachst alteren direct ab." Moreover, they afford no evidence of the accuracy of his conclusion, that the inner layer of the bud is com- posed of entoderm ; indeed, since this inner layer does not give rise to the alimentary tract alone, as he supposed, but to the nervous system jilso, the facts in Cristatella tend to weaken his hypothesis. In order to determine finally just what the origin of the stolon from which the inner layer arises is, it will be necessary to study the origin of tlie first- formed polypides. This I have not yet been able to do. Our present knowledge on the subject is still in an unsatisfactory state. Allman ('56, pp. 33, 34) has described and figured some stages in the 124 BULLETIN OF THE development of the egg, but without referring to gastrulation, or the layers involved in the first polypide. Metschnikoff ('71, p. 508) and Nitsche (75, p. 349) maintain that the outer layer of the embryonic " cystid " goes to form the inner layer of the primitive polypides, and that its inner layer forms the outer layer of the polypides. Eeinhard ('80% pp. 208-212) is more explicit concerning the early stages than preceding authors. Apparently the egg segments regularly, and undergoes embolic invagination. The blastopore closes. There is a circular groove in the anterior part of the embryo (Barrois's mantle cavity), and from the cap or "hood" which the mantle cavity sur- rounds, the wall of the " cystid " or colony-wall is subsequently formed. The embryo is already composed of three layers, " an outer, the tunica muscularis, and the entoderm." All three layers of the " hood " share in the formation of the polypides, but the fate of each layer is not clearly described. Haddon ('83, p. 543) suggests that the gastrula is to be regarded as one in which the alimentary tract is retarded in development, and that the enlarged ccolomic diverticula, such as occur in Sagitta, etc., line nearly the whole of the so-called archenteron. From the small mass of true entoderm at the pole opposite the blastopore the alimentary tract arises. This suggestion, unfortunately, has no positive facts for its support, and could be of service only upon the assumption that the alimentary tract of the first polypide is formed from the inner layer of the "cystid"; but this assumption is contrary to the observation of all who have written on this subject. Kraepelin ('86, p. GOl) has also observed the "gastrulation," but he believes that it is to be interpreted as the precocious formation of an enterocoel, in which case the invagination to form the first polypide is to be regarded as the true gastrulation, the inner layer of the cystid as mesoderm, and the inner layer of the bud as entoderm. By far the most satisfactory and complete account of the embryology of fresh-water Bryozoa is tliat of Korotneff, '89. The genera studied were Alcyonella and Cristatella. Since the development takes place inside of an ooecium, the use of the section method is necessary for the elucidation of the details of the embryological processes. Apparently the egg segments regularly and forms a blastula. Loose cells are given off from the inner surface at one pole of this blastula. These arrange themselves in an epithelium, lying immediately inside of the ectoderm, over a part only of its inner surface ; so that while the upper two-thirds MUSEUM OF COMPARATIVE ZOOLOGY. 125 of the embryo has two layers, the lower third is one-layered. The cavity of the lower third contains some scattered cells, which, the au- thor hints, may be representatives of the mesoderm, while the cavity in which they lie may represent an enterocoel. The author regards the in- ner layer of the upper two-thirds as true entoderm. The method of its formation recalls that of the entoderm of some Ccelenterata, as demon- strated by Metschnikoff. There is no epithelial invagination, such as Kraepelin maintained, and therefore the cavity which the inner layer lines cannot be regarded, says Korotnetf, as an enterocoel. Later, the entire embryo becomes two-layered by an extension of the inner layer. The two polypides arise from two distinct invaginations of the double- layered wall. Unfortunately, Korotneff does not demonstrate by figures the method of origin of the alimentary tracts of the first polypides ; but there is little reason to doubt that it is essentially like that in other buds. If it is admitted that the inner layer is entoderm, as Korotneff maintains, then the entoderm takes no part in forming the digestive epithelium ; but the latter is derived solely from ectoderm. In his discussion of the theoretical bearing of his results (p. 404), the author seems to niaintain that the polypide is to be regarded neither as an individual (Nitsche's view), nor, on the other hand, as an assemblage of organs homologous with organs of the same name in other groups; but rather as a new structure, developed upon the cystid, to aid in its nutrition. In criticism of Korotneff s view, that the loose cells given off from one pole of the blastula are entoderm, I may point out that this process bears quite as much resemblance to the process of " mesenchyme " formation (as described by Korschelt for the Echinoids), as it does to the origin of the entoderm in some Coelenterates. Compare Figs. 13 E and 182, in Korschelt und Heider's Lehrbuch der Vergleicheuden Ent- wicklungsgeschichte. Braem ('89^ pp. 676, 677) has shown that the primaiy polypide of the statoblast arises from the cell layers of the statoblast, exactly as the primary polypide of the egg embryo does from those of the ** cystid," and the alimentary tract is formed as in buds of Cristatella. To sum up : The outer layer of the colony-wall is ectodermal in ori- gin ; the inner layer arises by an embolic (]) invagination of the blas- tula, and would therefore appear to be entoderm, although the possibility of its being homologous with the mesoderm in other forms is perhaps not excluded. The first polypides so arise that their inner layers are 126 BULLETIN OF THE formed by an invagination of the outer layer of the colony-wall, and their outer layer from the inner layer of that wall. In EndoprorMf Seeliger ('90, pp. 176-187) has shown decisively that the inner layer of the bud is derived solely from the ectoderm, and that this inner layer gives rise to the digestive epithelium of the alimentary tract, to the nervous tissue of the brain, and to the outer layer of the tentacles. Here mesenchymatous cells, representing undoubtedly meso- dermal tissue, come secondarily to surround the polypide as a loose outer tissue. In Loxosoma the same is probably true. The conditions of budding in Gymnolaemata are more difficult to un- derstand. In Paludicella the bud seems to arise as in Phylactolsemata (Allman and Korotneff). The same is probably true for Alcyonidium (Haddon, '83, p. 523, Plate XXXVIII. Fig. 23). In the Cheilostomata, however, the fact of the great development of a loose mesenchyme-like tissue obscures the process, and makes it difficult of interpretation. This tissue, which is known under three probably homologous terms, — " Funiculargewebe," Nitsche, " Parenchjmgewebe " in part, Vigelius, and " Endosarc," Joliet, — is to be considered as representing the funicu- lar and coelomic tissues of Phylactola;mata. The most careful observa- tions on the origin of this tissue are those of Joliet ('77, pp. 249, 250, and '86, pp. 39, 40) and Vigelius (*84, p. 76). Both authors assert that this tissue is derived from cells given off from an epithelium at the distal end of the budding individual. Vigelius ('84, pp. 19, 79) believes that this epithelium is ectodermal, and that it is the sole rudiment of this layer; but Ostroumoff ('85, p. 291) and Pergens ('89, p. 505) have shown that the ectoderm persists and secretes in its cells the cal- careous ectocyst. It seems more probable, however, that the " funicu- lar tissue" arises from the inner layer of the body-wall (Nitsche, '71, p. 37, Plate III. Fig. 5, c), and is the equivalent of the coelomic epithe- lium of Cristatella. The fact that many of these mesenchymatous cells conglomerate in the formation of the polypide sufficiently accouuts for the origin of its outer layer of cells. The origin of the inner layer is problematical, if, as is asserted to be the case by several authors, the bud is not formed in the region of the body-wall. It will be premature to speculate upon the significance of the facts of budding in the Ectoprocta until we shall have gained a more com- plete knowledge of the ontogeny of the group, and of the relationship of the Cheilostomatous to the Phylactolsematous type through compara- tive agamogenetic studies. It may appear in the end, that, under cer- tain circumstances, undifferentiated embryonic tissue, derived from a MUSEUM OF COMPAUATIVE ZOOLOGY. 127 certain germ layer, can assume the task of building organs in budded individuals similar to those derived from a ditfcrent layer in the sex- ually produced individual. Whatever may be the truth of the conclusions reached by Haddou ('83, pp. 548, 549, 552) and by Joliet ('8G, pp. 54-5G), that the nervous system and the alimentary tract arise from two distinct layers, or kinds of cells, in the species studied by them (and their evidence is certainly not conclusive even for these), their attempts (Haddon, '83, p. 540, Joliet, '86, p. 57) to apply their results to the Phylactolaimata are not justified by the observations which are here presented, nor by those which have been made upon most Gymnola3mata and Endoprocta. 4. Origin of the Alimentary Trad. — I'here is a curious difference between the Endoprocta and the Ectoprocta in the development of the organs of digestion. Seeliger ('89, pp. 182-184) lias sliown for Pedi- cellina, that the oesophagus and stomach arise as an evagination of the oral wall of tlie young bud, which secondarily becomes connected with the proctodaeum. Haddon ("83, pp. 517, 518) lias shown for Flustra, Barrois ('86, pp. 73-86) for Lepralia, Braem ('89'', pp. 677, 678) for tlie statoblast polypides of Cristatella, and the present paper for the polypides in the adult Cristatella, that the oesophagus only is formed on the oral side, the stomach arising with the rectum on the anal side of the atrium. In all cases the oesophagus is formed first (Plate II. Fig. 13). A comparison of my Figure 18 with Figure 41, Plate XXX., of Hatschek ('77), shows a striking resemblance between the two. The form of the alimentary tract and the depression to form the ganglion are practically identical ; and were the tentacles to arise directly from the immature lophophore arm (6r. loph., Fig. 18), and from the circum- oral fold which has already appeared, it would be difficult to decide whether the anus opened outside or inside the circlet of tentacles, — whether, at this stage, the Cristatella polypide were ectopi-oct or endoproct. 5. Origin of the Central Nervous Si/sfem. — The only observations on the origin of the brain in Bryozoa relate to Phylactolsemata and Endo- procta. In buds of Pedicellina, the ganglion is formed, according to Hatschek ('77, p. 520), as an invagination of the floor of the atrium, tvhich later becomes cut off as a liollow sac. Harmer ('85, pp. 274, 275) has studied the origin of the ganglion in the bud in Loxos(mia. lie states that it is derived from the floor of the vestibular [atrial] cavity, and (apparently on purely theoretical grounds) that this latter is ecto- dermic. " In a longitudinal section through a fairly advanced bud 1'2S BULLETIN OF THE (Fig, 15) it is seen that a narrow slit-like diverticulum of the vestihulu passes behind the epistome. This diverticulum, which remains in vciv much the same condition throughout life, does not give rise in toto to the ganglion, which is merely formed by a dillerentiation of some of its ectoderniic cells." Harmer further doubts Hatschek's account of the formation of the ganglion in redicellina, and believes that the lumen of Hatschek's hollow sac is in reality the commencement of the fibi-ous tissue which occupies the centre of the ganglion in the adult, and which in optical sections might easily be mistaken for an empty space. "Similarly," he continues, "Nitsche has described the ganglion of^lAv/o- iLtlla as originating as a diverticulum from the tentacle sheath. 1 ix-gard it as probable that the explanation whirh I have suggested iur raJicrl- lina will hold also for Alci/onella" The conditions whicli every student of the embryology of Phylactolccmata has stated since MetschnikoiFs paper in 1871, and which my own residts reailii-m, do not warrant Ilar- mer's conclusions. The nerve fibres are very evident in the adult ganglion of Cristatella, and in addition to them there is a cavit}-, ontogenetically derived frt)m the atrium, wliicli, as Saefftigen ('88, p. 9G) lias also shown for I'hylactolaimata, contains no histological elements (Plate V. Fig. 52). 6. Orifivi of the Funiculus and Muscles. — The origin of the so-called funicular tissue in Gymnolajmata has been described already (page 12G). This same tissue also gives rise, according to Vigelius ('81, p{). 34, 35) and others, to the retractor muscles of the polypide. As 1 have already shown (pages 115-117, Figs. 22, 54), in writing of the origin of these tissues in Cristatella, the coelomic epithelium gives off cells, some of which take on an amoeboid appearance, and, uniting together, form that end of the funiculus -which is attached to the colony-wall. Other cells from the coelomic epithelium jiass directh' to the adjacent outer layer of the bud, to form the nascent retractor and rotator muscles. Both of these organs are, however, formed in part from cells composing the outer layer of the bud, — itself closely related ontogenetically to the coelomic epithelium. These facts would seem to confirm the conclusion which tlic similar relation of the two layoi's would suggest, nameh', that the coelomic epithelium of Phylactolamata is the homologue of the " endosarc " of Gymnola^mata. MUSEUM OF COMPARATIVE ZOOLOGY. 129 IV. Organogeny. 1. Development of the Ring Canal. — Nitsche ('75, p. 358) describes the ring canal as a farrow arising from the opening of each of tiie lopho- phoric pockets, and running towards the oral side of the bud. In a later stage, botli layers become deeply implicated in this furrow, and the ring canal is completed by a growing together of the edges of the furrow. Braem ('89'', p. 679) merely states that he cannot fully agree with Nitsche's description of the formation of the ring canal. As a result of my own studies on this subject, I have readied the conclusion that tlie circumoral branch of the ring canal makes its first appearance in the median ])lane in the oral region at about the time that the depressions of the lophophoric pockets ai'c first indicated. 'J'he formation of both organs is preceded by a pi-eliminary thickening of the inner layer of the bud (Plate IV. Fig. 20, br. loph., and Plate 111. Fig. 17, can. crc). It is only later, after tlie lophophoric pockets have at- tained considerable depth, that the groove of the incipient "ring canal" appears continuously on the side of the polypide, extending from the pre-oral region to the lophophoric pockets (Plate IV. Figs. 33, 35, 37, can. crc). As indicated in the successive stages of Figures 18 and 19, Plate III., the thickening of the inner layer anterior to the mouth is followed by a fold at this point involving both layers. The fold is deepest in the pre- oral part of the median plane, and becomes shallower as it proceeds pos- teriorly. Finally, the outer-layer cells of the lips of the fold approach each other and fuse, thus forming a true canal (Plate IV. Fig. 33, can. crc). Kraepelin ('87, p. 57, Figs. 72, 73, qh.) asserts that this canal does not communicate at its neural ends witli the coenocoel, but that it is always closed by a strong " Querbriicke '' comiecting the " Kampto- derm " with the alimentary tract. By making sections of the colony parallel to the sole, dozens of individuals are cut through the entire length of the circumoral ring canal. Although I have examined many individuals cut in this way, I have never succeeded in finding in Crista- tella this closing "Querbriicke"; but in both young and old specimens, sections nearly corresponding to Kraepelin's Figure 72 sliow a perfectly iniinterrupted semicircular space surrounding the cesoj)hagus, and open- ing freely into the coenocoel on each side of the brain (Plate IX. Fig. 78, can. crc). I must therefore conclude that in Cristatella the fluids of the cavities of the circumoral branch of the ring canal, and therefore VOL XX — NO. 4. 9 130 BULLETIN OF THE of the tentacles also, are in free comrnuTiication with the fluids of the conmion body cavity. As Figure 51, Plato V., shows, the posterior ends (if tiie ling canals open into a pair of cavities which are the bases of the Idplidjihoric ])ockets, and liy a coni])avison of Figures G1-G3, cxri. crc, am. or.', i'iate \' 1 1., it will become ajiparent that they each beconic CDnilticnt with a I'lirruw which ])asses up the lophdjihore arm, and from which tlie outer lophophoric row of tentacles is developed. Further, by a com- parison of can. or.", cati. crc.'", in Figures G1-G3, Plate A'll. (dcxtro- sinistral vertical sections), and Figure f)!), Plate V. (horizontal section, coinj)arc also Fig. 52, a sagittal section), it will be seen that from the tip of the lophoj)horic arm a groove (can. crc") passes down iipon the side opposite to the ascending groove ((•««. crc.'), and, reaching the base, fm-iis abiaiptly anteriorly {can. crc.'". Fig. 50), and finally, in .a later stage, becomes confluent with its fellow of the opj)osite side in the median plane, just behind the e])istome and above the l)rain. It w(jul(l be quite unnecessary for me to give figures showing the course of this supragangl ionic canal (cf f'^ig. 52, Plate V.). It has long been recognized, and is shown in Kraejielin's ('87) Figure GG, Taf. II. This is probably what Vcrworn ('87, pp. 114, 115, Figs. 20 a, 20 b, Taf Xll.) has described as a "segmental organ." Braem ('89'', p. G71)) has given to it the name " Gabelkanal." The " Piingkanal " of Nitsche is, then, to my mind, merely the circumoral portion of a groove which is elsewhere unclosed to form a proj)er canal and which lies at the base of all tentacles. My reason for avoiding another term for the unenclosed portion of the "canal" is, that I regard the whole as mor- phologically equivalent to the ring canal of Gynmohemata, which is said to bo closed throughout. 2. Development of the Lophojihore. — The early stages in the forma- tion of this organ are well known, both from the descriptions of Nitsche ('75, pp. 357, 358) and the earlier ones of Allman and others. I have already (page 114) shown how the cavities of the lophophoric pockets become confluent between the rectum and ganglion, and how their opposed walls, formerly passing over into each other through the floor of the brain, are now anteriorly continuous by means of the new floor of the atrium, and posteriorly are fused together. The union of the inner layers of the two opposed walls of the lopho- phore arms (Plate Y. Fig. 44, loj^h.') continues, however, for some dis- tance above the floor of the atrium, up to within a short distance of the tips of the young arms (Plate VIT. Figs. Gl, G2, lojth.'). As the arms grow longer, the relative extent of their free and fused portions remains MUSEUM OF COMPARATIVE ZOOLOGY. 131 approximately the same. The free ends of the arms are shown in Figure 99, Plate XL, just above loph'. The polypide figured here is only slightly older than that of Figure 77, Plate IX. The connection between the two arms is not one of contact merely, for in the region of fusion one can count roughly three layers of nuclei, whereas each of the two free portions of the same cell layer contains but one layer of nuclei (Fig. 99). Before the atrial opening is formed, a separation of the two arms begins to take place. This process commences at the base of the arms, and proceeds upward as the tentacles of the inner row successively reach a certain stage of development. As the work of separation pro- gresses, the cells of the connecting band lose their capacity for becoming stained and appear vacuolated. The vacuoles increase in size until the connection between the arms is reduced to a series of fine threads (Plate VIII. Fig. 75, lophJ), which arc probably sundered when the ten- tacles of the inner row (can. crc", Fig. 76) bend at right angles to their former position to become parallel to those of the outer row. In at- tempting to find an explanation of this process, it must first be ascer- tained how the arms of the lophophore grow in length. One is perhaps inclined to think of a terminal growth, but this does not take place. So far as I can judge from an examination of many longitudinal sec- tions of the arms, cell proliferation goes on throughout the whole length of the arm, and with nearly equal rapidity in all parts. The distance between the centres of the terminal tentacles is about the same as in the case of the more fully developed proximal ones, but they are closer together in the young arm than in the adult one. This being the case, there ought to be as many (incipient) tentacles in the young as in the adult, and I find that to be, so far as I can determine, very nearly or exactly the case. The horseshoe-shaped lophophore being characteristic of the Phy- lactolsemata, a study of its development is important, since it may be expected to throw light on the phylogeny of the group. We have in Cristatella, Plumatella, and Fredericella, a series in Avhich the arms of the lophophore are shorter and shorter, in correspondence with other changes, by which is effected a gradual transition to the Gymnolaemata, which have a circular lophophore. In Gymnolaemata, the ring canal lies at the base of all tentacles in the adult. The anus lies outside the circle of this canal. The brain lies within the lumen of the canal. Nitsche ('71, pp. 43-45) has given the best description extant of the 132 BULLETIN OF THE development of the lophophore in Gymnolsemata. At a very early stage, the rudiments of the tentacles, he says, are seen lying in a U-shaped line, surrounding the mouth in front, but unclosed behind. The same is true for Paludicella (Korotneff, '75, p. 371). The post- oral tentacles make their appearance at the posterior free ends of the row of tentacles. They are bent slightly downward, so as to be con- cealed by the tentacles above. At a later stage, the tentacles lying next to the anus gradually come to lie nearer to the anal side of the mouth opening, the nearly parallel lateral rows lose their compressed appearance, and a circular basin is formed whose walls are constituted by the corona of tentacles. In Pedicellina (Hatschek, '77, pp. 520, 521) the tentacles arise as five pairs of papilla-like processes in the upper part of the atrium. Two additional pairs are formed later nearer the anal opening. In tlie adult (Nitsche, '69, p. 21) the tentacles are arranged with bilateral symmetry, and so that the plane of symmetrj' passes through two inter- tentacular spaces, which are thus the only unpaired spaces ; they are also much broader than the others. One might be inclined to ask by what modification of the condition of the tentacles in Endoprocta we may suppose the condition in Ecto- procta to have arisen, but the question is not a fair one. I have already (page 127) shown that the young bud of Cristatella has many points of similarity to a well advanced Endoproct. This similarity leads me to the conclusion that the common ancestor of the Endo- procta and Phylactolsemata more nearly resembled the former than the latter group. But the Endoprocta are not that common ancestor ; rather they are themselves more or less modified descendants of it. The proper inquiry is, To what ancestral relation between tentacles and anal opening does a comparison of the ontogeny of Endoprocta and Ectoprocta point, and by what modifications of that ancestral type may the two divergent types of the present be derived 1 Eliminating for a moment the evidently coenogenetic character of the lophophore arm, an early stage of either Endoprocta or Ectoprocta reveals a U-shaped band from which tentacles are to arise. This band completely encircles the mouth, and passes posteriorly as far as the anus. This is the condition of the Endoproct bud, with only five of its seven pairs of tentacles formed ; it is also the condition of the Cristatella bud of Stage XIII. (compare Figs. 19, 44). Starting from this common condition, that of the adult Endoproct, on the one hand, was attained by the addition of two pairs of tentacles posteriorly, thus nearly completing the circlet MUSF.UM OF COMPAKATIVE ZOOLOGY. 133 behind the aims. The condition of the advdt Ectoproct, on the other hand, was reached by the carving oralwards, and the meeting of the free ends of the rows of tentacles between the moutli and anus, thns shuttiniT the anus outside of their circle. In evidence of tliis latter assertion, 1 submit the following comparative stiitement. As Nitsche has shown for Gymnolajmata, the tentacles on the ring canal are first arranged in two rows, placed l)i]aterally, and meeting in front, but not behind. Later tlie hindennost uf the tentacles move forwaid and toward tlie median plane, thus comiileting the circlet of tentacles at a point behind tlie inoutli, but in front of tlie anus. I l)c- lieve tlie circnnioral ring canal 5)lus tlie early invaginations of the lopho- jihoric arms in Phylactolremata to be homologous with the ring canal of Oymnokcmata in its early stage ; like the latter, it is closed in front, l)ut has two free ends behind. The difference lies in the greater devel- opment of the posterior ends of tlie canal, which lattei- have become thrown into a vertical fold to alford space for more tentacles. At tliis stage of development it would be difficult to say whether the anus opened within or without the corona of tentacles. As in Gymnola-niata the ciicle is completed by a movement inward of the posterior tentacles, so in Phylactolccmata the corona of tentacles is cmiijileted in front of the anus by the two anterior processes, can. crcJ" , Figure 50 (cf. Fig. 44), of the lo[)liophore arm, which come to imite just behind the epistome, Figui'es o2, 81, can. err.'" The lumen of this pi'ocess of the lophopliore arm thus forms tliat portion of the ring canal which, as I sh.ill show directly, is the morphological equivalent of the most ])osterior portion of tlie rin<>' canal in OvninoliL'iuata. The tentacles which arise from tiiis portion of tlie ring canal are ontogcnetically, and thei'cfore phylo- "eneticallv, the voimuest. As in (Ivmnohvmata, so liere the moviuu- foi-- ward of the most posterior tentacles obliterates the basin-like floor of the atrium, siicli as we see in Kndoprocta, and leaves the anal opening far outside the circlet of tentacles. The answer to the question, How may the horseshoe-shaped tentac- ular corona of riiylactolaDuiata l)e homologized with circular ones? is involved in the answer to the jireceding query. Nitsche C"-"), p. 3.')7) believed the lophophoric arms to be " primaiy tentacles," and the tentacles borne on tliem to l)e secondary tentacl(>s. '• ( lar iiicht ohne Weiteres niit den Tentakelu der Infundibulata von (Imunais v.w vcrglei- chen." The only evidence which he offers in suppoi-t of his theoi-y is tlie fict that the tentacles on the lophopliore arm arise latfi- than the arm itself 134 BULLETIN OF THE The tentacles of Phylactolsemata may be distributed into two groups. The first includes those which arise from the circuraoral branch of the ring canal. The ring canal, from which they spring, begins to be formed at nearly the same time as the lophophoric arms. These tentacles are undoubtedly homologous with those of the same region in Endoprocta and Gymnolsemata. The second group of tentacles includes those which are borne upon the lophophore arms and upon the supraganglionic ring canal. Are these comparable with the posterior tentacles of Gymnolae- mata? I believe they are, and for the following reasons. Nitsche's reason for supposing that they are not is unsatisfactory', since, if we re- gard the lophophore arms aa mere upward folds of the wall of the ring canal, we should expect to have the tentacles arise later than the arms. The fact that the tentacles of the lophophore arm arise much later than those of the circumoral region is what we should expect, since the pos- terior tentacles arise later than the circumoral ones in both Endoprocta and Gymnolsemata, — a criticism which Hatschek ('77, p. 541) has already applied. In direct support of my belief are the facts, ( 1 ) that the ring canal is continuous along two sides of the lophophore arms, which would be the case if they were mere upward folds of the wall of the ring canal ; (2) the structure of the tentacles is the same as that of the oral ones, and the relation of their intertentacular septse to the ring canal of the arms is the same as that of the septae of the oral ten- tacles to their ring canal, as Kraepelin ('87, pp. 55, 56) has shown. If both circumoral and lophophoric tentacles find their homologues in Gymnolsemata, we have only to conceive of an elongation of the postero- lateral angles of the lophophore of Gymnolsemata, after the forward movement of the posterior tentacles, to effect the condition which is found in Phylactolsemata. The significance of the fusion of the lophophore arms is difficult to determine. I had thought it might be possible to find a phylogeuetic explanation for it, by regarding the unfused tips of the arms in Crista- tella as homologous with the short arms of Fredericella. In studying Plumatella, however, where the length of the lophophore arms is inter- mediate between that of Cristatella and Fredericella, I have been able to find no trace of this fusion. It does exist, however, in Pec.tinatella. I have had no material of Lophopus, upon which it is important to study this point. The evidence so far seems to indicate that this fusion of the arms during the period of their development is a secondarily acquired adaptation to some condition concerning the nature of which I am ignorant. MUSEUM OF COMPARATIVE ZOOLOGY. 