Paper - The development of the neuraxis in the domestic cat to the stage of twenty-one somites

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Schulte HVW. and Tilney F. The development of the neuraxis in the domestic cat to the stage of twenty-one somites. (1915) Ann. N. Y. Acad. Sc. 24: 319-346.

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Contents

The Development of the Neuraxis in the Domestic Cat to the Stage of Twenty-one Somites

By H. von Schulte and Frederick Tilney


(31 March, 1915)

Presented In abstract by the senior authur at the meeting or ll May, 1914. under the title "Early Stages In the Development of the Brain in the Domestic Cat."

Manuscrlpt received by the Editor, 17 September, 1914.

Introduction

The present paper is 21 study of the morphology of the neuraxis in its early stages, with especial reference to an attempted interpretation of the forebrain in terms of the longitudinal zones of the neural tube. viz., the basal and alar lamina’, and the ganglionic crest. The theoretical problem can he-stated hriefl_V. Since the ganglionic elenients are included in the wall of the neural tube at the time of its separation from the ectoderm—Neumayer has shown this for reptiles, nncl it is also true of the cat——it is possible that in regions in which a ganglionic crest is not formed, homodyiiamous elements remain incorporated and fomi permanent constituents of the brain-wall. It ought not to be inferred, from absence of a discrete ganglionic crest in any region, that its equivalent is lacking, for it may simply have failed to separate from the neuraxis. On the other hand, with regard to the interpretation of the forehrain in terms of transverse segmentation, neuromeres, it would seem that a complete dorso-ventral sc,<.rment of the neural tube should contain ganglionic, alar and basal elements, and that failing any of these it is something less and other than what the term neuromere properly implies. Now we have no evidence of the existence of the basal lamina in advance of the nucleus of origin of the oculo-motorius, nor can we see the advantage of assuming its presence in front of the point at which evidence of its existence ceases. Further, the transverse segmentation of the neurasis is either the result of intrinsic factors, or what seems more probable. is at least in part secondary to the segmentation of the mesoderm. This is myomeric in the trunk, branchiomeric in the head. That a secondarv segmentation of the neuraxis thus ei‘l'eeted should result in a continuous series of meristic equivalents seems on the face of it somewhat improbable. These, in brief. are the questions we have had chiefly in mind in attempting an ontogenetic analysis of the forebrain of the cat. At the same time it seemed desirable to record the general data concerning the neuraxis in a close and fairly numerous series of young: embryos, for as yet the knowledge of these stages in mammals is far from extensive. The list of embryos is as follows:

Cat Embryos in Study
Prior to the appearance of somites Nos. 5129, 400.456. 550. 555
One pair of mesorlermic somites Nos. 554. 594
Two pairs of mesodermic somites No. 539
Three pairs of mesodermlc somites No. 593
Four pairs of mesodermlc somites No. 409
Seven pairs of mesodermic somites Nos :587. 588
Eight pairs of mesodermlc somites No. 530., 586
Nine pairs of mesodermlc somites No. 531
Ten pairs of rnesodermic somites Nos. 476, 532
Twelve pairs of mesodermlc somites Nos. 534, 547
Thirteen pairs of mesodermlc somites No. 86
Fourteen pairs of mesodermlc somites Nos. 188, 548
Sixteen pairs of of mesodermlc somltes No. 551
Seventeen pairs of mesodermlc somites No. 568
Nineteen pairs of mesodermlc somites No. 502
Twenty-one pairs of mesodermic somites No. 558


The embryos were cut into transverse sections and reconstructed ln the Born method at a magnification of two hundred diameters.

Casts of the lumina, as well as models of the neural tube, were made in each case and were found useful in controlling and interpreting the surface relief. The period of development of the ueuraxis covered by these embryos extends from the appearance of the medullary plate to the definition of the pallial anlage. It will be convenient, first, to record the conditions observed in the several embryos and then proceed to a consideration of the morphological questions outlined above.

Description of Embryos

Embryos prior to the appearance of intersomitic Clefts

(Plate XXII, Fig. 1.) Toward the end of this period when the axial mesoderm has thickened and is on the point of hecoming segmented. the mcdullar_v plate (5)2 is represented h_\' an oval tl1l('l\'t‘lllI1g of the cctoderm which peripherally passes into the somatic ectodcrm (7) l;_\' a gradual transition (28) without clearly defined limits. In some of our c1tthr\'os there were irregular depressions and elevations of the surface, espeL'iullA\' in its lateral regions, which presented some 1'csemhlancc to the Seitenl'in'clie, Randfurehe and Parietalzone of authors, but which. in View of the unit'orn1 outlines of the excellent eml)r_\'o here figui'e(l, we have not been able to convince ourselves were natural structures.

Embryos of One Somite

(Plate XXII, Fig. 2.). In these eniliryos one pair of complete intersomitic clefts is present. In addition. the axial mesoderm presents on each side two or three transverse constrictions which are evidently due to the inception of somites. In the eat it appears that several somitie constrictions are initiated .~'llIllllla!1(‘0lIs‘l_\‘. In our reckoning we have regarded onl_\' the complete clefts and the number of somites assigned to the ettil)r\'os of our series are estimated in terms of complete clefts. In the two emhrpvos which we have classed as having one pair of somites there are. in addition. two or three pairs of constrictions, one pair of u'hi('h is situated in front of the first complete intersomitic cleft. The medullar_\' plate (-3) tnerges laterally into the somatic ectoderm (7) by a gradual transition (28) as in the earlier stages. Cephalad its axial region is depressed. We would call attention to the fact that the summits of these incipient mc<lullar_\' folds correspond to the middle 7.one of the medu|lar_\' plate on each side.

Embryos of Two Somites

(Plate XXIII, Figz. 1.) The HIC<l|IlliH'\ plate has the same general characters as in the [)l'(.'('L'(llll,|_' emhr_vos. It is very broad and passes hy gradual transition (38) into the somatic ectoderm (7) at the sides. Craniad the mcLlullar_\‘ f_'I'U<|\'t‘ is .“H|llt'\Vllill«

‘The numerals In ltalics refer la it.» mtam.-.— ..r lllc Laurels In am Ilfiutrn


deeper and the medullary folds have partially erected themselves over a greater longitudinal distance. Craniad also a faintly marked furrow is present in the region where the medullary plate becomes thinner as the somatic ectoderm is approached. In addition, the lateral region of the plate presents slight undulations of the surface.

Embryos of Three Somites

(Plate XXIII, Fig. 2.) The l1€11l'tlXiB has the same general configuration as in the preceding embryos. The boundary between the medullary plate (5) and the somatic ectoderm (7) is still effected by a gradual transition (28). Craniad there is a faint furrow as in the embryo of two somites, and in this region also the margin is marked by faint undulations.

Embryos of Four Somites

(Plate XXVII, Fig. 1; Plates XXI VXXVI.) The neuraxis has not lengthened appreciably as compared with the embryos of three and of two somites. In its cranial half, however, the neural folds (6) have fully erected themselves and an abrupt bound ary has been established between them and the somatic ectoderm (7). Caudad this junction is still effected by a gradual transition; there is no furrow. The floor plate terminates craniad in a distinct thickening and elevation (2) which intervenes between the ventral extremities of the optic snlci (1). In front of them it is continuous with the ectal margins of the primitive optic vesicles. We have designated this elevation the tubercle of the floor. The optic snlci begin on each side of the tubercle of the floor where they are continuous with the angle that defines the floor-plate from the parieties. They describe an arc with the coneavity caudad and approximately parallel to the margin of the neural plate, but approaching the neurosomatic junction and becoming fainter as they are followed caudad. Corresponding to their arched segment there is a marked thickening and external prominence of the wall of the neural tube.

At a short distance eaudad to the optic snlci there are a pair of similar though shallower furrows, close to the neurosomatic junction. Their course is at first horizontal; their caudal portions turn ventrad and approach but do not reach the fioor—plate. Corresponding to their horizontal portion, there is a ridge-like projection between the summit of the neural plate and the somatic ectoderm. This ridge is the quintal anlage (3). That it is more intimately related to the neural fold (5 ) than to the somatic ectoderm ( 7) is shown both by the fact that the neurosomatic junction is dorsal to it and by the fact that it blends at both of its extremities with the thickened dorsnm of the neural plate. It is in no sense an element intermediate between neural plate and ectoderm. Just behind the quintal anlage is another, smaller fun-ow (1,) which is not accompanied by any definite local protrusion of the wall but simpl_\' grooves the medullary fold entally near the neurosomatic junction, becoming broad and shallow and disappearing before the floorplate is reached. This furrow corresponds to the acoustico-facial anlage in older embryos. We would note further that on the right side there is a small pit situated midway between the optic and quintal sulci having the same characters as the latter, except that it is unaccompanied by an external protuberance and is present only in three sections of 13.3 micra each. 'l‘l1is had no homologue on the left side. It is possible that this minute furrow represents the profundus anlage, although clear evidence of its presence is not found in our embryos until the stage of twelve somites is reached. A similar conformation was observed farther caudad on the left side in the region behind the last somite; it extended over three sections. Caudad the medullary folds divarieate and become lower, eventually disappearing in the region of the primitive groove. In contrast to the cranial extremity, it is to be noted that here the first portion elevated is the lateral and not the basal zone of the medullary plate. This is characteristic of this region in later stages as well. It is apparent, therefore, that, both craniad and caudad, the elevation of the neural folds is accomplished in two phases but that the order of events is reversed at the two ends of the embryo. _ Craniad the basal region first becomes vertical, then the lateral, while caudad the converse is true.

This is the only evidence we have been able to find of a morphologic difference between the basal and alar plates, for the suleus limitans is a late formation, if it is present at all in young embryos of the cat. Embryos of Seven S0miies.——Tl1e neural folds are separate in their entire length’ and in general show but little advance in comparison with the embryo of four somites. The optic sulci are strongly arched and the prominence of the primitive optic vesicles is slightly increased. The tubercle of the floor intervening between the two optic sulci forms a well defined cranial limit to the floor of the neural tube, and blending with the wall in front of the optic sulci forms the ventral lip of the neuropore. The quintal and acoustico-facial sulci consist of horizontal and obliquely descending segments, the latter in eat.-l1 case becoming broader and shallower as they approach the floor-plate. On the cctal surface of the neural plate faintly marked oblique elevations con-e.~apond to the oblique portions of these sulci. The quintal anlage fortn.-l an elongated ridge, extending a little farther craniad than the horizontal segment of the quintal sulcus. Its extremities now project free of the medullary plate, no longer fusing with it as in the embryo of four somites. This anlage enters into intimate relations with the mesenchyme of the head. the two tissues passing into one another by such gradations that it is diflicult and in some sections impossible to determine the precise limits of the ganglion.

