Paper - The development of the telencephalon in sphenodon punctatum
The Development of the Telencephalon in Sphenodon Punctatum
Hull Laboratory of Anatomy of the University; of Chicago, and the Labomtary of Zoology, Kings College, London
In a study of the brains of several reptiles and primitive mammals at University College, London, the attention of the writer was directed by Prof. Elliot Smith to the excellent collection of Sphenodon embryos belonging to Prof. Arthur Dendy of Kings College. This study was intended to be merely a preparation for the analysis of the ventro-lateral sector of the developing telencephalon in man. However, in the progress of that work several unrecorded facts were noted. These facts not only have a direct bearing upon the question of immediate interest to the writer, the method of telencephalic evagination, but also upon the development of the complex group of nuclei in man known as the corpus striatum. This study is, then, an anticipation of that partially completed for man.
To Prof. Elliot Smith for the introduction to this material, and for his kindness and interest in the progress of this study; to Prof. Arthur Dendy for the privilege of studying his unique collection in his laboratory and for his thoughtfulness while there and for the kindness of‘ other members of his staﬁ, the author is indeed thankful. To Prof. C. Judson Herrick, for his excellent criticism and advice While this study was in preparation, the author expresses a just indebtedness. To Professor Dendy’s, a excellent assistants, for the photographs from which the drawings; were made; to her own assistant, Sara W. Kiser, for aid in making the Wax models, and ﬁnally to Mr. Toda for the wash drawings, the writer is full of appreciation.
In 1899 Dendy gave an excellent outline of the general development of Sphenodon punctatum and in that study divided the embryos of his collection into groups which he called Stages A, B, etc. In several published articles (’O2, ’09, ’19) Elliot Smith has referred to particular portions of the telencephalon of the young and adult Sphenoda but in no place has he recorded a study of its development and completed structure. Gisi (’07) has given the most exhaustive review of the brain of the adult Hatteria. In this dissertation she has presented in an excellent manner a study of the external form, the relations of brain to skull, the ﬂexures, but has described only in part the morphology of the telencephalon.
There are also many pieces of excellent work upon the forebrain of reptiles. The initial investigations upon the forebrain are found in the classical studies of Edinger (’96), Meyer (’97) and C. L. Herrick (’90). These were followed some years later by two papers of De Lange (’11 and ’13), based principally upon Varanus and one by Unger (’ 11) upon the brain of Gecko. More recently in America there have appeared the analysis of the telencephalon of turtles by Johnston (’15, ’16) and of the alligator by Crosby (’17). However, not one of these studies, although approaching in a few details the scope of presentation intended in this paper, has dealt exclusively with the histological development of the telencephalon, followed by a description of its adult structure. The signiﬁcance of the ﬁrst part of the study may be further enhanced by a comparison with the development of homologous structures in man, where it is possible to know practically the ‘ﬁnal destiny of certain portions of this telencephalon.
Prof. Arthur Dendy of Kings College, London, put his unique and precious collection of developing Sphenodon into the author’s hands for study. Through this courtesy, the author has the privilege of making this small contribution to the study of forebrain morphology. This material had been carefully sectioned in series, 10 (1 in thickness and stained with hematoxylin and a counterstain. With the exception of a few of the older embryos, where the brain was sacriﬁced for the sake of the skull, the material is in excellent condition. These embryos are the ones which Dendy (’99) grouped in stages. To this paper the writer has had constant recourse. Some of these brains were drawn at a magniﬁcation of 44 or 33 with a camera lucida in London and wax models made of them in Chicago. Consequently the author has had the aid of the third dimension in the subsequent description. The material is arranged in tabular form for sake of clearness.
TABLE 1 Plane of Section in Stage (Dendy) No. (Dendy) Reference to Forebrain Modelled N 96 Sagittal N 14a Transverse + OP 35a Horizontal + P 3921 Transverse + P 45a Obliquely horizontal -— PQ 50a Transverse + PQ 51a Transverse —Q 52a Sagittal + R 141 I Transverse + R 144 Transverse ~ R 142 Sagittal ~ RS 1 Transverse S H Sagittal + S 9a Transverse — Adult I Transverse + Adult V Sagittal —
In studying the forebrain of the adult, the writer experienced a great deal of difficulty in tracing the ﬁber systems, because the sections were not electively stained. However, they were studied with high power and their existence duly recorded. Inasmuch as no Cajal, Golgi, or Weigert treated material was available, the author knows well that this report is incomplete; but she hopes that it will prove suﬂicient stimulus to those for Whom this material is more readily available to extend this description in a way worthy of the interest which this brain holds for students of comparative neurology.
DESCRIPTION Embryo, Stage N, No. 14a (ﬁgs. 1, 29, 30 and 31)
The development of the telencephalon of this embryo resembles point for point that of the 11.8 mm. no. 1121 human embryo belonging to the Mall collection described by the writer (’22). The telencephalic evagination is divided into two halves in its rostral part by a slight depression and in its caudal extension by a small hillock or outpouching of the midline tissue. The former is the region of the lamina terminalis and the latter of the paraphysis (ﬁg. 1, tel. r. pl., and ﬁgs. 30 and 31, par. ar.). This elevation is made up entirely of ependymal tissue. This type of midline differentiation is continuous by way of the velum transversum with the roofplate of the diencephalon. Immediately contiguous ‘laterally with non-nervous ependymal tissue in the telencephalon, running dorsally over the roof is a small portion of the massive forebrain wall which is differentiated from the remainder of that tissue by an outer clear zone and ‘an inner cellular line, the matrix layer. This region is the primitive hippocampus (fig. 1, prim. hip.) and resembles exactly that same area described by the writer for man (Hines, ’22, ﬁgs. 22, 23, 24, 25, man. l. and prim. hip. pp. 136 and 137).
Embryo, Stage P, N 0. 39a (ﬁgs. 2, 32, 33, 34)
The telencephalic vesicle of embryo, number 39a (ﬁg. 2) is easily divided into the four sectors after the manner of Herrick’s (’10) description and corresponds exceptionally closely to the development of the human forebrain afforded by the 14 mm. in the Mall collection (no. 940, Hines, ’22). Both of these developing telencephala resemble that of the adult frog in that their walls are divisible into morphologically distinct sectors. The sectors, however, are not exactly equivalent with those of the frog, for the dorso-lateral contains the general pallium and the ventro—lateral contains both pyriform lobe and corpus striatum. These sectors are, brieﬂy! the dorso-medial, which contains the primitive hippocampus (prim. hip.); the ventro-medial, separated from the dorso-medial by the sulcus limitans hippocampi (sul. lim. hip.), the septum; the dorso—lateral, which is a very small portion of the arc, whose ventral limit is seen best in ﬁgure 3 as the ventricular groove, the sulcus superstriaticus (sul. superstr, ﬁg. 3), the general pallium and part of the pyriforrn; the ventro— lateral, separated from the Ventro—medial by the angulus ventralis (ang. vent), the striatum. The striatum is divided into two portions by a shallow Ventricular sulcus, the sulcus interstriaticus (sul. z'nterstr.). The dorsal portion, the neostriatum, corresponds in part at least to Elliot Smith’s hypopallium, Johnston’s dorsalventricular ridge, Crosby’s dorso-lateral area and Kappers’ neostriatum. Ventral to the sulcus interstriaticus is the paleostriatum—a portion of the forebrain upon which investigators agree more or less. Because the dorsal ventricular ridge behaves in early development of the telencephalon of Sphenodon as it does in that of man; and in man actually becomes a part of the nucleus caudatus, the Writer has thought it less confusing to use Kappers’ term, neostriatum. At the present, however, its origin will not be considered.