135 3. Develojiinent of the Tentacles. — Nitsche ('75, p. 359) observed that both layers of the bud went to form the tentacle in Phylactolscmata, and that the inner layer was derived from the outer layer of the polyp- ide ; the outer, on the contrary, form the inner cell layer. He states, moreover, as already mentioned, that the oral tentacles arise first, then those of the outer row of the lophophore arms, of which the basal are fidlv formed before the terminal ones. The tentacles of the inner row, he says, are formed last, and in Alcyonella are yet lacking when tlie polj'pide is lirst evaginated. My own observations contirm in general those of Xitsche. The long- est tentacles in a polypide of about the age of that shown in Figure 77, Plate IX., are those arising from the region of transition from the circum- oral ring canal {(Xin. arc.) to the outer lophophoric ring canal (can. crcJ). Tlie tentacles lying near the median plane, and in front of the mouth, are somewhat shorter tlian these (75 /x: 52 /j). Tlie tentacles situated near tlie proximal extremity of the inner lophophoric ring canal (^can. crc") are still shorter (50 /a). Those situated at the tips of the lopho- phore arms are at this stage about 30 /x in length. The tentacles behind the mouth, arising from the supraganglionic part of the ring canal (can. C7r."'), are shortest of all at this stage (15 fj.). The tw'o layers which, as we have seen, go to form the upper wall of the ring canal in all its parts, are the ones which give rise to the ten- tacles. In Figure 74, ta.', Plate VIII. (compare Fig. 51, fa.'), young oral tentacles are cut transversely at different heights. The circumoral pai't of the ring canal is seen at a point (can. or.) near which it opens into the cavity of the lophophore arm. The plane of the section ])asses ob- liipiely upward and anteriorly from tliis point. The most posterior ten- tacle in the lower part of the figure is cut at the base. I'he calilire of the canal (including its wallsj is evidently much enlarged at this point. The enlargements of the canal at the base of the tentacles are seen also in Figure 78, can. or., Plate IX. The more anterior tentacles in Figure 74 show the two la3-ers well marked, but as yet enclosing no lumen. Since the tentacles arise from the ring canal at intervals only, the ring canal is a tube (or groove) whose lumen is alternately constricted and ex- panded laterally as well as vertically. The lumen is, indeed, often so small between the tentacles that the ring canal appears divided into separate chambers liy a series of transverse se^jtre, which, however, are always penetrated by an opening (Fig. 78, can. crc). Figm-es 73 and 77. ta.', show, in longitudinal section, successive stages in the development of the oral tentacles. The formation of tentacles begins by a rapid cell 136 BULLETIN OF THE proliferation at intervals in the upper wall of the ring canal ; thus a projection is formed at each of these points, which constantly elongates to form the tentacle. Figuies 70 and 60 (I'late VII.) are longitudinal sections of two later stages in the development of tentacles. The inner layer, ex. (Fig. 70), becomes gradually thinner as the tentacle grows older, and its cells finally become thiead-like (Fig. G9, ex.). Figure 81 (Plate IX.) shows the arrangement of tlio tentacles about the mouth and over the ganglion in a young polyjiide. The supraganghonic part of the ring canal is cut tangentially just beliind the e])istome (cu)/. crc.'"). I have often noticed that, in polypidcs of about the age of that of Figure 77, or older, certain of tlie nuclei seen in a cross section of a tentacle stain more deeply than the others. These nuclei are usually two or three in number on each of the lateral surfaces of the tentacles. The}' arc evident in Figure 81. I do not know what this difference in staining properties signifies. Yigclius ('84, p. 38, Fig. 23) descrilios and figures a condition of the nuclei in Flustra, as seen on cross-section, which is similar to that just described. The deeply staining nuclei in Flustra lie on the inner fiice of the tentacle, are larger than the otliers, and belong to cells which possess no cilia. Kitsche ('71, p. 43) described the development of the tentacles in Flustra as thouoh tluv wei'e derived cxclusivclv from tiie inner lavcrs of the bud ; but Kepiachofi" ('7^)^ pp. 138, 139, '75^ j). 1-52) showed that in Cheilostomes both cell layers ()f the bud took part in their formation, and he figures an early stage which is quite similar to my Figure 70. 4. Development of the Lophoplioric Nervea. — It has long been known that a large nerve passes along the middle of the upper wall of each lophophore arm, connecting proximally with the corresponding side of the ganglion. Xo observations have been made, so far as I know, upon the oriirin of this organ. Fvidently there are, a priori, two possibilities. Either (1) the lophophoric nerve is formed by a direct outgrowth of the ganglion, or (2) it arises in place from the inner layer of the bud, which, since it here forms the outer layer of the lopho])h()ric pocket, is the same as that fnm which the ganglion itself is constructed. By a careful stndv of this nerve in many stages of development, and from sec- tions in (lifTci-ent directions, I have come to tlie conclusion that it arises as an outgrowth of the walls of the ganglion, and that it penetrates between the outer and inner lavers of the arm. The facts which have led me to this conclusion are these. First, dur- ing the formation of the brain, soon after its lumen is cut ofi^ from its connection ^vith the atrium, its cells begin to divide rapidly (Plate V. MUSEUxM OF COMPAliATIVE ZOOLOGY. 137 Fig. 51, Plate VII. Figs. 63, 68) ; but that the new cells so formed do not all remain iu the brain is iudicated by the fact that the braiudoes uot in- crease very rapidly iu ssize. (Compare Plate 111. Fig. 19, and Plate IX. Fig. 77.) Tins rapid cell division would be inexplicable upon the as- sumption of an origin in. situ. .Secondly, at an early stage the lopho- plioric nerve is already seen extending from the brain to the adjacent inner layer, with which it remains in contact. A longitudinal section through the middle of tins nerve shows a prolongation of the lumen of the brain extending into it, so that its upper wall passes directly into the upper wall of the brain, and its lower wall into the corresponding part of tlie central organ (Plate Vil. Fig. 68, Iu. gn., n. luph.). The proxi- mal part of the lophoplmric nerve is thus to be regarded as a pocket of the brain. '1 he existing condition is not what we should expect if a ct)rd of cells derived from the outer layer of the lophophoric arm had secondarily fused with the brain. Thirdly, I have never found any good evidonce that cells were being given ott from the outer layer of the arm at its tip to form the nerve, where we should look for such a process, if anywhere ; on the contrary, the nerve is cpiite sharply marked off from the outer layer at this pniut, as will be seen l)y reference to Figures 64— 67 (Plate Vll.). ' These ligures I'epresent successive transverse sections from a young lophophore arm of about the stage of development of that shown in Figure 71. Figures 6.3-67 were drawn from one arm in about the position indicated by the lines 65-67 in Figure 71. Figure 64 was drawn from the opposite arm of the same individual, and in about the region of Figure 65. In Figures 64 and 65 there is a small space be- tween the nerve (ji. loph.) and the overlying cells of the inner layer (i.). This may be due to shrinkage, but in any event it indicates a complete indepeiuleuce between the twt) cell masses which it separates. Over the nerve the cells of the laver i are shorter than elsewhere. This mifrht be considered as an indication that the cells had recently divided in order to give up cells to the nerve, which, on this assumption, would be formed in situ. Three appearances, however, indicate that the cells of the layer i. have been rather subjected to crowding at this ])oint, as though by a mass of cells forcing their way between them and the layer ex., and gi'adually increasing in volume, (a.) The surface of the layer i. is raised above the general level directly above the nerve. (/>.) The cells of the layer /. are somewhat broader over the nerve than elsewhere, and the nuclei are shorter, but thicker. These arc the conditions which we should expect in an epithelium subjected to [)ressure by the intrusion of a mass of cells at its base, for in volume the crowded cells compare 138 BULLETIN OF THE fairly with their neighbors, whereas, if they had by division given rise to nerve cells, they should all be smaller, (t;.) In Figure 67, which is a section immediately in front of the advancing tip of the nerve, the po- sition corresponding to that opposite the nerve in the preceding sections is indicated by an asterisk (*). The nuclei are here crowded together, indicating pressure. Fourthly, there is a consideral)le diti'erence in size between the nuclei of the cells of the layer i. of the lophophore and the nerve cells. This is not what one would expect upon the assumption of the formation of the nerve directly from the overlying cells. Fifthly, a longitudinal section through the young lophophoric nerve (Plate \IL. Fig. 71) shows a more active cell division in it than in the walls of the arm (compare Fig. G-i, h. loph.), and a crowding together of nuclei of the outer layer of tlie arm, i, at its distal end, rather than a passage of nuclei into the nerve. The conclusion to which I have arrived from considering these facts is that the peripheral nervous si/stem in riiyladolcemata arises from the brain as an outgrowtli of its walls. 5. Development of the Epistoine. — The epistome was regarded by Lankester at one time ('74, p. 80) as homologous with the foot of Mollusca, and on another occasion (^^o, p. 43-i) as representing the preoral lobe of Annelids, — a view for which Caldwell ('83) first pro- duced evidence from comparative embryology. In view of such diver- gent opinions, and of the occurrence of an organ which is possibly its homologue, in quite aberrant genera, such as Phoronis, llhabdopleura, etc., a careful consideration of its origin and development is desirable. After the ganglion is fully formed, its oral face remains in contact in front with the posterior M'all of the oesophagus (Plate V. Fig. 52, Plate IX. Fig. 77), and on each side with the outer wall of the lopho- phoric pockets b}' means of the lopl:y3phoric nerves (Plate VII. Fig. 63, n. loph.). The outer layer of the bud penetrates between the gan- glion and rectum, but not between the ganglion and the oesophagus (Fig. 51,*). This layer also comes to lie between the floor of the atrium above, the ganglion below, and the lophophoric nerves on either side, having made its way in from behind as a double cell-layer enclosing a flat cavity (Plate V. Fig. 52, Plate VI. Fig. 5(S, Plate VIII. Fig. 74, can. € stm.). My description of the process by which tlic inner layer comes to envelop tlie ganglion above and behind differs considerably from Nitsche's, already quoted (page 114). As the ganglion becomes farther removed from the floor of the atrium, the cavity above it (m;/. e stm.) enlarges, and the two lateral walls of this canal, each composed of MUSEUM OF COMPARATIVE ZOOLOGY. 139 two layers of cells, both belonging to the outer layer of the bud, form the " Verbinduugsstrang des Ganglions mit dem Lophoderm " of Kraepe- lin ('87, p. G3, Taf. II. Fig. 59, vs.). (See Plate V. Fig. 51, Plate VI. Fig. 56, and Plate IX. Fig. 80,*.) This canal is the only one by which communication between the body cavity and the cavity of the epistome can occur. It may be called the epistomic canal (Plate V. Fig. 62, Plate VIII. Fig. 7^2, can. e stm.). The epistome proper arises at the point where the epistomic canal ends blindly, above and in front of the brain (Plate VIII. Fig. 73, Plate IX. Fig. 77, e stm.) ; it is a pocket, the outer wall of which is contin- uous on its under surface with the oesophageal epithelium, and on its upper surface with the floor of the atrium. The growth of this organ is disproportionately great after the first evagination of the polypide. That part of its wall which is turned towards the alimentary tract is then much thicker than the remaining part ; it forms the posterior wall of the pharynx (Plate VIII. Fig. 72, e stm. ; compare Plate IX. Fig. 81). Is the epistome innervated by fibres from the brain, as maintained by Hyatt ('68, pp. 41-43)'! I have not succeeded in finding such fibres, and the conditions of the formation of the epistome, cut off as it is from the brain at every point, make such a connection improbable. Allman ('56, Fig. 8, Plate XI.) and Korotneff ('75, p. 371) have shown for Paludicella, and Nitsche ('71, p. 44) has shown for Flustra, that an epistome-like fold occurs at au early stage of development, but is absent in the adult. Such an organ has been described by Allman ('56, p. 56) and other observers in Pedicellina, and it is still more prominent in Loxosoma, in which the relation of the epistome to the body cavity is similar to that in the Phylactolaematiu The constant occurrence of this organ in the development of Bryozoa, and its presence in so many aberrant genera which seem to be some- what allied to this group, can only be interpreted, it seems to me, as signifying that it is an ancient and morphologically important organ. The manner of its development in Cristatella seems to throw very little light, however, upon its significance ; it arises rather late, and does not become of any considerable size until the atrial opening is made. 6. Development of the Alimentary Tract. — The later development and histological differentiation of the alimentary tract have not been hereto- fore carefully studied. At the stage at which we left the alimentary tract (Plate III. Fig. 19) only two parts wei-e clearly differentiated, the oesophagus and the intes- 140 BULLETIN OF THE tine. In the next stage shown (Plate VIII. Fig. 73), further changes are seen to have taken place. The most prominent is the down-folding of the lower wall of the intestine at its middle region to form the coecum. Even at this early stage histological ditferentiation of the cells of this region has occurred to such an extent that the lumen of the coecum is nearly obliterated by the great elongation of some of the cells lining it. This condition of affairs will be understood by studying the cross sec- tion of the coecum at a later stage, as shown in Figure 94, Plate X. The cavity of the rectum has also enlarged, and its cells have taken on the regular columnar appearance which exists in the adult. At a still later stage (Plate IX. Fig. 77), the position of the cardiac and pyloric valves, separating respectively the oesophagus (ce.) from the stomach (ga.), and the coecum (cce.) from the rectum (rt.), is clearly in- dicated. The blind sac is still further elongated and well differentiated from both stomach and rectum. In order to attain the adult condition (Plate VIII. Fig. 72), the oral portion of the alimentary tract has merely to become divided, by a difference in the character of its cells, into pharynx {phx.) and oesophagus (as.), the stomach (ga.) to increase in diameter, and the blind sac (cce.) to elongate. The anus (an.) finally comes to lie at the apex of a small cone, or sphincter valve. The histological changes which the cells of the different parts of the alimentary tract undergo are considerable, and will be treated of in order, beginning with the (Esophagus. — At a stage a little later than Figure 77, the oesophagus, as is shown in Figure 84, Plate X., has a small diameter relative to that of the rest of the alimentary tract (cf. Plate VIII. Fig. 72, ce.), and its inner lining is composed of high columnar epithelium, like that of the oral groove. The shape of the cells is not greatly different in the adult ; but they become vacuolated, and since these vacuoles lie near the* base of the cells, and either nearer to or farther from the lumen than the nuclei, the latter acquire that irregular arrangement referred to by Verworn ('87, pp. Ill and 112). Stomach. — Figure 93 (Plate X.) represents a section across the stomach immediately below the cardiac valve, from the same individual as that from which Figure 84 was taken. The proximal ends of all cells stain more deeply than the distal ends, but the cells are all alike as far as re- gards receptivity to stains. Already, in certain regions, the cells are higher or lower than the average, and have even begun to group them- selves as typical ridge- and furrow-cells. Figure 82 is a section through the same region as Figure 93, but from an adult individual. The ridge- MUSEUM OF COMPARATIVE ZOOLOGY. 141 cells are distinguishable from those of the furrows by their greater height, their weaker attraction for dyes, and their vacuolated and gi-an- ular appearance. Moreover, the cell boundaries of this epithelium are gradually lost. Kraepeliu ('87, p. 51) has argued that the elongated cells are the true digestive cells, and that the deeply dyed cells of the furrows are, functionally, liver cells. Coecum. — Figure 94 is from a cross section of the coecum at the stage of Figures 84 and 93. The cells are more differentiated here than at any other part of the alimentary tract. They stain uniformly, however, except for a narrow light zone next to the lumen, and all reach to the muscularis. The digestive cells are swollen at their free ends ; the liver cells, on the contrary, are thickest at the base. Figure 83 is from a sec- tion of the proximal part of the caecum of an adult. The changes which the cells have undergone are of a similar character to those experienced by the gastric epithelium, only there has been an exaggeration in this region of the features shown by the stomach. Figure 85 represents a sec- tion near the blind end of the coecum of an adult. The diameter of the tube is smaller here thaif in the section last described, but the inner epithelium is thrown into still higher ridges and more profound furrows. Nearly all of the cells, however, seem to extend to the muscularis. The " liver " cells do not extend so far towards the blind end of the coecum as this region. The cytoplasm is not at all stained. Evidently, here the process of digestion reaches a maximum. The circular muscles of the muscularis are striped, and are developed here to an extraordinary de- gree, and the coelomic epithelium is greatly thickened, another evidence, it seems to me, of the intimate relation of this layer to the muscularis. The number of ridges is not constant in different parts of the alimentary tract of the same individual, and varies somewhat for the same region in different individuals. In sections corresponding in position to Figure 83, I have, however, usually found six ridges. 7. Development of the Funiculus arid Muscles. — It has already (page 117) been pointed out that the fixed ends of both the funiculus and muscles originate at a great distance from their position in the adult. Thus the funiculus originates upon the oral face of a young bild. As this bud grows older, the fixed end of its funiculus becomes gradually farther and farther removed from its neck towards the margin, until finally the funiculus is inserted upon the colony- wall at the margin, or even upon the sole. So the retractor and rotator muscles arise together on each side of the polypide and in the angle formed by the colony-wall and the radial partitions. Later (Plate V. Figs. 44, 45, mu. ret. + rot.) 142 BULLETIN OF THE they are found on the partitions immediately below the colony-wall. Still later (Piute VI. Fig. 59, viu. rot., viu. rd.) we see them on the lower portion of the partition, and linally (Fig. 5^), viu. rot., mu. ret.) they are found attached to the sole, at some distance, it may be, from the radial partition. The question arises at once, How do these changes of position take place] Examination shows that the union between the ccelomic epi- thelium and the cells of that portion of the funiculus which is attached to the roof is very slight after the funiculus has passed to some distance from the mother polyi)ide. Although occasionally I have seen the cells of the fixed end closely applied to the ca^lomic epithelium, tlie only comiection between the two is usually eflfected by means of ama^boid cells (I'liite V. Figs. 46-48, cl. mi.). On cross sections of the fixed end of the funiculus these cells (Fig. 49, cL vii.) are seen to surround it as a loose layer, and in longitudinal sections some of the anueboid cells are seen to be connected with the ccjehnnic epithelium. It is difticult to determine the origin of these cells, but they have the position and character of the cells of wliich.the funicidus was exclusively ct)mpo8ed before the entrance into it of the ectodermal plug described by Ihaem. The ouly explanation of the migration of the funiculus whicli occurs to me has been suggested by tlie facts given above ; namely, that the " migratory cells,"' by which the funiculus is attached to the ccelomic epithelium, change their position, carrying with them the funiculus. Remembering that the coenocoel is filled with a fluid in which the funiculus floats, and that by the growth of the funiculus it is elongated in proportion as the distance from its origin to the coccum increases, this hypothesis does not seem improbable, altl)ough its truth can hardly be tested by the study of preserved n)aterial. AVhen tlie funiculus has reached its permanent position its attachment to the ccelomic epithe- lium is more intimate. jNIeanwhile the end attached to the poly])ide has become more and more attenuated (Plate IX. Fig. 11., fun.), until, in the adult, I have usually been unable to discover any attachment. In any case, it must certainly be broken Avhen the polypide begins to degenerate. The migration downward of the ends of the muscles which are attached to the partition is even more difficult of explanation. During this migration their point of origin seems to be in the nniscularis of the partition itself. Tlie fixed point of the muscle in the adult is probably in the muscularis of the sole, since I have traced muscle fibres through the ccelomic epithelium, and to the muscularis (Plate YI. Fig. 58, mu. MUSEUM OF COMPARATIVE ZOOLOGY. 143 ret^. The insertion is iu the muscularis of the polypide (Fig. 56), but 1 have not been able to determine the precise relation between the muscle tibres of the great coelomic muscles and those of the muscu- laris. A comparison of Figures 44, 59, and 56 shows quite plainly that both the retractor and the rotator muscles originate from a common mass of muscle cells, and become distinct from one another by a separation of their points of attachment to the polypide. The re- tractor muscles (inu. ret.) are attached to the oesophagus immediately below the ganglion (Plate IX. Fig. 78) ; the rotator muscles {mu. rot.), on the contrary, to the lateral walls of the opening leading from the ccenoccel (coen.) to the cavity of the lophophore arms. These two re- gions are near to each other in the young polypide, but become con- stantly more widely separated with the growth of the lophophore. Compare Figure 78 with Figures 74 (Plate Vlll.) and 51 (Plate V.), which are younger stages, cut somewhat above the level of Figure 78, and more than twice as highly magnified. I have been able to obtain in thick sections various stages in the development of the muscle fibres, some of which are shown in Figures 89 to 92 (Plate X.). In the earlier stages, all parts of the muscle cell stain uniformly in cocliineal. Later, the cell body becomes diffei'entiated into two portions, easily distinguishable by their different receptivity to the dye. The more retractile portion becomes greatly elongated, highly refractive, and incapable of being stained. A mass of indifferent pro- toplasm, including the nucleus, still remains stainable (Fig. 90). The undifferentiated portion continues to diminish relatively to the whole mass of the cell, which has greatly increased in size, until little remains but tlie nucleus, placed on one side of the muscle fibre (Figs. 91, 92). Figure 92 is one of the retractor muscle fibres, in a partly contracted state. The end placed uppermost in the figure was that which abutted upon the muscularis of the oesophagus. Its more intimate relation to the muscularis could not be traced. 8. Origin and Development of the Parieto-vaginal Muscles. — These consist of two sets, the lower, or posterior, and the uj)per, or anterior. The posterior arise earlier. At about the time when the neck of the polypide begins to disintegrate in order that the polypide may become extrusible, a disturbance is seen in the cells of the outer layer of the kamptoderm immediately below tlie neck of the polypide, and in the coelomic epithelium opposite to them (Plate XI. Fig. 97, ma. inf.). As a result, several cells of each layer become organically connected with those of the opposite layer, and give rise to muscle cells. A later stage of 144 BULLETIN OF THE such a process is seen at Figure 98. - By the time the atrial opening ia established these cells have become plainly muscular (Plate IX. Fig. 79). Farther up in the angle of attachment of the kamptoderm to the roof of the colony, the coelomic epithelium and the outer layer of the bud are both seen to be somewhat disturbed (Fig. 97, inu. su.). At different points, a single one of tliese cells reaches across, and later becomes differentiated into a genuine muscle cell (Fig. 99, rrm. su.). Of these there may be three rows, 9. Disintegration of the Neck of the Polypide. — The neck of the polyp- ide, having fulfilled its function as the most important part of the stolon, must now give way to allow of the extrusion of the nearly developed pol- ypide. The Hrst indication of this process is the formation within the cells of the neck of a secreted substance (cp. sec.'), apparently like the se- creted bodies of the ectoderm. This metamorphosis first involves the outer and middle cells of the I'eck only (Plate XI. Fig. 97, cev. pyd.). Later (Plate IX. Fig. 77, of. atr.) a depression occurs in the ectoderm. This is due, I believe, to a cessation of cell proliferation at the centre, although it remains active at the edges of the neck. The depression gradually deepens until the atrium is closed by a thin layer of cells only (Fig. 98). The cells of the side of the neck do not disintegrate, but go to form the " Randwulst" of Kraepelin ('87, p. 40). The cells of this region remain unmetamorphosed. Only a thin layer of cells now stands between the polypide aud the outside world. This ruptures, as is shown in Figure 99, and by the relaxation of the muscularis, which is thickened about the atrial opening into a sphincter (Fig. 98, sjyht.), the polypide is ready to expand itself. 10. Development of the Body-wall.—-- A% already stated (page 117), Braem believes that the whole body-wall in Alcyonella is derived from the neck of the young polypide, after it has begun to give rise to daughter polypides ; and I have given my reasons for believing that in Cristatella a portion of it at least is derived from the margin. In addition to this, cells are undoubtedly added to the body-wall, as Braem states, after the time of origin of the buds. Particularly after the formation of the median bud, the neck appears to continue to furnish cells to the ectoderm. Figure 73,* Plate VIII., shows such a mass of cells. Later stages show that these cells secrete a gelatinous substance within their protoplasm {^cp. sec.', Plate XL Figs. 97, 98) ; they gradually in- crease in width and height from the neck outward (Figs. 97-99), and at the same time become more and more completely metamorphosed. The result of the addition of these cells from the neck of the polypide is to MUSEUM OF COMPAKATIVE ZOOLOGY. 145 carry the body-wall at the region of the atrial opening to a considerable height above the level of that portion of the roof lying between polyp- ides. (Compare Fig. 73, Plate VIII. ; Figs. 98 and 99, Plate XI.) This method of origin of the body-wall is of much less importance in Crista- tella than in Alcyonella, since the extent of the proper body-wall about the atrial opening is much less in the former than in the latter case. The development of the gelatinous bodies deserves further attention. Kraepelin ('87, p. 24) concluded, from a study of the condition in a statoblast embryo, that they are formed by a metamorphosis of the cell piotoplasra, beginning at the outer end of the cylindrical cell, and finally involving, in some cases, the entire cell, together with its nucleus. Some appearances which I have noticed in the ectoderm of Cristatella lead me to conclude that the origin is not always so simple as Kraepelin describes. Figure 79, Plate IX., shows at cp. ser. a niimber of small gelati- nous masses occurring at various regions in the protoplasm. Such an appearance is quite common, and must be interpreted, it seems to me, as the formation of the gelatinous balls by an intra-cellular metamor- phosis of the cytoplasm. The balls, flowing together, produce the larger masses. The metamorphosed matter from several cells may also fuse into one mass (Plate VI. Fig. 55, cp. sec). The final result of this pro- cess of cell metamorphosis in the ectoderm is a frame-work of old cell walls, having a thin layer of protoplasm and nuclei at its base, and in- closing the great gelatinous balls. Such a condition exists near the centre of the colony between adult polypides, and is shown in Figure 100, Plate XI. Summary. 1. Most individuals give rise to two buds, of which one forms a new branch, the other continues the ancestral branch. 2. The median buds migrate away from the parent polypide to a con- siderable distance before giving rise to new buds. 3. The descendants of equal age from common ancestors are arranged similarly in the same region of the colony. 4. New branches are formed upon either side of ancestral branches. 5. The greater the difference in age between the youngest and the next older bud, the greater the distance between the points at which they begin to develop. 6. In typical "double buds," both polypides arise from a common mass of cells at the same time. From the neck of old polypides a stolon- VOL. XX. — vo. 4. 10 146 BULLETIN OF THE like process of cells is given off" to form median buds. Between these two extreme types, intermediate conditions occur. 7. The alimentary tract is formed by two out-pocketings of the lumen of the bud in the median plane, one forming the oesophagus, the other the rectum and stomach. The blind ends of these two pockets fuse, and thus form a continuous lumen. 8. The central nervous system arises as a shallow pit in the floor of the atrium ; the pit becomes closed over by a fold of the iimer layer only of the polypide, which thus forms a sac, the walls of which become the ganglion. 