Embryos of Seven Somites

(Plate XXVI], Figs. 2, 3; Plates X.\'VlII-.\'.\'X.) While the neura.\is shows little if any increase of length in comparison with the preceding embryos, the closure of the neural folds has been initiated. 'l‘his is first effected in the region immediately in front of the quintal anlage (3), where the folds obtain their greatest height (en1bt‘_\’o No. 536). In entbryo No. 530 there is an additional point of closure immediately caudal to the quintnl anlage.

The optic sulci (I) have increased in depth and the optic vesicles form prominent, ellipsoidal.projections with nearly vertical axes. Evidently they correspond to the arched segments of the optic sulci in the younger embryos; the horizontal caudal continuations of these furrows are now reduced. (‘audal to the optic vesicle the wall of the neural tube is flat until near the quintal anlage (J), where a moderate dilation is present. This guitglioii has now a triangular form and is shorter than in the precedin,-1, embryos. It is attached to the dorsum of the medullary plate near its junction with the somatic ectoderm, corresponding to the interval between the two points of closure of the neural tube (embr_\'o No. 530) and at the summit of a slight constriction iitter\'eitiitg between two moderate dilatations. The first of these dilatations (J!) has already been lllclltlfllled: tllt‘ -'t’t'0INl (/J) ('0rI‘c>'po1nls to the oblique descending por 'tiou of the quintul sult-us, which as a whole in tlicse cu1br_\'os is under going reduction. The acoustico-facial ganglion (4) is small and has a shallow coi'respondin;_r sulcus. The ganglion occupies the summit of a constriction immcdiatcl_\' behind the quintal dilatation and is followed by a sliglitcr cnlar,t:emcnt (1.3) of the tube, into which its sulcus merges. It is then, in its sulcus and following dilatation, a repetition on a smaller scale of the quintal anla,<:c. The common gaugzlioiiic crest c.\'tends front the acoustico-facial ganglion, with which it is continuous to the level of the fourth pair of inesotlerinic sc_r_'nIeitts. In the caudal half of the neuraxis the neural folds are lower: at first parallel, the)’ di\'cr;:c in the region of the rhornboid fossa (.31). Here the neural plate of each side comprises a horizontal mesial re,-zion. and a smaller lateral erected portion, the two meeting at a rounded anpzlc. In the midline there is a \‘t'Stlg(.‘ of the pr|mltl\‘(' groove (Ii).

Embryos of Nine Somites

(Plate XXXI.) TlIL'1'ln.~'llI't' of the neural tube is atlvancing rapidly. It is not, liowc\‘cr. clfccted by a uniform ailvance in both directions from the region of carlic:-‘t closure, but on the contrary is incident at st-wral .~'cpal'al(‘, points of the ncuraxis, as was forcshadowed in the t?i,L’lll-.\‘UiIIlTc «-ml-r_\‘o. In addition to the anterior neuropore (8), there are three small gaps (.9) in the region of the quintal aulage (8) correspolltlillg to the hiatus there present in the embryo of eight somites. Another small orifice (III) is situated in the region of the acoustico-facial anlage (4). Caudal to this the folds unite t'or a eonsid~ erable distance but again separate (II) in the region of the fourth to the eighth somite: opposite tl1e ninth they are again united for a short distance when they finally separate and tli\'et'gc.

The anterior neuropore is markedl_\' diminished in extent. That this is caused by closure at its ventral as well as at its dorsal tip is shown by a comparison of the models in Plate X.\'l/ll, Fig. 3, and Plate .\'XXl, Fig. 2. In the eight-somite embryo the optic vesicle is open in its whole extent; in that ot' nine somites closure has been elfected in about half of its length. The prosencephalon shows a distinct advance bc_vond that of the eight-somite enibryon‘ both in size and in the complexity of its surface relief. It projects strongly vcntrad, its caudal margin forming approximatel_v a right angle with the floor of the neural tube. It is demarcated from the midbrain by a shallow anterior istlnuian sulcus (22) which is very obliquely inclined. The optic. vesicles form its \'entro—cranial region ; they are somewhat p_\'ril'0I'm with a pointed caudal e.\tremit_v and their long axes are inclined at an acute angle with the horizontal. llorsally the optic vesicle is defined by a shallow depression; between this and the anterior isthmiau suleus are two small elevations, one on the dorsal and one on the ventral aspect ot' the tube. The dorsal eminence is the thalanienceplmlon (Plate XXXI, Fig. 2, lb’) and is opened in its whole lengrth by the neuropore. It is somewhat triangzular in shape and its prominence diminishes ventrad where it is separated by a faint depression from the ventral eminence. This latter also has a triangular shape and is the first indication, in our series_ of the rnannnillary region (Plate X.\'Xl, Fig. '3, I7). Ventrally it is separated from the pointed extremity of the optic vesicles b_\' a slight ineisure which corresponds to a tliiekcningg and an inward projection of the tloor—plate. .\s this is interposed between the ventral extremities of the optic sulci. it is evidently the tubercle of the floor (Plate XXXIX, Plate XL. l"i-,r. I. J) of the earlier .~'l2t_L"t‘.~_ The dc» pressions defining the tlialamenccplmIon and the mannnillary t'('gluIt lorni an H-shaped system ot' l'urrow.~, while the twin elevations taken tu;_'ctlIcr form :1 segment which separates the priniitiw optic vesicles from the midbrain. That this S(‘§£Ylt('tlt is not a an-nromerc in an_\_' precise sense of the term is obvious from its de\clopnn-ntal liistorv tor the two cmincnces of which it is ('nI”tl[ltI.~'(‘tl fuse onli at a con.-i:lcrabl_v later period (sixteen soniitcsl. The [H't)."{'lI('(.']lllill(t|l In-s iminc«lIatc|_v in l'ront of the t'ore;zut and the entmlcrni is clost-l_v 2ll)|D|ll'll to thc lll5I|ttllIlllitl'_\' r-—_-_-ion. .\ comparison with the embryos of eight somites makes it evident that these changes are not wholl_v to be attributed to inequalities of growth, but that a profound remodeling of the cranial extremity of the neuraxis has taken place in addition, as is shown by the alteration in the direction of the optic axes, the inclination of the anterior isthmian salons and in the notable lengthening of the midbrain taken in connection with its diminished height. Embryos of Ten iS'o1m'tes.——-(Plate XXXII.) The union of the neural folds has progressed. The anterior neuropore (8) extends through the greater part of the length of the optic vesicles into the thalamencephalon (16), the somatic ectoderm being further adherent as far as the mesoncephalon (15). In this segment of the brain and for a considerable distance caudad the folds have closed and the neural tube has separated from the ectoderm. The roof-plate is concave and depends into the lumen as a longitudinal ridge. Somewhat in front of the middle of the neuraxis there is a considerable hiatus in the line of closure of the neural folds and behind this are three small areas in which the ectoderm is still adherent, though closure has been completed (11). The fossa rhomboidalis (21) falls into two portions of about equal length. Cephalad it is narrow and the neural folds are high, approximated and nearly parallel. The caudal region is broad and bounded by low folds erected only in their lateral parts; here the neural plate still passes into the somatic ectoderm by a gradual transition.

The prosencephalon joins the rest of the neuraxis at an acute angle. The anterior isthmian sulcus (22) is horizontal with ventral concavity. The mammillary region (17) is well marked, as is also the thalamencephalon (16); the axis of the optic vesicle is horizontal. A posterior isthmian sulcus (23) defines the midbrain caudally. In embryo N o. 476 this vesicle is obscurely divided into two segments b_v a shallow furrow. The hindhrain has three recognizable segments; the first (31) is triangular with its base ventral; the second (12) and third (13) are obliquely inclined and defined by shallow. oblique furrows. The quintal anlage (3) is attached in the interval between the second and third segments and the protundus anlage between the second and first (Plate XXXII, Fig. 2, 3a). This is the youngest embryo of our series in which the profundus element can definitely be made out. The acoustico-facial anlage (4) is separated by a short interval from the quintal anlage; it occupies the furrow which defines the third hindbrain segment caudally and is continuous with the ganglionic crest of the trunk (not shown in the model).

A second embryo of ten somites (No. 532) corresponds closely to the embryo just described. except for a somewhat greater degree of union of the med11llary folds. The anterior neuropore is reduced to a minute orifice situated ventrally at about the middle of the optic vesicles, thus affording, in comparison with the foregoing embryo conclusive evidence of progressive closure at the anterior lip of the neuropore and to this degree depriving the orifice of value in determining the morphologic cephalic extremity of the neural tube. Apart from this small opening the anterior neuropore is closed, but the ectoderm is adherent over the cranial portion of the optic vesicles and the adjacent region of the thalamencephalon. The midbrain shows no sign of division into two segments.

Embryos of Twelve and Thirteen Somites

(Plate XXXIII.) These embryos form a closely graded series, passing from the conditions described in the embryos of ten somites to those attained by the fourteen somite embryos of our series. In the region of the anterior neuropore (8) they show a considerable degree of variation in the closure. In embryo No. 534 of twelve somites, the neuropore has been (-omplctel_v closed. The ectodermis, however, adherent at the middle of the sagittal length of the optic vesicles and further over the region of junction of optic vesicle (I) and thalamencephalon (16). In embryo No. 86 of thirteen somites, the ectoderm is adherent in the whole length of the optic vesicle and there are three small orifices, one at the middle of the optic vesicle, one at its junction with the thaIamenreplmlon and one in the thalamencephalon itself. ’

There are three oblique segments in the hindbrain; their intersegmental constrictions give attachment to the profundns (3a), qnintal (-3,) and acoustico-facial (4) ganglia in the order named cranio-caudad. The acoustico—facial is continuous with the ganglionic crest (.20) which extends for somewhat more than half the length of the neuraxis. Following these oblique ganglionic segments is a series of vertical segments; their constrictions corresponding to the mesodermic somites are six to seven in number. It is thus seen that the vertical segments of the neuraxis correspond in location to the somites, but lag considerably behind them in number, which we take to mean that an interval in time elzipses between the formation of the mesodemiic somite and formation of the corresponding myelomere. It seems to us, therefore, that ontogenetically myelomeres are secondary to the mesodermic somites. The three oblique hindbrain segments. associated with the llH'ce large ganglia, are situated in advance of the somites. We would eniplmsi'/.e the difference in their disposition as evincing their independence of the myomeric segmentation.