The thickness of the telencephalic wall does not vary greatly in these four portions of the vesicle as in later stages, although it is noticeable that the septum and the paleostriatum are thicker. Here, however, the paleostriatum is more prominent in its extent and thickness than any other portion of the lateral Wall. The primitive hippocampus has the characteristic mantle layer, while within the pallial wall two layers, the matrix and outer cellular layer, are visible. In the ventro-lateral wall opposite the sulcus interstriaticus lies an area almost devoid of cells. This area anticipates the ingrowth of the numerous thalamic-striatal ﬁber systems. Immediately lateral to this is a layer of cells which in man is the lower limit of the pyriform lobe. This appears to be either pyriform or nucleus of the lateral olfactory tract.
In the outer margin of the septum appears a group of cells which the writer has called nucleus medialis septi for man, following Herrick’s (’10) nomenclature for the amphibian forebrain.
In ﬁgure 33 the median sagittal-View of this forebrain, the various structures of the telencephalon medium as outlined for the 19.1 mm. human embryo, number H 173 of the Chicago collection (Hines, ’22), can be seen. Beginning at the recess in front of the optic chiasma (rec. preop.), the following structures may be identiﬁed: the ventral massive portion of the lamina terminalis, pars crassa (p. c.); the dorsal slender division of the same, the pars tennis (3). t.); the angulus terminalis (any. term.), a small portion of the roof, from which no lateral plexus invagi— nates, the tela chorioidea telencephali _medii (tel. ch. tel. med.); the paraphysis (par. ar.); and the velum transversum (vel. trans.). Extending rostrally above the anterior and posterior limits of the paraphyseal arch (par. ar.) is the paraphyseal pouch (p. 39.), while ventral and lateral to this rnidline structure, the lateral choroid plexus (pl. ch. 2;. lat.) is visible. The roof of the third ventricle extends forward over the paraphyseal pouch and, although not markedly pleXus—like in morphology, is ependymal in structure. At its most caudal limit the pineal evagination is attached. In the floor, beginning at the recessus preopticus the following typical hypothalamic structures are found, the optic chiasma, the tuber cinereum, recessus infuridibuli, and the recessus mamillaris.
In ﬁgure 34, two ventricular ridges in the lateral wall of the hemisphere may be seen, the small, inconspicuous neostriatum, separated from the ventral, larger paleostriatum by the sulcus interstriaticus.
Embryos, Stage PQ, Nos. 50a and 51a (ﬁgs. 3, 4, 5 and 6)
The four ﬁgures taken for illustration of the development of the telencephalon in Dendy’s Stage PQ show at a glance that the cellular differentiation of the various sectors has progressed further than that seen in the preceding forebrains. The dorso— medial area or the primitive hippocampus (prim. hip.) lies now almost completely upon the medial wall with its outer layer distinctly cell free. The ventro-medial sector is unusually thickened, laterally, and contains the nucleus medialis septi (nuc. med. sept., ﬁgs. 3 to 5). Taking the sulcus superstriaticus (sul. superstr., ﬁgs. 3 to 6) for the ventral limit of the dorsolateral sector and the beginning of the cellular outer layer as its medial boundary, it is clear that this portion of the forebrain evagination is minute. This is the region which contains the dorsal portion of the pyriform, if Elliot Smith (’19) is correct, and whatever of general pallium Sphenodon possesses.
Lying between the angulus Ventralis (any. vent, ﬁgs. 3 to 5) and the sulcus superstriaticus is the enormous ventro-lateral area, the region of the striatum. That division of the striatum which lies between the sulcus interstriaticus and the sulcus superstriaticus, the neostriatum, is thicker and more massive than its development as seen in Stage P, number 39a. This enlargement of the dorsal ventricular ridge, the neostriatum, is very evident in the wax model of embryo number 50a, and extends rostrally as far as the enlargement of the vesicle itself.
The division of the telencephalic wall into three cellular layers is most evident in the neostriatum. Here the writer Wishes to point out that of these three, the middle layer is the thickest. The outer cortical layer is quite distinct from either the outer mantle layer or inner matrix. Further, the middle layer (mid. Z., ﬁgs. 3 and 4) of the developing neostriatum is continuous around the sulcus superstriaticus with the middle or intermediate cell layer of the small dorso-lateral sector. In a like manner, the continuity of the mantle layer or marginal zone of the pallium with that of the neostriatum (man. l., ﬁgs. 3 and 4) and the matrix of the pallium with that of the neostriatum may be followed around the superstriatal sulcus (mat. ﬁgs. 3 and 4). The ventral limit of what appears to be an outer cortical layer (man. Z.) is the space in the neostriatal wall into which the thalamic striatal ﬁber systems will ‘extend. This peculiar arrangement is not only characteristic of this portion of the forebrain wall in this embryo, but also in the development of the homologous region of the human (ﬁgs. 29 to 32, cor. str. lat., Hines, ’22).
In ﬁgure 6, a section through the foramen interventriculare, the typical arrangement of the invagination of ependymal tissue joining the lateral limb of the paraphyseal arch (p. a.) forms the plexus chorioideus ventriculi lateralis. Dorsal to the paraphyseal arch lies the cross section of the forward extension of the paraphyseal pouch, while ab.ove the latter is the balloon-like ependymal dorsal sac of the roof of the third ventricle. In this embryo, the pouch formation is similar to that found in H 163, the 39.1 mm. embryo of the Chicago collection, although the development of the lateral plexuses is that of a much younger embryo, the 16 min, no. 465 of the same collection. There is at no time in human development a real correspondence to the dorsal sac, although the postvelar tubules of the 40 mm. (H 163) resemble it.
Embryo, Stage Q, No. 52a (ﬁg. 7)
The development of Stage Q as compared with Stage PQ is marked by three changes, namely: a further evagination of the olfactory bulb, the hypertrophy of the :neostriatum, and the further differentiation of the three cellular layers in the dorso— lateral area and the dorsal area of the neostriatum. In ﬁgure 7 (a sagittal section) the mantle layer (man. I.) extends from the outer ventral angle of the neostriatum up and over the length of the telencephalic evagination to the hippocampal cortex, caudomedially. The middle layer (mid. Z.) is characteristic of the dorso~ lateral sector, and of the neostriatum, being continuous around the sulcus superstriaticus. In the neostriatum, this layer lies next the matrix.
The mantle layer is less distinct as a deﬁnite layer of cells in the extreme ventral portion of the section in question. The massive part of the neostriatum is formed by cells without deﬁnite arrangement lying ventral to the middle layer. Between the latter and this irregular mass is a cell-free region, the ﬁrst appearance of the thalamo-striatal ﬁber system. This system is more prominent in the great cell-free area lying deep in the wall of the ﬂattened paleostriatum. There are two other ﬁber systems which have appeared in the forebrain at this age of development, namely the fornix, in the cell—free zone of the primitive hippocampus (prim. hip.) and the lateral olfactory tract, at the ventral edge of the pyriform lobe.