9. The kamptoderm arises by the transformation of the columnar epithelium of the two layers of the wall of the atrium into pavement epithelium. 10. The funiculus arises from amoeboid cells derived from the coclomic epithelium. 11. 1'he retractor and rotator muscles arise together from the coelomic epithelium of both body-wall and bud, and in the angles formed by the radial partitions and the body-wall. 12. The wall of the colony grows by cell proliferation at its margin. 13. The radial partitions arise as follows: certain muscles of the muscularis at the margin of the colony leave the latter, and are carried into the coenocoel, taking with them a covering of ccjelomic epithelium. 14. Budding in Cristatella presents conditions transitional between direct and stoloniferons budding. 1.5. Throughout the group of Bryozoa, the youngest and next older buds are intimately related, and the place of the origin of the younger buds relatively to the older is determined by a definite law. 16. Cristatella difl'ers from Alcyonella in possessing a region of the colony-wall, — the tip of the branch, — which grows independently of the polypides. 1 7. Each of the layers of the younger bud arises from a part of the same cell mass as that which gave rise to the corresponding layer of the next older bud. 18. The digestive epithelium and the nervous tissue are both derived from one and the same layer of cells, the inner layer of the bud. 19. The alimentary tract of Cristatella at an early stage is similar to that of a young Endoproct. MUSEUM OF COMPARATIVE ZuOLOGY. 147 20. Harmer's conclusion, that the ganglion of Phylactolsemata arises exactly as in Endoprocta, is not confirmed. 21. Tlie "rincr canal" lies at the base of all tentacles. o 22. The circninGF„ .Tunc, 1890. 148 BULLETIN OF THE BIBLIOGRAPHY. Allman, G. J. '56. A Monograph of the Fresh-water Polyzoa, including all the known Spe- cies, both British and Foreign. London : Ray Society, 1856, pp. i.-viii. 1-119, 11 Pla. Barrois, J. '86. M6inoire sur la Metamorphose de quelques Bryozoaires. Ann. des Sciences Naturelles. Zoologie. 7* serie, Tom. I. No. 1, pp. 1-94, Pis. I.-IV. 1886. Braem, F. '88. Untersuchungen iibcr die Bryozoen des siissen Wassers. Zool. Anzei ger, XI. Jahrg., No. 2S8, pp. 503-509 ; No. 289, pp. 533-539. 17 Sept., 1 Oct., 1888. '89». Ueber die Statoblastenbildung bei Plumatella. Zool. Anzeiger, XII. Jahrg., No. 299, pp. 64, 65. 4 Feb., 1889. '89''. Die Eutwicklung der Bryozoencolonie im keimenden Statoblasten. (Vorlaufige Mitlh.) Zool. Anzeiger, XII. Jahrg., No. 324, pp. G75-679. 30 Dec, 1889. Caldwell, W. H. '83. Preliminary Note on the Structure, Development, and Affinities of Phoronis. Proc. Roy. Soc. Lond., Vol. XXXIV. pp. 371-383. 1883. Ehlers, E. '76. Hypophorella expansa. Eiu Beitrag zur Kenntniss der minirenden Bryozoen. Abhand. d. konigl. Gesellsch. d. Wiss. zu Gottingen, Bd. XXI. pp. 1-156, Taf. I.-V. 1876. Haddon, A. C. '83. On Budding in Polyzoa. Quart. Jour, of Micr. Sci., Vol. XXIII. pp. 516-555, Pis. XXXVII. and XXXVIII. Oct., 1883. Harmer, S. F. '85. On the Structure and Development of Loxosoma. Quart. Jour, of Micr. Sci., Vol. XXV. pp. 261-337, Pis. XIX.-XXI. April,'l8S5. '86. On the Life-History of Pedicellina. Quart. Jour, of Micr. Sci., Vd. XXVII. No. 101, pp. 239-264, Pis. XXL, XXII. Oct., 1886. Hatschek, B. '77. Embryonalentwieklung und Knospung der Pedicellina echinata. Zeitschr. f. wiss. Zool., Bd. XXIX. Heft 4, pp. 502-549, Taf. XXVIII.-XXX.. u. 4 Holzsch. 18 Oct., 1877. MUSEUM OF COMPARATIVE ZOOLOGY. 149 Hyatt, A. '68. Observations on Polyzoa. Suborder Phylactolaemata. Salem [Mass.]. Printed separately from Proc. Essex Inst., Vols. IV. and V., 1866-68, pp. i.-iv., 1-103. 9 Pis. Joliet, Lf. '77. Contributions a I'Histoire Naturelle des Bryozoaires des Cotes de France. Arch, de Zool. Exper., Tom. VI. No. 2, pp. 193-304, Pis. VI.-XIII. 1877- '86. Recliercbes sur la Blastogeuese. Arch, de Zool. Exper., 2® serie, Tom. IV. No. 1, pp. 37-72, Pis. II., III. 1886. Korotneff, A. A. '74. Eo^KOBaHiE Paludicella. Bull. Roy. Soc. Friends of Nat. Hist. Moscau, Vol. X. Pt. 2, pp. 45-50, Pis. XII., XIII. 1874. [Russian.] '75. [Abstract of Korotneff, '74, by Hoyer, in Hofmann u. Schwp.lbe's Jahresber. Anat. u. Pliys. f. 1874, Bd. III. Abth. 2, pp. 369-372. 1875.] '89. Sur la Question du Developpement des Bryozoaires d'Eau douce. Me- moires de la Societe des Naturalistes de Kiew, Tom. X. Liv. 2, pp. 393- 410, Tab. v., VI. ' 21 Oct., 1889. [Russian.] Kraepelin, K. '86. Ueber die Phylogenie und Ontogenie des Siisswasserbryozoen. Biol. Centralblatt, Bd. VI. Nr. 19, pp. 599-602. 1 Dec., 1886. '87. Die Deutsclien Siisswasser-Bryozoen. Eine Monographie. I. Anato- misch-systematischer Teil. Abhandl. der Naturwiss. Verein in Hamburg, Bd X.,'l68 pp., 7Taf. 1887. Lankester, E. R. '74. Remarks on the Affinities of Rhabdopleura. Quart. Jour. Mic. Sci., Vol. XIV. pp. 77-81, with woodcut. 1874. '85. [Article.] Polyzoa. Encyclopaedia Britannica, Ninth Edition, Vol. XIX. pp. 429-441. 1885. Metschnikoff, E. '71. Beitrage zur Entwickelungsgeschichte einiger niederen Thiere. 6. Al- cyoneUa. Bull, de I'Acad. Imp. Sci. de St. Petersbourg, Tom. XV. pp. 507, 508. ]871. Nitsche, H. '69. Beitrage zur KenntTiiiss der Bryozoen. I. Beobachtungen iiber die Ent- wicklungsgeschichte einiger chilostomen Bryozoen. Zeitschr. f. wiss. Zool., Bd. XX. Heft 1, pp. 1-36, Taf. I.-III. 1 Dec, 1869. '71. Beitrage zur Kenntniss der Bryozoen. III. Ueber die Anatomie und Entwicklungsgeschichte von Flustra Membranacea. IV. Ueber die Morphologic der Bryozoen. Zeitschr. f. wiss. Zool., Bd. XXI. Heft 4, pp. 416-498, Taf. XXXV.-XXXVIII. and 4 Holzschn. 20 Nov., 1871. [Also separate, pp. 1-S3.] '72. Betrachtungen liber die Entwicklungsgeschichte und Mor])hologie der Bryozoen. Zeitschr. f. wiss. Zool., Bd. XXII. Heft 4, pp. 467-472, 2 Holzschn. 20 Sept., 1872. 150 BULLETIN OF THE '75. Bcitrage zur Kenntiiiss dcr Brjozoeu. V. Ueber die Kuospung der Bryozoeu. A. Ueber die Knospuug der Polypide der phylactolfemeu Susswasserbryozoeu. B. Ueber deu Bau uud die Kuospung vou Loxo- soma Kefersteiuii Claparede. C. Allgenieiue Betraclitungeu. Zeitsclir. f. wiss. Zool., Bd. XXV., Supplementbaud, Iloft 3, pp. 343-402, Tal'. XXIV.-XXVI. 22 Dec, 1S75. Ostroumoff, A. '85. Reiiiarques relatives aux Kecberches dc Mr. Vigelius sur des Bryozo- aircs. Zool. Auzcigcr, VIII. Jalirg., No. 193, p]). 2'JO, 291. 18 'i\Iai, 1885. '86**. Contributions a I'Ktude zoologique et morphologique des Bryozoains du Golt'e de Sebastopol. Arch. Slaves de Biologie, Tom. II. pp. 8-25, 184-190, 329-355, 5 Pis. 18S6. Pergens, E. '89. Uutcrsucliungcn an Sccbryozoen. Zool. Auzciger, XII. Jahrg., No. 317, pp. 504-510. 30 Sept., 1889. Reichert, K. B. '70 Verglciclicnde anatomisclie Uutersucliuiigcu iibcr Zoobotryon pellucidus (Ehrenberg). Abliaudluiigen der kouigliclieu Akadcuiie der Wissenscharteu zu Berlin, aus dcni Jahre 1869, II., pp. 233-338, Tat". I.-VL, Berlin, 1870. Reinhard, W. W. '80^ Ziir Kenutniss des Siisswasser-Bryozoeu. Zool. Auzciger, III. Jahrg., No. 54, pp. 208-212. 3 May, 1880. '80''. Enibryologischc Uutcrsuchungen an Alcyouella fuugosa uud Cristatella muccdo. Vcrliaudl. d. Zool. Sect. VI. Vers. Russ. Naturf. Abstract by Br.\xdt, a., in Zool. Auzciger, III. Jahrg., No. 55, pp. 234, 235. 10 May, IS SO. '82. " Skizze des Banes uiid der Eiitwickelung dcr Siisswasser-Bryozoeu." Charkow, 1882. 7 Tnf. [Russian.]- '88. " Skizze des Bancs uud der Entwickelung dcr Siisswasser Bryozoeu." Arb. Naturf. Gesellsch. Charkow, Bd. XV. pp. 207-310, 7 Taf. 1888. [Russian.] Repiachoff, W. '75». Znr Eutwickclungsgcschichtc der Tendra zostericola. Zeitsclir. f. wiss. Zool., Bd. XXV. Ilcft 2, pp. 129-142, Taf. VII.-IX. I Marz, 1875. '75^. Zur Natnrgeschichtc dcr Chilostonicu Sccbryozoen. Zeitsclir. f. wiss. Zool., Bd. XXVI. pp. 139-160, Taf. VI.-IX. 8 Dec., 1875. Saefftigen, A. '88. Das Nervcn.system dcr phylactolsemen Siisswasser-Bryozoeu. (Vor- liiufige Mittlicilung.) Zool. Auzeiger, XL Jalirg., No. 272, pp. 96-99. 20 Feb., 1SS8. Seeliger, O. '89. Die niigeschlcchlliclic Vrrmehrnug dcr eudoprokteu Bryozoen. Zeit- MUSEUM OF COMPARATIVE ZOOLOGY. 151 sclir. f. wiss. Zool., Bd. XLIX. Heft 1, pp. 168-208. Taf. IX. u. X., 6 Holzschn. 13 Dec, 1889. Verworn, M. '87. Beitrage zur Kenntnis der Siisswassserbryozoen. ZeitscLr. f. wiss. Zool., Bd. XLVI. Heft 1, pp. 99-130, Taf. XII. u. XIII. p Nov., ] 887. Vigelius, W. J. '84. Die Bryozoen, gesammelt wahreud 3. u. 4. Polarfahrt des " Willera Barents" in den Jahren 1880 und 1881. Bijdragen tot de Dierkuude. Uitgegeven door het Genootschap Natura Artis Magistra, te Amsterdam, lie Aflevering, 104 pp. 8 Taf. 1884. EXPLANATION OF FIGURES. All figures were drawn with the aid of a camera lucida from preparations of Cristatella mucedo. ABBREVIATIONS. An. Anal side of poiypidfe. mu. Muscularis. an. Anus. mu. inf. Inferior parieto- vaginal atr. Atrium. muscles. hr. loph. Lopliophore arm. mu. Ig. Longitudinal muscle fi- can. crc. Ring canal, circumoral bre of muscularis. part. mu. ret. Retractor muscle "f can. crc' Ring canal, outer lopho- polypide. piioric part. mu. rot. Rotator muscle of pol- can. crc." Ring canal, inner lopho- ypide. plioric part. mu. su. Superior parieto-vaginal can. crc."' Ring canal, supra-gan- muscles. glionic part. mu. tr. Transverse (circular) can. e stm. Epistomic canal. muscle fibre of mus- cao. loph. Cavity of lopliophore cularis. arm. n. loph. Lophoplioric nerve. cev. pjjd. Neck of polypide. nu. nd. Nucleus of muscle fibre- cl.fun. Young cells of funiculus. t. (Esophagus. cL mi. Migratory cells. of. atr. Atrial opening. cl. mus. Young muscle cells. om. Ovum. coe. Coicuni. Or. Oral side of polypide. can. Coenocoel. or. Mouth. cp. sec. Secreted bodies of ecto- pam. atr. Floor of atrium. derm. pam. gn. Floor of ganglion. eta. Cuticula. phr. Pharyn.x. di Sep. Intertentacular septum. pyd. [i., ii.^ , &c.] Polypide. di Sep. r. Radial septum of colony. pyd.Jili. Daugliter polypide. ec. Ectoderm. pyd. ma. Mother polypide. e stm. Epistome. rt. Rectum. e t. cal. Coelomic epithelium. sol. Sole. ex. Outer layer of bud. spht. Sphincter. fun. Funiculus. sul. or. Oral groove. ga. Stomach. ta. Tentacle. gn. Ganglion. ta.' Oral tentacle. i. Inner layer of bud. tct. Roof of colony. kmp. drm. Kamptodorm. tct. gn. Roof of ganglion. loph.' Place of union of arms vac. Vacuole. of lophophore. vlv. cr. Cardiac valve. In. gm. Lumen of the bud. vlv. py. Pyloric valve. hi. gn. Lumen of the ganglion. Davknport. — Ciistatella. PLATE I. Fig. 1. A portion of the lateral rim of a colony. An optical section taken just below the roof of the colony, showing the arrangement of polypides. X 72. " 2. Origin of the stolon (I.) from the neck of a mother polypide of about Stage XII. (Fig. 18). Sagittal section of mother polypide. The margin of tiie colony is to the left. X 390. " 3. Earliest stage in the origin of a bud from a young mother polypide. Sagittal section. Margin to left, x 390. " 4. Origin of a bud from a mother polypide of about the age of that of Fig. 3. Sagittal section. Tiie margin of the colony is to the right of figure. X 390. " 5. Sagittal section of a double bud. Margin of colony to the left, x 390. " 6. Later stage in bud formation of same type as Fig. 4. Sagittal section. X 390. " 7. A part of the right side of a polypide of a stage of development interme- diate between those of Figs. 19 and 73. Seen from the sagittal plane. The cut surface lies to the right of the sagittal plane, and passes through the orifice of the right lophophore arm. The alimentary tract thus lies immediately above the plane of the paper, x 150. ' '" ■■ NPORT - CRISTATELU 1 PL. I \ i /v- .'/ A.-. '•( V :. \, \ -\ ''■'^■4U''®®lo^. '"^•^'- 7^®^. T®S7^% v47 •na .^ 1! I 12 cfroel. ^\. hnp.rim hrlnph ./// Of: Davenport — Cristatella. PLATE II. All figures are magnified 300 diameters, and are from sagittal sections. Fig. 8. Stage II. in the same series as Fig. 2. The funiculus, fun., has moved farther from the mother polypide. Margin to left. " 9. Stage IV. The inner layer, /., of the hud is definitely formed, and the external layer is greatly thickened. Margin to left of figure. " 10. Stage V. The cells, ('., have arranged themselves in a layer, and begin to form an invagination. Margin to right. " 11. Stage VIII. The first indications of the alimentary tract appear as a depression in the inner layer, rt. The funiculus, cl.fun., has begnn to form, as is indicated by a disturbance of tiie ccelomic epithelium. Daughter bud forms Stage VI. in a series beginning with I.,. Fig. 3. Margin to left. " 12, 13. Successive stages in the formation of the alimentary tract. " 14. Stage VI. The two cell-layers are now definitely formed, and a lumen has begun to appear in the inner. Margin to right. " 15. Stage III. in the stoloniferous type of budding. Stolon has elongated greatly, and active cell division is taking place at its distal (i. e. mar- ginal) end. A' PlU /'■ />;.', pyrin to. 12. & «4 • ■ filter. - ■'©; ■ ■ .-4 ' ® ■■%t 0^ -^'c^^^ ■'■■■■■ ^-^ PYd.mri n Or :■ J mil cy; .fpc // /-f /- • ' ■• ]' V -!> ,T). et : 0/ _^^ ; /©,«?; ^ © / - "^^ ^ .T('. /"-/' ■ liifjiii e frdf/ 15. r ■J .-.•?^^.-.-, ^•^^:^M ,^^ ■;-:i^ S''>. rrm.nr ■ t¥- - ■ v r-r:r r'r trim - • * pnmut • • • . ■■ ' :. ,>, ■).. ,. • c.l.'": rrnli}f>l>. hr.loph. - ->:em ■- ■ At ->.x "/ or nn. tin _ " .":' • .r,.D,;s;o:-. Davenport. — Cristatella. PLATE VI. Fig. 5o. Young funiculus, showing its connection with polypide. x 390. 64. Origin of muscles. The section passes diagonally across a partition at the left, di sep. r., and cuts the polypide taneentially at the right. X 390. 55. Section including a radial portion, showing the position of the muscles in the partition near the margin of the colony. X 390. 56. Section through tjie retractor and rotator muscles of a polypide of about the age of that shown in Fig. 77. X 390. 57. Young funiculus, whose upper end is free from the coelomic epithelium of the roof of tiie colony, x 390. 58. Section through the sole, showing the relation between the muscle cells and the muscularis of the sole. X 600. 59. Section across a radial partition, and both rotator and retractor muscles which are migrating from the roof to the sole. >' 390. 00. Section at right angles to the wall of the colony, showing the elongated and unmetamorphosed cells of the margin, x 390. [i.WKXP'JKi **. L.V!. kl ,).j >>^N a //;.■„ ^^ 1/-^,^ rli j'f/i I /•Onp, stall. '■'IV -.■•,■,■/■ A " . '3- i ■ >(V. ■miLlvt ,jli:ii rfi r' 'I Hi roi (/> .wpr inn ifj. till. nil' < ', ■'(■/ cr A(>l I)a\ KNi'oin — CiistaUII I- PLATi:*VIll. ^''ig. T'J. Sagittal section of an ailnlt polypide. Tlio lophopliorc has been onntteil. Outlines with camera lucida. Nuclei put in free haiui. X 175. " To. Sagittal section of huil. St^ige XIV. Tiie margin of colony to loft. * Ectodermal cells derived from neck of polypide. x ."jyo. " 74. Nearly horizontal section of a IhkI a little older than that sliowii in Fig. 73. The plane of section passes obliquely upward and forward. Tiie tentacles are cut at different heights, x .j90. " 7;'). Transverse section of lophophore arms befort; separation. Tlu- coiniectiiig band, lajih.', is reduced to threads. Tlie polyjiide has already evagi- nated. The section figured, is the seventh from the distal end of the arms, — about 40 m distant. X 390. " 76. Transverse section of lophophore arms innnediately after separation. The tentacles arising from can. cr< .'' were previously fused, x 300. r.'AVEXPORT - GRISTATELL\ 7J PkVlII. lonpeirtTi. 'y 1'^ aJf» ••• • •• /<: e.s'.m (rinm IjW^-so. f» m /7 ''.;■ tni}/i f.ccet. ■rr,:,/,r ■y^" », \\ 74 / ^p vlvcr afr ^ •Q,<,v ,5^ jp*^i.'ia«^ Mnrs/m .V /(«s4' canr XJV. ife'-^^ 70 J®soe- s5~=^;?>^-.. Davenport. — Cristatella. PLATE IX. Fig. 77. Sagittal section through a polypide, of which the atrial opening {of. atr.) has already begun to form, x 390. " 78. Horizontal section through the circumoral part of the ring canal, can. crc, showing its free communication with the coBnocoel (cceti.). Adult. X 175. " 79. Vertical section through the roof of the colony (to the left) and the kamptoderm (to the right), showing their connection b^' the inferior parieto-vaginal muscles (mu. inf.) at an early stage of their develop- ment. X 600. " 80. Horizontal section in position marked 80, Fig. 72, Plate VIII., showing epistomic canal, can. e stm., and supra-ganglionic part of ring canal, can. crc.'" X 390. " 81. Section cutting lophophore at base of tentacles. The arm of the right side only is shown entire. Stage of Fig. 77. x 175. Davexport - Cristatf.lla PlIX of.alr br Inph mm. 1) ll^i^'^'^l'""-- mn mi. ss^«^^^.^- <£® 81. 0 can rrr ",,■£«'.,'■ „ . -■. • ■. '^;'-^d&^:^ a' raw, ^# i«H --'1 ■ --^ > !• ..«s e^ •'sr @ ^ ^ I'o'.'rv'''. . '■ B?■•■ '/O. 91. \ nil. ml m. ■/n' >^ «.-.""■--: - ii"- '%%'^XQ- kmprln <.W loph.' cpser '>^' c B I) In: B 'i^teisei.iiin.Boslon Xo. 5. — The Eijcs in Blind Crayfishes. By G. H. Parker.^ In the fall of 1888 Mr. Samuel Garman placed at my disposal several crayfishes "^ which had been collected by Miss Rutli lloppin in the caves of Jasper County, Missouri. The specimens were given to me with the su<>-"-estion that I slumld asceilain the extent to which their eyes had degenerated, for, judging from external api)earances, these organs had become as rudimentary as the eyes of the blind crayfish, Cambarus pellucidus, Tellk., from Mammoth Cave. In order to establish compari- sons it was desirable to study the eyes in C. pellucidus, and for this purpose specimens of this species were kindly furnished me from the collections in the Museum of Comparative Zoology. These specimens, as well as those collected by Miss lloppin, were preserved in strong aleohi)]. Mv study of this material was carried (ui in tiio Zoological Laboratory of the Museum, under the direction of Dr. E. L. Mark. Notwithstanding the general interest which zoologists have shown in the blind crayfishes there have l)een very few publications on tlie minute structure of the eyes of these animals. The earliest contribution to this subject was from Newport, who, in discussing the ocelli of Anthojihora- bia, incidentally described the structui-e of the eye in Cambarus pellu- cidus. According to Newport's account ('uo, p. lG-1), tlie eyes in this species would seem to be only jxirtially degenerated, for although the retinal I'egion is not pigmented, the corneal cuticula is nevertheless divided into irregular facets, or " corneales," as they are termed, "and the structure [hypodermis] behind these into chambers t(t which a small but distinct optic nerve is given." The second investii>'ator who studied the eves of l)lind crayfishes was Leydig ('83, pp. 36 and 37). The material whicli was accessible to him was unfortunately so poorly preserved that it was of little value for his- tological purposes. He nevertheless satisfied himself that the cuticula in the corneal region was not facetted. He also cpu:)ted from an abstract 1 Contributions from tlio Zoological Laboratory of tlie Museum of Comparative Zoology, under the direction of E. L. Mark, No. XX. ■2 These crayfishes had previously been submitted to Dr. Walter Fa.xon for determination. They have since been described by him as a new species, under the name of Cambarus sctosiis, an account of which will be found in Mr. Carman's recent paper ('80, p. 237) on "Cave Animals from Southwestern Missouri." VOL. X.X. — NO. 5. 154 BULLETIN OF THE of Newport's paper, to the effect that the eye is " ohne Hornhaut, Pig- ment uud Nervenstabe." Tlie phrase "ohne Hornhaut" means, I be- lieve, that Vi. facetted cornea is not present ; at least this seems to be the interpretation placed on it by Leydig, for the quotation is shortly fol- lowed bj' this sentence : " Dort wo man eine gefeldorte Coi'nea zu suchen hatte — am Gipfel des Kegels — zeigt sich die Haut von der gewijhn- lichen BeschafFeuheit." There was greater reason for Leydig's regret that he could not consult Newport's original paper than Leydig himself appreciated ; for, although he probably had no reason to consider the abstract incorrect, if his quotation from it is exact, it differs at least in one respect from Newport's account. Newport described the cornea as facetted ; Leydig's quotation from the abstract states that it was not facetted. I have been unable to discover where this abstract was pub- lished, but, since Leydig quotes directly from it, the probabilities are that the discrepancy between his quotation and Newport's actual state- ment is to be attributed to an error in the abstract. Aside from this difficulty, it must be borne in mind that Leydig and Newport in their observations on the cornea by no means agree ; for while Newport really describes the cornea as facetted, Leydig states from his own observa- tions that it is without fiicets. According to Leydig, then, the eye of C. pellucidus is more completely degenerated than the observations of Newport would lead one to suppose. Tlie latest account of the eyes in blind crayfishes forms a part of Packard's paper on " The Cave Fauna of North America" ('88, pp. 110 to 113). Newport and Leydig studied C. pellucidus; Packard had the opportunity of studying not only this species, but also C. hamulatus, Cope and Packard, from Tennessee. In both species according to Pack- ard the cornea was without facets, and the hypodermis was not thick- ened in the retinal region, but an optic nerve and ganglion were present. The results obtained by Packard thus confirm those given by Leydig. From this brief historical review it will be observed that one of the principal questions concerning the eyes of blind crayfishes deals with the extent of their degeneration. This change has not only affected the finer structure of the retina, but it has also altered tlie shape of the optic stalk. I shall therefore begin witli a description of the external form of the stalks. The optic stalks of blind crayfishes are not only proportionally smaller than those of crayfishes which possess functional eyes, but they have in the two cases characteristically different shapes. In crayfishes with MUSEUM OF COMPARATIVE ZOOLOGY. 155 fully developed eyes the stalk is terminated distally by a hemispherical enlargement; in the blind crayfishes it ends as a blunt cone. This cone-shaped outline is especially characteristic of C. pellucidus (Fig. 2). It will be observed that in this species the optic nerve («. ojjt.) termi- nates in the hypodermis immediately below the blunt apex of the cone. In C. setosus (Fig. 1) the termination of the optic nerve is also at the apex of a blunt cone. In this case, however, the axis of the cone does not coincide with the axis of the stalk, as it does in C. pellucidus, but the two axes meet each other at an angle of about forty-five degrees, and in such directions that the conical protuberance at the distal end of the stalk is directed forward and outward from the median plane of the animal. The protuberance is rather more blunt in C. setosus than in C. pellucidus (compare the regions marked r. in Figs. 1 and 2). Through the kindness of Dr. Walter Faxon I was enabled to examine two specimens of C. hamulatus. In this species the stalks also termi- nate in blunt cones. They are not so pointed as in C. pellucidus, but approach the more rounded form of C. setosus. The three species, C. pellucidus, C. hamulatus, and C. setosus, are the only blind crayfishes thiis far known in North America, and, as they agree in having a conical termination to the optic stalks, a peculiarity not observable in crayfishes with functional eyes, it may be concluded that the conical form is characteristic of tlie stalks in blind crayfishes. Unquestionably, this conical shape is coupled with the degenerate con- dition of the retina. In describing the finer anatomy of the eye it will be m^)re convenient to begin with the condition found in C. setosus. Figure 1 is drawn from a longitudinal horizontal section of the optic stalk in this species. The plane of section passes through the region where the optic nerve and hypodermis are in contact. This region (Fig. 1, r.) corresponds to the retina of other crayfishes. The optic stalk is covered with a cuticula (Fig. 1, ct.), which is of uniform thickness and which resembles the cuticula of the rest of the body. In this respect the stalk differs from that of decapods with well developed eyes, for in these, although much of the stalk is covered with ordinary cuticula, the retinal region is pro- vided with a thin flexible cuticula. This has been named by Patten the corneal cuticula ; it cannot be said to be differentiated in C. setosus. In optic stalks with functional retinas the corneal cuticula is usually facetted, but in C. setosus no indication of facets is discoverable. The undifferentiated condition of tlie cuticula leads one to antici- pate a simple condition in its matrix, the hypodermis. The latter is a 156 BULLETIN OF THE continuous layer of cells (Fig. 1, hd.) with its distal face applied to the cuticula and its proximal face bounded by a fine but distinct basement membrane (inb.'). The layer is throughout very nearly uniform in thick- ness ; at least it is not thicker in tlie region of the retina than at many other places, and the slight variations in its thickness are not in signifi- cant regions. The only feature of the retinal hypodermis which would sutrgest that it was unlike the rest is the somewhat closer crowdinjr of its cells. This manifests itself in the arrangement of the nuclei in two or three irregular rows, instead (jf a single one. In otiier respects the nuclei of the retinal region and the surrounding hypodermis are essen- tially similar. The optic nerve (Fig. 1, n. opt.) consists of a poorly defined bundle of nerve-fibres wiiich extend from the optic ganglion to the hypodermis. The nerve-fibres are doubtless intimately connected with the cells in the hypodermis, for tlic basement membrane is interrupted where the nerve and hypodermis are in contact. It is probable that the basement mem- brane is reflected from the hy})odermis to the optic nerve, although I have not been able to oliservc this with clearness. Recent investigations support the conclusion that the retina in the Crustacea is derived from tlie hypodermis. In C. setosus that portion of the hypodermis from which tlie retina would be derived is scarcely distinguishable from other parts of the same layer. The retina in this species, tlierefore, has so completely degenerated that it has at last returned to the condition of almost undifferentiated hypodermis. That the optic nerve still retains its connection with the retinal area is, on tlie whole, not so significant a condition as one might at first sup- {)ose. It is probable that the optic nerve arises in this species as it does in the lobster. I have elsewhere (Parker, '90, p. 43) attempted to show tliat in the lobster it is not an outgrowth from either the optic ganglion or the retina, but that, as the ganglion was differentiated from the hypodermis, the optic nerve remained as a jirimitive connection be- tween these two structures. So long, then, as an optic ganglion should be differentiated one might expect an accompanying optic nerve ; but the nerve would be present as a ])assive connection between hypodermis and ganglion, rather than as a structure which had retained that posi- tion by virtue of its continued functional importance. The foregoing account of the eye in C. setosus is based upon obser- vations on three individuals of this species. Two of these measured, from the tip of the rostrum to the end of the telson, G cm. ; the third, 4.2 cm. In the three individuals the eyes presented essentially the MUSEUM OF COMPARATIVE ZOOLOGY. 157 same condition. Figure 1 is taken from one of the larger individuals. In this specimen the cuticula was somewhat thinner and the hypoder- mis ratlier thicker than in the other two. This I believe was due to the ftict that the animal had recently moulted. So far, then, as the eye of (J. setosus is concerned, although the optic ganglion and optic nerve are present, the retina has uudei-gone a com- plete degeneration, and is now represented by a layer of undifferentiated hypodermal cells. The eyes of Cambarus pellucidus present a somewhat different condi- tion from that described in C. setosus. A longitudinal horizontal sec- tion of the optic stalk of C. pellucidus is shown in Figure 2. The outer surface of the stalk is covered with a cuticula {d.) of uniform thickness, and there is no indication of facets. Excepting at the apex of the stalk, tlie hypodermis (hd.) is composed of a remarkably uniform layer of cells. As in C. setosus, it is bounded on its deep face by a deli- cate basement membrane (i/iL). Both an optic ganglion (y;i. opt.) and nerve {a. opt.) are present, the latter being connected with the hypo- dermis. In all these respects C. pellucidus resembles C. setosus, but when the retinal part of the hypodermis in the two species is compared a striking difference can be seen. The retinal hypodermis in C. se- tosus (Fig. 1, r.) is, as we have seen, substantially like the i-emaining hypodermis of the optic stalk. The retinal hypodermis in C. pelluci- dus (Fig. 2, r.) is much thicker than the hypodermis of the stalk. With this thickened region of the hypodermis the optic nerve is connected, and there is no question, therefore, that this thickening represents the rudimentary retina. Omitting minor details, the form of the thick- ening is that of a plano-convex- lens, the curved sui*fiice of which is applied to the concave inner face of the cuticula at the distal end of the stalk. The optic nerve is attached to the central part of the flat face of the thickening. When the retinal thickening is carefully studied by means of radial sections, one can see that it differs from the neighboring hypodermis not only in thickness, but also in the fact that it contains two kinds of substance: a protoplasmic material uniform with that of the rest of the hypodermis, and a number of relntively large granular masses (Fig. 3, con.). These granidar masses contain two, three, four, or sometimes five nuclei, and nuclei are also to be found scattered through the xmdiffer- entiated protoplasmic substance. The nuclei in the granular masses are slightly smaller than those in the surrounding portion of the hypo- 168 BULLETIN OF THE dermis ; they are, moreover, round in outline, while the other nuclei are usually somewhat elongated. The same features can be observed in tangential sections (Fig. G). Here, however, the outlines of the larger nuclei no longer appear oval, since these nuclei are now cut in a plane at right angles with their elongated axes. The nuclei \n the hypodermis which adjoins the retinal thickening resemble the larger oval nuclei of the thickening. Nowhere in the adjoining hypodermis have the granular masses with their smaller nuclei been observed. It is therefore clear, that in C. pellucidus the retinal hypodermis is dis- tinguished from the neigliboring hypodermis, not only by its greater thickness, but also by the fact that it is composed of two kinds of sub- stance, each with its special form of nucleus. Since the protoplasmic material of the retinal region contains nuclei which resemble those of the surrounding hypodermis, it is probable that this material represents hypodermis which has remained unmodified after the differentiation of the granular bodies. As shown in Figure 3, the granular bodies are for the most part limited to the deeper portion of the retinal thickening, and the oval nuclei occupy the more superficial part. If these oval nuclei represent undifferentiated hypodermal cells, it is only natural that they should occupy a superficial position, for it is there that the function of such cells, namely, the secretion of cuticula, could be most advanta- geously carried on. In tangential sections of the retinal thickening, both the nuclei of the undifferentiated hypodermis and the outlines of the cells to which they belong are distinguishable (Fig. 5). These cells when compared with those from the hypodermis of the sides of the stalk (Fig. 4) are seen to be much smaller than the latter. Like those from the sides of the stalk, however, they present no definite grouping. This accoi'ds with the fact that the cuticula presented no special mark- ings, such as fiicets, etc., for such markings could of course result only from some special grouping of the secreting cells. It is difl[icult to say what the granular bodies with their contained nuclei are. Doubtless they represent some element in the retina of the functional eye reduced by degeneration to this form. The ommatidium or structural unit in the retina of a crayfish consists of five kinds of cells. These are as follows : first, two cells in tlie corneal hypodermis, lying next the cuticula ; second, four cone-cells directly below the corneal hypodermis ; third, two pigment-cells, the distal retinulae, flanking the cone-cells ; fourth, seven pigment-cells, the proximal reti- nulpD, surrounding the rhabdome ; fifth, a few yellowish accessory pig- ment-cells limited to the base of the retina. Excepting the accessory MUSEUM OF COMPAKATIVE ZOOLOGY. 