Embryos of Fourteen Somites

(Plate XXXIV.) The two embryos of this stage in our series show a close correspondence in the neuraxis, save only that embryo No. 548 is in most respects slightly in advance of its fellow, No. 188. The neural folds are united, except in the short, narrow rhoinboid fossa, in the caudal portion of which the neural plate still passes by a gradual ‘transition into the somatic ectoderin. 'l‘he anterior nenropore (S) has closed, but the ectoderm is still adherent front the middle of the optic vesicle to the tliahuiienceplialon. l'}mb1'_\'0 No. 188 is more advanced in this respect, for the ectoderm is free of the neum_\;is iii the whole lengih of the prosencephalon. lt is, liowever, adherent in the iiiidliiie t'roni the level of the profundus anlage to that of the acoustico-t'-aciul gaiiglion. A comparison of these two einbryos affords a. striking e.\ainple of the i»rregularit\' incident to the whole process of closure of the neural tube and its separation from the ectoderm in the cat, and seems to justify the attachment of less iiiiportance than is usually ascribed to the point of ultimate closure. 'l‘lie"iietii-axis is bent \'(3lltl'tt(l at the posterior istliiniaii sulcus (.3-3). ’l‘his is the second actual tiexure observed in our series, for a comparison with the figures of the precedin,I_{ enibryos shows that the earlier projection ventratl of the prosencephalon was associated with a remodeling of the midhrain and an inclination of the anterior istlmiian sulcus, while in the stage now under discussion the bend is accentuated at the posterior isthmus. The bintlbrain forms aiqtiiitle arch passing into the straiglit in_\'elencephalou. in the t'orebrain iinportant new conditions are initiated. 'l‘he optic \"esicle (I) is now not only relatively but absolutely smaller than in the younger embryos and an ccloplic zone begins to emerge from its periphery. As yet these changes are conspicuous only dorsally between the optic vesicle and the thalaiueiicephalon (I6), and to -it less degree ventrad iiuniediately in front of the mammillary region (17). 'l‘he dorsal element is the telencephalou (19), the ventral corresponds in general to the int'undibulai' region (1.9). The niainniillary region and the thalamenecphaion have increased in size and form well-marke.d triangular proiniuenccs in lateral view. The niidbrain (I5) is a wellniarked dilatation, triaugrular in torui, defined by conspicuous isthniian furrows 3:, J. ) which all but iucct ventrally in the angle formed by the second fle.\'ure of the neuraxis. The arched form of the liindbrain has been mentioned; its vertical diameter is increased by a ventral projection at the level of the quintal ganglion (3), the pontine angle. Thus it appears that a projection of this region long antecede-‘ the development oi’ the pontiue flexure. in lateral view, the three segments with their oblique sepa rating: t'urrows are more conspicuous than in the younger eniliryos. '|‘hc_\' are especially prominent ventrad, which would seem to imply that their increasing deiinitcncss is associated with the bending of the liimlhraiii. The _JiItt_LIli.’l are atrar-hcrl (lorsally in the furrows: the profnndus (311) between the first and seeolltl segments (J1, 12); the quintus (3) between the set-ontl and tllll'tl (1.2, Io’) ; the aeoustieo-fat-ial (1,) immediately behiml the thircl. \'entrullv the second segment shows signs of subdivision. Barely seen in etub1'_vu X0. 188, in .\'o. 548 it is marked by a vertical furrow ascending to the attaelnnent of the quiutnl ganglion and dividing the segment it1to an oblique ernniul portion (Ila), extending completely across the neural tube and it triangular caudal fraction (1%) which forms the bulk of the pontine angle but is confined to the ventral region of the neural wall, being e.\'t-lutletl from the alar portion by the oblique eotnplete thirtl segtttetit. The tn_\'elent-ephalon has seven neuromeres. These contrast sharply with the foregoing by their vertical position. The oblique metent-ephalie segn1ent.~:- are now eontinuous with the neuronteres of the tn_veleneephztlon; that they form a hontotlynatnous series is cert-uinl_\' not supported by the e\‘i<lenee of their development in the eat, in fact the |1etet'o;_'eneit_v of these elements seems us clearly given by their ontogeny as by the <livet~.:it_\' ol' the peripheral nerves with which the’ are znzsneiatetl in the adult.

Embryo of Sixteen Somites

(Plate XXXV.) The neural tube is closed and completely sepurutt-«l from the ('t'l0(leI'nI ext-ept in the region of the fossa rhonihoitlalis. The tle.\'ure ut the po.<tet’im' i.<tlnnian :~'lll('lls (33) has increased and the llll('lltll bend is now present. In the himlbrain the region of the SCC()l1(l (I!) .~'egtnet1t projects ventmlly ztml forms the pontine angle. The optie vesicles (I) are still further I'l‘(l|l('l.‘(l in size, z1bsolutel_\' as well as reluti\'el_\', and there is formed both ventral and dorsal to the vesicle a considerable zone which rt'pre.~‘etIt.~' the exterision of the telencephalon (I9) and int'umlihu|ur regzion (I8) ol' the preceding eInln'_\'o.<. The ('()tllt'.~‘('l‘lll‘t‘ between the lll2llll|lllll2ll‘_\' region (17) illlfl the thalnmeneeplmlon (Iii) lms itit-t'eu.<e:l: and the_\' now form a \V(‘ll-IllzlI‘l\'('<l .~‘t‘g’[t1L‘Ilt between the tnitlbrnin unrl the (leriv;\ti\'t_-R of the nptie \'e.\‘ieles (2‘).“ The 1ne.<em-eplmlou (E5) is triangular and marketlly t-ottiprcssetl ventratl. The .~‘e,<:tttet1t.~' ot' the lllllllllflllll are le.~':~' oblique than in preceding ernbr_\'o.u. The tirst (.21) is lur_r_:e. the .~‘(‘.('()n(l (Ila. Iflb) forms the protninenee of the pontine :u1_:|e, is V['lllI‘illl>\’ :~‘lll)(ll\'l(l('ll nnzl its second segtnent now e.\len(l.~‘ further :lor.~'u<l. The IlllI‘(l se,-gnu-nt (1-1’) is narrow. The }_,r:Lu_x_rli:t I't'li|lll their priinitixe it|ll‘I‘.\(‘_:IllIt.’lll:ll pos.itions_

I:‘Inbr_I/0 of ."eI‘r,'I:Im'u .\'mmIr.~ tl’lnlv .\.\.\'\'l I Tilt‘ |‘I"llN'1i"lI Hf flit? flptie vesiele (I) ('0HIinlte.~‘_ The t(‘ll_'ll1'l‘|lll2Il()ll (/21) tornis al prominent convexity in front of the optic vesicle. Near its upper limit there is a. point where the ectoderm is still adherent (8). The area which we have termed infundibalar region (18) is very large and has a pointed apex directed caudad. The mammillary region (17) has increased in size but otherwise closely resembles that of the fourteen somite embryos. The same is true of the mesencephalon (15). The hindbrain is markedly enlarged in its vertical diameter. The pontine angle has increased in prominence. The first hindbrain segment (31) is broad and prominent; the second (12) is subdivided by a deep sulcus; its posterior moiety (1212) has a considerable vertical extent but is now fusing dorsally with the third segment (13) which remains narrow but has increased greatly in height, forming indeed the apex of the pontine angle. The quintal ganglion (3) retains its intersegmental position, but_ that of the profundns (3a) is beginning to shift caudad and is now in part attached to the second segment (12a). The cranial extremity of the myelencephalon has increased markedly in vertical diameter and is beginning to be assimilated to the hindbrain; it shows a prominence of the roof at its commencement which is separated by a depression from the remainder of the roof-plate. This embryo showed some degree of side-to—side compression with resulting diminution of the relief of its lateral walls and an exaggeration of the projections in the dorsal and ventral midline.

“ In this re.-apeet this r-mhryn (‘III'fi'.‘4]IIII|ll.~' t-t...~.»I_\- In the hull‘ mtlltlm-I4-r slneep 4‘In'>r.\’0 Yl£.'lll'ed |>_\' .\'r-uninys-r» -Htotllon zur l‘:IH\\'l<'lU‘lllH£N|:('H1'lIll'lIl(‘ dc-:4 Co-Iilrnm (lor Rainierthlr-rr-. [<‘a.~zt.-zehrtft zum Nl(‘l|9n7,Ig,~‘I|||| t:u|»urt-tau: ron (‘nu-I \'un kuppfer. 'l‘n!. I\'l’.Vl|l. Hz. 4. Jenn. moo.


Embryo of Nineteen Somites.——(Plate XXXVII.) The forebrain is defined by a well marked anterior isthmian constriction (2.?) which is practically in line with the venter of the hindbrain. The thalamancephalon ( 16) has enlarged and is separated from the telencephalon (19) by a shallow but definite furrow (£76) extending from the dorsal midline, obliquely over the lateral surface of the brain to the depressed area immediately behind the optic vesicle where the thalamcncephalon merges into the relief of the regio mamxnillaris (1 '7), The latter has not increased in size and is separated from the regio infundibnlaris (18) by a shallow furrow; it is also distinguished by its greater lateral prominence. The remainder of the forebrain comprises the optic vesicles and an ectoptic zone surrounding them. The latter has greatly enlarged in its dorso— cephalic portion, the telencephalon (19), which now forms the extremityof the brain. The regio infundibularis shows but a moderate increase in size. New conditions are initiated ventral to the optic vesicles, which now begin to retreat from the margin of the brain, leaving a narrow strip of tissue through which the infundibular region and telencephaion are continuous. This condition is more clearly shown in the model of the lumen.