Embryos, Stage R, Nos. 141, 142 and 144 (ﬁgs. 8 to 13)
The telencephalic wall has become markedly thickened, so that the lateral ventricle is almost negligible. In fact the forebrain resembles more closely than in a11y other stage the appearance of the adult. "There is nothing here to remind the writer of any stage in the development of the "human brain. Nevertheless the four sectors of the previous description are easily recognized. The dorso-medial sector (ﬁg. 9), now relatively small, is the hippocampus, in which a narrow matrix and a broad cortical layer, the mantle layer, are evident. The dorso-lateral area "is now comparatively insigniﬁcant when compared to the enormous ventrolateral area. The mantle layer of the former is continuous with that same layer in the hippocampus and in the pyriform. Lying in the brain wall near the sulcus superstriat-icus is the short middle layer which is continued as a layer of cells close to the matrix of the neostriatum. This middle layer is not found in the paleostriatum, although the outer or what has been called the mantle layer is seen in its dorso-lateral portion. However, it appears that this layer curves above the angulus ventralis next the matrix, forming the nucleus accumbens of Kappers (nuc. ac., ﬁgs. 9 and 12). Opposite this nucleus in the extreme ventral part of the wall is a row of cells, the tuberculum olfactorium (tub. olf., ﬁg. 9). Both these nuclei encroach upon the septum proper, which, now much thickened, contains two nuclei, a medial and lateral, the nucleus medialis septi (nuc. med. sept., ﬁgs. 9 and 12) and the nucleus lateralis septi (nuc. lat. sept., ﬁgs. 9 and 12), respectively. T
The clarity of separation between the various layers as they appear in the developing neostriatum in this brain becomes more or less confusedyin the caudal levels of the telencephalon. In both ﬁgures 10 and 11 there seems to be but one layer which at the sulcus superstriaticus divides into two, a lateral or pyriform lobe complex and a medial or neostriatal layer. In the extreme ventral extent of the paleostriatum is a triangular nucleus, which in its morphological relationships resembles the nucleus ventro— medialis (nuc. vent. med., ﬁgs. 10, 11, and 13) described by Crosby in the alligator.
The septum ependymale has thickened medio-laterally, as well as lengthened anterio-posteriorly. These two enlarged regions are joined dorsal to the foramen interventriculare by the telachorioidea telencephali medii. Within the tissue itself two nuclei mentioned above are found, the medial and lateral nuclei of the septum. This condition may be seen in the stage of development reached by a 60 mm. human embryo. However, when the same region is examined at the level of the commissura pallii posterior, two triangular nuclei appear lateral and ventral to the commissura, the nuclei of the commissura aberrans (nus. c. ab., ﬁg. 11).
In the wax model of the forebrain of this embryo, the preoptic recess has increased in depth; the pars crassa of the lamina terminalis has encroached upon the pars tenuis; the paraphyseal pouch has elongated dorsally, while the tela chorioidea telencephali medii remains unchanged. This development resembles that outlined for these same structures in man (Hines, ’22).
The approximation of the middle and mantle layers in the brain wall about the sulcus superstriaticus maybe easily demonstrated in ﬁgure 8 taken from a sagittal series. The other features are similar to the descriptions for number 141. In ﬁgure 12 from embryo number 144, two new circular nuclei besides the nuclei medialis et lateralis septi lie in the septum. The more ventromedial of the two lies dorsal to the commissura hippocampi and may be seen in the same position in a 60 mm. human embryo; the dorsal-lateral one, lying between the medial and lateral nucleus, may also be seen in the same human embryo. In ﬁgure 13, the nucleus ventro-medialis extends dorsomedially to the sulcus interstriaticus, which as a group of cells may be seen about that sulcus, in the more rostral levels of this stage of development of the telencephalon. This nucleus may be said to have two limbs, a dorso-medial, which lies along the interstriatal sulcus and a ventro-medial, found in the extreme ventral angle of the forebrain vesicle. These two limbs become conﬂuent in the caudal levels of the telencephalon.
Embryo, Stage RS, N 0. Sit (ﬁg. 14)
The development of the telencephalon of embryo number Sii resembles in all points that of the adult, except that the neostriatal ridge is not as large as it is in the adult. When the dorsal wall of the forebrain vesicle is lifted off two huge ridges appear; the lateral neostriatum, which extends from the anterior portion of the olfactory stalk to the most ventrolateral expansion of the telencephalon, and the medial septum. The latter is conﬁned to the medial wall only and does not swing lateral in the vesicle with the hippocampal formation whose most posterior end almost meets that of the pyriform, encircling the ventro—lateral-posterior tip of the neostriatum. If the neostriatal hillock is pushed aside, a small ridge, the paleostriatum, may be seen, making its way, from about the posterior third of the neostriatal formation, anteriorly into the basal portion of the olfactory stalk.
The one ﬁgure of this brain (ﬁg. 14) is cut sagittally through the vesicle in such a manner that the greater part of the ridge shown is neostriatal. The anterior portion_ of the neostriatum contains the deﬁnite stratiﬁed arrangement of cells; in the posterior part the same cells are more diffuse. Ventral and posterior to the sulcus interstriaticus is a nucleus, which by its position seems to be the lateral anterior limb of the Ventr0medial nucleus of Crosby (’17). Immediately ventral to the hippocampal cortex, lying among the ﬁbers of the fornix are a few cells, the remnant of the primordium hippocampi. Arching from the pyriform lobe, anteriorly, to the hippocampus, posteriorly, is the irregular cortical layer, the mantle layer of the general pallium.
Summary of the Development of the Telencephalon of Sphenodon Punctatum
The telencephalic evagination of the earliest stage presented resembles that of a 11.8 mm. human embryo. The two halves of the forebrain are joined in the midline, anteriorly by a comparatively slender piece of septum ependymale, posteriorly by a very thin ependymal area in the form of an arch known as the paraphyseal arch. In the dorso-medial portion of the wall of the incipient hemisphere the primitive hippocampus is distinguished from the rest of the vesicle by a relatively clear cell-free outer layer.
The primitive hippocampus characterized by this same peculiarity of cell arrangement is found in the dorso-medial sector.
Here it remains, until there appears within it a deﬁnite layer of cortex in Stage R. At this time there is slight indentation or trough in the medial wall below this formation, a structure homologous to the ﬁssura hippocampi.
Very early the dorso-lateral sector (i.e., lying between the hippocampus and the sulcus superstriaticus) shows two deﬁnite cortical layers and then three in Stage Q. These three become more or less fused in such a manner that only one may be deﬁnitely distinguished from the matrix. This region contains whatever general pallium the Sphenodon possesses.
The ventro-medial sector, or the septum, follows closely the sequence of development outlined for man (Hines, ’22). It increases in its ventro-lateral measurement as well as in its antero-posterior extent. This development seems to take place within the septum ependymale itself, in such a manner that the tissue thus distinguished decreases while the lamina terminalis increases. At ﬁrst there are two layers of cells, a matrix and an outer group- Then, those of the outer group arrange themselves in the form of a nucleus, the nucleus medialis septi, while between this nucleus and the matrix lies the nucleus lateralis septi. Besides these two large groups of cells, the small circular ones appear in Stage R. The writer does not know what they are, although she has seen them in a 60 mm. human embryo. A row of cells about the angulus terminalis appears in Stage PQ, the nucleus accumbens of Kappers. In the most ventral part of the brain wall, lies a layer of deeply staining cells, the tuberculum olfactorium. This structure is prominent in Stage R.
The development of the ventro-lateral sector, although following in the main the same sequence of developmental events as seen in man, is most characteristic for this animal. The lateral wall of the ventricle shows early an extreme ventral hillock, the paleostriatum, separated from a more dorsal but at this stage (P) minute ridge, the neostriatum, by the sulcus interstriaticus. This ridge in the forebrain wall of Stage PQ is separated from the overlying pallium by a ventricular groove, the sulcus superstriaticus. The neostriatum grows larger until it almost fills the cavity of the ventricle. During this process the middle cellular layer of the telencephalon becomes more prominent. About the sulcus superstriaticus, now much deepened, this layer of cells seems to be continuous with the outer or mantle layer. These were distinct until the Stage R. Up to that time in development the mantle layer in the lateral part of the ventro-lateral area formed the cortex of the pyriform lobe whose lower extent was the nucleus of the lateral olfactory tract. These structures remain more or less separate from the nuclei which compose the neostriatum in the rostral half of the forebrain evagination and become markedly compressed in the caudal half. There is change from a regular arrangement to an irregular one in this middle layer of the posterior third of the neostriatum, which shows itself in Stage S for the ﬁrst time.