159 pigment-cells, all the cells in an omraatidium are ectodermic in origin ; the accessory pigment-cells are probably derived from the mesoderm. Of these five kinds of cells, the granular bodies probably do not repre- sent the accessory pigment-cells, for in fully developed eyes the latter lie on both the distal and proximal sides of the basement membrane, whereas the granular bodies are found only on the distal side of that structure. The granular bodies, then, more likely represent one of the four remaining elements, all of which naturally occur only on the distal side of the membrane. It is not probable that the granular bodies represent the cells of the corneal hypodermis, for these produce the cu- ticula of the retinal region, and if they have any representatives, those representatives must be the distal layer of unmodified hypodermal cells already indicated in the retinal thickening. The position of the granular bodies, therefore, precludes their representing corneal hypodermis. If then the granular bodies are not accessory pigment-cells nor corneal hypodermis, they must be either distal .or proximal retinula; or cone- cells. In a previous paper I have given reasons for considering the proximal and distal retinulse as both originating from a common group of cells, the retinuke. These are essentially sensory in function, as con- trasted with the cone-cells, which are merely dioptric. Tlie question then narrows itself to this: Are the granular masses clusters of dioptric cone-cells or sensory retinulte] In determining to which of these two groups of cells the granular masses belong, the relation which the latter sustain to the fibres of the optic nerve would doubtless be of great importance, for the nerve fibres in fully developed eyes are known to terminate in the retinulae, not in the cone-cells. Unfortunately, the histological condition of my material was such as to preclude the possibility of determining this question. The fact that each granular mass contains several nuclei clearly indi- cates that it consists of several cells. The number of cells in each mass, judging from the number of nuclei, varies from one to about five, the more usual number being three or four. When one compares the condition of intimate fusion which the cells of each mass present with the normal condition of the retinulae and cone-cells, the masses must certainly be admitted to resemble more closely the cone-cells. More- over, the number of cells in each mass, although variable, is nearer to that of the closely united cone-cells than to that of the retinula;. Not only do the number of cells involved and the intimacy of their fusion favor the idea that each mass represents a degenerate cone, but the 160 BULLETIN OF THE granular substance of the mass also closely resembles the granular ma- terial of a cone. For these reasons it seems probable that the granular nucleated masses in the retinal region of C. pelhicidus are the degen- erate representatives of the cones in normal eyes. The fact that, of all the ectodermic elements of the retina, only the granular nucleated masses continue to be differentiated, tlirows them into strong contrast with the surrounding structures. The retention of these masses may mean that on account of their extreme differentiation they have had time to respond only incompletely to the influence of degeneration ; or it may imply that phylogcnetically they were among the earliest retinal structures differentiated. Admitting them to be degenerated cone-cells and merely dioptric in function, one can scarcely conceive how tliey could have been differentiated before the sensory cells which they serve. But even if they cannot be regarded as more primitive structures than retinulsc, their retention still may be signifi- cant, as an indication that the ommatidia of primitive crustaceans con- tained cone-cells as well as retinula). Former studies have led \ne to believe that the difference in the ommatidia of various crustaceans could be explained on the assump- tion that the number of elements has been gradually increased from lower to higher forms by cell-division. The simplest conceivable rep- resentative of an ommatidium in the Crustacea might then be a sin- gle cell. This would be of course a sensory cell ; V)y its division, the more complicated ommatidia might subsequently be derived from it. In such an event, the cone-cells must be modified sensory cells ; but the fact that these cells persist in so rudimentary a retina as that of C. pellucidus points rather to the conclusion, that they are probably almost as old, phylogcnetically, as the retinulse themselves, and that primitive ommatidia consisted of at least two kinds of cells, sensory cells or retinuhie, and cone-cells, derived not from degenerated sensory cells, but from the undifferentiated hypodermis. As I have already shown, the results which Newport, Leydig, and Packard arrived at are not always in agreement. This might be ex- plained by the fact that the organ under consideration is a degenerated one, and consequently subject to considerable individual variation. This supposition, however, is not supported by anything I have observed. The preceding account of the eye in C. pellucidus is based npon the examination of three individuals. These were respectively 6.5 cm., 5.6 cm., and 4.4 cm. long. Figure 2 was drawn from the optic stalk of the shortest individual. In all essential features the eyes of the two MUSEUM OF COMPARATIVE ZOOLOGY. 161 other crayfishes presented the same condition as that shown in Figui-e 2. In the specimen 5.6 cm. in length, the granular bodies were less dis- tinct than in the other two, but they were nevertheless recognizable, and the retinal thickening was as pronounced in this as in eitlier of the other specimens. The fact that these three individuals show so little variation leads me to believe that the condition of the eye in the blind crayfish is not so variable as I at first supposed it would be. The same constancy is also true of C. setosus. Hence it seems to me improbable that the differences between Newport's observation and those of the later investigators are due to individual variations in the specimens studied. The fact that Newport's work was done before the develop- ment of present methods of research offers, I believe, a more natural explanation of some of his results, than the supposition of individual variations. That the metliods of his time were imperfect is evident from tlie fiict that Newport himself seems to have overlooked the gan- glion of the optic stalk, a structure readily discoverable by means of serial sections. (Compare Newport's Figure 13 ['55, p. 102] with Figure 2 in this paper.) Leydig's observations, so far as they extend, are fully con- firmed by my own. Packard's account differs from mine in only one par- ticular, but that is of considerable importance ; he states that there is no retinal thickening in the two species studied by him. This difference may possibly be due to individual variations in the crayfishes. Unfor- tunately, Packard does not state the number of specimens which he examined, and consequently one is nncei'tain how mucli weight to give to his general statements. The conclusions to be drawn from the foregoing account may be summarized as follows. In both species of crayfishes studied, the optic ganglion and nerve are present, and the latter terminates in some way not discoverable in the hypodermis of the retinal region. In C. setosus this region is represented only by vmdifferentiated hypodermis, com- posed of somewhat crowded cells, while in C. pellucidus it has the form of a lenticular thickening of the hypodermis, in which there exist multi- nuclear granulated bodies. These I have endeavored to show are degenerated clusters of cone-cells. If Packard's observations are correct, the retina in C. pellucidus may be reduced in some individuals as much as it is in C. setosus, which I have studied, but my own examinations do not render this view probable. Cambridge, February 24, 1890. 162 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. BIBLIOGRAPHY. Leydig, F. '83. Uiitersucliungen zur Anatomic und Ilistologie der Thicre. Bonn, Emil Strauss, 1883. 174 pp., 8 Taf. Newport, G. '55. On tlie Ocelli in the Genus Anthopliorabia. Trans. Linn. See, Lon- don, Vol. XXI. pp. 1G1-1G5, Tab. X., Figs. 10 to 15 incl. Read, April 19, 1853. Packard, A. S. '88. The Cave Fauna of North America, with Remarks on the Anatomy of the Brain and Origin of the Blind Species. Mem. Nat. Acad. Sci., Vol. IV. Pt. 1, pp. 1-156, 27 Pis. Read, Nov. 9, 1886. Parker, G. H. '90. Tlie Histology and Development of the Eye in the Lobster. Bull. Mas. Comp. Zool. at Harvard Coll., Vol. XX. No. 1, pp. 1-60, 4 Pis. 1890. Garman, S. '89. Cave Animals from Southwestern Missouri. Bull. Mus. Comp. Zool. at Harvard Coll., Vol. XVII, No. 6, pp. 225-240, 2 Pis. Dec, 18S9. Parker. — Blind Crayfishes. EXPLAXATIOX OF FIGURES. ABBREVIATIONS. con. cone. 7)1 6. ba?cnieiit membrane. ct. cuticula. nl. con. nucleus of cone-ceil. (jn. opt. optic ganglion. nl. hd. nucleus of hj'podcrmis hd. liypoderniis. n. opt. r. retina. oj)tic nerve. The specimens from which the following figures were talcen were killed and preserved in strong alcoliol, and stained in Czoclier's alum-cochineal. The cray- fish from the optic stalk of which Figure 1 was drawn was 6 cm. long. That from which the remaining figures were made was 4.4 cm. long. Fig. 1. A longitudinal horizontal section through the right optic stalk of Cam- barus setosus, Fa.xon. The liistological detail is "^iven in the hy- podermis only. The optic ganglion and the optic nerve are tinted. Between these structures and the hypodermis tlie space is filled with a loose connective tissue. X Go. " 2. A longitudinal horizontal section tlirough the right optic stalk of Cam- barus pellucidus, Tellk. This drawing was made in the same manner as Figure 1. X 65. " 3. An enlarged drawing from the distal end of the section which immediately follows that from which Figure 2 is taken. This figure shows the details in the retinal enlargement of the hypodermis. The space between this enlargement and the cuticula was artificially produced. X 275. " 4. Tangential section of the hypodermis from the side of an optic stalk of Cambarus pellucidus. X 275. " 5. Tangential section of the superficial portion of the retinal thickening in the eye of Cambarus pellucidus. X 275. " 6. Tangential section of the deep portion in the retinal thickening of the eye of Cambarus pellucidus. This section is taken from the same series as the one from which Figure 5 was drawn. X 275. Parker -Bund PL I % gn.opf. . > . ' A' ^ //,/ ■^ m7. the stage described, usually on the seventh day, the external conditions still remain nearly the same, the ocular tentacles being perhaps a little more prominent, and the concretions in the shell gland more numerous. The cells of the primitive entoderm, which surround the yolk, form a striking feature of the condition at this stage. These entoderm cells are very large, vacuolated, and only slightly stainr.ble. They contain large ovoid nuclei, which are crowded to one margin of the cells by the nutritive contents accumulated in the cells. Each nucleus contains one large deeply stained nucleolus, and a network of chromatic substance (Plate I. Fig. 2). The ectoderm, except over the nutritive sac, consists of elongated cells, whose nuclei are so arranged as to give the appear- ance of two or more layers. The ganglionic cells at this time closely resemble the mesodermic cells, and this makes it difficult to distinguish between the two (Plate I. Fig. 7). The internal ends of the primitive nephridial organs are situated one on each side of the head, immediately above and back of the ocular tenta- cles. These organs pass at first forwards and upwards, then in an arch backwards over the nutritive sac, and finally downward and forward. Their external openings are far back in the lateral walls of the body, behind the head region. The organs are readily distinguished in sec- tions by their large slightly stained cells, which are arranged in a single layer around an oval lumen. The large nuclei contain each a single deeply stained nucleolus (Plate I. Figs. 2 and 6). The primitive en- toderm and the nephridial organs retain this histological condition throughout the embryonic stages. The cerebral invaginations at first appear as shallow depressions in VOL. XX. — NO. 7. 12 178 BULLETIN OF THE the ectoderm at the base of the ocular tentacles, at a point immediately below the nephridial organs. At the same time that the infolding takes place, cells, whose nuclei are larger than those of the mesodermic cells, are being proliferated from the deep surface of the invaginating portion of the ectoderm (Plate I. Fig. 6). In the region of the ventral wall of the foot referred to in the stage previously described, there are in the ectoderm of each side of the body two groups of ganglionic cells {prf. x)d., Plate I. Fig. 7), one behind the other. These cells project into the cavity of the foot, and reach nearly to another small group of cells situated not far from the ventral wall. The cells of the latter group (there is one group on each side of the body) have nuclei simdar to those of the cells still connected with the ectoderm. Each group lies in the position subsequently occupied by the pedal ganglion of its side of the body, and is undoubtedly the beginning of that ganglion, for the cells in the ventral wall of the foot continue to be proliferated during several days, and are found in some individuals to be in direct continuity with the ganglia after the latter have at- tained considerable size. In the individuals shown in Figures 7 and 9 (Plate I.), the right otocyst (Fig. 7) is seen as a closed vesicle, which is not yet wholly detached from the ectoderm. The otocysts undoubtedly vary in regard to the time of their detachment, as will be seen by a glance at the left otocyst of the same individual, which has entirely lost its connection with the ectoderm (Fig. 9). All the other ganglia, with the exception of the one near the olfactory organ and the buccal ganglia, arise by cell proliferation from ectoderm which lies between the foot and the head region, either at or a little above the posterior angle formed by the body wall with the dorsal sur- face of the foot, or along a depression which runs forward from this point. This angle marks the posterior limit of a furrow which passes obliquely forward and downward, partially separating the head and visceral mass from the foot. This depression will be designated as the pleural groove. Of the remaining ganglia, only the visceral have begun to be foi-med at this time. The cells destined to form these ganglia are situated immediately above the angle produced by the pleural groove (Plate I. Figs. 8 and 9). Some of those of the left ganglion are wholly detached from the ectoderm, but those of the right (Fig. 7) are still continuous with the ectoderm, though pro- jecting into the body cavity. The cells have large, round, faintly stainable nuclei, each containing one large nucleolus, which takes a deep stain. MUSEUM OF COMPARATIVE ZOOLOGY. 179 Twenty-four hours later, about the eighth day, the pulsating sac of the foot has become still larger, and the oral sinus has extended backward and downward as a very narrow tubular passage, — the oesophagus, — which follows the surface of the nutritive sac for some distance, and subse- quently opens into it. The peculiar ciliated cells of great size and spongy appearance, which occupy a linear tract along the middle of the roof of the mouth and oesophagus, are at this time very prominent (Plate I. Fig. 2, loph. cil). These cells form what Fol ('80, pp. 190, 191) has called the "ciliated ridge." They persist until after the completion of the nervous system. The ingrowth of the ectoderm to form the rectum is now composed of a compact group of small cells, which shows a small lumen in its central portion, but is still closed at both ends. The cei'ebral ganglia remain in neai'ly the same condition as that last described. About twelve hours later, between the eighth and ninth days, Figure A. — The right face of a section parallel to the sagittal plane from an embryo of the nUith day X 220. gn. pd. Pedal ganglion o cy. s. Left otocyst. the two cerebi-al invaginations have become deeper, and the two groups of cells which form the main portions of the corresponding ganglia con- tain a greater number of cells. (Plate II. Fig. 15.) The pedal ganglia are also now composed of many more cells than in the previous stage. Each ganglion is usually pear-shaped, and tapers to- wards the posterior end of the foot. They both continue to receive ac- cessions from the ectoderm (Figure A), and at the same time are rapidly increasing in size by division of the cells already in position. The nuclei are larger and more easily distinguished than in the previous stage from those of the mesodermic cells, the latter being more spindle-shaped than 180 BULLETIN OF THE before. The cells of the mesoderm form a continuous layer along the inner surface of the ectoderm, except where cell proliferation is taking place (Figure A). As yet nothing is to be seen of the pleural ganglia. The visceral ganglia have increased in size (Plate I. Figs. 10, 11, 12); they are still connected with the ectoderm (Figs. 10, 12), although a few cells with large nuclei have become detached from it (Fig. 11). The ganglion and the otocyst of the same side of the body lie in nearly the same sagittal plane. Each ganglion is situated just above the angle caused by the pleural groove. The right visceral ganglion (Fig. 10) is somewhat farther forward and more dorsal than the left (Figs. 11, 12). About in the median plane of the body, and above the angle made by the pleural groove, are the cells which form the abdominal ganglion (Plate I. Fig. 13). The greater part of them are still embedded in the ectoderm. Although in some regions they project into the body cavity, they are nowhere wholly separated from the ectoderm. The abdominal ganglion seems to be at first more intimately connected with the left visceral ganglion tlian with the right, but a connective is formed with both of them a little latei-, and the abdominal ganglion thus appears to occupy the place of a direct commissure between the two visceral ganglia. As development proceeds, the abdominal ganglion becomes closely fused with both the visceral ganglia. Quite an advance in external conditions is made by the ninth day. But individuals of the same age vary so much in the degree of develop- ment attained by both their external and internal organs, that the age assigned can be taken only as an approximation to the average condition at the time indicated. The tentacles appear as protuberances, the labial tentacles being much smaller than the ocular ; the shell gland contains more concretions, the mantle is larger and V)ends backward over the dorsal surface of the foot. The radula sac makes its appearance and extends backward into the foot, where it ends blindly immediately back of the pedal ganglia. In trans- verse sections it appears flattened dorso-ventrally ; its lumen is oval, and the ectoderm lining it is more than one cell deep. The cerebral invaginations (Plate II. Figs. 15, 19, Plate III. Figs. 25, 26) are much deeper, the infolding ectoderm is greatly thickened, and the incipient ganglia receive accessions from ectodermic depressions be- tween the rudiments of the upper lips and the labial tentacles (Plate II. Fig. 21). The cerebral commissure (Fig. 21) is also being formed, the MUSEUM OF COMPARATIVE ZOOLOGY. 181 cells of the mediau portion of each ganglion growing out to meet the corresponding cells from the opposite ganglion. The commissure at this stage is composed of a small number of cells, which are very much elon- gated. The fibres resulting from their elongation already make a con- tinuous bridge from one ganglion to the other. The pedal ganglia (Plate II. Fig. 20, Plate III. Fig. 27, Plate V. Fig. 60) consist of two small groups of cells, situated about midway be- tween the sole of the foot and the posterior end of the radula sac. They are a little below and behind the pleural groove and the otoc^'sts, and they are farther from each other than from the lateral wall of the foot. There is a slight indication of a commissure (Plate III. Fig. 27) joining their anterior portions to each other. The commissure is formed in the same manner as the cerebral commissure, the individual cells composing it being spindle-shaped, with their nuclei somewhat elongated in the direction of the fibres. The otocysts (Plate II. Fig. 20, Plate III. Fig. 27, Plate V. Fig. GO) are on a level with the lower margin of the radula sac, and are nearer the pedal ganglia than in the preceding stage. On each side of the body above the pleural groove is a group of a few cells, which are in all probability the first indications of the pleural ganglia (Plate II. Figs. 1-i and 20). The centre of each cluster is seen on cross sections (Fig. 20) to be nearly on a level with the lumen of the radula sac. The cells at this stage are very small, and so loosely associ- ated that it is difficult to distinguish them from mesodermic cells. I have not satisfactory evidence of their origin directly from the ectoderm, for, although I have found them at times very near to the ectoderm (Fig. 20), I have never found them at any stage continuous with it. On the other hand, I have not seen conditions which would warrant the conclusion that the ganglia were the result of outgrowths from either of the pre-existing ganglia. A little before the ninth day the cells detached from the ectoderm to form the visceral ganglia (Plate II. Figs. 17, 18) increase rapidly in size, and the diameter of their nuclei often becomes four or five times as great as that of the ectodermic nuclei. The ganglia consist of elongated groups of such cells, still attached to the ectoderm above the pleural groove (Figs. IG, 18). The want of symmetry in the positions of the right and left ganglia is more conspicuous than in the ])receding stage, the ganglion of the riglit side being considerably more dorsal and far- ther back than that of the left side (Plate II. Fig. 23, Plate V. Fig. 60). Owing to the infolding of the ectoderm on the right side of the body to 182 BULLETIN OF THE form the respiratory chamber (I'kite II. Figs. IG, 24), the region from which the ganglionic cells arise is now located on the ventral and median walls of the infolding. The ganglia have also grown forward, and lie between the nepliridial organs and the nutritive gac (Plate II. Fig. 23). In an individual cut crosswise, the posterior portion of the ganglia is found to be two or three sections back of the otocysts. Both the ganglia may be traced through live or six sections. A little behind the visceral ganj^lia, and to the left of the median plane of the body, are the {jrominent cells of the abdominal ganglion (Plate II. Figs. 24, ah.). All of these ganglia still consist of groups of loosely associated cells. Later they become more compact, and arc surrounded by connective- tissue cells. The buccal ganglia (Plate II. Fig. 22), first seen with certainty at this stage, arise, one on each side of the radula sac, at the angle between it and the oesophagus. It is to be seen from cross sections that the cell ])i'oliforati()ns from which they spring take place from the dorsal wall of the neck of the sac, where its lumen begins to be separated from that of the oesophagus. This is also their permanent position ; they are later joined together by a commissure, which i-esults from outgrowths of the cells composing the two ganglia. On the teii/h day the external appearance of the embryo remains nearly the same as before, with the exceptitm that there is an increase in the size of the embryo, and especially of its j)ulsating sacs. The sac of the radula has become more elongated, and the anal opening (Plate III. Fig. 31, an^ is formed. The cerebral invaginations still appear, in sections parallel to the sagittal plane (Plate III. Figs. 28 and 29), as shallow depressions. The number of cells in each ganglionic group (Plate IV. Fig. 58, Plate V. Fig. G3) has increased perceptibly. At the same time the groups have extended backward, and show indications of the cerebro-plenral connect- ives. In specimens cut in the sagittal plane, the cerebral commissure cut crosswise may be seen above the oral opening (Plate III. Fig. 30). The pedal ganglia (Plate IV. Figs. 54, 58, Plate V. Fig. 63) have in- creased in size. Their anterior borders now reach as far forward as the plane of the pleural groove, and they extend backward into the foot much fartlier than before. In cross sections (Plate IV. Fig. 54) they appear as roiuidcd groujis of cells, which are fiir apart and not yet very compact ; tliev still continue to receive accessions by the proliferation of MUSEUM OF COMPARATIVE ZOOLOGY. 183 ectoclermic cells from the walls of the foot (Plate IV. Fig. 57, 58, 7^r/.). The first decided evidence of a pedal commissure makes its appearance during this stage. It consists (Fig. 54) of a few very much elongated nerve cells, which stretch across from one ganglion to the other a little posterior to the region of the otocysts. The commissure may be traced on about half a dozen successive sections, or for a distance of some 50 or 60 fx. From its position it evidently is the beginning of the anterior commissure. The thickness (10 yy.) of a single section contains only three or four cell?, the nuclei of which have the chromatic substance so concentrated into a single niicleolus as to make the nuclei appear clearer than those of the surrounding connective-tissue cells. There is at present no trace of a posterior commissure. The otocysts are now nearer the ganglia (Plate IV. Fig. 58, Plate V. Fig. 63) than at any previous stage. The pleural ganglia (Plate V. Fig. 63) are still inconspicuous, being composed of only a few scattered cells, which lie nearly dorsal to the otocysts, about midway between the visceral and the cerebral ganglia of the same side of tlie body. Many of the cells are elongated in the direction of the ganglia between which they are located, and appear to form the beginning of a connective between them. The visceral ganglia (Plate IV. Figs. 58 and 59, vsc) are still con- nected with the ectoderm, but project more prominently from the wall of the body, and extend forward more than before. The right (Plate IV. Fig. 59) is larger, and still lies more dorsal, than the left (Plate V. Fig. 63). The cells which compose the ganglia are numerous and large, and the nuclei of those which form the centre of the ganglion are conspicuously larger than those at the periphery. In cross sections of a stage possibly a little less developed than the one last described, the ganglia (Plate IV. Figs. 53, 56, 57, 55) lie, one on each side of the body, immediately above the pleural groove, a little below and inside the external orifices of the primitive nephridial organs. On the right side of the body the ectoderm which constitutes the anterior wall of the infolding to form the mantle chamber is seen in sagittal sections (Plate IV. Fig. 58) to be much thicker in the region adjoining the pleu- ral groove than in that which forms the deeper portion of the infold- ing. The transition from the thick to the thin ectoderm is very abrupt, and is marked by a pockct-likc depression. The right visceral ganglion is situated at the side and in front of this depression. Some of the cells in the anterior portion of this ganglion (Plate IV. Fig. 5G) are trncealile toward the median iiLuie of the body. Tlie left visceral ganglion 184 BULLETIN OF THE (Figs. ^6, 57) is not yet as large as the right, and it consists of fewer cells. The position of the connective between the visceral and pleural gan- glia (Plate V. Fig. G3) is indicated by the presence of spindle-shaped cells with fibrous projections. The connective is at this time long, and the cells and fibres composing it are only joined to one another loosely. As the abdominal ganglion increases in size, it extends more toward the right side of the body (Plate V. Fig. 61), and the connective be- PlGURE R. — The loft face of a section parallel to the sagittal plane finm an embryo of the eleventh day. X 73. ab. -vsc. s. Left abdominal-visceral connective. j)d. Tedal ganglion. ceb, s. Left rerebral ganglion. jie.