The midbrain (I5) is little changed as yet but is beginning to show a lengthening of its dorsum which becomes marked and characteristic in later stages. In the hindbrain the pontine angle has increased in prominence. The first segment (31) is very large, the second (12) is ventrally subdivided into two, as is also the third (13). The snlci corresponding to these subdivisions extend dorsad only so far as the ganglia. Behind the third of the original segments of this region is one small projection confined to the ventral portion of the neural tube. This is followed by the vertical myelomeres corresponding to the mesodermic somites.

The large ganglia are shifting their points of connection with the neural tube and are now attached somewhat above the middle of its lateral wall. The profundus (3a) has shifted to the middle of the second segment, lying at the extremity of the sulcus which marks its ventral subdivisions. The quintal ganglion (3) has lost its primitive intersegmental position and adheres to the cephalic part of the third segment. These two ganglia are thus beginning to approach one another. The acoustico-facial (4) occupies the interval between the fourth segment (11,) and the first myelomere (21,).

Embryo of Twenty-one Somites

(Plate XXXVIII.) The forebrain differs from that of the preceding embryo notably in the enlargement of the infundibular region (18) as well as in the increase of the ectoptie zone as a whole. The optic vesicles have receded further from the ventral margin and fl broader strip connects the infundibular region with the telencephalon (19). The cctoderin is adherent to the latter at a point corresponding to the sotnewlmt angular junction of the ventral and cranial margins of the pallitnn. On either side of this line of adherence the neural tube gives rise to projections (27), not quite syrumetrieal, which bear the same relation to the medullary plate as the large ganglia at their inception. The element of the right side, which is somewhat the larger. contains in its interior two small cavities which, however, do not communicate with the lumen of the neural tithe. We are unable to offer an_v suggestion as to the significance of this structure, nor have we found in _vounger or older (.‘Hll)I‘_\'I)s' of the cut any corresponding structure.

The thalamencephalon (In) that Iunininllla1‘_y region (Ii) togetlter form a well-defined segment or triangular outline interposed between the foregoing structure a.nd the midbrain (15). The latter shows an increase of length in its dorsal zone. The hindhrain (litters but little from that of the preceding entbryo. Its first segment (31) is sonic-what compressed; the second (12) is subdivided ventrally and the profundus ganglion (30) is attached at almut its \'t-rtical iniddlc. close to its caudal margin. -The third segment is also subdivided ventrally; the attachment of the quintal ganglion (3) extends close to its cranial border. These two ganglia are tlms approxinmted and on the point of union which is ultimately effected in einhryos of t\veut_\'-si.\' somites. The third segment is followed by a slight prominence (I4) confined to the ventral region of the neural tube and less clearly niarked otf t'ron1 the m_velencephalon than in the preceding embryo. Above it is the acoustico-t'acial ganglion (4). The relief of the following neuroineres is faint and to be made out only with great dittieulty in this embr_\;o. The model of the lumen (Plate XLI) corroborates the description we have given of the external relief, but in addition presents one or two details which are not perceptible from the surface. The plica ventralis encephali by its broad summit forms the floor of the 1I1i(ll)l'zlillA Its anterior angle, the tuberculum postero-superius of authors, juts l'oI'\\'a1'(l prominentl_v. linmediatel_v below is the recess of the mannnillar_v region (17) bounded ventrally by the tuberculum postero-inferius (2). Ventral to this again is the large triangular cavity of the int'undibular region (18). its ventral wall shows a moderate thickening. torus postoptieus, in front of which is the shallow preoptic recess, from which the lamina terminalis extends forward and upward to a point at which the ectoderm (7) is adherent.


We have now completed the record of our objective findings on the basis of which we propose to discuss the problem stated in our opening paragraph; we shall endeavor in the course of this discussion to compare our results with those of other students only in so far as they have dealt with mammalian cinbrvos of corresponding stages of development. I'nfortunatelv the number of detailed descriptions of such einbryos is not large. We have not, tlieretore, attempted any general comparison of the ontogeny of the mainmulian ncura_\'is with that of better lmo'u'n forins, except in a few instances when it has a direct and important bearing upon our problem.

It has already been stated Ihitt thc nicdullai'_v plate primitively lacks a precise lmundary and passes h_v a gradual thinning into the somatic <-‘V‘Wl0"ll (Plate XXll. l’i,<,:. 1. Jo‘). Prior to closure. lioivever, and this is true of the In-ad as well as the trunk. an abrupt demarcation is established and the somatic ectmlerm joins the inedullarv plate at its domemedinn angle (Plates .\'.\'\'lll-.\'.\'.\'). 'l‘his reinodeling of the neurosoluatic junction is pi'o;.rI'c.~'.~‘i\'c ccphnlo-cnudnd. and is coniple‘.ed in each region before closure occurs. (‘oiueident|_v the Iuedullary plate narrows markedly so that it is probable that the primitive region of transition is thinned out and added to the somatic ectoderni. When closure is effected the eetoderin is closely applied to the concave dorsum of the neural tube and is continuous from side to side, without median attachinent to the neuraxis, and with no interveuin,<_r cells (Plate Xl.l). We wouhl emphasize these facts, for they patentl_\-' exclude the interpretation of the gan_4lionic crest of the trunk as an element intermediate between the general ectoderm and neural tube, and primitivelv imlependent of the latter. In this point our observations are in accord with the sections of the cat ligured by fleiselimann‘ and of the human cnibr_vo by l<'elix"' and agree with Xeumayer's° observations upon the trunk region of reptiles. It would seem to follow, therefore. that the g2ln,I_{lionic crest of the cat is a demative of the neural tube, and this view acquires an antecedent probnbilitv from the occasional retention of afferent ganglion cells in the ueuraxis ol the adult, as in Amphioxus and Teleosts. and in the mesencephalic root of the trigeminns of nunnmuls.

Ganglionic crest

In the cat, the ganglionic crest is formed in the trunk by it simple delamination of the dorsal less regularly arran_:ed t-ells at the summit; of the neural folds. Shortly after closure a minute cleft appears on each side and atlvances towards the midline. until the crests are attached only by a narrow niediun strand. Again our results are concordant with those of .\'euma_ver for reptiles.

Sometimes, and not always s_\'nnnetri(-all_v, prior to closure, a faint furrow appears on the ental surface of the neural l'old close to the junction with the somatic ectodcrm. 'l‘his we have taken to indicate sortie small degree of lateral movement of the cells at this point as though to form an evagination. 'l'he process is abortive but siiggests that in the derivation of the ganglionic crest from the neural tube delumination may have been substituted for eragrination. and a solid anlagze may have replaced a hollow one, as elsewhere in the ontogeny of forms rich in cells.

Farther cephalad the eva,t.rination bccolnes c0Il:4picll0lls_ In the acoustico-facial region (Plates XXVI and .‘{X.\') there is a shullmr oblique furrow unaccompanied by 1-va;_riuutiou and the ,-ran,-zliou seems in a;_rree


Tat’. III. figs. '5-1'1. \\'1e2xbnden. 18319. -" W. I-‘r:i.r_\': "Dle Entwlrkeluniz «Ir-r llarn und (ie:4cl|lrrlll:4uI'L:-III(- In Krllk-I und .\tuII_'

..,.y '0'

Hnndbuch I-Intwlckelnngzstresclilclite des .\[r-nschen. I-‘lgs. .n__. --—-I. .‘>'_'.'s—.'::I0_ 19|1_ “ L. ‘.\'i-‘,l'.\I.n'I:n: "Zur Miirpliologle dc.-I ('entrnl .\'ei'vens_\-stems der (‘In-lnnlor und (‘rm-u

dllies.“ Ans Voeltzkow Reise In Ostntrtkn in den Jnhn-n_ H103-1!\n.'u. “and l\‘_ 1914 Cher den Hchlnsn der Sekilndnren !\l«-dullnrfurche und die Gen:-so der .\'enrnlIelste_ \'er hnndl. Annt. Gen. 22. 1913. "”l-‘"0K<'nese und .\[nr|)|IoIl¢'m’-"*’ 0°-‘ l""'ll’l“""'" -Vvrwn systems. (ler flplnnltznngllen und dos .\'r-rvns s_\'mpullIo-th-u.-1." Ilunrllmcli ch-r \er;:|--lch, und expel-lint-nt. I-Intwlrlu-Iunizslr-hreder \\'lrIu-Itlil--re ll:-rtutpz. I'll"?

with the more caudal ones in owing its origin to delamination from the medullary fold. In the quintal anlage the sulcus is conspicuous and its horizontal por tion is associated with a ridge-like projection (Plates XXV and XXIX) in the angle between neural plate and somatic ectoderm, which yet reveals its closer affinity to the neural plate by blending with it at both of its extremities in the four-somite embryo. It then appears subtended by a longitudinal furrow, and differs more in size than in any essential character from the more caudal ganglionic crest.

Finally, the optic vesicle, a pure evagination, presents at an early stage much resemblance to the quintal anlage (Plates XXIV and XXVIII), from which it differs chiefly in size and in the more ventral position of its sulcus.

We see in these anlages a series of structures, passing by gradations from the delaminated ganglionic crest through the acoustico-facialis and quintus to the optic vesicle, which is formed by evagination alone. It seems to us, therefore, that the primitive neural plate in the cat gives rise both to the neural tube and to the ganglionic crest, the latter being a derived and secondary element and not a coordinate intermediate between the medullary plate and the somatic ectoderm. In the forebrain elements equivalent to the ganglionic crest are retained in the wall of the vesicles and constitute the dorsal region of the neural plates, for it’ the crest SOC0n(ltlI'il_\‘ separates from the neural tube, in regions where -such separation fails to occur, it is more probable that the crest is included in the brain than that it has been absolutely suppressed. Acr-arrlingI_:/. UH’ mml_z/sis of the prosem-cp/ialon is not to be atmnptcd in (max of the basal aml alrzr plates a/onc. as It/is been ru.s-lonm.r_// stare His, but nzust inri’mlr- (I 4]r;rs(l/ or crtal strip cquira/en! [0 HM g(1n_r//ionic crest along its ronrr.ri _:/, (mil tliis mus! in.r/udc at least as much of the brain trail as lies Mal to the optic sulcus. This ganglionic element (the primitive optic vesicle) at four somites forms the cephalic extremity of the neural fold. and arches ventrad to the floor. If our argument is correct. the optic vesicle and the ectoptic szructures, whether above, in front or below the optic region. must be considered of ganglionic equivalency, a conclusion which entails a revision of His’s analysis of the brain.