In each of these sectors by the time Stage S of development is reached the nervous structure resembles that of the adult.
THE TELENCEPHALON OF THE ADULT (figs; 15 to 25)
The cell masses in the forebrain
The olfactory centers. The hippocampus contains one cortical layer, the derivative of the outermost of the three layers which appear in the embryo. The middle layer or the nuclear layer of the mantle zone is not only negligible in the adult, but was at no time an important layer of Sphenodon’s pallium. Pyramidal and granular cells are both present. The primordium hippocampi consists of a few scattered cells lying ventral to the hippocampal formation in the medial Wall. It may be separated from the latter by a plane passing through a ventricular groove, the sulcus limitans hippocampi (sul. lim. hip., ﬁgs. 15 to 24) perpendicular to the outer contour of the medial wall. In all the ﬁgures with one exception (i.e., ﬁg. 18) this tissue occupies an appreciable amount of the medial wall. In the more caudal levels it is particularly large and contains the nuclei of the hippocampal cornmissure and forms the bed for the commissura aberrans. The small notch in the medial wall, more or less characteristic of this region, lying immediately ventral to the hippocampal cortex is the ﬁssura hippocampi. ‘
The septum (Sept, ﬁgs. 15 to 20) contains two nuclei, the nucleus medialis septi (nuc. med. scpt.) and the nucleus lateralis septi (mac. lat‘. sept.) poorly deﬁned, when compared to their diagrammatic outlines in the developing Sphenodon, and traces of the nucleus accumbens of Kappers (nuc. ac., ﬁgs. 17 and 18) and the nucleus of the commissura hippocampi (nuc. hip. com., ﬁgs. 19 and 20). The cortex of the tuberculum olfactorium (tub. 0lf., ﬁgs. 16 to 18) is present as an ill-deﬁned layer of cells, not at all like the beautifully marked nucleus to be found in Stage R, number 141 (ﬁg. 11).
The striatal complex. The paleostriatum (paleostr., ﬁgs. 16 to 21) is the slight ventral elevation which lies between the angulus ventralis (ang. vent, ﬁgs. 16 to 21) and the sulcus interstriaticus (sul. int_erstr., ﬁgs. 16 to 21). In the region of the olfactory stalk the ridge fuses with that of the septum and both come to form the ventral sector of the evagination. Caudally the ridge becomes relatively unimportant, so that approximately within two—thirds of the total distance from the base of the olfactory bulb to the tip of the posterior pole of the hemisphere this ridge has disappeared.
It is composed of scattered cells without deﬁnite nuclear groupings. In its rostral extent a few cells curve about the angulus ventralis next the diminutive layer of matrix, the nucleus accumbens of Kappers. As a nucleus it is not as prominent as it is in Stage R. In its more caudal position another nucleus curves about its dorsal limiting groove, the sulcus interstriaticus. This nucleus appears to be a part of the amygdaloid complex.
The neostriatum (neostn, ﬁgs. 15 to 25) forms the major portion of the striatum complex and extends as a part of the dorso-lateral wall of the olfactory stalk to‘ the most caudal limit of the telencephalic evagination. It ﬁlls the lateral ventricle almost completely, and protrudes medially far above the paleostriatum. It is composed of three layers of cells, the inner and much thinned matrix (mat, ﬁg. 21), a middle layer (maid. l., ﬁg. 21) which appears to be continuous with mantle layer of the pallium proper, and an outer scattered group of cells. The latter appear to be of the same variety of cell types as those‘ which compose the paleostriatum. In the rostral extent of this ridge, that is, anterior to the levels of the di—telencephalic union, the middle layer of this area appears to correspond to the intermediate cell layer of the mantle zone of the cortical layer of the pallium. In its caudal extent, however, this same neostriatal layer appears to be in direct continuity with the cortical layer of the pallium. This layer forms a distinct band or row of cells only in its central region. Both anteriorly and posteriorly, it is broken into groups which, although they show the same dark blue staining in hematoxylin and bear the same relation to the ventricular contour of the neostriatum as the central portion does, nevertheless cannot be considered a cortical layer by any stretch of the imagination, as the former have been described.
The pyriform lobe. The pyriform lobe (pyr. l., ﬁgs. 15 to 25) and lateral olfactory nucleus (nuc. olf. lat., ﬁgs. 16 to 22) occupy the most lateral border of the neostriatal complex. In the anterior levels (ﬁg. 15) it is diflicult to differentiate the two, since they appear as a group of cells lateral to the neostriatum. Slightly posterior (ﬁg. 16) these cells may be divided into a dorsal layer which is a continuation of the pallial layer and a ventral nucleus, whose ventral limit may be ﬁxed by passing a plane through the sulcus interstriaticus and the dorsal extent of the lateral olfactory tract. This division into two parts is particularly diagrammatic in the right hemisphere in ﬁgure 17. In the region of the massive part of the di—telencephalic union, the lateral olfactory nucleus, as a small insigniﬁcant group of cells just dorsal to the lateral olfactory tract, is most marked (nuc. olf. lat., ﬁgs. 19, 21 and 22). Moreover at these levels the pyriform lobe, if it has a cortical structure at all, must lie in that part of the telencephalic layer which is continuous with the main layer of the pallium dorsally and the middle layer of the neostriatum ventrally. There is no method for adequate differentiation between the pyriform and the nucleus olfactorius lateralis in these levels (ﬁgs. 23 to 25) other than the obvious fact that the more ventral part of the formation is probably nucleus.
In the extreme ventro-lateral corner of posterior portion of the telencephalic evagination a triangular nucleus is found. It resembles in position and certain ﬁber connections the nucleus described by Crosby (’17) in the alligator as the ventro-medial nucleus (nuc. vent. med., ﬁgs. 22 to 24). It is in all probability a part of the amygdaloid complex.
The ﬁber systems
The minute and accurate description of the origin and termination of the ﬁber systems of Sphenodon punctatum must be the privilege of another. The present writer may only describe the position and relationship to cell masses of those ﬁbers, prominent enough‘ to show themselves where the brain as a whole is stained with hematoxylin. Therefore the following must be considered tentative only. However, it offers in a few instances a classiﬁcation of certain regions into divisions, whose homology with those in other reptiles has a slight bearing upon some of the more important questions of forebrain morphology, and this, the writer thinks, is reason sufficient for the incomplete discussion which follows.
The nomenclature used in this description is that nowgenerally accepted for man and corresponds therefore to that used in Miss Crosby’s excellent memoir on the forebrain of Alligator mississippiensis.
The olfactory tracts. The tractus olfactorius medialis (tr. olf. med., ﬁg. 1:5), probably Gisi’s tractus bulbo-epistriaticus, is found in the medial wall of the bulb and stalk. Some of its ﬁbers turn into the region of the tuberculum olfactorium. No tractus olfactorius intermedius can be distinguished here. ..
Within the lateral wall of the bulb and stalk the ﬁbers of the lateral olfactory tract (tr. olf. lat., ﬁgs. 16 to 23) pass to the nucleus olfactorius lateralis, the pyriform lobe and possibly to the nucleus ventro-medialis (nuc. vent. med., ﬁgs. 22 to 25) of Crosby.
In ﬁgures 17 and 18, the diagonal ban.d of Broca (dtag. b. Br.) lies on the surface of the tuberculum olfactorium and joins the medial olfactory area with the lateral.