^. Font. cnch. Shell gland. sac. rid. Railula sac. 0 cy. Otocyst. la. Ocular tentacle. tween it and the right visceral ganglion, which is hardly perceptible at this stage, is much shorter than that to the left visceral ganglion. The buccal ganglia remain in the same condition as in the preceding stage (Plate 11. Fig. 22). By the eleventh day the embryo has increased greatly in size (Figure B) ; the tentacles are prominent, and the pulsating sac of the foot is very large. A narrow slit-like infolding of the ectoderm (compare Plate VIII. Fig. 101, gl. pd.) has arisen in the median plane of the body at the an- terior end of the foot, into which it extends backward a short distance. It is the beginning of the foot gland. The salivary glands also make MUSEUM OF COMPAKATIVE ZOOLOGY. 185 their appearance during this stage as a pair of evaginations of the lateral walls of the CESophagus, immediately above its communication with the radula sac, and a little in front of the buccal ganglia (Plate VI. Figs. 77-80). The cerebral invaginations still open broadly at the sides of the head (Plate III. Figs. 32-34, and Figure C). They are, however, quite deep, and in a series of sagittal sections the depression becomes deeper and deeper as one approaches the median plane, and at the same time the orifice which leads to the depression becomes narrower and narrower, PiouRE C. — The posterior face of a transverse section from an embryo of the eleventh day. x 73. ab. Abdominal ganglion. cav. mt. Mantle cavity cnck. Shell gland. iv. ceb dx. Right cerebral invagination. mt. Mantle. rod. Radula sac. sul. plu. Pleural groove. until it is almost slit-like (Figs. 32-40). The deep ends of the invagi- nation are turned a little towards the median plane. These invagi- nated portions of the brain are composed of small, closely packed ceJls, whose nuclei stain deeply. The proliferated portions of the cerebral ganglia, which are deeper than the sacs (Plate V. Fig. 64, Plate VI. Figs. 70, 71), extend toward ^ach other in the median plane, and back- ward and downward toward the pedal ganglia (Fig. 71). They have now become differentiated into a fibrous central part (Fig. 71), in which 186 BULLETIN OF THE are lodged the larger scattered cells with their very large nuclei, and a peripheral part, where the cells are crowded together and the nuclei are smaller (Fig. 70). They are loosely enveloped by spindle-shaped, very much elongated, connective-tissue cells (Fig. 71). Immediately above the oral cavity is the cerebral commissure (Plate VI. Fig. 80'). It can be traced from one side of the brain to the other, and its cross section appears as a very small round patch of fibrous substance, sur- rounded on the dorsal side by a layer of Hat cells. The cerebro-pedal connectives are indicated (Fig. 71) by a few cells extending from the ventral-posterior ends of the cerebral ganglia to the anterior ends of the pedal, a little in front of the cerebro-pleural con- nectives (Fig. 70). The latter extend from the posterior ends of the cerebral to the anterior ends of the pleural ganglia, thus diverging somewhat from the cerebro-pedal connectives. There are found in the ganglia many cells which are in different stages of division. It is owing to this cell division that the ganglia increase rapidly in size, especially after they are wholly cut off from the ectoderm ; cells in the commis- sures and connectives are also found in process of dividing in planes perpendicular to the direction of their fibres. The principal change in the pedal ganglia (Plate YI. Fig. 71) is due to an increase in size, particularly in the antero-posterior direction. The central portion of these ganglia has the same fibrous appearance as that described for the cerebral ganglia, and the pedal nerves can be traced for a considerable distance toward the tip of the foot (compare Figure E, page 191). The anterior commissure (Plate III. Fig. 44, Plate VI. Fig. 74) is now somewhat shorter than in thp previous stage, and con- sists of a greater number of cells. Cell proliferation is still taking place from the ectoderm of the ventral wall of the foot (Plate VI. Fig. 71), and the ganglia continue to receive accessions from these sources. More highly magnified views of the regions of proliferation are given in Plate VI. Figs. 72 and 73. The pleural ganglia (Plate VI. Fig. 70) are now easily recognized. Each ganglion is formed of a triaugular group of cells, occupying a posi- tion immediately above and anterior to that part of the pleural groove which is nearest to the otocyst. The cells composing the ganglion are fewer than those of any of the other pairs of ganglia, but resemble theni in their histological conditions ; they are only loosely connected, and their fibres are elongated in the directions of the three connectives. At this stage the ganglia are not closely enveloped in connective tissue. The pleuro-visceral connectives are well developed, especially the left MUSEUM OF COMPARATIVE ZOOLOGY. 187 one (Fig. 70) ; the right one is much longer and more attenuated, since the right visceral ganglion is farther from the pleural than the left vis- ceral. The ganglia are most distinctly seen in specimens cut in a sagittal direction. The visceral ganglia (Plate V. Figs. 67-69, Plate VI. Fig. 70) are much larger and more elongated in the direction of the pleural ganglia — i. e. downward, forward, and outward — than they were during the pre- vious stage. They are still connected with the ectoderm at their pos- terior dorsal ends, while the opposite ends are much drawn out toward the pleural ganglia (Figs. 69, 70). The right visceral ganglion (Figs. 67-69) is larger than the left, and its longest axis has a dorso-ventral direction (Fig. 68). The fibrous pi'olongations continue into the pleuro- visceral connectives (Fig. 71). The abdominal ganglion (Plate III. Figs. 43, 44, 46, 47, Plate VI. Figs. 75, 76), although still connected with the ectoderm, is also larger, and projects more into the body cavity than on the tenth day. A large portion of it still lies to the left of the median plane of the body (Plate VI. Figs. 75, 76), and the connective to the left visceral is well devel- oped (Plate III. Figs. 41, 42, Plate V. Fig. 68); that to the right is less complete (Plate III. Figs. 45, 51). The buccal ganglia (Plate V. Fig. 62, Plate VI. Fig. 77) are now very distinct ; the dorsal wall of the radula sac still contributes to their increase in size. Cell proliferation takes place from the ectoderm bordering the en trance to the respiratory cavity. A few cells, which probably form the olfactory ganglion, are seen at this stage to be separating from the ecto- derm in this region. For the next twenty-four to thirty-six hours (^twelfth mid thirteenth days) the external appearance of the embryo remains nearly the same as on the eleventh day. In the living embryo the larval heart may be seen pulsating, and the foot gland extends somewhat farther towards the posterior extremity of the foot. The cerebral invaginations appear simply as long narrow sacs filled with a coagulated substance ; the inner ends of these sacs have, grown upward as well as backward (Plate VII. Fig. 94). The proliferated portions of the cerebral ganglia (Fig. 94) are mucli larger, and have now assumed more nearly their ultimate positions (Plate III. Figs. 48, 49; Plate VII. Figs. 81, 82, 94). The central portion of each has become more fibrous (Fig. 81). 188 BULLETIN OF THE The connectives, both to the pedal (Plate VII. Fig. 81) and to the pleural ganglia (Plate III. Fig. 48), are well developed, and are both thicker and shorter than in the stage last described. The pedal ganglia do not differ materially from the condition de- scribed for the eleventh day. The anterior end has increased in diam- eter, and has grown a little farther forward (Plate III. Fig. 50, Plate VII. Figs. 81, 91). Both commissures are now present ; the anterior (Fig. 92) is a little behind the otocysts (compare Fig. 92 with Fig. 91), and the posterior (Fig. 90) is directly above the blind end of the foot gland, and about 0.2 mm. back of the anterior commissure. The pleural ganglia (Plate III. Fig. 48, Plate VII. Figs. 82, 83, 88) are very near the cerebral ganglia, as may readily be seen in sagittal sections (Figs. 48, 82), and the fibrous connectives to the other ganglia are plainly to be distinguished. The ganglia have become more com- jjact and rounded, and occupy a position nearer the middle plane of the body (Figs. 86, 88). The visceral ganglia (Plate III. Fig. 49 ; Plate VII. Figs. 83, 84, 86-89), although they have increased greatly m size, are still connected with the ectoderm which forms the anterior wall of the mantle chamber (Figs. 88, 89). They have also moved inward and forward. The right ganglion (Figs. 49, 83, 87-89) is especially well developed, and much farther forward than in the previous stage. Its axis is prolonged into a nerve, which runs upward and backward, probably to the olfactory ganglion (Figs. 84, 87). The connective from the right visceral to the abdominal ganglion passes backward and inward (Plate VII. Figs. 83, 84). Where the connective leaves the visceral ganglion (Fig. 83), the nuclei of the ganglionic cells are very large, and the fibres are very much elongated in the direction of the connective. In specimens cut crosswise the nerve which forms the dorsal prolon- gation of the axis of the visceral ganglion is found far forward, in front of the anterior face of the abdominal ganglion ; it passes upward and inward (Plate VII. Figs. 87, 88), and is connected with the ectoderm that forms the wall of the small infolding from the respiratory cavity (Fig. 88) referred to in the account of the tenth day. This region is at the same level as that with which the abdominal ganglion is connected farther back (Plate VII. Fig. 93). The ectodermic cells to which this nerve is distributed form the lining to an irregular infolding from the MUSEUM OF COMPARATIVE ZOOLOGY. 189 median face of the respiratory cavity, and the lumen of the infolding connects by a narrow orifice with the respiratory chamber (Fig. 88, cav. mt.). I believe this is the organ first described by Lacaze- Duthiers. A little farther forward the right visceral ganglion sends to the right side of the body a nerve (Plate VII. Fig. 89 n.), which passes between the wall of the mantle chamber and the primitive sexual duct, probably to be distributed to the right half of the mantle. At this time the greater portion of the abdominal ganglion (Plate VII. Figs. 81, 82, 85, 86, 93) lies on the right side of the median plane, al- though it is joined to the left visceral by a large and prominent connect- ive (Plate III. Figs. 50, 52, Plate V. Figs. 65, 66, Plate VII. Fig. 93). Since the visceral ganglia have grown inward and forward, the ab- dominal ganglion now occupies a position considerably posterior to them (Plate VII. Figs. 83, 86) ; it lies above the right side of the radula sac. Its posterior dorsal margin is still continuous with the ectoderm of the wall of the respiratory cavity (Fig. 93), but farther forward it is entirely separated from the ectoderm (Fig. 85), and is surrounded by a layer of connective-tissue cells. All the other ganglia are similarly en- veloped in connective tissue except where they are continuous with the ectoderm. The connective to the left visceral ganglion (Plate A"II. Fig. 93) passes downward, forward, and outward to the left side above the radula sac. The buccal ganglia (Plate YII. Fig. 81) are larger than on the tenth day, but are closely applied, as before, to the walls of the radula sac. Their commissure (Plate V. Fig. 65) is embraced in the angle between the (Esophagus and the neck of the radula sac, and in sagittal sections presents a circular outline. On the fourteenth day the upper lips as well as both pairs of tentacles are very prominent, and the foot gland has grown backward still farther into the foot (Plate VIII. Fig. 102). The salivary glands have now be- come elongated into tubular organs with a circular lumen and thick walls consisting of a single layer of epithelial cells (Plate VIII. Fig. 106). They reach a little farther back than the buccal commissure ; in passing forward they lie on either side of the oesophagus, about on a level with its lower border. They pass along the dorsal side of the buccal ganglia, and then suddenly bend downward to open into the oesophagus. The cerebral invaginations (Plate VIII. Fig. 96) present the same gen- eral appearance as in the stage last described, but the lumen of the sacs I'JO BULLETIN OF THE is smaller (Plate X. Figs. 121, 12G) ; in cross sections (Fig. D) it ap- pears oval. The walls are thick, being composed of spindle-shaped cells arranged perpendicularly to the axis of the sac and so crowded that the nuclei are three or four deep. The proliferated portion of the cerebral ganglia (Plate IX. Fig. 114) re- tains its pear-shaped condition, but is shorter and thicker. A ventral and Figure D. — Posterior face of a transverse section from an embryo of the fourteenth day. X 190. cnch. Shell gland. cr. Heart. gn. ceb. Cerebral ganglion. hp. Liver. hp. dx. Right lobe of liver. in. Intestine. iv. ceb Cerebral invagination. mt. Mantle. Nephridial organ. aSsophagus. Pericardium. Pleural ganglion. sac. rad Radula sac. ia. dx. Right ocular tentacle. ta'. Labial tentacle. vsc. Visceral ganglion. nph. a'. pi. cr pin. median poi-tion of each ganglion forms a small rounded lobe (Figure E). These lobes are near the bases of the upf)er lips, and in sagittal sections appear almost completely separated from the larger part of the ganglia by ingrowths of connective tissue. It is from these lobes that the pedal connectives arise. The connectives to the pleural ganglia emerge from MUSEUM OF COMPARATIVE ZOOLOGY. 191 the larger portion of the ganglion ; they are thicker and shorter thau the cerebro-pedal connectives, from which they are separated by only a narrow space. The cerebral comniissnre is much shorter than before (Plate X. Fig. 126), but it has not increased much in thickness (Plate VIII. Fig. 101). lu sagittal sections it is seen to be composed of a central portion made up of nerve fibres cut crosswise and a peripheral layer of nuclei ; but the nuclei are wanting on tlie face of the conmiissure which is in contact with the dorsal wall of the 03sophagus. Figure E. — The left surface of a section parallel to the sagittal plane from an embryo of the fourteenth day. X 73 art. pd Pedal artery. plu -pd. Pleuro-pedal connective. ceb. dx. Right cerebral gan ;lion. phi -r.sc Pk'uro-visceral connective ceb.-pd. Cerebro-pedal conuectlve. vd. d.r. Right pedal ganglion. cnch. Shell gland. rt. Rectum. lab. Lip. ta. Ocular tentacle. n. Nerve ta'. Labial tentacle 0 cy. Otocyst. vsc. Visceral ganglion. plu. Pleural ganglion. The pedal ganglia (Pkite VIII. Figs. 97-100, Plate IX. Figs. 114, 118, 119) lie between the radula sac, which is above, and tlie foot gland whicli is below them. They are nearer together than on the twelfth day, and their anterior ends arc more rounded (Fig. 111). Their pos- 192 BULLETIN OF THE terior ends are elongated and continued as two large nerves far back into the foot. In specimens cut crosswise these nerves appear as rounded patches of fibres, situated one on each side of the body, above the plane of the foot gland and about midway between it and the lateral walls of the foot. Each is surrounded by a layer of connective-tissue cells. As one approaches the pedal ganglia in passing from behind forward, the nerves increase in size and lie nearer to each other. In the region of the posterior commissure (Plate IX. Fig. 119) the ganglia are nearly as broad as in the region of the anterior commissure (Fig. 118), but they are not much more than half as thick in the dorso-ventral direction. In front of the posterior commissure they are separated by a narrow space, which is wider behind than in front, where it is terminated by the ante- rior commissure. The commissures are both well developed (Plate VIII. Figs. 101, 102, Plate IX. Figs. 118, 119), and owing to the approxima- tion of the ganglia have become shorter than in the last stage. The nuclei in the region of the posterior commissure (Fig. 119) are of nearly uniform size; but in front of it each ganglion (Figs. 114, 118) contains a fibrous central portion immediately surrounded by the greatly enlarged nuclei of cells which form the most of the fibrous substance. The pleural ganglia (Plate VIII. Fig. 106, Plate IX. Figs. 114, 116, Plate X. Figs. 123, 125, and Fig. E) have increased considerably in size, and are more compact. They have moved downward and inward ; and each now lies in contact with the posterior face of the corresponding cerebral mass (Plate IX. Fig. 114), and below and in front of the ven- tral portion of the corresponding visceral ganglion (Figs. lOG, 12.3, 125). They are much smaller than either the cerebral or visceral ganglia. The nuclei of their central cells are, as in the pedal ganglia, much enlarged. The visceral ganglia (Plate VII. Fig. 95, Plate IX. Fig. 114, Plate X. Figs. 123, 125) are now entirely detached from the ectoderm, and have moved downward, forward, and inward. The left ganglion (Plate VIII. Fig. 106, Plate X. Fig. 125) is smaller than the right, and more closely connected with the left pleural (Fig. 125) than in the previous stage. Its dorsal surface is slightly above the level of the dorsal wall of the radula sac, and its connective with the abdominal ganglion (Plate VIII. Fig. 104) is much broader than before. The right visceral ganglion (Plate VII. Fig. 95, Plate VIII. Figs. 102 and 106, Plate IX. Fig. 114, Plate X. Fig. 123) is much larger than in the last stage ; it is also closely connected with the right pleural ganglion (Fig. 123). It extends dorsally much farther than the MUSEUM OF COMPARATIVE ZOOLOGY. 193 left visceral, and also uearer to the mediau plane (Plate YII. Fig. 95, Plate VIII. Fig. 102, Plate IX. Fig. 120). It is iu contact witli the lower surface of the right end of the abdonaiual ganglion (Plate VII. Fig. 95). The abdominal ganglion (Plate VII. Fig. 95, Plate YII I. Figs. 101, 102, 104, Plate IX. Figs. 115-117, Plate X. Fig. 123) is entirely un- connected with the ectoderm, and has moved forward, so that there is a considerable space between it and the pleural groove, but its posterior face extends farther back than that of the right visceral ganglion (Plate VIII. Fig. 102). The greater portion of the ganglion is now situated on the right side of the body, immediately above and to the right of the radula sac (Plate VIII. Fig. 104, Plate IX. Figs. 115-117). It is elongated, and its chief axis is directed obliquely across the body, the right end being considerably higher atid a little farther back than the left end. In passing downward and forward to the left side of the body, it lies between the oesophagus aud the posterior part of the radula sac. Its left end is prolonged into a connective, which passes forward and outward to join the left visceral ganglion (Plate VIII. Fig. 104, Plate X. Fig. 123). A large nerve, which passes upward and backward to be dis- tributed to the viscera, emerges from the most dorsal portion of the abdominal ganglion on the right side of the body (Plate VIII. Fig. 104, Plate IX. Fig. 117). The histological condition of the abdominal gan- glion is similar to that of the previously described ganglia of this stage. The fibrous portion, as well as the enlarged cells and nuclei, are espe- cially prominent in the portion of the ganglion which lies to the right of the median plane of the body (Plate IX. Fig 117). The buccal ganglia (Plate VII. Fig. 95, Plate VIII. Figs. 102, 106, Plate IX. Fig. 120, Plate X. Fig. 121) have become larger, and with their commissure (Plate VIII. Fig. 101, Plate IX. Fig. 120) stretch across the dorsal wall of the neck of the radula sac, to which they are still closely united. The nuclei immediately surrounding the central fibrous portion of the ganglion are already slightly enlarged, though the cells are not so far advanced in their histological differentiation as are those of the other ganglia. A single pair of connectives passes obliquely forward, downward, and outward, to join the buccal to the cerebral ganglia (Plate X. Fig. 12\. By the sixteenth and s.eventeenth days, besides a general increase iu size of the external orgaiis, the foot gland extends backward much farther VOL. XX. — NO. 7. 13 194 BULLETIN OF THE than the pedal ganglia (Plate VIII. Fig. 107), and the viscera lie rather more to the left side of the body (Figure G). The central nervous system (Figure F) now consists of five well devel- oped pairs of ganglia and an azygos ganglion (Figure G). The cerebral ganglia with their commissure form the dorsal portion of three nerve rings, the remainder of which are completed respectively, (1) by the cerebro-pedal connectives, tlie pedal ganglia, and their commissures ; (2) by the cerebro-pleural connectives, the pleural ganglia, the pleuro- FiouRE F. — Posterior face of a transverse section from an embrj-o of the sixteenth day. X 70. art. ce. Cephalic artery. ce. CEsophagus. art. pd. Pedal artery. phi. Pleural ganglion. com. ceh. Cerebral commissure. plu.-pd. Pleuro-pedal connective. dt. sx. pr. Primary sexual duct. pd Peilal ganglion. gl. sal. Salivary gland. sac. rod. Radula sac. gl. pd. Pedal gland. ta. dx. Right ocidar tentacle. n. pd. Pedal nerve. ta. s. Left ocular tentacle. n. ta. Tentacular nerve. visceral connectives, the visceral ganglia, the viscero-abdominal connect- ives, and the abdominal ganglion ; (3) by the cerebro-buccal connectives, the buccal ganglia, and their commissure. The first and second rings are further joined to each other by means of the pleuro-pedal connect- ives. Each of these three rings encircles the cesophagus. The posterior end of the radula sac in the earlier stages, up to the present one, is usually found to occupy a position above the pedal ganglia and their MUSEUM OF COMPARATIVE ZOOLOGY. 195 commissures ; but with a greater concentration of the nervous ganglia toward one another, the sac is forced to occupy a position helow the pedal ganglia and their commissures. The relations of the dififerent ganglia to each otlier is even more definite than before, and can be more readily understood from transverse sections than from sagittal ones. The peripheral nerves from the cerebral, pedal, visceral, and abdominal ganglia are well developed ; the principal changes from this time until hatching are histological. The cerebral invaginations have become narrow and shorter, but are still open to the exterior (Plate X. Fig. 124, iv.). The deeper portion of the invagination, that in contact with the proliferated portion of the cerebral ganglion, has become a solid and rounded mass (Plate X. Fig. 122, loh. lat.), which is intimately connected with the ganglion by means of fibrous outgrowths from its ganglionic cells. It is com- posed of small deeply stained cells, which have undergone no such histological change as those which compose the proliferated portion of the brain. It forms a lobe on the antero-lateral face of each cerebral ganglion (Plate X. Figs. 122, 124, 127). From this time forward the principal change in the cerebral sacs consists in the gradual obliteration of the lumen of the invagination. This is usually completed somewhat later in the embryonic life ; but, as previously stated, the sacs have in one instance at least been found open several days after hatching. Be- sides this, there is no other connection now remaining between the ectoderm and an}' of the ganglia, except such as is effected by means of the peripheral nerves. The median proliferated portions of the cerebral ganglia now extend dorsally farther than in the last stage, and their commissure is much shorter (Plate A^III. Fig. 105, and Fig. F). The pedal ganglia (Plate VHP Figs. I03^ 109-113) have moved forward, and are broadly in contact with the pleural ganglia. They have become more compact, and rather more triangular in shape, than before. From the ventral portion of each ganglion emerge four" or five large nerves, which terminate in the ventral wall of the foot ; from the dorso-lateral region two nerves are given off to the lateral walls, and the antero-ventral part of each ganglion tapers off into a stout nerve running forward to the anterior wall. The connectives with the cerebral ganglia are well developed (Plate VTII. Figs. 103, 107). The pleural ganglia (Plate VIII. Figs. 103% 111-113) are nearer to the median plane than previously. The^ ventral posterior face of each is closely joined to the corresponding pedal ganglion (Figs. 103% 112), 196 BULLETIN OF THE the dorsal median face to tho visceral ganglion (Figs. 103% 112, 113), aiid the anterior face to the cerebral ganglia (Fig. 107). JN'o nerves arise from the pleural ganglia. The visceral ganglia (Plate VIII. Figs. 103% 110-113) have also moved nearer to the median plane. The left ganglion is directly below Figure G. — Posterior face of a transverse section from an embryo of the seventeenth day. X 60. ab. Abdominal ganglion. art. ce. Cephalic artery. art. pd. Pedal artery. dt. SI. pr. Primary sexual duct. gl. sal. gl pd. hp. mt. Salivary gland. Pedal gland. Liver. Mantle. Nerve. n. pd. Pedal nerve. npk. Nephridial organ. nph. dx Right nephridial organ. o cy. Otocyst. pd. s. Left pedal ganglion. pi. cr. Pericardium. ret. ta. Retractor muscle of right ocular tentacle. ret. ta s. Retractor muscle of left ocular tentacle. vsc. s. Left visceral ganglion. the oesophagus (Fig. Ill and Figure G), since the latter occupies a posi- tion more to the left side of the body than before. The right visceral ganglion still remains larger, and extends farther dorsally than the left (Figs. 103% 111, 112). It is nearer the median plane than in the MUSEUM OF COMPARATIVE ZOOLOGY. 197 stage last described (Figure D., page 190) ; it lies in front and only a little to the right of the abdominal ganglion (Fig. 111). The abdominal ganglion (Plate VIII. Figs. 109-111) is less elongated than in the last stage (Fig. 104). It is wedge-shaped, and appears as though crowded in between the two visceral ganglia from behind and above. It is so intimately connected with these ganglia that it almost appears to form a part of them (Fig. 111). But the presence, between the ganglionic masses, of connective-tissue cells, which reach nearly to the connectives, enables one to make out with some certainty the extent of each of the three ganglia. Since the planes which separate them are oblique to the transverse plaues of the body, these boundaries are not always readily seen in cross sections. The right and left visceral gan- glia have no direct commissural nerve fibres uniting them ; they are joined only by such fibres as pass through the abdominal ganglion. The buccal ganglia (Plate VIII. Fig. 108, Plate X. Fig. 124) are now entirely separated from the dorsal wall of the radula sac, from which they arose, and are surrounded by a layer of connective-tissue cells. The differentiation of their ganglionic cells is well advanced. Summary. 1. In Limax maximus the whole of the central nervous system arises directly from the ectoderm. 2. The cerebral ganglia originate in part as a pair of true invagina- tions, one on each side of the body in front of the pleural groove and behind and below the bases of the ocular tentacles. In the course of their development, the neck of each invagination becomes a long, nar- row tube-like structure, which remains open throughout the period of embryonic life. The main part of the cerebral ganglia is formed from cells which are detached at an early period from the deep ends of these cerebral invaginations, or from neighboring ectoderm ; the portions which persist as the walls of the infoldings finally form distinct lateral lobes of the brain. 3. All the other ganglia originate by cell proliferation from the ecto- derm without invagination. 