The question of the substitution of mesectoderm for a Aneurogenic ganglionic crest in the protitie region in mammals can hardl_v, we believe, receive an attirmative answer in view of the conditions observed in the cat, for we find quite generally the separation between ectoderm and neural tube clean cut, and the space between these structures unoccupied by cells. The nmaectodertn of the iehthyopsid and sauropsid embryo, described by Neal,’ Johnston,“ Neuinayer,“ and others, is, perhaps, not altogether fortunately designated ganglionic crest, for these investigators describe it as taking origin from an intermediate element, interposed between neural tube and ectoderin and derived from both of these structures. On the other hand, the ganglionic crest in the trunk in sauropsids (Neuma_\'er) as well as in the cat is a derivative from the neuraxis, as in the latter form are also the cranial ganglia. It would seem, therefore, that having slightly different derivations the mesectodemi and the ganglionic crest ought not absolutely to he homologized. Neumayer has formulated this standpoint clearly, as follows:

“So zelgt slch lm Aufbuu (les cerebrulen Teiles der Ganglienlelste. sowelt ale dem Arehencephalon und dem praeiitlschen Geblet des Hlrus angehiirt, eln Verhnlten, das slch nmrphogeuetlsch wesentllch von den elnfacheren Verminnissen lm postotlschen und splnalen Gebiet unterscheldet. Ich kann mlch fiber die Vorgtlnge hter lm Anschluss und die oben (11. 452) gemachten Angnben in Ktlrze tassen. Entsprechend dem lm splnalen Geblete anders ertolgenden Verschlusse des Medullarrohres wtrd das zuin Aufbuu der Gangllenlelste notwendige Zellmaterial bet Crocodiles -nmdagescarmtsés und Em:/8 {maria nur voxn Dnche der Medullaranlage gellefert; die so entstandene postotlsche Gangl1enleiste lst demnach wesentllch verschleden von der cerebralen pruciitlschen. Sle entbelirt des primtiren, nus Exoderm und Mednllarwand entstummenden Antells und enthfilt nur jene Iilemente. welche in die cerebrale pi-aeiitlsclie Ganglienlelste sekundilr elntreten.

“Hlehzu komint nucli eln,L'n,ters(-hied in den Leistungen der belden Gangllenlelsten: von lhnen llefert die cerebrule. praeiitlsche in glelcher Welse Nerven und mesodermules Gewebe fiir das priiotlsclie Kopfgeblet, wilhrend das splnale, postotlsehe Gangllenlelstensystem einzlr: Nervengewebe aus slch her vorgelien liisst."

In the cat, the paraxial mesoderm of the head is abundant and Very early becomes loosely arranged. extending dorsad and forcing its way into the cleft between the ectoderm and the dorsum of the neuraxis. We have stated that at the time of separation of these two structures, their demarcation was sharply defined and without intervening cells, nor were we able to find mesoderm at any time in this situation, which was not continuous ventrally with the general iuesodcrin of the head. This lack of evidence of the fonnation of mt-set-tnrlerrn in the cat inclines us strongly to accept .\'eumayer‘s distinction of a primary and secondary prootic ganglionic crest, the former (iuesectoderm) being undeveloped

7 H. V. NEAL‘. "The Segmentation of the Nerroiis System in Squalus Acantlilafl." Bu” Mus. Comp. Zool. Cambridge, Mass, Vol. XXXI. .\'o. 7. 1898. ’J. B, J0}{Ns1‘o_\1; The Nervous System of Vertebrates. l'hIladelphla. 1906. "The

Morphology of the Forebraln Vesicle ln Vertt-brnle.~I." Jmlr. 1.‘umr>- -\'¢'III'0l- Mid l'9.VC'1°'-

in the cat, the latter arising from the neuraxis by delamination com~ bined with the evagination constitutes the cranial ganglia, and at the extreme cephalic pole, failing, we believe, to separate remains incor porated as the primitive optic vesicle.

Closure of the Neural Tube

At the extremities of the axis the elevation of the neural folds may be resolved into two acts. Cephalad the median or basal portion is first elevated, while laterad the plate has still a horizontal direction. This condition is present in the embryos of two and three somites (Plate XXIII, Fig. 2). The condition seems to be the effect of modeling of the ectoderm upon the paraxial accumulation of mesoderm. At four somites (Plate XXVII, Fig. 1) the fold is erect in its whole extent and rises well above the mesoderm, the somatic ectoderm being closely applied to the neural plate in its dorsal half. Caudad the process differs; the lateral part of the plate is first elevated at some distance from the median line (Plate XXVI], Fig. 1, and Plate XXVII, Fig. 2) and forms a low wall for the broad rhomboid fossa. When the tube closes here, its diameter is much less than the width of the fossa would lead one to expect. This is suggestive of the possibility that the lengthening of the tube is not due alone to axial growth, but may be assisted by a rearrangement of the material of the neural plate in the sense of a shift towards the median line so that the plate is extended c"a‘udad as it narrows. The elevation of its lateral margins is associated with the moderate ent_vp_v of the blaatoderm and the early completion of the amnion at its caudal end.

The neural folds first meet in the region of the future mesencephalon, but their closure is not simply progressive from this point in both directions. On the contrary, it is incident simultaneously at several points which may be rather widely aeparated. In the eight-somite embryo, in addition to the closure of the midbrain, which extends from the optic anlage to the quintal ganglion, there is a second closure between the qnintal and acoustico-facial anlages; a.nd again, after an interval. at a third point the folds seem on the verge of meeting (Plate XXV”, Fig‘ 2). There is also some fusion cephalad at the ventral margin of the neuropore. This is of some theoretical importance and diminishes the significance of the neuropore as a morphologic landmark. The gaping of the tube in the region of the optic and quintal anlages suggests that such structures in some way delay closure. but as the neurosomatic junction is now sharply defined, it is difliciilt to believe that this is in the interests of a hypothetical inrolling either here or in the optic vesicles.

At nine somites the neuropore is shorter, by reason of closure at both its ends but chiefly caudad. Its extremities now lie in the same vertical plane. The quintal hiatus is closed at two intermediate points, presenting three small orifices (Plate XXXI, Fig. 1). Caudad to it a short segment of the tube has closed. A small hiatus is present in the region of the acoustico-facial ganglion. This embryo, therefore, gives additional evidence of the retardation of closure in regions of large ganglia.

Further irregularities are shown in Plate XXXII, Fig. 1, and Plate \XXXIII, Fig. 1, in the trunk region, and in Plate XXXIII, Fig. 2, and Plates XXXIV-XXXVI in the anterior neuropore. As this latter closes ventrally as well as caudally, and irregularly in the intermediate portion, it seems diflicult to consider that its region of latest obliteration has any fundamental morphological importance or can at all properly be used to determine the cephalic extremity of the neuraxis, which is certainly deflected ventrad. Further, if the wall of the neural tube is divided into basal and alar plates, its cephalic pole ought to be the most cephalic point in their line of union, i. e., in the terminology of His, the cephalic extremity of the sulcus limitans, when this sulcus can be recognized. It would seem, therefore, that wherever this point is localized it cannot be situated in the raphé which throughout its length is assumed to be a suture between the summits of the alar plates, or, if our interpretation prove correct, in the forebrain, between retained ganglionic zones. To accept the last point of attachment of the ectoderm marked by the recessus neuroporicus as the extremity of the axis, implies that the raphé below this point is a suture between the basal plates, although it has never been shown that they were primitively cleft; further it would seem the necessary consequence of the acceptance of this landmark (recessus neuroporicus) that the mammillary and infundibular regions and the ventral half of the optic vesicles themselves were derived from the basal laminae. To accept the recessus neuroporicus as the ontogenetic pole of the brain seems, therefore, to disregard the ventral deflection of the neuraxis and the composition of its wall of basal and alar plates. In His’s three months’ embryo (No. 7 of Ziegler’s series of models) the sulcus limitans passing forward in the midbrain is continuous with a furrow which arches ventrad and reaches the midline immediately in front of the oculomotor nucleus. This, we believe, is actually the sulcus limitans demarcating the alar and basal lamina,and reaching the midline where the latter ceases to give evidence of its existence, 1'. e.. immediately in front of the oculomotor nucleus, the most 338 .41v.r.u.s NEW roar: .4c.4n1my or sermons

cephalic structure which by its derivatives can be assigned to the basal plate. A second furrow, not connected with the foregoing, extends across the wall of the thalamencephalon, nearly horizontally, to the foramen of Monro. This sulcus of Monro, His interpreted as the continuation of the sulcus limitans. The interpretation we have suggested is at least as concordant with his observations and does not entail morphologic impossibilities.

In order to facilitate the comparison of these divergent interpretations we subjoin two schemata; the first (Fig. 1) is based on the well-known


Fro. 1.—8ohenIa of the composition of the eucephalon In terms of basal and alar plates of Ills

1. Basal plate. 2. Alar plate. 3. sulcus llmltans.

figure of His; the second (Fig. 2) illustrates the region of the neuraxis which must be assigned to the ganglionic crest on the basis of our interpretation. We have retained from His’s figure the sulcus limitans as it appears in later stages of development for the purpose of defining the basal plate, although in the period of development of the cat covered by our series of embryos this furrow is not even indicated.

Prosencephalon

The elevation of the neural folds at their cephalic extremity is accomplished in two phases, affecting first their basal, later their alar portions


(Plate XXII, Fig. 2; Plate XXIII, Fig. 2, and Plate X.\L\'ll, Fig. 1). The process is completed in the 8Illl)I'_\‘0 of four somites. Two conspieu~ ous landmarks are now p1'eseut—the tubercle of the floor and the optic sulcus. Corresponding to the latter is a tliick-walled evagination, the optic vesicle (Plate XXIV). This anlage resembles those of the quintal and acoustico-facial ganglia and is peculiar only in the course of the sulcus, which here approaches the floor at its cephalic and not its caudal extremity, as in the case of the other ganglia. The tubercle of the floor intervenes between the terminal portions of the optic sulci. Ventrad it


fig. 2.—8chema of the compoamon of the encephalon in terms of basal and alar plates and ganglionic zone

1. Basal plate. 2. AIM plate. 3. Sulcus llmltnns. -I, Ganglloulc zone.

is in relation with the blind extrernity of the foregut; the stomodzeuui approaches, but hardly reaches it, from in front. By means of these relations the tubercle is easily recognized in succeeding stages. when ultimately it forms a transverse ridge intervening between the manimillary and infundibular regions.