From the region of the base of the olfactory stalk to the posterior part of the neostriatum, a minute ﬁber system extends, the tractus bulbo-epistriaticus (tr. b. eptstr., ﬁgs. 19 and 27). This is the system of that name described by Gisi‘, Edinger and others.
The tractus parolfacto—corticalis (tr. parolf. cort., ﬁgs. 17 to 19) joins the medial nucleus of the septum to the hippocampus. With these ﬁbers are those of the tractus tuberculo-corticalis (tr. tub. cort., ﬁg. 17). From the ventricular surface of the hippocampus two ﬁber systems pass either to the nucleus lateralis septi, the tractus cortico-parolfactorius (tr. cart. parolf., ﬁgs. 18 to 20), or to the hypothalamus joining the tractus parolfacto-medialis, the fornix.
The tangential ﬁbers (tang. f., ﬁgs. 19 to 24) lie along the outer margin of the pallium fraying out from the hippocampus toward the pyriform lobe. On the ventricular surface these regions are connected by the alveus (alv., ﬁgs. 17 to 25).
The commissures are three in number. In the anterior, Gisi describes a middle portion containing ﬁbers arising in the ganglion basale of Cajal (the epistriatal division here), and a few unmyelinated ﬁbers and a ventral part, the olfactory division. The commissura hippocampi (c. hip, ﬁgs. 19 and 20) is perhaps Gisi’s commissura fornricis. The commissura aberrans (c. ab., ﬁgs. 22 and 23) was identiﬁed ﬁrst by Elliot Smith and described again by Gisi.
The stria medullaris includes all the Various systems of ﬁber tracts which connect the secondary and tertiary olfactory centers with the habenula.
a. The cortico—habenular system is a part of the fornix.
b. The tractus olfacto-habenularis of Crosby connecting the ventro-medial nucleus with the habenula.
c. A system of ﬁbers joining the pyriform lobe and the nucleus olfactorius lateralis with the habenula.
The olfactory projection tract (fig. 28) originates in the ventro— medial nucleus, the ventral part of the nucleus olfactorius lateralis and possibly the pyriform lobe, passes over the forebrain bundles into the hypothalamus. No ﬁbers could be traced to the corpus mamillare.
The medial forebrain bundle, described also by Gisi, is seen ﬁrst in the rostral part of this brain lying in the midst of the septum. It passes through the thalamus Ventral to the nucleus medialis, into the hypothalamus and the midbrain, the tractus olfacto—hypothalamicus and tractus olfacto-peduncularis. The lateral forebrain bundle contains both ascending and descending ﬁbers between the thalamus and striatum. A part of this system connects the nucleus medialis thalami with the core nucleus of the anterior part of the neostriatum. Some of these ﬁbers pass ventrally into the hypothalamus, the tractus strio-hypothalamicus.
The telencephalon medium and adjacent structures
The sequence of growth and differentiation of the telencephalon medium in Sphenodon is Very similar to that of man. The ventral and thicker portion of the lamina terminalis grows at the expense of the thinner, dorsal part. The paraphyseal arch is quite long in the younger stages; its anterior and posterior limbs approximate each other as the embryo growsfrom Stage P to Stage R. The lateral invaginations form the thin ependymal tissue lying in the lateral wall between the lateral limb of the paraphyseal arch and the sulcus limitans hippocampi; the lateral choroid plexus invaginations take place in a manner similar to that of man, except that they are neither so well developed nor so prominent in the ventricle at any time in Sphenodon as in the young telencephalon of man. However, the development of the paraphysis is further advanced in Stage P than it is in the 40 mm. human embryo. A large and signiﬁcant pouch may be seen in Stage P. The development of the lamina terminalis as a bed for the anterior and hippocampal commissures in the adult is that which the 60 mm. human embryo has reached.
The medial subcorttcal centers
The septum increases its size by growth medio-laterally and rostro-caudally. As this growth continues the ﬁrst change is found in the appearance of a deﬁnite line of cells in the outer layer, the nucleus medialis septi. Then follows, the proliferation of the matrix from which the lateral nucleus is developed. This process proceeds dorso-caudally at the expense of the thin septum ependymale, therefore constantly reducing it. .In Stage R, besides these nuclei two others may be seen, which taken with the marked appearance of the nucleus accumbens of Kappers make this stage in Sphenodon’s development resemble that of the same tissue in the 60 mm. human embryo. The nucleus just dorsal to the commissura hippocampi persists in the adult. The writer has been unable to identify the second circular one in the adult. All of these septal nuclei are more marked in Stage R than they are in the adult.
All of these structures are found in the medial wall ventral to a plane which might be passed through that wall along the sulcus limitans hippocampi, at right angles to it. Not only do the septal nuclei lie below this plane but also that rather large mass of tissue in which the fornix ﬁbers are embedded and through which the commissura aberrans passes. This might be considered the caudal continuation of the septum and has been called by Johnston the primordium hippocampi. However, if. the tissue dorsal to this plane be examined, the author ﬁnds that there are scattered cells which in no way have any relation to the cortical layer of the hippocampus. This is also primordium hippocampi or that part of the medial wall dorsal to the sulcus limitans hippocampi, in which cortical structure never develops. This is alsotrue for this region in the development of the human telencephalon. The amount of this tissue varies slightly in the more caudal levels. It is the tissue through which the ﬁmbria ﬁbers pass from the hippocampus to the bed of the fornix. Within this region in the human embryo the fascia dentata develops.
This tissue begins at a small easily delineated area in the dorsal roof of the telencephalic evagination. It is distinguished from the remainder of the hemisphere wall by a narrow matrix and a cell-free marginal velum. This diﬁerentiation is seen next in the dorso-medial sector. It is not until Stage Q, that a few cells appear in the otherwise cell-free layer. It is found in‘ the following embryos in the same position and arrangement as in the adult, a. real cortical layer lying in the middle of the dorso-medial wall. Only this one layer of cells differentiates out of the matrix in this animal. In the later stages of development of the hippocampus in man (that is 40 mm. to 60 mm.) two cortical layers appear, an outer and an inner. This latter layer is never seen in the dorso-medial wall at any stage in its development in Sphenodon.
There is immediately anterior to the level of the lamina terminalis the beginning of a groove which is continued up over the medial wall. This groove is not a total infolding of the whole wall, although the pallial layer lies in the dorsal lip, while its ventral lip borders the primordium hippocampi. The Writer has shown for man, as Levi has for the mouse, that the fascia dentata develops in the ventral lip of a similar groove in the primordium hippocampi, and therefore calls the groove ﬁssure hippocampi. The groove in Sphenodon is probably homologous.
The general pallium
The general pallium can be deﬁned as the region in the telencephalic wall which receives ﬁbers from the thalamus, the thalamocortical radiations. Quite possibly the larger pyramidal cells lying between the pyriform laterally and the hippocampus medially, form a part of the general pallium. However, nothing deﬁnite can be affirmed concerning the vault of the hemisphere until ﬁbers in a silver impregnated brain can be followed. It is folly to discuss whether it be subiculum or neopallium until we know deﬁnitely the ﬁbrous relationships.