4. The ganglia arise separately, and, with the exception of the ab- dominal and mantle ganglia, in pairs, one on each side of the body. Their connection with each other is the result of a secondary process in the development, — the outgrowth of nerve fibres. In advanced stages, tlie central nervous system consists of five pairs 198 BULLETIN OF THE of ganglia and an azygos ganglion. Together these form three complete rings surrounding the oesophagus. The relative positions of the ganglia are best appreciated from cross sections. In passing from behind forward, they are encountered in the following order: (1) the pair of pedal ganglia, which lie under the radula sac, and are joined to each other by an anterior and a posterior comniissure ; (2) one abdominal ganglion a little to the right of the median plane ; (3) a pair of visceral ganglia occupying the posterior angle formed by the outgrowth of the radula sac from the oesophagus. They are separated by the abdominal ganglion, from which connectives pass to them ; (4) a pair of pleural ganglia, not joined by a com- missure, and not giving off nerves. They are united by means of con- nectives to the pedal, visceral, and cerebral ganglia of the same side ; (5) a pair of cerebral ganglia, with their supra-oesophageal commissure and connectives to the pleural, pedal, and buccal ganglia ; (6) a pair of buccal ganglia, with a commissure under the oesophagus posterior to its connection with the sac of the radula. The mantle ganglion lies far back, and is joined to the abdominal ganglion by a large nerve. It seems as if there could be no doubt that the infolding of the ecto- derm of the anterior wall of the respiratory cavity on the right side of the body gives rise to the special-sense organ discovered by Lacaze- Duthiers ('72, pp. 483-494). It corresponds in its position and its connection with the right visceral ganglion to his description of the adult, and also to Fol's description ('80, pp. lGG-168) of the origin and position of that organ in the aquatic pulmonates. As is well known, Limax belongs to that group of Gastropods in which all the nerve centres, except the cerebral and buccal, lie on the ventral side of the intestinal tube ; not to the group in which the connection between the right pleural and right visceral ganglia passes above the oesophagus, and in which that of the left lies below it. Limax, there- fore, is not directly referable to Von Jhering's group of Chiastoneura, although the want of symmetry in the position of its ganglia does not allow one to say that it is orthoneuric. The Gastropod in which the details of the origin and fate of the nervous centres have been most carefully studied is Bithynia, a chias- toneuric form, in which Sarasin has found that the abdominal ganglion is joined to the right visceral ganglion only, and is located at the fundus of the gill cavity. The relation is different from that found in Limax MUSEUM OF COMPARATIVE ZOOLOGY. l'J9 maximus, where the abdominal ganglion is intimately fused with the riuht visceral, and is also in close connection with the left visceral ganglion. As was to have been anticipated, the abdominal ganglion of Limax corresponds more nearly in position to tliat in Lymneus and other fresh-water pulmonates, as described for the adult by Lacaze-Duthiers ('72, pp. 437-500). Of the authors wlio have studied the origin and development of the cerebral ganglia in Mollusks, Fol ('80, pp. 168, 169, 193-195) is the only one wlio has pursued his investigations on Limax maximus. He says ('SO, p. 1 93) : " Vers I'epoque de la fermeture de la vesicule oculaire, se montrent deux autres enfoncements de Tectoderme. L'un des deux, assez vaste et situe a la base du tentacule, a son bord interieur, est I'origine du ganglion cerebroide ; je le decriai plus loin. L'autre enfoncement, plus petit, est situe au-dessous de ce dei'nier, a la base du pied, et mene a la constitution de la vesicle auditive." As to the method by which the cerebral ganglia originate, this agrees in part with that which I have found ; but as to the time of origin, my investigations lead me to a difterent conclusion. The otocysts are pres- ent as small groups of cells (Plate I. Fig. 4), and the cellular elements which go to form the beginning of the pedal ganglia are also being pro- liferated (Fig. 3), before there is a trace of the invaginations which go to form the cerebral ganglia (Fig. 2). A little later the otocysts assume the form of closed vesicles, uncon- nected with the ectoderm (Plate I. Fig. 9), while the cerebral invagi- nations are now seen as shallow pits (Fig. 6). Therefore, in Limax maximus the formation of both pedal ganglia and otocysts precedes that of the cerebral invaginations. Sarasin ('82, pp. 1-68) maintains that in Bythinia tentaculata there are no invaginations to form the cerebral ganglia. They arise as thick- enings of the ectoderm, one on each side of the body, which he calls die SiiinesjAatte. In the recent researches of the Sarasin brothers ('87, pp. 600-602, '88, pp. 59-69) on Helix "Waltoni, of Ceylon, it is asserted that each of the cerebral ganglia is at first represented by a group of cells derived from the part of the ectoderm called " Sinnesplatte " before there is any invagination. There are two groups of these cells, one on each side of the body. Somewhat later two infoldings arise from each Sinnes- I)latte, one above the other. These infoldings become long, narrow 200 BULLETIN OF THE ' "cerebral tubes," the deep ends of which are enlarged ('88, Fig. 24). From their inner ends a rapid cell proliferation takes place, the prod- ucts of which join the cerebral cells already in position. The invagi- nated portions later form the " accessory lobes " of the brain. At a late stage only one pair of tubes remains open to the exterior, and the openings to these are closed before the end of embryonic life. The Sarasins ('88, p. 61) do not know the precise time at which they are closed, but are certain that the openings do not persist. They express their belief that the cerebral tubes are homologous with the organs of smell in Annelids, which, according to Kleinenberg's studies on Lopado- rhynchus, also originate as invaginations of the Sinnesj)latte, and by cell proliferation furnish a part of the material for tlie brain. Prior to any knowledge of the investigations on Helix by the Sarasins, I found very similar conditions in Liniax maximus. In this case, how- ever, there is but one invagination of the ectoderm on each side of tiie body. It corresponds in position to those described in Helix, being perhaps the equivalent of the upper or larger invagination in that species. The invaginations in Limax have the form of shallow pits before any other ganglionic cells are to be seen. The cell proliferation, which re- sults in the production of the main portion of the ganglia, takes place during their ingrowth. Possibly the proliferation from the depression between labial tentacle and upper lip represents what was originally a true invagination, and corresponds to the lower of the two invagina- tions described by tlie brothers Sarasin. In Limax maximus the ex- ternal openings persist until a late stage, and occasionally even after hatching. Here, also, the invaginations form a lobe of the brain, exactly as in the case of Helix (Sarasin, '87, p. 601). Two well developed " Seitenorgane " were found by the Sarasins ('88, p. 54) in Helix Waltoni, situated near each other in the " sense- plate " ; and they think (p. 60) that these may correspond in position to the cerebral tubes of later stages. The groups of cells embedded in the ectoderm, from which, in my opinion, the greater part of the nervous system in Limax maximus takes its origin, resemble both in the arrangement of the cells and their histological condition the " Seitenorgane " described by the brothers Sarasin ('88, pp. 53-57). But I have never observed bristles, or other terminal structures, projecting toward the outer world. Moreover, in Limax unmodified ectodermic cells usually lie between these groups of large cells and the outer surface of the body. MUSEUM OF COMPARATIVE ZOOLOGY. 201 The Sarasins ('88, p. 57) consider these clusters of cells homologous with the " taste-buds " and " lateral organs " of vertebrates, and say that they are to be found in and at the margin of the Sinnesplatten, and along the sole of the foot, — more rarely on the sides of the foot. I think these organs are probably the same as those which I have seen in Limax, and to which I attribute simply the function of contributing to the formation of the ganglia. Salensky ('86, pp. 685-G90) describes the cerebral ganglia of Verme- tus as arising from a pair of ectodermic thickenings, which early show pocket-like invaginations, and become deeper and narrower. From tlie inner ends of these invaginations are formed the main portion of the ganglia. The latter are united to each other by a very small commis- sure, composed of fibrous prolongations of the ganglionic cells surrounded by other nerve cells. The principal difference between the method of development in Ver- inetus and that in Limax maximus consists in the fact that the detach- ment of the deep portion of the invaginations to form the ganglia in Vermetus is not etfected until the invaginations have reached their ulti- mate size, whereas in Limax the detachment of cells from the invagi- nated area begins as early as does the invagination, and accompanies it during the whole of its formation. Kowalesky ('83% pp. 1-54) found in Dentalium two deep invagina- tions, which he calls the " sincipital tubes," one on each side of the head region, a little ventral to the middle of the velar area. From the posterior deep ends of these sacs the cerebral ganglia are subsequently formed ; but he is uncertain whether all the cells concerned in the in- volution share in the formation of the ganglia. If his Figure 65 is compared with Figures 27 and 33 A in Salensky's paper, the close resem- blance in the method of origin of the cerebral ganglia in the two types becomes apparent. Fol ('80, pp. 169, 170) asserts that the pedal ganglia of the aquatic pulmonates appear as condensations in an already formed mesoderm, and that they are nearer the pharynx than the ectoderm when they begin to be discernible. " One may therefore sa}"^," he adds, " that these ganglia arise from the mesoderm without prejudging the unsettled ques- tion, viz. from which of the primordial layers arises the mesoderm which forms them." Of the pedal ganglia of the terrestrial pulmonates, he says that they are diffentiated en lieu et place in the midst of the mesodermic tissues of the foot. 202 BULLETIN OF THE With this I cannot agree, although I admit that at the time when the groups of cells which form the ganglia begin to be proliferated from the ectoderm, it is extremely difficult to distinguish them from the mesodermic elements (Plate I. Fig. 5). It is to be observed, however, that Fol considers it an unimportant distinction, whether the ganglia are formed from groups of mesodermic cells which have themselves recently originated from the ectoderm, or by a proliferation of cells directly from the ectoderm. I am unable to reconcile the account of the development of the pedal ganglia in Bithynia given by P. Sarasin ('82, pp. 47-49), with the conditions seen in Limax ; nor can I think it probable that any consid- erable difference exists between nearly related mollusks in regard to the place whence the ganglionic cells arise. Sarasin maintains that in Bithynia the pedal ganglia arise from a single median thickening of the ectoderm of the dorsal wall of the foot, in the region where that wall bends over to become continuous with the posterior wall of the visceral sac. Anteriorly, in the region of the oral mvagination, this median band of cells forks, and each branch becomes joined to the correspond- ing cerebro-pleural cell mass by a slender cord of cells. Subsequently, the posterior unique portion of the proliferated cell mass is completely divided into lateral branches by a separation which progresses from in front backward. It seems to me that, according to this account, both the pedal ganglia must be regarded as arising from a common mass of cells, and that they are not from the beginning wholly separate, as I maintain for Limax. The relative positions of pedal ganglia and otocysts present, to my mind, a serious objection to Sarasin's view, which may not have seemed so important to him on account of his uncertainty about the origin of the otocysts. I believe it is sufficiently evident that the otocysts do not arise, as Sarasin thinks probable, from the cerebro-pleural prolifera- tions, but independently, and from the dorso-lateral wall of the foot in the region of the " pleural groove." They ultimately lie immediately dorsal to the corresponding pedal ganglia. If Sarasin's view as to the origin of the pedal ganglia as a median dorsal proliferation were correct, the ganglia would have to migrate to a lower plane than that occupied by the otocysts. But there is no evidence either in Limax or the figures given of Bithynia which would confirm such a supposition. As further corroboration of my opinion that the pedal ganglia arise from the ventral and lateral walls of the foot, I would cite the conclusion reached by Salensky ('86, pp. 691, 692) for Vermetus. He has shown that the MUSEUM OF COMPARATIVE ZOOLOGY. 203 pedal ganglia originate from the ventral wall of the foot, in a region and by a method corresponding to that seen in Liraax maximus, as will be seen by comparing his Figures 21 C to 23 with my Plate I. Figs. 5 and 7, and Plate IV. Fig. 57. The only important difiFerence between Vermetus and Limax lies in the fact that, in the case of the former, the cells forming the ganglion remain from the beginning a more compact mass than they do in the latter. jS'o one except Lacaze-Duthiers ('72, pp. 456, 457) has mentioned the existence of more than a single pedal commissure. He maintains that there are in Lymnseus as many as three. After speaking of the cerebral ganglia as being connected by one commissure, he goes on to say (p. 456), " Au contraire les ganglions pedieux ont trois commissures reelles." He seems, however, uncertain as to whether the vao?,t j^osterior ouoht to be considered a true commissure : "La troisi^me commissure merite-t-elle bien ce nom? elle est constante dans les Pulmones et se presente sous la forme d'uu petit nerf grele transversal naissant a pen pres a la hauteur du troisieme nerf pedieux inferieur ; elle donne vers son milieu naissance a un filet nerveux tres-delie, impair median que Ton suit dans les tissus de la fosse pedieuse sans trop pouvoir definir et limiter exactment son role." (p. 457.) His investigations were made exclusively upon the adult. In Limax maximus two commissures are certainly distinguishable during a greater part of the embryonic life ; no trace of a third has been seen. The adult has not been studied. None of these authors, with the exception of Sarasin, say anything conclusive concerning the origin of the remaining ganglia, although Salensky (86, p. G97) speaks as if the pleural ganglia of Vermetus originated in the cerebro-visceral connectives, which are shown in his Figures 31 B to 31 F. Sarasin asserts ('82, pp. 46, 47) that in Bithynia the pleural ganglia originate as pai't of the " Sinnesplatte," from which the cerebral ganglia arise, and that these ganglia, cerebral and pleural, are so closely fused with each other in the later stages of development as to form on either side of the body a single mass. I believe that they arise in Limax maximus by cell proliferations from the lateral walls of the body, behind the cerebral ganglia, and just above the pleural groove ; they are closely connected (not fused) with the cerebral ganglia only in late stages. Sarasin ('82, pp. 50-52) says that the visceral ganglia in Bithynia 204 BULLETIN OF THE arise by cell proliferation from the dorsal margin of the ventral wall of the head or trunk region, above that which I have called the pleural groove. Further, that the right visceral (or supra-intestinal) ganglion is connected by a nerve fibre to the olfactory ganglion under the gill cavity. Farther back than the visceral ganglia he finds a median pro- liferation of cells lying at the ventral margin of the gill cavity, from which the abdominal ganglia arise. He asserts that there are two ab- dominal ganglia, — one connected with the supra-intestinal ganglion, the other with the sub-intestinal ganglion. In Limax maxiraua the visceral ganglia and the abdominal ganglion arise by the same method as that described by Sarasin ; but the former are produced from the lateral walls of the head region, above the pleural groove, one on each side of the body. The right ganglion in later stages is more dorsal than the left. It appears to be formed in part from the inner wall of the respiratory cavity, to which it remains connected by a nerve. It is in this region that is developed an organ which I believe to be the olfactory organ of Lacaze-Dutiiiers. There is only one abdominal ganglion ; this takes its origin a little to the left of the median line of the body, from the anterior margin of the body wall immediately above the pleural groove. Sarasin ('82) is the only author who gives attention to the origin of the buccal ganglia. He describes them as arising in exactly the same manner, and in the same situation in relation to the walls of the radula sac and the oesophagus, that they do in the case of Limax maximus. Cambkidge, November, 1889. MUSEUM OF COMPARATIVE ZOOLOGY. 205 BIBLIOGRArHY. Bobretsky, N. '76. Studieu iiber die embryonale Eutwickluug der Gasteropoden. Arch. f. mikr. Auat., Ed. XIII. pp. 95-169, Taf. VIII.-XIII. 1876. Butschli, O. '77. Entwickluiigsgeschichtliche Beitrage. Zeitscbr. f. wiss. ZooL, Bd. XXIX. pp. 216-254, Taf. XV.-XVIII. 1877. Fol, H '80. Developpement des Gasteropods Pulmones. Arch, de Zool. exp. et geu., Tom. VIII. pp. 103-232, Pis. IX.-XYill. 1877. Jhering, H. von. '75. Ueber die Eatwicklungsgeschichte von Helix. Zugleich ein Beitrag zur vergleicbenden Anatomie und Phylogenie der Pulmonaten. Jena. Zeitscbr., Bd. IX. Heft 3, pp. 299-388, Taf. XVIII. 1875. '77. Vergleicbende Anatomie des Nervensystems uiid Phylogenie der Mol- luskeu. Folio, x + 290 pp., 8 Taf., 16 Holzscbiiitte. Leipzig, 1877. Jourdain, S. '85. Sur le Systeme nerveux des Embryons des Limaciens et sur les Relations de rOtocyste avec ce Systeme. Compt. Rend. Acad. Sci. Paris, Tom. C. ])p. 383-385. Kowalesky, A. '83. Embryogenie du Chiton Polii (Pbillippi), avec quelques Remarques sur le Developpement des autres Chitons. (Odessa, Dec, 1882.) Ann. Musee Hist. Nat. Marseille, Zool., Tom. L, Mem. No. 5, 46 pp., 8 Pis. 1883. '83». :^tude sur TEmbryogenie du Deutale. Ann. Musee Hist. Nat. Mar- seille, Zool, Tom. I., ^lem. No. 7, 54 pp., 8 Pis. 1883. Lacaze-Duthiers, H. de. '60. Memoive sur I'Anatomie et I'Embryogenie des Vermets. 2^ Partie. Ann. Sci. Nat., 4 serie, Tom. XIII. pp." 266-296, Pis. VII.-IX. 1860. '72. Du Systeme nerveux dos Mollusques Gasteropodes pulmones aquatiques et d'un nouvel Organ d'Innervation. Arch, de Zool. exp. et gen., Tom. I. pp. 437-500, Pis. XVII.-XX. 1872. '85. Le Systeme nerveux et les Formes embryonales du Gardinia Garnotii. Comp. Rend. Acad. Sci. Paris, Tom. C. pp. 14G-151. Langerhans, P. '73. Zur Entwickeliing dor Gastropoda Opisthobranchia. 'Zeitscbr. f. wiss. Zool., Bd. XXIII. pp. 171-179, Taf. VIII. 1873. 206 BULLETIN OF THE Lankester, E. R. '73. Summary of Zoological Observations, etc. Ann. Mag. Nat. Hist., 4tb series, Vol. XI. pp. 85-87. 1873. '74. Observations on the Development of tlic Pond Snail (LymnEeus stag- nalis), and on the early Stages of other MoUusca. Quart. Jour. Micr. Sci., Vol. XIV. pp. 3ba-:391, Pis. XVI., XVII. 1874. Rabl, C. '75. Die Ontogenie der Svisswasser-Pulmonaten. Jena. Zeitscbr., Bd. IX. Heft 2, pp. 19.5-240, Taf. VII.-IX. July, 1875. '79. Ueber die Entwicklung der Tellerscbnecke. (Wien. Mitte Feb., 1879.) Morph. Jabrbuch, Bd. V. Heft 4, pp. 562-655, Taf. XXXII.-XXXVIII., 7 Ilolzschnitte. 1879. '83. Beitriige zur Entwickelungsgcschichte der Prosobranchier. Sitzungsb. der K. Acad, der Wissensch. Wien, jMatb.-naturw. CI., Bd. LXXXVII. Abtheil. III. Heft 1, pp. 45-61, Taf. I., II. 1883. Salensky, W. '72. Beitriige zur Entwicklungsgeschiclite der Prosobrancliier. Zcitschr. f. wiss. Zool., Bd. XXII. pp. 428-154, Pis. XXXV.-XX XVIII. 1872. '86. Etudes sur le Dcvcloppemcnt du Vcrmet. Archives de Biologic, Tom. VI. pp. 655-759, Pis. XXV.-XXXII. 1886. Sarasin, P. B. '82. Entwicklungsgcschichte der Bithyiiia tcntaculata. Arbeit, a. d. Zool.. Zoot. Inst. Wiirzburg, Bd. VI. Heft I, pp. 1-6S, Taf. I.-Vil. 1882- Sarasin, P. und F. '87. Aus der Entwicklungsgcschichte der Helix Wultoni Reeve. Zoolo- gischer Anzeiger, Jahrg. X., No. 265, pp. 599-602. 21 Nov., 1887. '88. Aus der Entwicklungsgcschichte der Ileli.x Waltoni Reeve. Ergeb- nisse naturwissenschaftlichcr Forschungcn auf Ceylon in den Juhreu 1884-1886, Bd. I. Heft 2, pp. 35-69, Taf. VI.-VIII. 1888. Wolfson, W. '80. Die embryonale Entwickelung dcs Lymnseus stagnalis. Bull. Acad. Sci. St. Petersbourg, Tom. XXVI. No. I, pp. 79-99, 10 Figs. 12 Mars, 1880. MUSEUM OF COMPAJiATIVE ZOOLOGY. 207 EXPLANATION OF FIGUEES. All the figures were drawn with the aid of the camera lucida, and were made from preparations of Limax maximus. INDEX TO STAGES. The Roman numerals indicate Plates. The Arabic numerals, Figures ; those which are enclosed in a parenthesis belong to the same specimen. Skeleton num- bers on the plates refer to the number of tlie section in its series. 6th d 7th 8th 9th 10th 11th 12th 14tli 16th 17th ((M.)' VuMa)^ Wo. 17, t J' ^337^3^7/' ^Ti)' ^U, 10. I'.i^ / III. IV. V. \ ^2S-U' 58, yy' 61, do'' r III. V. VI. \ / V. VI. \ ^32-47, 5l' 62,04,67' 7U-7o^' ^GS, O'j' 77-80/ ( ^iL V. Vn. \ / IIL VII. \ / VI. \ / VII.\ M8~4y' 65, (jti' 81, b-;/' Wo, 02' SO-'.U/' ^4-76, 80'/' \83-S4/ /VII. VIII IX. \ / nil. IX. X. ^ 11.5 ' 96, 101, 102' 114''' W.17-10U, 104, 106' 115-120' 121, 123, 125, 126 VIotT')' fe)" / VIII. X. \ \10:i, 10J=, 100, 105-113' 122, 127/ )• 208 BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. ABBREVIATIONS. The right side of the animal is indicated by the tetters dx., the left side by s. These letters are usually affixed to one or mure of the abbreviations used to designate organs. The skeleton figures immediately under the number of a fgiire on the plate indicate the number of the section in the series to which the figure belongs. Consult also " Index to Stages " (p. 207). ah. Abdominal ganglion. lab. ab.-vsc. Abdomino-visceral connective. Ins. an. Anus. lob. lat. buc. Buccal ganglion. loph. cil. cav. mt. Mantle cavity. mt. ceb.-buc. Cerebro-buccal connective. n. ceb. dx. Right cerebral ganglion. npli- ceb. s. Left cerebral ganglion. oc. ceb.-pd. Cerebro-pedal connective. 0 cy. ceb.-plu. Cerebro-pleural connective. cr. cnch. Shell gland. pd. com. a. Anterior pedal commissure. pes. com. buc. Buccal commissure. phi. com. ceb. Cerebral commissure. pht.-pd. com. pd. Pedal commissure. pIll.-VSC. com.pd.a. Anterior pedal commissure. Prf com. pd. p. Posterior pedal commissure. rod. dt. sx. pr. Primary sexual duct. ret. ta. dx. Right. B. en. Entoderm. sul. plu. gl. pd. Pedal gland. ta. gt. sal. Salivary gland. ta.' gn. Ganglion. vsc. iv. ceb. Cerebral invagination. vsc.-plu. Upper lip. Lens. Lateral lobe of brain. Ciliated ridge. Mantle. Nerve. Nephridial organ (primitive kid- Eye, [ney.) Otocyst. OEsophagus. Pedal ganglion. Foot. Pleural ganglion. Pleuropedal connective. Pleuro-visceral connective. Cell proliferation. Radula sac. Retractor muscle of tentacle. Left. Pleural groove. Ocular tentacle. Labial tentacle. Visceral ganglion. Viscero-pleural connective. HENCHMAN, ^Nervous System of Litnaz. PLATE I. All the figures of this plate were made from material killed in Perenyi's fluid, and all except Fig. 1 are magnified 250 diameters. Fig. 1. A small portion of Fig. 5 more highly magnified to show the cell prolifer- ation for the right pedal ganglion. " 2. Posterior face of a transverse section from an individual about six days old. The section passes anterior to the "pleural groove," and through the region where the cerebral invaginations subsequently arise; the left side is cut a little anterior to the right. Stained in alcoholic borax-carmine. " 3. A section from the same individual posterior to the pleural groove in the region of the cell proliferation for the pedal ganglia. " 4. A section from the same, still farther back. " 5. Transverse section from an embryo a few hours older than the preceding, in the region of the proliferation to form the pedal ganglion. Stained in alcoholic borax-carmine. " 6-9. The left surface of sections parallel to the sagittal plane from an em- bryo of the seventh day. Figs. 6, 8, and 9 represent respectively the 11th, 16th, and 18tli sections of the series, and are from the left half of the embryo. Fig. 7 is from the right half, and passes through the right otocyst. Stained in Czoker's cochineal. " 10-13 exhibit the riglit surface of sections from another individual (between the seventh and eighth days) cut parallel to the sagittal plane, the anterior portion a little in advance of the posterior. Fig. 10 is a section passing through the proliferation forming the right visceral ganglion. Figs 11 and 12 are two successive sections passing through the left visceral ganglion ; the latter also passes through the left otocyst. Fig. lo shows the region of the forming abdominal ganglion. Stained in picro- carminate of lithium. In both these individuals the left ganglia and otocysts are more developed than the right. HKN'C IjMAK.- NERVOVS 5YETi:M Pi, ' yfS\ 7 » '1 9 t>rf pU-fix «aa ■%•. *tf ■ ^.•..->^ >^^'' ■';■•' ■^fj" -® // t v^=.' idi n. Henchman. — Nervous System of Limax. PLATE II. All the figures of this plate are magnified 250 diameters. Fig. 14. A section parallel to the sagittal plane from an individual of the eighth day. It passes through the cerebral and pleural ganglia of the left side of the body, and also sliows four cells of the left otocyst poste- rior to the pleural groove. The material was killed in 0.33% ciiromic acid, and stained in alcoholic borax-carmine. " 15. The left surface of a section cut parallel to the sagittal plane from an embryo of the seventh day (but more advanced than in Figs. 6-9), pass- ing througii the cerebral invagination and a group of cells belonging to the proliferated portion of the cerebral ganglion of the right side. The material was treated as in that of Fig. 14. " 16. A section from the same individual as Fig. 14, passing through the cell proliferation to form tlie visceral ganglion of the right side. " 17 and 18 are from the same individual as Fig. 15. " 17. A section passing through the visceral ganglion and the external opening of the nephridial organ of the right side. " 18. The second section nearer the median plane than Fig. 17, showing the cell proliferation to form the right visceral ganglion. " 19. From the same mdividual as Fig. 14, showing tiie cerebral invagination and proliferation of the right side, and also a cross section of the primitive kidney " 20-24. Tlie anterior surfaces of transverse sections from an embryo of the ninth day. Material killed in Perenyi's fluid, and stained in alcoholic borax-carmine. " 20. Portion of a section which passes through the proliferation of cells form- ing the pleural ganglion (dorsal to the pleural groove), and through the pedal ganglion and the otocyst of the left side. " 21. The 37th section, which passes through the cerebral commissure and shows the proliferation of cerebral cells on the left side. " 22. The 51st section, which passes through the buccal ganglion of the right side. " 23. The 69th section, showing the unsymmetrical position of the visceral ganglia and a cross section of the right nephridial organ. " 24. The 75th section of the series, passing through the abdominal ganglion and the invagination to form the mantle cavity. It is in the region where the anterior portion of the embryo is bent backward over the foot by the nutritive sac; the foot is not represented in the figure. Hemchmak- NERvora System of Umax. PL. I!. ,,1k I 'I cfb.s. ^ ^'■ 4^ P^P fj , .■Iptu. '-t&r f>, ■'■V^i ir.crbjdj ' />■■ jG». - II ph . pFf.vsrdj. rnc/i , J6. &> ^ ■■■! ; yS^Jr ' ' ' ^ o ■' /.9. /7. ^ ,J >^ :xy nttt ^// r.-. m . .*7^.^tj'ii^ A.^tt.d-1. Hekchman. — Nervous System of Limax. PLATE m. Figs. 25-27 are from the same individual {ninth day) as Figs. 20-24 on Plate II. They are magnified 83 diameters. " 25. This section passes through the left cerebral gangUon, and the cerebral invagination of the right side. " 26 shows, in addition to the cerebral invagination, that of the right eye (at the left of the figure). " 27. Section passing through the pedal ganglia and their anterior commissure. It shows a cross section of the oesophagus, the radula sac, and the right otocyst. " 28-31. The left surface of sections parallel to the sagittal plane from an em- bryo of the tenth day. (Figs. 58, 59, Plate IV., and Figs. 61, 63, Plate v., also belong to this series.) This individual was killed in Perenyi'a fluid, and stained in picro-carminate of lithium. X 100. " 28 and 29 are successive sections passing through the invaginations of the cere- bral ganglion and the eye of the left side. " 30 shows a cross section of the cerebral commissure. " 31. The second section nearer the median plane than Fig. 30, showing the cerebral commissure, the position of the right visceral ganglion, and the anus already open to the exterior. " 32-47, 51. The left surface of sections cut parallel to the sagittal plane, from an embryo of the eleventh day, magnified 100 diameters. Killed in Perenyi's fluid, stained in Czoker's cochineal. (Sections shown in Plate V. Figs. 62, 64, 67, and Plate VI. Figs. 70-73, also belong to this series.) " 32-38. Seven successive sections passing through the invagination for the cerebral ganglia of the left side. " 39. The second section nearer the median plane than Fig. 38, showing the inner sac-like end of the invagination. " 40. The next section, showing the blind end of the invagination and the proliferated portion of the ganglion. " 41 and 42. Successive sections (32d and 33d of the series) passing through the left pedal ganglion and the connective between the abdominal and left visceral ganglia. " 43. Section cutting the abdominal ganglion crosswise. " 44. The 35th section shows, in addition to the abdominal ganglion, a cross section of the anterior pedal commissure. (The 36th, 37th, 39th, and 40th sections of the series are sliown in Figs. 47, 46, 45, and 51, respectively.) " 45. Section passing through the connective between the abdominal ganglion and the right visceral ganglion. " 46 sliows the abdominal ganglion still connected with the ectoderm. " 47. The next section to that shown in Fig. 44, passing through the abdominal ganglion. " 48, 49. The left surface of sections cut parallel to the sagittal plane from an individual of about the twelfth day, magnified 100 diameters. Killed in (See obverse.) PLATE III. (continued.) 0.33% chromic acid, and stained in alcoholic borax carmine. (Sec. tions shown in Plate V. Figs. 65, 66, and Piute VII. Pigs. 81, 82, also belong to tiiis series.) Pig. 48. A section passing through the cerebral and pleural ganglia of the right side, and also through the abdominal ganglion. " 49 shows the proliferated portion of the cerebral ganglion and the visceral ganglion of tlie right side. " 61. (See explanation of Pigs. 32-47.) The section following that shown in Fig. 45. It passes through the connective between the abdominal ganglion and the visceral ganglion of the right side. " 50, 62. The posterior surface of transverse sections of an embryo at the same stage of development (twelfth daj) as that represented in Figs. 48 and 49. The ventral portion and the right side cut a little in advance of the dorsal portion and the left side, magnified 100 diameters. Killed in 0.33% chromic acid, stained in alcoholic borax-carmine. Figs. 85-94, Plate VII., continue this series. The following shows the sequence of the sections : — Section 77, 101, 10.3, 109, 110. 112, 1 13, 117, 125. 126. 128, 146. Figure 90, 92, 91, 93, 50, 52, 85, 86, 88, 87, 89, 94. " 50. This section passes through the left pedal ganglion and the abdominal ganglion where the latter is still attached to the ectoderm. The mantle cavity open to the exterior. " 52. The second section in front of Fig. 50, showing that in this region the ab- dominal ganglion is free from the ectoderm. Hemchman-Nervoi'S System okLim.«. g6. n'.cct>dx. 4 -ff Pl.IG. ss. ..■nph-...,-:\ 50. ack. ,' .13. ,ad. /f'^' I 30. nf*h. npk .... %■ ■A y^ k:!^ rfZ''- .irS?5r-"'^«- "ri. . -o-f&is '-■ cnek. 40. 23 \ com.fi.t. '.^ lajix. V-.,. ./ ■■jLjiI,:. ^2. J3 -^. 50. 50. 45. 4S. 36 49. rk.l ^ .-/. -^ 47. 4W. •IS'i-Jl. ^^^ — nf^ .'