The wall of the optic vesicle is divided by the sulcus into a ventral portion adjoining the floor plate and an ectal zone extending to the neurosomatic junction, and therefore forming the summit of the medullary fold and later the lateral lip of the neuropore. Ventrally this zone joins the tubercle of the floor, which so constitutes the ventral neuroporic lip. It is obvious, but important to note, that the optic vesicle at this stage forms the cephalic extremity of the neuraxis. Preoptic structures in consequence must be derived secondarily, either from the ectal zone of the optic vesicle, or by inrolling of somatic ectoderm at the neuropore.

Of the latter process there is no evidence in the cat, and this altemative seems excluded by the abrupt character of the neurosomatic junction. We may, therefore, confine our attention to the optic vesicles and summarize the changes by which they give rise to an ectoptic zone which includes the anlages of the thalamencephalon, telencephalon and the infundibular region. Most striking is the progressive and absolute reduction in size of the optic vesicles, which are actually smaller at the stage of sixteen somites than they were at eight. Coincidently they cease to occupy the whole vertical extent of the wall of prosencephalon and become relegated to a ventral position. The nature of these changes, especially the reduction in size of the optic vesicle, permits of but one interpretation, namely, that the ectoptic zone is formed at the expense of the vesicle. A similar remodeling of the ventro—caudal portion of the vesicle gives rise to the infundibular region.

As the tubercle of the floor constitutes the extremity of the floor-plate and at the same time the primitive ventral lip of the neuropore, it is of prime importance‘ to ascertain its position in subsequent stages of development. It is easily recognized by its thickness and its interposition between the ventral ends of the optic sulci. At first, also, it is abutted upon by the cul-do-sac of the foregut which, however, rapidly recedes from it, at eight somites only reaching its caudal extremity and at ten somites terminating beneath the midbrain.

In Plate XXXIX, Fig. 1, the ental surface of the brain is shown by a mid-sagittal section of an embryo of eight somites. The tubercle of the floor (9) forms the ventral lip of the neuropore; its cephalic extremity is connected with the suprasulcal portion of the optic vesicle of each side. From the parieties it is separated by the shallow prolongation of the optic.sulcus which terminates in a depression of the floor, immediately behind the tubercle and above the foregut. There is, as yet, no corresponding elevation of the ectal surface; the recess is the first evidence, in our series of the mammillary region (17). In the embryo of ten somites (Plate XXXIX, Fig. 2) these fundamental relations are still recognizable, although important changes have supervened. Coincidently with the ventral deflection of the extremity of the neuraxis, the tubercle of the floor (2) has assumed a vertical position. A considerable degree of closure has been effected ventrally in the anterior neuropore, so that the tubercle no longer constitutes its ventral lip. The mammillary recess (17) is better defined, the mammillary eminence projects laterad but scarcely as yet forms a prominence in the inid-ventral line. As a consequence of closure ventrad in the neuropore and of the ventral deflection of the optic vesicles, a prominent angle is formed immediately below the tubercle of the floor, which now intervenes between the two recesses, the marnmillary above and the one just described, the infundibular (18), below. The optic sulcus meets the floor—p1ate in the infuridibular region, but a shallow prolongation can still be followed beside the tubercle of the floor. In the embryo of twelve somites (Plate XL, Fig. 1), slight changes have superveued. The floor as a whole is thinner, which may be taken as an expression of the expansion of the cavit_\', and the tubercle of the floor (2) is no longer a conspicuous thickening. The infundibular region_ (18) has increased in size and its cavity is more widely confluent with that of the optic vesicles. Coincidently the teriiiinal portions of the optic sulci undergo reduction and lose their intimate relation to the tubercle of the floor. A very faint furrow may he followed from the optic vesicle across the parieties just above the remnant of the tubercle, while the main line of the optic sulcus extends into the infundibular region. The tubercle of the floor is now losing its demarcation from the parieties with the etfacement of the primitive ventral segment of the optic sulcus, and from this period appears as a transverse ridge intervening between the mammillary and infundibular regions. It is, therefore, evident that the mammillary region arises from the cephalic extremity of the primitive floor—plate and that the infundibular region is a derivative of the primitive optic vesicles.

Not only ventrally but also dorsally the periphery of the optic vesicle undergoes a remodeling and important new conditions are established. First, a prominence is formed immediately in front of the anterior isthmian sulcus, the thalamencephalon, and subsequently the telencephalon emerges in front of this. The two elevations are separated by a slight depression, the first indication of the velum transversum, from the stage of thirteen soinites, the earliest period at which the teleneephalon is recognizable. Both of these structures appear in the lip of the anterior neuropore prior to its closure in their respective regions, and are accompanied by a recession of the optic vesicle from the margin of the medallary plate and, what is of major importance, an absolute diminution in the size of the vesicle. This is well marked in the period of from ten to sixteen somites when the thalamencephalon, telencephalon and infundibular region are well defined and the optic vesicle reaches the margin of the neuraxis at only a single point between the infundibnlar region and telencephalon. From this period the ventral pole of the vesicle slowly recedes from the margin, and the telencephalon becomes continuous with the infundibular region. There has thus been formed from the periphery of the vesicle a series of derivatives which constitute an ectoptic arcade and in each case the process has been the same, a remodeling of the primitive optic vesicle, so that its central region ultimately constitutes the definitive vesicle, while its periphery becomes allotted to the anlages of the other elements of the prosencephalon which are, therefore, ectoptic in their arrangement and cannot be reduced to a linear series of neuromeres referred to the longitudinal axis of the neural tube. We have now summarized the ontogeny ot' the forebrain as it appears in our series of embryos of the cat. lt is hardly necessary to add that we are not offering these conclusions as an explanation of the phylogeny of the mammalian brain.

Mesencephalon

Closure of the neural tube and its separation from the ectoderm are accomplished first in the midbrain. As elsewhere, the resulting roof is concave. The concavity is present in the whole length of the neuraxis but its degree increases cephalad and is most conspicuous in the forebrain. In the mesencephalon it is marked and persists to the stage of sixteen somites: it is associated with a sagittal ridge which depends into the lumen entally. In section the ridge is often constricted at its base of attachment and occasionally a fragment of it is found separate as a small group ot' cells within the neural canal. In the embryo of eight somites the mesencephalon (15) is closed in half of its extent and forms the highest region of the neural tube (Plate XXVII, Fig. 3). At nine somites the closure is complete, the midbrain has lengthened and its height has markedly diminished (Plate XXXI, Fig. 2, 15). A comparison of these two embryos throws some light upon the nature of these changes. The distance between the quintal anlage and Sessel’s pocket is the same in both models, but the interval between the optic vesicle and the quintal ganglion has markedly increased, the axis of the optic vesicle has altered and the forebrain has come to project strongly ventrad at right angles to the rest of the neuraxis. These facts taken in conjunction with the diminished height of the midbrain cannot be adequately interpreted on the principle of unequal growth alone. It must be taken as the expression of a remodeling of the whole region, in particular a lengthening of the dorsal portion of the midbrain without corresponding increase of its ventral parts, in consequence of which the optic vesicle has not only been displaced but rotated through 90° and the neuraxis has been hent ventrad in the region of the forebrain. There are no signs of compression in the mesencephalon but the slight bulge of the mammillary region is, perhaps, the result of flexure.


ln embryos of ten to twelve somites (Plates XXXII and XXXIII), the mesencephalon acquires a triangular profile, demarcated from the hindbrain by a shallow constriction which gradually becomes pronounced and has a vertical direction (the posterior isthmian sulcus 23). The anterior isthmian sulcus (2:2) is horizontal and forms a sharp boundary against the mammillary region but becomes shallow cephalad near the pole of the optic vesicle. A slight transverse depression of the roof separates the midbrain from the thalamencephalon (16). The walls are convex and entally show nothing which can be taken for the sulcus limitans of His. Ventrad the isthmian sulci converge to the angle made by the forebrain with the floor of the neuraxis.

In the thirteen and fourteen somite embryos (Plate XXXIV), the mesencephalon is gradually bent ventrad and comes to form the most cephalic point of the brain. This bend is associated with clear evidences of compression in the floor of the hindbrain. In the sixteen-somite embryo (Plate XXXV), the mesencephalon has been carried slightly beyond the crown of the cephalic arch and conjointly with the prosencephalon (19) makes a right angle with the hindbrain. The isthmian sulci now converge at an acute angle and the midbrain reaches the ventral margin only by its pointed extremity. At. this stage, a nuchal bend is well defined and the effects of compression upon the mesencephalon are at a maximum. From this period to that of twenty-one somites, the midbrain lengthens in its dorsal segment and chiefly in a cephalic direction, as is shown by the alteration in the angle at which the anterior isthmian furrow meets the floor as well as by the increased flexure of the forebrain (Plates .\'X.\"\'l-.\'XX\'III). We have described the midbrain as a single segment without subdivision into neuromeres, for, though we have searched for evidence of a constriction, we have been able to find none in any of our embryos save that of ten somites, No. 476, ‘and here with less certaintlv than could be wished. A faint concavity was present between the isthmian furrows and i-nclined so that its continuation would have bisected their angle and divided the midbrain into a sliglitly larger cephalic and smaller caudal portion. It was confined to the dorsal portion of the neural plate without, however, causing a depression in the roof. The brain in this embryo was somewhat spirally twisted to the right and the depression in question was not quite symmetrical on the two sides. For this reason and because in all our other embryos the midbrain attains its greatest width precisely in the region where this exceptional furrow appears. we are inclined to attribute its presence to the unusual twist of the head.