The pyriform lobe
The pyriform lobe may be seen ﬁrst in Stage P where it appears as a row of cells limited ventrally and medially by an open space, in which the thalamo-striatal ﬁbers will soon appear. Following its differentiation as a distinct region, a small bulge appears in the lateral wall, delineated ventrally by what may be called the endorhinal ﬁssure and dorsally the rhinal ﬁssure. Immediately dorsal to the former a narrow band of ﬁbers appears, the tractus olfactorius lateralis. Rostrally, this tissue is separated from the outer layer of the developing cortex in the dorso-medial direction. If it be true that the lateral olfactory tract is limited as previously described, and the dorsal indentation in the lateral wall is the rhinal ﬁssure then the pyriform lobe lies opposite the developing neostriatum. Furthermore if the middle cortical layer is characteristic of this latter region and becomes more or less fused with the pyriform about the sulcus superstriaticus midway between the two extremities of the telencephalon in the older Sphenodon embryos, it is impossible to believe that the apparent neostriatal bulge is formed by the proliferation of the lower end of the pyriform cortex. This same arrangement of cell layers may be seen in the developing telencephalon of a 20 mm. human embryo, where it is possible to see that the more dorsal ridge of the caudate nucleus is not the product of pyriform proliferation. The appearance of direct continuity of pyriform cortex with the stratiﬁed layer of the neostriatum in the telencephalon of adult Sphenodon is of secondary formation——and happens only after the neostriatum is completely differentiated. The nucleus of the lateral olfactory tract is closely associated with the pyriform and lies as a nuclear mass ventral to it. In this material, the writer cannot determine the relationship of these areas to the neostriatum or to the nucleus ventro-medialis. However, most of the typical connections can be found for this region, except those with the nucleus preopticus.
The riddle of the ventro—lateral area is now before us. Does this analysis present any new evidence for aiding its solution? This region is known as the corpus striatum and until an adequate deﬁnition of that complex structure, i.e., the analysis of its cellular types, their connections and other relationships, is accomplished, any attempt to homologize its parts in any particular vertebrate with the same in man will meet with more or less deserved criticism. Now if it is possible to take the region in question in Sphenodon and in a critical and tentative manner compare it with studies upon other reptiles, and other vertebrates, even mammals, such an analysis may be of signal service. But may it be known that this presentation is not ﬁnal, since the writer intends to study the development of this region in minutia in the brain of that most interesting of vertebrates, man.
For the purposes of the present discussion, the writer has followed Kappers’ classiﬁcation of the striatal complex into paleostriatum, neostriaturn, and archistriatum or amygdaloid complex. The paleostriatum is the ancient portion of the striatum in which various types of impulses were synthesized and discharged into the hypothalamus, subthalamus or midbrain. The neostriatum is that portion of the striatum of which the characteristic connections are neothalamic. The archistriatum is that area in the ventro—lateral wall other than the pyriform cortex and its associated lateral olfactory nucleus, into which secondary or tertiary olfactory impulses discharge and from which the olfactory projection tract originates. In Sphenodon, however, the neostriatal ridge contains the olfactory and nonolfactory portions. or the archistriatum and neostriatum respectively. In the developing telencephalon of this reptile, the paleostriatum grows more rapidly in the early stages, While later the neostriatal ridge outstrips the more ancient elevation so as to become the prominent ventricular feature of the forebrain.
The paleostriatal ridge in Sphenodon shows little differentiation. In its more rostral part it contains a group of large triangular cells arranged about fascicles of ﬁbers which seem to become a part of the lateral forebrain bundle and with it may be distributed to the hypothalamus. Gisi (’07) described such a bundle, the tractus strio—hypothalamicus. Besides this there are smaller cells, which make up the bulk of this area and can be distinguished from the nucleus accumbens of Kappers. The latter is more clearly visible in the later stages of development than in the adult (compare ﬁgs. 4 and 6 with 9 and 12), and in that particular resembles the same nucleus in the developing paleostriatum of man.. In the developing ventro-lateral wall of man, the ventricular portion becomes incorporated into the nucleus caudatus, while that portion lying beneath becomes the globus pallidus. The relation of the latter to the surrounding structures, its larger triangular cells and its contribution of fibers to the lateral forebrain bundle makes it similar to part of Johnston’s caudate nucleus (’15) or his bed of the stria terminalis (’23), Crosby’s (’17) small»-celled ventro-lateral area, the ancient nucleus basalis region of De Lange (’11) and Kappers (’O8, ’21), and the ventrolateral quadrant of Herrick’s (’10, ’21) studies on the forebrain of amphibians.
The neostriatum proper in Sphenodon is the anterior division of the neostriatal ridge and is connected with the thalamus through the lateral forebrain bundle. The ﬁbers of this tract penetrate the irregular nuclear mass of its core and cannot be traced to the so-called cortical layer. This ridge develops as an enormous proliferation of the initial groups of cells which emigrate from the matrix layer. The so—called cortical layer develops later as asecondary separation from this nuclear mass and becomes continuous with the pyriform cortex secondarily around the sulcus superstriaticus. The marked extension of this ridge into the ventricle is the product of active proliferation of neuroblasts within the core, plus an increase in the supporting tissue. These cells lie between the lower extremity of the pyriform cortex and the inner layer of the neostriatum. It is not due, then, to proliferation of cortical layers inward but rather to greater energy potentialities of the area opposite the lowermost edge of the pyriform. This derivation cannot be spoken of as cortical, unless all the tissue which forms the striatum over and above its most ancient components, which do develop from what seems to be an area in the ventricular side opposite the pyriform cortex dorsal to the nucleus interstriaticus, be of cortical origin. If, however, all tissue dorsal to the plane passed from the sulcus interstriaticus of Sphenodon or the homologous sulcus in other reptiles to the ventral limit of the pyriform be cortical this neostriatum can be thought of as cortical in origin. Here there is no proliferation or inbending of general pallial areas, which become included secondarily within this large ventricular mass, as described by Crosby for the alligator. Here it is rather a development in situ. Nor, on the other hand, can it be justly said that the origin of this new striatal tissue is striatal in the sense of De Lange and Kappers. It is, rather, new and as such develops in certain deﬁnite relations to the more ancient cell masses and ﬁber systems. This is all that can be said, except as it is possible to determine the point or position in which the active proliferation of cells is taking place. What we do ﬁnd in the phylogenetic study of various areas is that coincident with a change in anatomy of the animal in question, there are also changes in the central nervous system, which become more startling in the higher centers; and as the new tissue of the highest centers are studied we ﬁnd a change in the next highest center. In fact, new elements arise, not out of the activity of older ones, but rather as an addition to them.
The anterior part of this more dorsal ridge contains the representatives of not only the lateral portion of the nucleus caudatus in J ohnston’s (’23) use of the term, but also part at least of the putamen, if the similarities of development in man and Sphenodon be reliable guides. For in man as in this reptile the primordium of this portion of the lentiform is seen lying between the anlage of the pyriform cortex in the lateral wall and the matrix on the ventricular side of the developing neostriatal ridge. There is no evidence here by which the age of the lentiform may be discerned. The position of derivation of its main mass of cells, however, seems to indicate that at least a portion of it may be developed simultaneously with the great increase of the neostriatum in reptiles.