^ 1;2 A /•/«.. A?H.d8:. BMeisel.iitn.Boslon Henchman. — Nervous System of Limax. PLATE IV. All figures of this plate magnified 250 diameters. Figs. 53, 66, 57, and 65 are four successive sections from the same embryo. This was seven days old, but much more advanced than the embryos represented in Figs. 6-13, 15, 17, and 18. Killed in 0.33% chromic acid, stained in alcoholic borax-carmine. " 63. Posterior face of a transverse section passing through the visceral ganglia, the external opening of the nephridial organ of the right side, and the opening into the mantle chamber or respiratory cavity. (Compare Figs. 5G and 55.) " 54. The posterior surface of a transverse section passing through the anterior pedal commissure. Embryo of about the same stage of development as the preceding, and prepared in the same way as that. " 55. Portion of the third section following that shown in Fig. 53; it passes tlirough the pedal and visceral ganglia of the right side. " 56 shows both the visceral ganglia. " 57. Portion of section showing the left pedal ganglion and cell proliferation from the ventral wall of the foot ; the visceral ganglion and the ex- ternal opening of the nephridial organ of the left side are also seen. " 58, 59. Consult explanation of Figs. 28-31, Plate III. " 68. Sagittal section passing through the proliferated portion of the right cerebral, pedal, and visceral ganglia and right otocyst. Shows cell proliferation from the ventral wall of the foot, and that the visceral ganglion is attached to the ectoderm at the margin of the mantle cavity. " 59. A small portion of the section following Fig. 68 to show the right visce- ral ganglion and its attachment to the ectoderm. Henchman- Nervous System ofLimax. PlIV. ic'f s '.-.^ i^w^ Tfjph. ■■ * V ■ :ie'- « ':*»',.. '■ sitl.pht. ^ rad. Pfi f / . /f ■\ \'^c-di. -• com., a. ' & f ;..<^L ^ I ^- 3^ e>- ^<-- ; •fe- ,J --.-•'*. ■ :,><3' • : ■^■ -E<<» ■^ •irrj .rylii- t • . ••• ■i.o. b '*eise..i!iK.Sc*tor Henchman. — Nervous System of Limax. PLATE V. All figures magnified 250 diameters. Fig. 60. The left surface of a section cut parallel to the sagittal plane from an embryo of the ninth day. The section passes through a few cells of the cerebral, the pedal, and the visceral ganglia of the right side of the body, and also siiows a section of the right otocyst. Killed in Perenyi's fluid, and stained in alcoholic borax-carmine. " 61. (ConsultexplanationofP'igs. 28-31, Plate III.) Sagittal section (the 22d) to show the abdominal ganglion, which lies embedded in the ectoderm anterior to the pleural groove " 62. (Consult explanation of Figs. .32-47, Plate III.) The 32d section of the series, showing a transverse section of the cerebral commissure and a portion of the left buccal ganglion. " 63. (Consult explanation of Figs. 28-31, Plate III ) The 15th section of the series; it passes through the left cerebral, the pedal, the pleural, and the visceral ganglia, and the wall of the left otocyst. " 64. (Consult explanation of Fig. 62.) The 24th section ; it shows the inter- nal end of the cerebral invagination and the cell proliferation to form the larger part of the brain of the left side. " 65, 66. Compare explanation of Figs. 48, 49, Plate III. " 66. The 91st section of the series, showing transverse sections of the buccal commissure and the connective between the abdominal and the left visceral ganglia. " 66. The 102d section of the series ; it passes through the abdominal ganglion. " 67. (Consult explanation of Fig. 62.) A section through the right visceral ganglion. " 68. The posterior surface of a transverse section from an embryo of the eleventh day. It shows the right visceral and the abdominal ganglia. Killed in Perenyi's fluid, stained in Czoker's cochineal. " 69. The second section anterior to Fig. 68, passing through the visceral ganglion of the right side. (Additional sections from this specimen are shown in Figs. 77-80. Plate VI.) HKKCHMA.y.- 'i7:ii-. ••- c'v_ Pi 61. (iU crh.ilx. ^^-^ ^>^ 'Z-r-^ .. fi''£^- % pj. ,&^ T* • fA cam eeb. .- ^ ;-. \^t^\ 62. f1 — nph. ^ prf. crh comittr. rrrd .s-O^HJ ,,..i-abrvsc.s. 65. rtld. 67. 6-'/. . /•-/«■. (7/.. j^,t> •'Sul.phi. .'-^ ■■> ^ C.li. 68. \-sr lij- ".•iUl .plij , ■■rnti . c.n. Henchman. — Nervous System of Limax. PLATE VI. All the figures of this plate were made from material killed in Pcrenyi's fluid, and all except Figs. 72, 73, and 77 are magnified 250 diameters. Figs. 70-73. Consult explanation of Figs. 32-47, Plate III. " 70 shows the left cerebral, pleural, and visceral ganglia in the region of the cerebro-pleural and pleuro-visceral connectives. " 71. Section passing through the otocyst and the cerebral, pedal, and visceral ganglia of the left side. The visceral ganglion is connected with the ectoderm, and the pleuro-visceral connective is much more elongated than on the ninth day. " 72. A portion of the ventral wall of the foot from the 32d section, to show the cell proliferation for the pedal ganglion. X 605. " 73. Same as Fig. 72, but from the right side of the body. " 74-76, 80*. The posterior surface of transverse sections from an individual of tlie twelfth day. The right side cut slightly in advance of the left. Stained in picro-carminate of lithium. " 74. The 81st section, which passes through the otocysts and both pedal ganglia in the region of their anterior commissure. " 75 and 76. Tiie 59th and 60th sections of the same series, showing a part of the left side of the embryo in the region of the abdominal ganj^lion. " 77-80. (See explanation of Fig. 68, Plate V.) Posterior faces of four sue cessive transverse sections through tlie mouth of the left salivary duct where it connects with the cEsophagus. Fig. 77 shows also the buccal ganglia. " 80». (See explanation of Figs. 74-70.) The 107tli section of the series. It shows the cerebral conmiissurc, a few cells of the right cerebral ganglion, and tlie right eye. a.' CHMAX- NERVOUS SYSTEM OFLIJ'IAX. \ S-'X. /•^■N .0^ ^lu i.''..'»frS. ^ '^M .?., „,.,y. sn " »C" Cfi- 106. ;rsf.iz. inc. eeb. -x. ^ \f Cfh's 'l07. . ' . ncy. rrb./ui. -■ pd. ■fl ai. - S4- pd-ix. too. .'6 V hur..^. earn Hu . _,^f^_.V MJtt (rh tob.lau, net.... . OCX- nl.Jid. liJ. no. \ tl2. ■ rsc.dz. a^-v.-Tf^ 108. 2^ IIS. ■<».. fl. .pd. ■?i{at\. EKBSel.IitK.Bgtion Henchman. — Nervous System of Liiuax. PLATE IX. All the figures of this plate are magnified 250 diameters, and were made from material killed in Perenyi's fluid, and stained in picro-carminate of lithium. Fig. 114. (See explanation of Fig. 95, Plate VII.) Tiie 66th section; it passes through tiie cerebral, the pedal, the pleural, and the visceral ganglia of the right side in tiie plane of tiie cerebro-pedal and cerebro-pleural connectives. It also shows the right otocyst. " 115-120. See explanation of Figs. 97-100, Plate VIII. " 115. Tiie 109th section; it shows a portion of the abdominal ganglion at the right of tiie radula sac. " 116. The Ultli section ; it passes through tiie abdominal and right pleural ganglia. " 117. The 113th section; it shows the abdominal ganglion where it passes above the radula sac, and a portion of tlie right pleural ganglion. (Compare Fig. 104, Plate VIII.) " 118. Tlie 102d section; it passes tlirough the pedal ganglia in the phine of tlieir anterior commissure. It also shows tlie riglit otocyst. " 119. The 87th section. The pedal ganglia in tlie plane of their posterior commissure. " 120. Tiie 121st section, which passes tlirough the visceral and buccal ganglia of tlie right side, and shows a portion of the buccal commissure. HF.KCHKy.N.- Nervous system of Umax. PL IX. ifi. <-^l..U x?. — ' -- ( 'h-.^a rrojjf plu , ocy. li:. nh. in;. X } "Iti-dx-. 118. ■02 rad . -^:. 0- r \ pfi-dy ah. 117. pfu.eijc "fi" 119. com.hirc- ''prU^-' 120. :?Hdei. BJfes«I.!jiJ>3««m'. Henchman. — Nervous System of Liraax PLATE X. Fig. 121. (See explanation of Figs 97-100, Plate VIII.) Tiie 127tii section; it passes through the left cerebral invagination, also through the left cerebral and buccal ganglia, and tlieir connective, x 237. " 122. (See explanation of Figs. 103, 103^ Plate VIII ) The 217lii section of the series. It passes through the right cerebral ganglion and its lat- eral lobe; it also shows the ocular tentacle and the wall of the riglit eye, transverse sections of the oesophagus, salivary glands, radula sac, and primary se.xual duct, x 237. " 123. (See explanation of Figs. 97-109, Plate VIII ) Tliis section passes tiirough the right pleural and visceral ganglia, a small portion of the left pleural and visceral ganglia, and the abdominal ganglion, which lies between the cesopiiagus and radula sac. X 250. " 124. Tlie posterior face of a transverse section from an embryo of the six- teenth dnij. The left side is cut a little in advance of the rigiit. The section passes through tiie cerebral invagination, — still open to the exterior, — the cerebral ganglion of the left side and its lateral lobe, and the left buccal ganglion It also shows in cross section the duct of the salivary gland, and a small portion of tlie wall of the radula sac. X 237. Perenyi's fluid ; picro-carminate of lithium. " 125, 126. See e.xplanation of Figs. 97-100, Plate VIII. " 125. The l'22d section; it shows the left pleural and visceral ganglia, with the pleuro-visceral connective, and a small portion of the left cerebral ganglion. X 250. " 120. The 139th section ; it shows in section the left cerebral invagination, the cerebral ganglion, and the cerebral commissure, x 250. " 127. (See e.xplanation of Figs. 103, 103^ Plate VIII.) The 225th section; it .shows the cerebral invagination, and the right lateral lobe of the brain. (Compare Plate VIII. Fig. 108.) x 237. r ..>y>- .y- gl.sai ^^ hph r,l. 'Mify No. 8. — The Parietal Eye in some Lizards from the Western United States. By W. E. RiTTEK.i With a single though notable exception, the numerous authors who have written on the parietal organ in vertebrates since the papers of de Graaf ('86^ and '86^) and Spencer ('86 and '87) appeared, have agreed that the structure is, or at least was in ancestral vertebrates, an eye. This belief is based entirely on the structure of the organ, no physiological experiments or observations on the habits of the animals possessing it having yet been produced in proof of its function. Leydig ('89) alone, in a recent preliminary paper on the subject, has denied its optical nature, and has assigned to it an entirely diti'erent function : though in a second preliminary, still more recent ('90), he expresses his denial with considerably less confidence. He rejects the eye hypothesis, however, on the same grounds that have led others to adopt it ; namely, on the grounds of its structure, and especially of its relation to the brain. He believes that what is generally held to be an optic nerve is in fact merely a string of connective tissue. Among those who believe the organ is or has been an eye, there are important differences of opinion as to its present value. By Ahlborn ('84), de Graaf, Spencer, and several other more recent writers, it is believed to be degenerate and entirely functionless in all living verte- brates. Rabl-Riickard ('86) has expressed the opinion that the organ may still be of use in furnishing its possessors with a more delicate means of detecting differences of temperature than exists elsewhere on the body. Beraneck ('87) believes that, while the structure is probably of an optical nature in some vertebrates, it has become so secondarily ; that the primitive function of the epiphysis, common to the brains of all vertebrates, was something entirely unknown to us now, though not concerned with vision ; but that in the Cyclostomes, the Amphibians, and the Reptiles it has taken on, secondarily, the function and form of an eye. 1 Contributions from the Zoological Laboratory of the Museum of Comparative Zoology, under the direction of E. L. Mark, No. XXII. VOL XX. — NO. 8. 14 210 BULLETIN OF THE But even were it established beyond question that the organ is a degenerate eye, there would still remain several quite distinct and very interesting problems to be solved. The most fundamental of these is pn)bably that of its homology. Much has been written on this ques- tion by the various recent authors, but even less unanimity of opinioo has been reached here than on the question of its structure and func- tion. The question why the organ has remained so well developed in a few systematically widely separated groups of vertebrates, while in all others the process of degeneration has gone so far as to leave but a mere trace of the proximal portion of the epiphysis, has not been much discussed. It is not my purpose in the present paper to enter upon a discussion of the theoretical questions involved, and they are here adverted to merely to point out tlie need — as indicated by their importance and the discordance of the opinions now held with regard to them — of a larger body of facts on the subject than we yet possess. For the present, I confine myself to a presentation of the facts observed, and my interpretation of them as bearing npon some of the minor conclusions reached by other writers, hoping to be able to pursue the subject further in the near future, when situated in a region where au abundance and a variety of material, adult and embryonic, can be ob- tained, and where observations on the habits of the animals can be made. The present work was undertaken at the suggestion of Prof. E. L. Mark. I wish here to acknowledge my indebtedness to Mr. G. H. Parker, of the Museum of Comparative Zoology ; to Mr. J. J. Rivers, Curator of the Museum at the University of California ; and to Mr. T. C. Palmer, of the United States Department of Agriculture, Wash- ington, for material used ; and also to Mr. S. Garman, of this Museum, for assistance in determining the species studied. A word as to technique. For studying the structure of the retina it is very desirable to remove the great quantity of pigment that inva- riably obscures the histological elements in this region. Neither nitric nor hydrochloric acid, nor the alkalies, have any visible effect on this pigment, but the desired result was reached by the use of chlorine gas. The mounted, unstained sections were covered by a film of ninety per cent alcohol, and placed in a tight glass chamber, in which was also confined a small vessel containing a mixture of potassium chlorate and hydrochloric acid for generating the gas. By being careful that the slide on which the sections were mounted occupied a perfectly horizon- tal position, and was so placed that the film of alcohol could not be MUSEUM OF COMPARATIVE ZOOLOGY. 211 drawn off by capillary attraction, the film soon became saturated with tb^ gas, and did not need renewing. From forty-five minutes to an hour, depending on the quantity of pigment, was sufficient time in which to accomplish the work. Considerable difficulty was found in removing the chlorine from the sections. As it had thoroughly pen- etrated the tissue, simple washing, even though prolonged, did not wholly remove it ; but by washing carefully, and then leaving the whole slide immersed in ninety per cent alcohol for twelve or fourteen hours, the gas was entirely removed. A good quality of Schallibaum's fixative held the sections perfectly through all this and the subsequent staining. For decalcifying and hardening the tissues I have found Perenyi's fluid more satisfactory than anything else tried, the two processes being accomplished at the same time by this reagent. Of the several species of lizards which I have studied I shall describe the structure in only three, namely, Phrynosoma Douglassii, P. coronata, and Uta Stansburiana, these being the only ones that have presented anything new or of special interest. Phrynosoma Douglassii. 1. External Appearance. — Conceraing the external appearance of the organ little need be said, since it differs in no essential particular from what has been amply described and illustrated in numerous other liz- ards. The scale marking the position of the eye is quite conspicuous, especially in very young individuals, where it is of a rather lighter color and larger size, relatively, than in the adult. In old individuals the great development of the surrounding scales and tubercles renders it somewhat less noticeable than it otherwise would be, but it is always readily distinguished, not only by its median position, but also by the absence of pigment and by its translucent appearance. 2. IVie Parietal Vesicle. — Figure 1, drawn from a sagittal section through the dorsal wall of the head, shows the form of the vesicle and its position within the parietal foramen and with reference to the external and internal surfaces of the wall. It lies within the parietal foramen, though extending somewhat above the dorsal surface of the parietal bone, firmly embedded in connective tissue, so that when the wall of the head is separated from the brain the vesicle always goes with the former. The tissues composing the dorsal wall of the head are, excepting the corneous layer of the skin, quite different immediately over the vesicle from those of the surrounding regions. The epidermal layer of the skin 212 BULLETIN OF THE elsewhere sends down irregular cone-shaped masses, which penetrate and become lost in the underlying connective tissue, thus finely uniting the two layers. Over the vesicle, however, these processes are wholly wanting, the under surface of the epithelial layer being even, and sharply limited from the connective tissue. These processes are espe- cially well developed immediately beyond the margin of the disk of the vesicle, where they carry the cells of the epidermal layer (e'drmJ) considerably deeper than their general level. The connective tissue between the vesicle and the epidermal layer is composed of fibres con- siderably finer and looser than those found in other places, and, further- more, the fibres are here disposed at various angles to the surface of the skin, whereas elsewhere they are approximately parallel to this surface (cmiH. tisJ). Pigment, which is found in great abundance in the skin in all other regions of the bo^y, is always entirely absent here. It will thus be noticed that each of the tissues over the vesicle is considerably more penetrable to light than are the corresponding ones elsewhere. The connective-tissue fibres immediately around the vesicle are arranged concentrically to its surface, and are, especially in the proximal two- thirds of their extent, considerably finer and closer than elsewhere. A kind of capsule for the vesicle is thus formed, and it is this alone ■which separates it from the cranial cavity. The fibres of a string of tissue extending from the distal end of the epiphysis can be traced, though with some uncertainty, to this capsule, but I find no indication of their passing through it, or even entering it, though I have given special attention to this point. The internal surface of the cranial wall in the region of the vesicle presents a depression, which is much less marked, however, than a cor- responding one in P. coronata, to be referred to hereafter. Running through the connective tissue at the bottom of this depression, and hence near the deep surface of the vesicle, are found a number of blood- vessels of considerable size and well filled with blood corpuscles (va. sng.). The vesicle itself is elliptical in sagittal section, the major axis, 258 /a long in the specimen figured, having the direction of the long axis of the head. In transverse section it is slightly elongated dorso-ventrally, and measures in this axis 171 /x. The cavity in sagittal section shows a triangular outline, the base of the triangle being on the dorsal or lens side. From this outline in the sagittal section the form gradually changes to that of an ellipse in the last sections on each side that cut the cavity ; so that the form of the cavity is approximately that of a broad, flat cone, the base directed MUSEUM OF COMPARATIVE ZOOLOGY. 213 outward aud the apex inward. The base of the cone is slightly con- cave, corresponding to the convexity of the inner surface of the lens. The wall of the vesicle is very distinctly differentiated into lens (his.) and retinal (rtn.) portions, the latter forming about two thirds of the whole. The lens is slightly biconvex, the two convexities being very nearly equal. The line of demarcation between the lens and the retina is a sharp one, though the two portions are plainly continuous. The cells composing the lens are large and distinct in outline, each one extending entirely through its thickness (Plate II. Fig. 5, cL Ins.). Their nuclei are large, easily stainable, and somewhat granular ; they are uniformly situated near the internal ends of the cells. The lens is entirely with- out pigment. Figure 5 represents a highly magnified portion of a longitudinal vertical section of the vesicle taken from near the median plane. In the retinal portion six regions or zones may be distinguished. Passing from the external surface toward the cavity, we find (1) a basement membrane {mb. ha. ex.). This is very thin, but uniform in thickness, and is of a structureless nature. From many points on this membrane fine processes radiate into the connective tissue enveloping the vesicle (Plate I. Fig. 3, ^^rc. r.). These processes do not appear to be of a mus- cular nature, but rather the same in structure as the basement membrane from which they arise. (2) A zone containing a iew scattered nuclei (nlJ), and fine-grained sparsely but evenly distributed pigment (pig.)- No cell boundaries can be made out in this zone. . The nuclei, few in number, form a single layer, and are situated near the basement mem- brane. They are very nearly round, exhibiting no tendency to elongate m the radii of the vesicle. Areas in their centres, which are somewhat more deeply stained than the rest of the nuclei, and which are probably nucleoli, are to be seen. (3) A zone (z.") in which are distinguish- able neither cells, nuclei, nor pigment; only a uniform, fine-granular, slightly stainable substance, of much the same nature, apparently, as the cell svibstance in those regions of the retinal portion in which cell boundaries can be distinguished. Whether or not this zone repre- sents the centrally directed ends of a layer of cells, the nuclei of which are the ones found in zone 2, I am unable to say, but it probably does. (4, 5) The next two zones are distinguished from each other only by the difference in the elements composing them, no distinguish- able line of separation existing between the two. The most obvious diflference between the constituent elements of these two regions is in the shape of the nuclei, those in zone 4 being approximately spherical 214 BULLETIN OF THE (rd.")., while those in zone 5 are much elongated in the radii of the ves- icle (nl.J"). The suggestion at once comes that this difference is due solely to the crowding togetlier of the cells nearest the internal surface of the retina, and hence that the two zones should in reality be re- garded as but one. If, howiwer, the difference in shape of the nuclei were the result solely of such crowding, we should find a complete gradation from the spherical to the elongated form in passing from without inward ; but such a gradation is not found in fact. Further- more, on close examination with high powers, it is found that the nuclei differ in structure as well as in form. An irregular stellated area can be detected in the centres of some of the spherical ones which does not exist in the elongated ones ; also, the entire substance of the former is slightly more granular than that of the latter. In the fifth zone cell boundaries (though not well shown in the figure) can be quite dis- tinctly traced to the internal basement membrane : but how the cells of the fourth and fifth zones are related I have been unal)le to deter- mine, since cell boundaries in the fourth zone cannot be traced. (G) The last layer may be designated as an internal basement membrane {mb. ba. i.), though it differs somewhat ii) structure from the external base- ment membrane, being of a granular nature. It extends over the surface of the lens, as well as over the retina, and is rather more com- pact in the former than in the latter region. Projecting into the cavity of the vesicle from the retinal portion are found certain structures con- cerning the nature of which I am not quite sure, but believe them to be secretions from the cells of the fifth zone. They are in general elongated, and pointed at their free ends, though their outlines are ragged and indefinite. They always stain most deeply at their internal free ends. In many ca.ses, as at *, they are seen to be continuous with the cells of the fifth zone through the internal basement membrane. These structures may correspond to v/hat de Graaf has described and figured as existing on the internal surface of the retina of Anguis, and has called " Staafjeslaag," but which Spencer and otliers believe to be merely a coagulum from the fluid that probably filled the cavity in the recent state. It is, however, scarcely possible to account for the structures here under consideration in this way, as is to be seen from my description and figures of them ; furthermore, a coagulum {cog^ does exist in addition to these. Within the substance of the retina (Fig. 5, va. rfn.) are found a num- ber of cavities varying in diameter, as measured in the plane of the sections, from 5.5 /a to 22 /x. The sections of these cavities are never MUSEUM OF COMPARATIVE ZOOLOGY. 215 quite circular, but are never much elongated. In many, though not in all, an exceedingly thin endothelial lining can be seen, and in a few in- stances blf-od corpuscles are found in the cavities (Plate I. Fig. 4, eiiHh. va. and cp. sng.). Although none of these cavities were found to extend through more than four or five sections, each 7.5 /a in thickness, and although in no instance was it possible satisfactorily to trace a connec- tion between them and tlje blood-vessels lying outside the vesicle, it still seems quite certain that they form a network of tine blood-vessels rami- fying through the substance of the retina. Owing to the fact that in some instances no lining membrane to these cavities can be found, and that their outlines are not sharply marked, the possibility of their having been artificially produced by the removal of pigment masses suggests itself j but the definiteness of the outline of many others and their endothelial lining membranes, in which much-flattened nuclei are found, strips this conjecture of its plausibility. If these are really blood-vessels, it might appear that some of them would be seen cut longitudinally ; and while it is true that in many cases focusing shows the cut walls to be very oblique to the plane of the section, still no sure instance of a vessel cut lengthwise has been seen. When, however, one considers the exceeding delicacy of the endothelial lining, and the fact that no differ- ential staining takes place, it does not seem impossible that such sec- tions may exist, and yet escape detection. These cavities have no regularity of arrangement, but are for the most part confined to zones 2, 3, and 4. In no instance has one been seen confluent with the cavity of the vesicle. These may possibly correspond to what Owsjannikow mentions as having been seen by him in Chameleon vulgaris. He says: "Am hin- tem Rande der Retina findet sich an einigen Schnitten das Lumen eines Rohrs, von dem nicht mit Bestimmtheit gesagt werden kann, ob es einem Blutgefiisse oder einem anderen Gewebe angehort." (Owsjan- nikow, '88, p. 16.) 3. The Epiphysis. — Figure 9 (Plate III.) represents a sagittal sec- tion of the epiphysis, and so much of the brain as is necessary to show the relation of the former to the latter. The entire structure, or, more properly, the combination of structures that must be considered at this time, presents the form of a curved cylinder, one end of wliich is pro- duced into a cone, while the other end has a hopper-shaped excava- tion. In keeping with the usual method of designation, I shall call the whole structure the epiphysis, though, as the sequel will show, it is doubtful if this is justifiable. The excavated end is proximal, the 216 BULLETIN OF THE excavation being the continuation of the cavity of the third ventricle into the epiphysis. The conical end, then, is distal, and rises somewhat above the level of the cerebral hemispheres. The curved axis forms very nearly a segment of the circumference of a circle, and is directed upward and forward from its point of origin from tlie brain. Continu- ing anteriorly from the apex of the cone is a string of connective tissue (con't. tis.), which passes to the region of the parietal vesicle, and in the distal portion of its course comes close in contact with the dura mater of the brain. The axis of the cylinder, if we consider it as continued to the anterior termination of this connective-tissue string, describes very nearly a semicircumference. The most anterior point in the con- nection of the epiphysis with tlie brain is at the junction of the cere- brum with the optic thalamus, somewhat anterior and dorsal to the superior commissure (com. su.). For a short distance above its connec- tion with the brain in this anterior part, the epithelial nature of the epiphysial wall is less distinct than at a higher level, where the wall becomes thicker, and is composed of a single layer of more or less cuboid nucleated cells, which stain readily in borax carmine or hasmatoxylin (Plate III. Figs. 8, 9, e^th.). Also at this level the wall becomes thrown into a highly complicated system of folds ; and it is this folded epithe- lium, containing within its folds great quantities of blood corpuscles, that forms a large bulk of the whole epiphysis (Figs. 8 and 9, e'th. and cp. sng^. In the section represented in Figure 9 no connection exists between the epithelium of the posterior portion of the epiphysis and the brain, and it is doubtful if such connection exists here in any of the sections of this specimen ; at any rate, if it does exist, it is exceedingly thin and limited in extent. There is, however, an undoubted j^onnection in this region in P. coronata, which will be described later ; but even in this latter species the posterior wall of the epiphysis is much less de- veloped than the anterior wall. The exceedingly thin epithelium that forms the posterior wall in P. Douglassii would, as is evident from its position and from comparison with P. coronata (Plate IV. Figs. 1 1 and 12), form a connection with the brain roof had not a separation taken place, either artificially or as a result of degeneration. This wall is closely applied to the anterior, concave side of the blood sinus to be presently described, and at a considerable distance above the brain is continuous with the anterior wall of the epiphysis. The space included by these walls is the hopper-shaped excavation in the proximal end of the cyl- inder already mentioned, — an extension of the cavity of the third MUSEUM OF COMrAUATIVE ZOOLOGY. 