Rhombencephalon

The large ganglia of the quintus, acoustico-facialis and profundus, which develop in the order named, are asociated in their inception with sulci. These, beginning near the neurosomatic junction, and at first parallel to it, eventually turn ventrad pursuing an oblique course across the medullary fold. Each furrow is thus composed of a cephalic segment, intimately concerned in the formation of the ganglia and a caudal portion which, becoming broad and shallow, occasions a dilatation of the neuraxis. As the tube closes three oblique segments are formed. In the embryo of ten somites (Plate XXXII, Fig. 2), where we were first able to recognize the profundus aulage distinctly, four segments were present. The first (31) is triangular in form with base ventrad, extending from the posterior-isthmian sulcus to the oblique furrow at the summit of which is the profundus ganglion (3a). This element has no ganglion associated with it developmentally but owes its demarcation to the establishment of a caudal boundary of the midbrain in the posterior isthmian sulcus. It is followed by three oblique ganglionic segments which are the expression of the oblique caudal portions of the ganglionic sulci in the interior of tube. Externally three oblique intersegmental constrictions are present, so that the third ganglionic segment lacks a caudal boundary and merges into the relief of the myelencephalon. The ganglia are situated at the summits of these furrows and are accordingly intersegmental in their points of attachment, as was first pointed out by Miss P1att.‘°


This configuration is retained to the stage of thirteen somites. In these embryos, and very vaguely in the more advanced embryos of twelve somites, the surface of the neural tube behind the ganglionic segments becomes marked by alternating constrictions and dilatations. These, in marked contrast to the ganglionic segments, are vertical in position and correspond to the mesodermic somites which abut upon the neuraxis in the intervals between the dilatations. These vertical segments are undoubtedly the myelomeres of McClure“ and are widely different structures from the oblique ganglionic segments of more cephalic position. Primitively the two series are separated by a considerable interval which is not effaced until the stage of fourteen somites, the most cephalic myelomeres being relatively late in appearance. Their retardation, we believe, is due to the small size of the iuesoderniic somites in this region, so that they are slower in producing an effect upon the modeling of the neuraxis. There is besides some evidence in the cat that somites are added at the cephalic end of the series, which also would serve to explain the retardation of the corresponding myelomeres. We are assuming that the segmentation of the neuraxis into myelomeres is ontogenetically secondary to the segmentation of the mesoderm, a view which receives support from the fact that the number of myelomeres always lags behind that of the somites, as well as by the fact that where the somites are small and possibly retarded in appearance developmentally as in the region just considered, there also the myelomeres are late in appearing.


“’ J. B. PLA'|"l‘: “A Contribution to the .\IorphnIog_v of the Vertebrate Head based on a study of Acanthlas Vulgsrls.“ Jour. .\iorph.. Vol. ‘v’. 1891.

“C. F. W. Mccwns: “The Prlmltlve Segmentation of the Vertebrate Bi-nln." Zool.


It is possible, therefore, to recognize two principles of segmentation in the deuterencephalon; the first incident to the formation of the cranial ganglia, the second associated with the segmentation of the mesoderm, for it is to be noted that the appearance of the ganglionic segments long antedates the branchiomeric segmentation. When, therefore, the series of myelomeres becomes continuous with the ganglionic segments the result is not a meristic series of.equivalents but comprises structures diverse in their genesis and heterogeneous in their products.


In the stage of fourteen somites, the boundary between these two series is gradually efiaced and important changes supervene in the ganglionic segments. The first and second of these become subdivided ventrally. This is initiated in the first segment at the stage of fourteen somites, in the second segment at nineteen somites. The third maintains itself as a small dilatation immediately in front of the first somite. These changes coincide with the formation of the pontine angle, the surface of which is marked by five elevations corresponding to the third ganglionic segment and the subdivisions of the first and second. In addition a small prominence, corresponding to the first myclomere, is situated immediately caudal to the last of these elements and is also recognizable at twenty-one somites (Plates XXXVII and XXXVIII). If we now add to our reckoning the pre-ganglionic segment abutting upon the posterior isthmian sulcus, a total of seven elevations is reached for the hindbrain, a number within the limits of the count given by students of the region in mammals, variation in which might well depend upon the age of the embryo studied. It would seem, therefore, that these elevations correspond to the neuromeres of authors. We have endeavored to show that they are secondary and heterogeneous.


Annotations of Leaders on All Plates

1. Optic sulcus or vesicle. 2. Tubercle oi.’ the floor. 3. Quinta] sulcus or ganglion. 8a. Profundus ganglion. 4. Acoustico-facial sulcus or ganglion. 5. Meduliary plate or medullary fold. 6. Primitive groove. 7. Somatic ectoderm. 8. Anterior neuropore or its vestige. 9. Qnintal hiatus. 10. Acoustico-facial hiatus. 11. Other hiatus in line of closure. 12. First ganglionic segment. 12a. First ganglionic segment; its cephalic portion. 121). First ganglionic segment; its caudal portion. 13. Second ganglionic segment. 18a. Second ganglionic segment; its cephalic portion. 181). Second ganglionic segment; its caudal portion. 14. Third ganglionic segment. 15. Mesencephalon. 16. Thalamencephalon. 17. Mammillary region. 18. Infundibulnr region. 19. Telencephalon. 20. Ganglionic crest. 21. Rhomboid fossa. 22. Anterior isthmian sulcus. 23. Posterior isthmian sulcus. 24. First myelomere. 26. Velum transversum. 27. Excrescence associated with vestige of anterior neuropore. 28. Region of transition between somatic ectoderm and neuraxis. 29. Mesoderm. 30. Entoderm. 31. Preganglionic segment of deuterencephalon.


Plate XXII Sections of Embryos before and After Segmentation

Fm. 1.—;.[‘ransve1-so section of an embryo prior to the appearance of intersomitic ciefts. Columbia Collection No. 550, slide 3, row 4,‘section 7. X 150.

5. Meduiiary plate. 7. Somatic ectoderm. 28. Region of transition. 29. Mesoderm. 30. Entoderm. Fm. 2.—Trnx_1sverse section of an embryo of one somite.

Columbia Collection No. 594, slide 5, row 2, section 6. X 150.

5. Medullury plate. 7. Somatic ectoderm. 28. Region of transition. 29. Mesoderm. 30. Entodenn. /2.... :1. ./.Z:.. 1.3.2.. IX:

Plate XXIII Sections of Embryos with Two and Three Somites

Fm. 1.—T1‘ansver5e section of un embryo of two somites. Columbia Collection No. 539, slide 5, row 4, section 4. X 150.

5. Meduiiary plate. 7. Somatic ectoderm. 28. Region of transition. showing presence of a shallow furrow. 29. Mesoderm. 30. Entoderm.

Fm. 2.—Transverse section of an embryo of three somites. Columbia Collection No. 593, slide 5, row 4, section. 4. )< 150.

5. Medullary plate. 7. Somatic ectudeum. 28. Region or transition. 29. Mesoderm. 30. Entoderm. _ 4 . w .1114.


Plate XXIV TRANSVERBE snc.-mom, szmnmo or mun

Transverse section of the embryo of four somltes, Columbia Collection No. 409. slide 5, row 6, section 10, passing through the optic vesicle. X 150.

1. Optic sulcus. 2. Tubercle of the floor. 5. Medullary told. 7. Somatic ectodenn. \'m,t'.\u>: .\'X1\', 1’|,.\n; XXXV


Transverse section of the embryo of four somltes, Columbia Cnxlle.-cum: No. 409, slide 5, row 5, section 5, passing through the quintal unlnges. X 150.

3. Quinta! sulcus. 5. Medullury fold. 7. Somatic ectoderm. \u1.I'.\n: X.‘\'l\'. l'x..n‘F. XXV


Plate XXVI rnansvsnss sE(.‘Tl0.\', ICMBRYO or mun Somites

Transverse section of the embryo of four somites, Columbia Collection No. slide 6, row 2, section 10, passing through the acoustico-fnclal anluges. X 150.

4. Acoustico-facial sulcus. 5. Medulinry tom. 7. Somatic ectoderm. fiwzv. ..«,r<._._ ..:.zr.Z :,.._._...» /. 3.1. .. /. /,._4..//2. PLATE XXVII nzconsrnucrlon or NEURAXIB or nzumwos or rouu [mu mam‘ sn.urrEs

Fm. l.—Reconstruction of an embryo of four sumltes. Columbia Collection No. 409. ,V, 210. R(’(lIl(f(’d 7%.

lI‘1o. 2.—Iieconstruction of the ncuruxis of an embryo of eight somitcs. Columbia Collection No. 530. Dorsal view. X 210. Reduced $4.

Fm. 3.—Same reconstruction, lateral view. X 210. Reduced $5,.

1. Optic vesicic. 2. Tubercic of the floor. 3. Quinta] aniugc. 4. Acoustico-facial aninge. 5. Meduiiury told. 6. Primitive groove. 8. Anterior neuropore. 9. Quintui hiatus. 12. First ganglionic segment. 13. Second ganglionic segment. 15. Mesencephalon. 21. Bhomboid fossa. 31. Pregangiionic segment of deuterenceph alon. A\\'.u..~x N. Y. Amn. Sm.


TRANSVERSE SE(."l‘Xl)l\', EMURYO OF EIGHT SOMITES

Transverse section passing through the optic vesicles of an embryo of eight

somltes. Columbia Collection No. 530, slide 5, row 3, section 3. x 150.

1. Optic sulcus. 5. Medullary plate. 7. Somatic ectoderm.


Plate XXIX TKANSVEHSE SECTION, muumro 01-‘ muxrr Somites

Transverse section passing through the unluge of the qulntal ganglion or an embryo of eight somites. Columbia Collection No. 530, slide 6, row ,2, section 5. X 150.

' 1. optic" sulcus. 3. Quintal ganglion (the suléus is still present). 5. Medul lnry plate. 7. Somatic ectoderm.

I'l.A'1‘E XXX 'rnANsv1«:ks1-: Sl<)C'l'ION, 1-:MnIn'o 01-‘ 1«:1un'r .~mm'rI-:s

Transverse section passing through the unlages or the acoustlco-tnclal ganglia of an embryo of eight somites. Columbia Collection No. 586, slide 4, row 2, section 1. X 150.