The archistriatum, or that portion of the striatum whose connections are predominately olfactory, are the nucleus ventromedialis, its dorso-medial extension about the sulcus interstriaticus and the posterior portion of the neostriatal ridge. The nucleus ventro-medialis sends ﬁbers into the olfactory projection tract. Whether or not ﬁbers connect the two nuclei through the commissura anterior cannot be determined in this material. Fibers from the lateral olfactory system also seem to send a few branches into it. A system of ﬁbers connects the anterior olfactory area with the posterior portion of the neostriatum. Dart (’20) has described the nuclear group about the sulcus interstriaticus as the nucleus amygdalae proprius, the ventro-medial, as the nucleus tractus taeniae and the posterior portion of the neostriatal ridge as the hypopallial element of the amygdaloid complex. The writer was unable to identify any connections of Dart’s nucleus amygdalae proprius, that is, of the writer’s dorsal limb of the ventro—medial nucleus; while those of the latter’s ventral limb resemble most closely those of the medial large—celled nucleus of Johnston or of Crosby’s ventro—medial nucleus. One of the important connections of the posterior part of the neostriatal ridge is the system of ﬁbers connecting it with base of the olfactory bulb. This ﬁber group is similar to Meyer’s (’98) lateral olfactory tract, which ends in that author’s nucleus sphaericus, or to Unger’s tractus bulbo-epistriaticus which is lost in the nucleus sphaericus or to certain ﬁbers arising from the base of the bulb, Edinger’s tractus parolfactorio-epistriaticus, and travel to the dorsoposterior region of the neostriatum. The tractus olfactorius lateralis penetrates the dorso-lateral part of the dorso-lateral area of Crosby in the alligator. The system which Herrick (’21) has described in Amphibia as the ventro-lateral olfactory tract is the nearest homology which the writer can make for that group. The amygdaloid complex, described by Herrick (’21) as having connections with the medial olfactory areas, habenula, hypothalamus, olfactory bulb, opposite amygdala and adjacent pyriform can be seen in part, differentiated into particular areas with special connections in the reptile under discussion. In other words, the olfactory projection systems, the stria medullaris and the connections with the pyriform, distinguish the nucleus ventro-medialis from the posterior part of the neostriatal ridge, which is bound to rostral portion of the telencephalon. This separation of these two components of the archistriatum seems to be rather fundamental, when it is remembered it is the former nucleus and connection which is characteristic of the amphibian amygdaloid complex, while the olfactory neostriatal ﬁber system appears to be a new relation established for the ﬁrst time phylogenetically in the reptiles.
The writer, therefore, is in complete accordance with Johnston (’23) when he says that De Lange is right in maintaining that the nucleus amygdalae of mammals is the equivalent of the epistriatum in so far as that part of his epistriatum enters into the formation of the amygdaloid complex. However, in Sphenodon there is no evidence that the manner of development of the caudal portion of the neostriatal ridge differs from that of the rostral. The cells migrate from the matrix of that area in the same manner as they do anteriorly. Nevertheless, it is the caudal portion which is connected with the anterior olfactory area, which shows no regular inner layer and is coextensive with the nucleus ventro-medialis. If the last fact denote an origin for the posterior division of the neostriatum different from its anterior, and it is not coextensive with this nucleus, then J ohnston’s (’23) distinction of origin of the two parts of the dorsal ventricular ridge may apply to Sphenodon. If such be not the case, and both parts are derived from matrix lying opposite the pyriform they cannot be distinguished here by origin. However this may be, their adult relationship and ﬁber connections are sufficient to warrant J ohnston’s distinction between the two portions of the neostriatal ridge.
1. The primitive hippocampus can be distinguished from the remainder of the telencephalic evagination in developing Sphenodon at Stage N, by an outer cell-free marginal velum. This tissue lies upon the dorsal surface in Stage N. ' Later in development it occupies the dorso—medial division of the hemisphere. The former stage of differentiation is a period of development of the telencephalon which resembles that of a human embryo 11.8 mm. in length, no. 1290, or that of a 9 mm. human, no. 163 mentioned by the author in 1922. There is a small undifferentiated primordium hippocampi.
2. The telencephalon medium in this reptile contains a broader piece of the ependymal tissue than that found in man. The paraphysis and its pouch are especially well developed. The pars crassa of the lamina terminalis encroaches upon the pars tenuis in growth. The growth of the former provides a bed for the anterior and hippocampal commissures.
3. The septum grows thicker as the medial and lateral septal nuclei are differentiated at the expense of the matrix. The nucleus accumbens is very prominent in Stages R and S.
4. The neopallium or general pallium is found probably between the pyriform and hippocampus, the region from which arise the tangential ﬁbers.
5. The ventro-lateral area of the cerebral vesicle in Sphenodon,
the corpus striatum, is separated from the general pallium above
by a ventricular sulcus, known as the sulcus superstriaticus, and below from the septum by the angulus ventralis. This area is further divided into two ventricular ridges, a dorsal and a ventral, the neostriatum and paleostriatum respectively by the sulcus interstriaticus. In the earlier stages of growth the latter is greater and for a short period (Dendy’s Stages 0, OP, and Q) seems to eclipse the dorsal ridges. However, after Stage P the neostriatum begins a period of marked acceleration, which ceases only upon the completion of growth.
Within the paleostriatum no discrete nuclei are found. Fibers which join the more medial fascicles of thelateral forebrain bundles may be traced from these cells.
The neostriatum may be divided into the following cellular groups; the intermediate cell mass or layer, capped by a stratiﬁed layer, Elliot Smith’ s ‘cortical’ layer, which may be further divided into an anterior and a posterior region, and the ventromedial nucleus of Crosby, which at the ventro—posterior pole of the telencephalon joins an irregular group of cells lying ventral to the medial limit of the stratiﬁed layer. The posterior region of the neostriatal ridge and the ventro-medial nucleus of Crosby form the archistriatum.
The intermediate layer of the neostriatum is characterized by two types of ﬁbers, the lateral forebrain bundle connects its anterior part with the thalamus and the tractus bulbo—epistriaticus connects its posterior portion with the olfactory bulb. The ventromedial nucleus of Crosby sends ﬁbers into the olfactory pro-. jection tract and the stria medullaris. It may receive impulses from the lateral olfactory nucleus and the pyriform lobe.
6. The pyriform lobe cortex and the lateral olfactory nucleus are connected with the lateral olfactory tract, with the stria medullaris, the diagonal band and probably with the ventromedial nucleus of Crosby.
As a layer of cells, the cortex of the pyriform lobe is continuous anteriorly with the general pallium; in the middle portion of the telencephalon with the so-called cortical layer of the neostriatum and ventrally with the nucleus of the lateral olfactory tract.
The primordium of this cortex appears as a group of cells in the marginal velum dorsal and lateral to an area in the ventrolateral Wall, into which the strio-thalamic ﬁbers will grow. Between this layer and the matrix of the neostriatal ridge appear many cells having no deﬁnite arrangement. In later stages of development this intermediate layer contains many mitotic ﬁgures. The number in other regions of the telencephalon appears few by comparison.
The stratiﬁed layer, i.e., the ‘cortical’ layer of the neostriatum, cannot be the result of the proliferation of the ventral tip of the pyriform cortex. It is formed, rather, secondarily from medial cells of the intermediate layer or directly from the matrix. The continuity of the layer in question with the pyriform cortex as it appears at certain levels in the adult forebrain is secondary and is not found until the reptile has passed Dendy’s Stage Q.
The elements composing the dorsal portion of the ventralateral area, the pyriform cortex, the lateral olfactory nucleus, and both the so—called cortical and intermediate layers of the neostriatum arise from the matrix of the ventricle between the sulcus superstriatus and the sulcus interstriaticus.