217 ventricle (vnt.^) into the epiphysis. Intimately connected with the dis- tal end of tlie portion of the epiphysis tluis far described is found a vesicle (epk. vs.), the thick walls of which are composed of columnar epithelium, and thus differ markedly from the folded epithelium of the anterior wall previously described. This vesicle is much flattened antero-posteriorly, its longest axis lying very nearly in the axis of the cylinder to which the epiphysis as a whole has been compared. That the structure here described is a separate vesicle, and that its cavity is not continuous with the cavity already described as a continuation of the third ventricle, admit of easy and satisfactory demonstration, not only in this particular instance, but also in all other individuals both of this species and of P. coronata of wiiich sections have been made. In passing through the entire sei'ies of sections, it is easily seen not only that the two cavities nowhere approach more nearly to conflu- ence than in the one represented in the figure, but also that the walls of the vesicle and those of the more proximal part of the epiphysis with which they are in relation are clearly distinct. The sejjarateness of these two structures will appear more clearly when we come to consider the same parts in P. coronata. Passing upward and forward from the distal end of this vesicle is to be seen a bundle of connective-tissue fibres which becomes blended with the string of connective tissue already described as runnitig from the apex of the cone to the region of the parietal vesicle. There is no indication that the epithelial wall of the epiphysial vesicle, as it may be called, passes into this string. Covering the whole postero-dorsal convex side of the portion of the epiphysis thus far described, and even extending considerably beyond its distal extremity, is an immense blood sinus fully distended with blood corpuscles (Fig. 9, sn. sng., and Fig. 8, cp. sng.). Phrynosoma coronata. 1. General Description. — Figure 2 (Plate I.) represents a transverse section of the dorsal wall of the head, passing through the middle of the parietal eye of P. coronata. The description of the external appearance and of the vesicle and its surrounding structures given for P. Douglassii requires modification in only a few points to become applicable to this species. The depression mentioned as existing on the internal surface of the wall of the brain-case immediately under the vesicle in P. Doug- lassii becomes in this species a deep pit. To correspond with this pit the external surface of the wall immediatel}' over the vesicle forms 218 BULLETIN OF THE a low, broad cone, a condition which gives quite a different general ap- pearance to the sections in the two species. In P. coronata the vesicle is situated somewhat nearer the external surface of the cranial wall than in P. Douglassii ; and the intervening connective tissue differs less, both as regards the fineness and direction of its fibres, from the adjacent tissues, than in the case of P. Douglassii. The vesicle, with its con- nective-tissue capsule, protrudes into the bottom of the pit considerably. The pit is bridged over by the dura mater of the brain, and thus a chamber is formed in which a great quantity of blood corpuscles is found {cp. snff.). It will be remembered that no such blood sinus in this region exists in P. Douglassii, but that numerous blood-vessels do occur here. In P. coronata, however, the sinus replaces the ves^ls. 2. The Parietal Vesicle. — With regard to the vesicle itself, the only points in which it differs very essentially from that found in P. Douglassii are the absence of the cavities in the retina regarded as blood-vessels, and the far less perfect development of the structures projecting from the internal surface of the retina into the cavity of the vesicle. The latter difference I am inclined to think duo to the probably somewhat greater degree of degeneration of the retinal cells which secrete these struc- tures. That this portion of the retina is more degenerated in P. coro- nata may be supposed from the fact that we find liere considerably more pigment than in the corresponding region in P. Douglassii. How- ever, too much stress must not be laid on the greater or less quantity of pigment, since the quantity is quite variable even within the same species. In one individual of this species pigment was found, though in small quantity, in the lens. 3. The Epiphysis. — Although this structure does not differ in any essential particular from what we have already seen in the preceding s|)ecies, the fact that several of the points which go to make the study of the epiphysis of much interest are here well brought out, has made it seem best to describe and illustrate the organ in detail. Figures 10, 11, and 12 (Plate IV.) present vertical longitudinal sections from the same animal at different planes to the left of the median plane, Figure 12 being very nearl}' median, and Figure 10 farthest removed from it. It should here be said, however, that the sections are not quite vertical; so that, while the epiphysial vesicle is situated more to the left than to the right side of the sagittal plane, yet it is less so than would be inferred from the way in which it appears in the figures. The form of the epiphysis, as a whole, is nearly the same as that found in P. Douglassii, and it is composed of the same parts ; — namely, a proximal MUSEUM OF COMPARATIVE ZOOLOGY, 219 part with an anterior much-folded epithelial wall, and a posterior not folded and thinner epithelial wall ; an epiphysial vesicle ; a blood sinus; and a string of connective tissue extending from the distal end of the vesicle and blood sinus to the region of the parietal vesicle. In the anterior wall of the proximal portion the folding extends down some- what nearer to the brain than is the case in P. Douglassii, and just at its junction with the brain a large blood-vessel is found filled with blood corpuscles (Fig. 12, cp. sng.). As already said in describing the posterior wall in P. Douglassii, the connection (opposite the letters vnt.^) with the brain is here complete and very evident, though the roof of the third ventricle {id. till, opt.) appears in the section to constitute a part of this wall. The cells composing the walls of the proximal pan are about two or three deep, but not arranged in layers. They are small, distinctly nu- cleated, and the nuclei are apparently perfectly round. They stain readily. On the outer surface of this wall is found, throughout most of its extent, a very thin layer of tissue, the cells of which are much flattened. This layer becomes continued from the apex of the epiphysis as the connective-tissue string (con't. tis.) already mentioned as passing to the region of the eye ; another portion of it also becomes continuous with the pia mater of the brain. Figure 10 represents a section through the longest portion of tiie epiphysial vesicle. In this plane the proximal portion of the epiphysis has not yet appeared, and is not found till we pass to a section in which the long axis of the vesicle has become considerably shortened. In the wall t^f the vesicle three zones or layers are found. The external one is similar to — in fact, on the posterior surface is continuous with — the thin external layer mentioned in the proximal portion. The second zone, comprising more than half of the entire thickness of the wall, is composed of cells apparently of the same nature as those described as forming the chief portion of the wall of the proximal part ; but the layer is considerably thicker here than there, and on tlie whole rather more compact (e'th., Figs. 10 and 11). The third and most internal zone is a deeply pigmented one (pig.). This pigment is so dense that when destroyed no distinguishable structure remains. In the presence of this pigment the species now under consideration differs entirely from P. Douglassii, where no pigment in this region is found. Again, however, attention is called to the fact that great importance cannot be attached to the presence or absence of pigment. Figure 1 1 shows the relation between this vesicle and the proximal portion of the 220 BULLETIN OF THE epiphysis. In this section it will be seen that a distinct line of demar- cation exists between the true epithelial portions of the two walls where they come in contact. This distinctness is maintained through- out the entire series of sections. When the median section is reached, the vesicle has entirely disappeared. From the distal end of the vesicle the connective-tissue string extends forward to the region of the eye, as in the case of the proximal portion (con't. tis.). The blood sinus (Fig. 12) does not, in this species, come in contact with the epiphj'sial vesicle, but occupies the same position on the proximal part as in the case of P. Douglassii. It is much smaller in P. coronata, but in other respects is of the same nature. Whether or not this epiphysial vesicle may be homologized with the secondary vesicle in Petromyzon "(Ahlborn, '83, Beard, '89, Owsjannikow, '88, Wiedersheim, '80) can be proBtably discussed only after its development has been studied. So far as the condition in the adult is concerned, there is little to indicate such a homology. I mention here an observation which may be of significance in con- nection with this complicated structure of the epiphysis. In both spe- cies and in all the individuals of Phrynosoma of which I have made sections favorable for exhibiting the entire dorsal surface of the brain, I have noticed that the pia mater appears to form a jimction with the coimective-tissue string described as passing from the distal extrem- ity of the epiphysis to the region of the parietal eye, and also that it is thrown into several folds on the dorsal surface of the cerebellum. The membrane where folded is considerably thicker than elsewhere, contains within its folds numerous blood-vessels, and is com.posed of a single layer of cells very regular and distinct in outline and of a de- cidedly epitheloid appearance. The condition reminds one strongly of the folded portion of the wall of the epiphysis. Uta Stansburiana. As I have had but two specimens of this species, both preserved in alcohol, and hence not in the best histological condition, my study of it has been less satisfactory than that of the species of Phrynosoma. A few points, however, have been observed which are of some interest; but these can be presented without entering into a detailed descrip- tion of the structure. Figure G (Plate II.) represents a portion of a sagittal section through the dorsal wall of the head and the parietal vesicle. The parietal foramen, too broad to be embraced in the figure. MUSEUM OF COMPARATIVE ZOOLOGY 221 is much larger here than in Phrynosoraa, and the vesicle can scarcely be said to be embedded in the connective tissue of the brain roof, as in the case of Phrynosoma, but rather is suspended from the under side of the wall in a connective-tissue capsule. The most striking features about this vesicle, as seen in the section, are its dorso-ventral flattening, and the entire separation of the lens from the retina. The lens, a well defined structure, composed of much elongated, almost fibrous, non-stainable cells, has its margins widely separated from the retina, and the intervening space is occupied by a uniformly fine granular substance (coff.T), which also occupies the nar- row space corresponding to what would be the cavity of the ve'sicle, were the lens and retina continuous at the margins of tlie former. The retina shows no structure beyond two deeply pigmented layers, cor- responding to its external and internal surfaces, connected at short but irregular intervals by pillars of pigment, between which are seen a few scattered nuclei. This distinct separation of the margins of the lens from the retina is the only undoubted case of the kind, so far as I know, that has been seen, and if normal may be of significance in connection with the theory of the origin of the eye recently advanced by Beard ('89). I am, however, inclined to believe, notwithstanding the fact that the condition here found is apparently confirmed by the sections of my second specimen of this species, that the separation is in reality due to the extreme differentiation of the two structures, by means of which the connection between them was weakened, and then to artifi- cial rupture by the flattening of the vesicle. The point certainly needs confirmation in more carefully preserved specimens. I was unable to study the epiphysis in the material which I had, but no trace of anything like a nerve or even like a connective-tissue string extending from the parietal vesicle could be detected, nor were there any indications of blood-vessels or sinuses corresponding with those existing in Phrynosoma found here. Conclusions. The general bearing of the facts here presented I discuss at present only in connection with the question of the function, past and pres- ent, of the parietal organ. I concur in the opinion held by most of the persons who have written on the subject, that the organ is a de- generate eye, although my observations furnish, perhaps, no evidence in addition to what has been presented by former writers, in support 222 BULLETIN OF THE of the belief. From the morphologist's point of view, the evidence that would remove all doubt as to the correctness of this opinion would be that the vesicle regarded as the eyeball should be composed of ele- ments essentially similar to elements found somewhere in organs known to perform the mechanical part in the act of vision ; and, isecond, that this vesicle should be connected with the brain by a nerve comparable with the optic nerve of some known functional eye. I think no one familiar with the structure of the vesicle as it exists in many Lacertilia and in Petromyzon, will refuse to accept as satisfactory the evidence on the first point. The evidence on the second point is less conclusive. In many cases where the vesicle is well developed, as in Phrynosoma, it is certain that nothing which can be justly compared to an optic nerve exists. Spencer ('86 and '87) and several succeeding writers have held it as beyond doubt that in several species, notably of the genera La- certa, Ilatteria, and Varanus, there is a nervous connection between the brain and vesicle. Leydig ('89), however, in his preliminary, based on his study of Lacerta ocellata, Varanus elegans, and other forms, says " der von Spencer beschriebene Nerv ist kein Nerv sondern das stning- artig ausgehende Ende der Zirbcl." Lacerta ocellata is one of the forms in which Spencer ascribes, with least question, a nervous nature to the structure under consideration ; but apparently Leydig has not examined either of the species of Varanus, viz. gigantea and Bengalen- sis, which Spencer studied ; while, on the other hand, V. elegans, Leydig's species, is not mentioned by Spencer as having been studied by him. This denial in toto of the existence of the nerve as described by Spen- cer, Leydig practically repeats in his most recent contribution to the subject (Leydig, '90), and adGs, as further confirmation of his opinion, that he has studied Hatteria (he does not tell us what species) and finds that here also the so-called nerve is of the nature of connective tissue. He also comes to the conclusion in this communication, that, while from the structure of the vesicle alone the organ must at least be put among the sense organs, it is yet "as good as itripossible to do so while it is recognized that in the parietal structure of all the animals investigated by me not one contains a nerve, for we must hold fast to the proposition that for the equipment of a sense organ the peripheral end of a nerve is necessary." It appears to me, however, that we are not compelled to relinquish the belief that the organ was originally an eye, even though we accept Leydig's statement, as against Spencer's and others, regarding the nature of the supposed nerve in the cases which both have exam- ined ; or even should it appear that in no case does the nervous con- nection noiv exist. MUSEUM OF COMPARATIVE ZOOLOGY. 223 It seems to me that Leydig has not given sufficient prominence to the possibility, not to say great probability, that the nervous connection has been lost by the raoditication and degeneration which the whole structure has certainly undergone ; and especially must we hesitate in rejecting this explanation, when we remember that by so doing we are compelled to seek another. To be obliged to ascribe a function other than that of vision to a structure entirely like an organ of vision in most of its essential parts, and differing widely from one in no essential point, is requiring us to accept a conclusion that would throw suspicion on all our morf)hological reasoning. Should it be shown conclusively that the vesicle never has, in any vertebrate, either in the adult or dur- ing its ontogeny, nervous connection with the brain, then we should be obliged to abandon the optical explanation of its origin, and turn to the exceedingly difficult task of finding another. But until such knowledge is at hand, it seems to me we must suppose that the organ was produced as an eye, that in some way entirely unknown to us it lost its optical function, and that, in the consequent modification and degeneration, the optic nerve degenerated more rapidly in some cases than did the optic vesicle ; and that in this way the separation which we now find took place.^ In previous discussions of the nature and function of the parietal organ, I believe sufficient attention has not been given to the structure and development of the epiphysis and its relation to the parietal ves- icle, and especially its relation to the so-called choroid plexus. I have designated the entire structure found in connection with the roof of the thalamencephalon as the epiphysis ; but, as already said, I have consid- erable doubt as to the wisdom of so doing. For the sake of precision it would seem best that the term epiphysis should be limited to the structure which arises as an evagination from this portion of the brain. Certain it is that the large blood sinus which I have described as a part of the epiphysis in Phrynosoma cannot be regarded as forming an essen- tial portion of the structure, and I think it quite possible that what I have called the epiphysial vesicle is not a portion of the epiphysis, should 1 Concerning the nervous connection between the eye and the epiphysis in Anguis fragilis, Strahl and Martin say ('88, p. 154), " Der Nerv der nach hinten am Vorderrand der Epipliyse scheinbar verschwindet, tritt von unten her in das Auge ein." Francotte ('88, p. 782) also describes essentially the same condition in this species. But such a condition would be so anomalous that C. K. Hoffmann ('88, p. 1991), notwithstanding the agreement of these independent statements, has. It seems to me with reason, expressed doubt as to the trustworthiness of the observations. 224 BULLETIN OF THE the term be limited as I have suggested that it ought to be. The dis- tinctness of the eitiphysial vesicle from the proximal portion of the epi- physis in the adult Phrynosoma is without exception^ so far as my observations have gone ; and if it is regarded as having been derived from the epiphysis, then we have two vesicles instead of one that have arisen in this way, and the difficulty of explaining the nature and function of the whole structure is correspondingly increased. Jn his recent paper, Leydig ('90) has expressed the belief that there are two forms of parietal organs. He says : " From the posterior por- tion of the embryonic thalamencephalon (Zwischenhirn), especially in Lacerta agilis, two thick-walled vesicles (Blasen) bnd out just in the middle line, lying one behind the other and springing from a common root (eincm Wurzelpunkte). The anterior vesicle gives rise to the parietal organ, and the posterior one constitutes the epiphysis (Zirbel)." It is only, he says, from the anterior of these two vesicles (Dlasen) that a vesicle (Blase) becomes cut off, and attains an eye-like character; the posterior one ends in the expanded blind terminal portion of the epi- physial thread (Zirbelfaden). But Selcnka ('90) informs us, in a still more recent communication, that, after studying the development of the brain in a large number of reptiles and other vertebrates, he is unable to confirm Leydig's statement as to the origin of the parietal eye. He does find, however, in all cases, an evagination from the dorsal wall of the fore brain very similar to the one that forms the epiphysis from the roof of the thalamencephalon ; also that the two structures elongate pari passu, the epiphysis becoming directed upward and forward, while the anterior evagination, which he calls the " paraphysis," becomes directed upward and backward. After the parietal vesicle is cut off from the epiphysis, the distal end of the paraphysis grows in between the vesicle and the end of the epiphysis from which it was detached, and the vesicle comes to lie on the paraphysis as on a pillow. The relation of the two structures in the adult he does not know. C. K. Hoffmann ('85) has also described an evagination from the roof of the brain at the place of transition from the fore brain to the thala- mus, which he calls the ependyma, — the beginning of the choroid plexus, — and he says that in the grown animal "it comes to take a not inconsiderable part in the formation of the epiphysis." Although there is nothing in the brief papers of either Leydig or Selenka to indi- cate whether or not the additional more anterior evagination seen by them is the same as that described by Hoffmann, yet, since all have studied the same forms, viz. of the genus Lacerta, it seems quite prob- MUSEUM OF COMPARATIVE ZOOLOGY. 225 able that they have all observed the same structure. Wiiether or not any portion of the epiphysis as I have found it in Phrynosonia cor- responds to the paraphysis of Selenlia, or tlie ependynia of Hoti'mann, can of course be determined only by studying tiie development of this portion of the brain. Bearing in mind the highly vascular condition of all parts of the parietal organ, the numerous large blood-vessels surrounding the vesicle in P. Douglassii, and the great sinus in the same region in P. coronata, the sinuses of the epiphysis in both species, as well as the great quan- tity of blood contained in the much, folded anterior wall of the epi- physis, it seems to nie impossible to escape the belief that, in this genus at least, the organ must have some physiological significance. Leydig ('89) has expressed the opinion that it belongs primarily to the l3'mph system. From what has ah'eady been said, it is evident that I cannot accept this conclusion ; but it does appear to me highly probable that the structure has become secondarily of such a character. From the numerous instances of change of function in the animal organism to which attention has been directed by Dohrn ('75), Kleinenberg ('86), Lankester ('80), Weismann ('86), and others, there are certainly no a priori objections to such a view, and it seems to afford more nearly a satisfactory explanation of the present condition of the organ than does any other. Cambridge, August 15, 1890. VOL. XX. — NO. 8. 15 226 BULLETIN OF THE BIBLIOGRAPHY. Ahlborn, F. '83. Uutersucliun<]^cn iiber das Geliirii der Petromyzonlen. Zeitschr. f. wiss. Zool., Bd. XXXIX. pp. 191-294, Taf. XlIl.-XVII. '84. Ueber die Bedeutuug der Zirbeldriise. Zeitschr. f. wiss. Zool., Bd. XL. pp. 331-337, 1 Taf. Beard, J. '87. Tlie Parietal Eye in Fishes. Nature, Vol. XXXVI. pp. 246-248 and pp. 340, 341. '89. Morpliological Studies. — I. The Parietal Eye of the Cyclostome Fishes. Quart. Jour, of Micr. Sci., Vol. XXIX. pp. 55-73, Pis. VI. and VII. B^raneck, Ed. '87. Ueber das Parietalauge der Reptilien. 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Ueber das Parietalauge der Saurier. Anat. Anzeiger, Bd. I., pp. 148, 149. EXPLANATION OF FIGURES. All the figures are raniera ilrawings excepting wliere otherwise indicated in tlie explanations. RiiTER. — Parietal Eye. ABBREVIATIONS. cav, e'phy. Cavity of the epipliysis. mtic. opt. cbl. Cerebellum. ceb. Cerebrum. ml. ba. ex. chs. opt. Optic cliiasm. cl. i. Cells of zone 5 of the retina. ml>. ha. i. d. Ins. Cells of tiie lens. nl. coy. Coagulum. n/'. com. a. Anterior commissure. nl". com. p. Posterior commissure. nl'". com.su. Superior commissure. os par. con't. tis. Cwinective tissue. pig. cp. sng. Blood corpuscles. pre. r. e'Jrm. Ectoderm. en'lh. va. Endothelium of retinal blood- vessels, rtn. eph. vs. Epithelium of the epiphysial sn. sng. vesicle. tct. thl. opt e'th. Epithelium. thl. opt. fa. trm. Lamina terminalis. va. rtn. Ins. I/cns. vnt.* lob. opt. Optic lobes. i'«. m. scu. Scale of the parietal eye. z". Spot marking the position of the parietal organ. External basement mem- brane, [brane. Internal basement mem- Nucleus. Nuclei of zone 2 of retina. Nuclei of zone 4 of retina. Nuclei of zone 5 of retina. Parietal bone. Pigment. Processes radiating from the external basement mem- brane. Retina. Blood sinus. . Roof of the optic thalamus. Optic thalamus. Retinal blood-vessels. Third ventricle of brain. Epiphysial vesicle. Second zone of retina. PLATE I. Fig. 1. Left face of a section through the dorsal wall of the head of Phrynoroma Doiifjlassii in the sagittal plane, and consequently passing through the middle of the parietal organ. Diagrammatic in unimportant details. X 140. " 2. Transverse section through the dorsal wall of the head and middle of the parietal organ of P. coronata. Diagrammatic in unimportant de- tails. X 140. " 3. Section of a small portion of the deep wall of the parietal organ and the enveloping connective-tissue capsule, to show the processes radiating from the external basement membrane. X 1060. " 4. A transverse section of one of the retinal vessels, in which a blood corpuscle is seen. X 1060. Bitter- I^hieialEye- Pl.]. co/t'f./ut. rnn ' /•', './."'/(, e'dn asprir vrtsiuj. Hn. In.^ rHri pia \ rnn'l iis \. os.fkir •srirj Hn (J).STlq C'l fhvn ip sn// :,l ')S^mr. pip. ,,l' rfit. lim'l lis RiTTER. — Parietal Eye ABBREVIATIONS. cav e'phy. Cavity of the epiphysis. mac. opt. cbl. Cerebellum. ceb. Cerebrum. mb. ba. ex. ths. opt. Optic chiasm. d. t. Cells of zone 5 of the retina. mb. ba. i. rl. Ins. Cells of the lens. nl. cog. Coagulum. nV. com. a. Anterior commissure. nl". com. p. Posterior commissure. nl'". com. su. Superior commissure. OS par. con't. tis. Connective tissue. P^9- cp. sng. Blood corpuscles. pre. r. e'drm. Ectoderm. en'th. va. Endothelium of retinal blood- vessels. • rtn. eph- vs. Epithelium of the epiphysial sn. sng. vesicle. tct. thl. opt e'th. Epithelium. thl. opt. la. trm. Lamina terminalis. na. rtn. Ins. Lens. vnt.^ lob. opt. Optic lobes. vs. m. scu. Scale of the parietal eye. z". * Processes secreted from the inner su Spot marking the position of the parietal organ. External basement mem- brane, [brane. Internal basement mem- Nucleus. Nuclei of zone 2 of retina. Nuclei of zone 4 of retina. Nuclei of zone 5 of retina. Parietal bone. Pigment. Processes radiating from the external basement mem- brane. Retina. Blood sinus. Roof of the optic thalamus. Optic thalamus. Retinal blood-vessels. Third ventricle of brain. Epiphysial vesicle. Second zone of retina. PLATE II. Fig. 5. A portion of a section near the median plane, through the same eye as that represented in Figure 1, more highly magnified. X 570. " 6. Sagittal section of the dorsal wall of the head, with the parietal organ of Uta Stansburiana. Diagrammatic in unimportant details. X 012. " 7. External view of the parietal eye and surrounding structures in Uia Stansbunana. X 8. R.'TTER.- RAJtiETAI, EYE. 7&e><^- &0OS,r iso^a^ ~%P ^••:"r (XMt'l h.s rin Pi'J cl Ins icjipi. inMu. ibhai. d. mh.hai coa. % vn.rin d.i. nl. ■' elf. 111. •■ I /' rIn . I'ifl Hn. bVitise'.l'ui.SostoA RiTTER. — Parietal Eye. ABBREVIATIONS. cav. e'phy. Cavity of the epiphysis. mac. opt. cbl. Cerebellum. ceb. Cerebrum. ml. ba. ex. chs. opt. Uptic chiasm. cl. i. Cells of zone 5 of the retina. mb. ba. i. cl. Ins. Cells of the lens. td. cog- Coagulum. nl'. corn, a. Anterior commissure. nl". com. p. Posterior commissure. nl'". com. 8U. Superior commissure. OS par. can't, tis. Connective tissue. pig- cp. sng. Blood corpuscles. pre. r. e'drm. Ectoderm. en'lh. va. Endothelium of retinal blood- vessels. rtn. tph. vs. Epithelium of the epiphysial sn. sng. vesicle. let. thl. opt, e'th. Epithelium. ihl. opt. la. trm. Lamina terminalis. va. rtn. Ins. Lens. vnt.» lob. opt. Optic lobes. vs. m. sat. Scale of the parietal eye. z". PLATE III. Spot marking the position of the parietal organ. External basement mem- brane, [brane. Internal basement mem- Nucleus. Nuclei of zone 2 of retina. Nuclei of zone 4 of retina. Nuclei of zone 5 of retina. Parietal bone. Pigment. Processes radiating from the external basement mem- brane. Retina. Blood sinus. Roof of the optic thalamus. Optic thalamus. Retinal blood-vessels. Third ventricle of brain. Epiphysial vesicle. Second zone of retina. Fig. 8. Left face of a sagittal section through a portion of the epiphysis, a short distance aliove its connection with the brain in P. Douglassii. It is in part diagrammatic, though the outlines of the figure as a whole, and of most of tlie foldings of the epithelium, were drawn with the camera. From the same individual as Figure 1. X 312. " 9. Similar view of a sagittal section from the same individual, to show the relation of the epiphysis to the brain and the blood sinus. X 30. RlTTER- PaRIET'SJ^EYE. s. r tJi fvii'i.lis. ..CpAny ■'^'J cell ^'■■' MM.opt. '-- coma /a /mi c/is oil/ 171/ '■ lob.opt ci.l. RiTTBR. — Parietal Eye. ABBREVIATIONS. cav. e'phy. Cavity of the epipliysis. cbt. Cerebellum. ceb. Cerebrum. chs. opt. Optic chiasm. cl. I. Cells of zone 5 of the retina. cl. Ins. Cells of the lens. COIJ. Coagulum. com. a. Anterior commissure. com. p. Posterior commissure. com. su. Superior commissure. con't. tis. Connective tissue. cp. sng. Blood corpuscles. e'drm. Ectoderm. en'th. va. Endothelium of retinal blood- vessels. eph. vs. Epithelium of the epiphysial vesicle. e'th. Epithelium, la. trm. T