4. Acoustico-facial ganglion (the sulcus is still present). 5. Medullary plate. 7. Somatic ectoderm. VOLUME: XXIV. I‘LATl-2 XXX

1 A<‘_u.\ .-,. PLATE XXX! nscoxsrnvonon or NEURAXIS or nunnvo or NINE sommas

l<‘m. 1.—l)o1'si\l view. Columbia Collection No. 531. X 210. Reduced %.

Fm. 2.—Lateral view.

1. Optic vesicle. 3. Quintal ganglion. 4. Aconatico-facial ganglion. 7. Somatic ectoderm. 8. Anterior neuropore. 9. Quinta! hiatus. 10. Acoustico-facial hiatus. 11. Another hiatus in line or union of the meduiiary toids. 12. First ganglionic segment. 13. Second ganglionic segment. 14. Third ganglionic segment. 15. Mesen— cephaion. 16. Thalamencephalon. 17. Mammiilary region. 20. Gangiionic crest. 21. Rhomboid fossa. Vo1.L'.u: XXIV, I'I.A'n-: XXX]

Plate XXXII RECONSTRUCTION OF NEUBAXIS OF EMBRYO OF TEN SOMITES

Fm. 1.~—Dorsa1 view. Columbia Collection No. 476. X210. Reduced 1%.

The acoustico-facial ganglion and the ganglionic crest are omitted in this model.

Fm. 2.—Lntera1 view.

1. Optic vesicle. 3a. Profundus nnlnge. 3. Quiutnl aulage. 8. Anterior neurupore. 11. Hiatus in line of fusion of the medullury folds. 12. First ganglionic segment. 13. Second ganglionic segmt-nf. 14. 'l‘i2ir<l ganglionic segment. 15. Mesenreplmluu. 16. Thulaunencopliulon. 17. Mammlllary region. 21. Rllomboid tossu. 22. Anterior isthmian sulcus. 23. Posterior isthmlnn sulcus. 31. Pregnngllonlc segment of deutereucephulon.


PLATE XXXII


Plate XXXIII RECONSTRUCTION OF NEURAXIS OF EMBRYO OF TWELVE SOMITES

Fm. 1.—Dorsu1vlew. Columbia Collection No. 534. X 210. Reduced V4. The ganglionic crest is omitted.

Fro. 2.—Laternl view.

1. Optic vesicle. 3a. Profundus unlnge. 3. Quintal anlnge. 4. Acoustico-incinl anlage. 8. Vestiges of anterior neuropore. 11. Vestige

of hiatus in line of iusion of medullary folds. 12. First ganglionic segment. 13. Second "ganglionic segment. 14. Third ganglionic segment. 15. Mcscnceplmlon. 16. Thulnmenccphnlon. 17. Mummillnry region. 18. lnfundibulnr region. 20. (langlionic crest.

21. Rhomboid fossn. 22. Anterior isthmlnn sulcus. 23. Posterior istixmiun sulcus. 31. Pregxmglionic segment of deuterencepimlou.


Vowsn: XXIV, l'u'n: XXXIII PLATE XXXIV nEcoNs'rnUc'rIoN or NEURAXIS or EMBBYQ or rounrnm somTr:s Lateral view. Columbia Collection No. 548. X 210. Reduced V3.

1. Optic vesicle. 311. Profimdus ganglion. 3. Quintai ganglion. 4. Acousticafncial ganglion. 8. Anterior neuropore, represented by :1 linear udhesion of the ectoderm. 12a. First ganglion segment; its cephalic portion. 121». First ganglion segment; its caudal portion. 13. Second ganglionic segment. 14. Third ganglionic segment. 15. Mesencephaion. 16. '1‘halamencephalon. 17. Mammiilnry region. 18. Infundibuinr region. 19. Telencephnlon. 20. Ganglionic crest. 24. First myeiomere. 31. Pregnnglionic segment of deuterenceplmlon. Vol.1: ,\u~: XXIV‘. PLATE XXXJV

Plate XXXV RECONSTRUCTION or NEUBAXIS or nunmro or srx-men Somites

Lateral view. Columbia Collection No. 551. x 210. Reduced 1,4.

1. 0pti_c vesicle. 3a. Protundus ganglion. 3. Quinta! ganglion. 4. Acousticafacial ganglion. 12a. First ganglionic segment; its cephalic portion. 121.. First ganglionic segment; its caudal portion. 13. Second ganglionic segment._ 14. Third ganglionic segment. 15. Mesencepimlon. 16. Thalamencepimlon. 17. Mammillary region. 18. Infundibular region. 19. Telemcephaion. 20. Ganglionic crest. 22. Anterior isthmian sulcus. 23. Poste rior isthmian suicus. 3_1. Preganglionic segment of deuterencephalon. \'oLL-M1-: XXIV, P141‘: XXXV

Plate XXXVI RECONSTRUCTION or NEURAXIS or mmnvo or ssvssmzu Somites

Lateral view. Columbia Collection No. 568. X210. Reduced 1,5. The embryo is well preserved but deformed by lateral compression incident to sectioning.

1. Optic vesicle. 3a. Proftmdus ganglion. 3. Quinta! ganglion. '4. Acousticafacial ganglion. 8. Vestige of anterior neuropore. 12a. First ganglionic segment; its cephalic portion. 121). First ganglionic segment; its caudal portion. 13. Second ganglionic segment 14. Third ganglionic segment. 15. Mesenoephalon. 16. Thalamencephalon. 17. Mammillary region. 18. Inftmdibular region. 19. Telencepimlon. 20. Ganglionic crest. 31. Pre ganglionic segment 01' denterencephalon. FVVHNN fl._.<..F.— .>~NM H2240? VI. ._<..r. A /. /..:.z.z.a~ PLATE XXXVII

llHCON8’l'HU(,"l'l0N (_)F NEURAXIS 01-" EMBRYO U19‘ NINETEEN SOM!'l‘!£S

Internl view. Columbia Collection No. 502. x 210. Reduced IA».

1. Optic vesicle. 3a. Proftmdus ganglion. 3. Quintnl ganglion. -i. Acoustico facial ganglion. 8. Vestige of anterior neuropore. 126. First gnnglionic segment; its cephalic portion. 12b. First ganglionic segment; its caudal portion.‘ 13a. Second ganglionic segment; its cephalic portion. 130. Second ganglionic segment; its caudal portion. 14. Third ganglionic segment. 15. Mesencephalon. 16. Thalamencephalon. 17. Mammlllary region. 18. Intundibnlar region. 19. Telenceplmlon. 20. Ganglionic crest. 24. First myelomere. 26. Velum tmnsversum. 31. Preganglionic segment of deuterencephalon. mm.

Plate XXXVIII Hi-ICONBTBUCTION or mrunaxxs or l'2Miiii\'U or 'i'\Vi'.‘N'i'Y-ONE Somites

iternl view. Columbia Collection No. 5534. V. ‘.:iU. Reduced I/J.



Optic vesicle. 3a. Prom s gnngiion_. Ii. Quintnl ganglion. -1. Acoustlcofacial ganglion. 8. V tig 1’ anterior nouropore. 12a. First ganglionic segment; its cephalic rtio . 12b. First ganglionic segment; its caudal c segment; its cephalic portion. 131). Sec ‘ ts caudal portion. 14. Third ganglionic segment. 15. Mesencephaion. 16'. Thaianiencephaion. 17. Mammiilary region. 18. infundibuiar region. 19. Teiencepimion. 20. Gangiionic crest. 22. Anterior isthmlan suicus. 23. Posterior isthmiun suicus. 27. Excrescence associated with the vestige of anterior neuropore (see page 331). 31. Pre I ganglionic segment of (ieutereuccplmion.

Plate XXXIX RECONSTRUCTION OF PORTIONS OF NEURAXIS OF EMBRY08 OF EIGHT AND NINE SOMITES

Fro. 1.——Reconstruction of the cephalic extremity of the neumxis in median section to show relief of ('u\'lt_\' of the for-ebraiu from an embryo of eight somites.

Columbia Collection No. 530. X 210. Reduced ‘,5.

1. Optic sulcus. 2. Tubercle of the floor. 7. Somatic ectodenn. 17. Mammillary region.

Fig. 2.—Reconsttuction of the cephalic extremity of the neurnxis in median section to show relief of the cavity of the forehrnin from an embryo of nine somites.

Oolumbia Collection No. 531. X 210. Reduced 1/J.

1. Optic sulcus. 2. Tubercle of the floor. 7. Somatic ectoderm. 17. Mammillary region. 18. Infundibulur region. \‘u1.L'.\u-; XXIV, PLAN: XXXIX


Plate XL RECONSTRUCTION OF PORTIONS OF ANEURAXIS OF EMBRYOS OF TWELVE AND TWENTY ONE SOMITES

Fro. 1.—Reconstruction of the cephalic extremity of the neuruxis in median section to show relief of the cavity of the torebrain from an embryo of twelve somites.

Columbia Collection No. 647. X 210. Reduced ‘,4.

1. Optic vesicle. 2. Tubercle of the floor. 7. Somatic ectoderm. The cut surtace _between these two leaders is the region of adherence of the ectoderm to the neural tube. 17. Mammiilary region. 18. Infundibnlar region.

Fm. 2.—-Reconstruction of the cephalic extremity of the neuruxis in median section to show relief or the ¢‘m'it_v of the forebruin from an embryo of twenty-one somites.

Columbia Collection No. 558. x 210. Reduced 54.

1. Optic vesicle. 2. Tubercle or the floor. 7. Somatic ectoderm. 17. Mnmmiilnry -region. 18. Intundibuinr region. XL

VOL: .\I E XXIV,

Plate XL! TBANSVEBSE srzcuom 011‘ EMUIIYO or SIXTEEN SOMITES

Transverse section of an embryo of sixteen somites, showing the closure of the neural tube completed prior to the appearance or the ganglionic crest. Columbia Collection No. 551, slide 4. row 4, section 11. X 150.

5. Neural tube. 7. Somatic ectoderm. 3» uk¢:_._ 5:3. M2940.»



Cite this page: Hill, M.A. (2019, July 22) Embryology Paper - The development of the neuraxis in the domestic cat to the stage of twenty-one somites. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_development_of_the_neuraxis_in_the_domestic_cat_to_the_stage_of_twenty-one_somites

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