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Ang. vent, angulus ventralis
C.a., commissura anterior
C .ab., commissura aberrans
C'.hab., commissura habenularis
C.hip., comrnissura hippocampi
C.mam., corpus Inamillare
Corp. str., corpus striatum
Diag.b.B., diagonal band of Broca D'ien., diencephalon
Di-tel.gr., di—te1encephalic groove Do7‘s.s., dorsal sac
Ep.ev., epiphyseal evagination F'£s.e'ndorh., ﬁssura endorhinalis
Fz's.hip., ﬁssura hippocampi
Fom'nt., foramen interventriculare F'is.rh., ﬁssura rhinalis
For.M., foramen Monroi
Gen.pal., general pallium
H ab., habenula
H yp.a.l., hypophysis, anterior lobe
I nf., infundibulum
Lam.term., lamina terminalis
Lat.f.b., lateral forebrain bundle
La,t.v., lateral ventricle
M (m.l., mantle layer
M ed.f.b., medial forebralin bundle Mesen., mesencephalon
M id.l., middle layer
N eostr., neostriatum
N eopal., neopallium
Nuc.ac., nucleus accumbens of Kappers N uc.c.ab., nucleus commissurac aber rantis Nuc.com.hip., nucleus commissurae hippocampi Nuc.hi;o.com., nucleus hippocampi commissurae N uc.lat.hypQth., nucleus lateralis hypothalamicus
N uc.lat.sept., nucleus lateralis septi
N uc.magnac., nucleus magnocellularis
N 'uc.med.sept., nucleus medialis septi
Nuc.olf.lat., nucleus olfactolius lateralis N uc.'pem'v. , nucleus periventricularis Nuc.preop., nucleus preopticus
N uc.vent.med., nucleus ventro-medialis of
0p.ch., optic chiasma
Op.ch.b., optic chiasma bed
0p.ev., optic evagination
Par.ar., paraphyseal arch
P.c., pars crassa
Pl.ch.v.lat., plexus chorioideus ventriculi
P. of R., pouch of Rathke
P.p., paraphyseal pouch
Pm'm.hip., primordium hippocampi
P.t., pars tenuis
Ree.op., recessus opticus
Rec.preop., recessus preopticus Rec.postop., recessus postopticus Rec.mam., recessus mamillaris
Sul.dors., sulcus dorsalis
Str.med., stria medialis
Str.term., stria terminalis
Sul.interstr., sulcus interstriaticus Sul.lz'm.h'ip., sulcus limitans hippocampi Sul.med.vent., sulcus medio-Ventralis Sul.super.str., sulcus superstriaticus Sul.vent.ant., sulcus ventro-anterior Tang.f., tangential ﬁbers
Tel.r.pl., telencephalic roof plate
Tvxbulbo-epi., tractus bulbo-epistriatus Tr.cort.parolf., tractus cortico—paro1fac‘ torius ’1'r.hab.ped., tractus habenulo-peduncularis Tr.olf.lat., tractus olfactoriu lateralis Tr.parolf.cort., tractus parolfactorio-cob ticalis Tr.sm'o-hypo., tractus strio-hypotha1amicus
Tr.tub.cort., tractus tuberculo-olfactorius V el.trans., velum transversum
PLATE 1 EXPLANATION or Frennns
1 to 25 These ﬁgures are pen-and-ink drawings copied from microphotographs of the forebrain of various specimens of Sphenodon punctatum. The embryological brains are those belonging to embryos Whose development lie between stages N and S cut either transversely or sagittally; the adult brain was cut transversely. All sections are 10 [L in thickness.
1 Through the telencephalon of embryo no. 1421, stage N. X 23.
2 Through the telencephalic vesicle, showing the four sectors. Embryo no. 392., stage P. X 22%. .
3 and 4 Through the telencephalic vesicle, anterior to the optic chiasma: through the optic chiasma, stage PQ. X 20.
PLATE 2 EXPLANATION or meurms
5 and 6 Through the telencephalon anterior to the lamina terminalis, showing the four sectors of Herrick: through the paraphyseal pouch, the dorsal sac of the diencephalon and the corpus striatum. Embryo no. 51a, stage PQ. X 20.
7 Sagittal section through the telencephalon of embryo no. 5221., stage Q. X 20. The developing cortical layers may be easily traced in the brain wall around the superstriatal sulcus.
8 Sagittal section through the telencephalon of embryo no. 142, stage R. X 21 %. The cortical layers which proved as distinct columns of cells in the preceding ﬁgure 7 are now"more or less fused about the sulcus superstriaticus.
PLATE 3 EXPLANATION OF FIGURES
9 to 11 Transverse sections through the telencephalon of embryo no. 141, stage R.
9 Through the endbrain anterior to the lamina terminalis showing a well developed layer in the neostriatal ridge and also the cortex of the tuberculum olfactorium. X 20.
10 Through the di—te1encepha1ic junction and the doral part of the lamina terminalis. X 19 1/5.
11 Through the commissura aberrans and the posterior end of the nucleus ventro-medialis. >< 191/5.
PLATE 4 EXPLANATION or FIGURES
12 and 13 Transverse sections through the telencephalon of embryo no. 144, stage R.
12 Through the lamina terminalis and the septum. X 20.
13 Through the thalamus and the posterior part of the telencephalie vesicle. X 20.
14 A longitudinal section through the telencephalon of embryo Sii, stage R, which shows the difference in the anterior and the posterior arrangement of the cortex-like layer of the neostriatum. X 20.
PLATE 5 EXPLANATION or FIGURES
15 to 25 Transverse sections taken from the telencephalon of an adult Sphenodon punctatum, no. 1. X 10.
15 Through the most anterior portion of the cerebral vesicle.
16 Through the septum anterior to the lamina terminalis.
17 Through the septum, posterior to the level of ﬁgure 16.
18 Through the septum, posterior to ﬁgure 17.
PLATE 6 EXPLANATION or FIGURES
19 Through the septum, the hippocampal commissure and the optic chiasma. 20 Through a level slightly posterior to that of ﬁgure 19.
PLATE 7 EXPLANATION or FIGURES
21 Through the foramen of Monro and the di-telencephalic junction. 22 Through the foramen of Monro and the commissura aberrans, showing the posterior extent of the neostriatum.
23 Through the commissura aberrans and the diencephalon posterior to the foramen. The nucleus ventro-medialis and the nucleus of the lateral olfactory tract seem coextensive.
24 Through the telencephalon and the diencephalon, posterior to the di-telencephalic junction.
25 Through the most posterior end of the telencephalon and the central part of the diencephalon. In the latter division of the brain, the medial and lateral nuclei as well as the habenula may be seen.
PLATE 9 EXPLANATION o1<‘ FIGURES
26 to 28 These three pen-and-ink sketches were taken from line drawings, made with a camera lucida at a known magniﬁcation, from the brain of adult Sphenodon.
26 Longitudinal section through the lateral part of the telencephalon, showing the tractus bulbo-epistriaticus and the tractus thalamo-striaticus. X 5%.
27 Longitudinal section through the medial part of the telencephalon and the olfactory bulb. The neostriatal ridge and the tractus bulbo-epistriaticus and the tractus striato-thalamicus. X 5%.
28 Transverse section showing the di-telencephalic junction in the transverse series of the adult no. 1. X 9.
PLATE 10 EXPLANATION OF FIGURES
29 to 31 These are wash drawings of a wax model of the telencephalon of embryo no. 143, stage N. X 30.
29 The lateral wall of the telencephalic vesicle. The optic evagination, the anterior lobe of the hypophysis and the epiphysis are the characteristic structures visible.
30 The medial sagittal section of the forebrain.
PLATE 11 EXPLANATION or FIGURES
31 The telencephalqn as seen from above. 32 to 34 These ﬁgures are Wash drawings of a wax model of the forebrain of embryo no. 39a, stage P. X 30. 32 The lateral surface of the forebrain.
PLATE 12 EXPLANATION OF FIGURES
33 The medial sagittal aspect of the forebrain.
34 The lateral wall of the telencephalon, as seen from the ventricle, showing the paleostriatum and the neostriatum. The medial wall has been removed with the exception of that attached to pars crassa of the lamina terminalis.
Cite this page: Hill, M.A. (2019, May 25) Embryology Paper - The development of the telencephalon in sphenodon punctatum. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_development_of_the_telencephalon_in_sphenodon_punctatum
- © Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G