walls of the vesicles. Thus the corpora striata are formed in the floor of the hemisphere vesicles, whilst the principal mass of each hemisphere is formed from the roof and sides (mantle) of those vesicles, and the olfactory lobes are hollow out

H

Vom

Fig. 68.-FORE-PART OF THE EMBRYO SHOWN IN FIG. 38,
VIEWED FROM THE DORSAL SIDE. . (From Kölliker.),

F, fore-brain; e, ocular vesicles; M, mid-brain; H, hindbrain; h, part of the heart seen bulging to the right side: Vom, omphalo-mesenteric or vitelline veins entering the heart posteriorly; Mr, medullary canal, spinal part: p, proto-vertebral somites.

growths from them. The cavities of the hemisphere-vesicles become the lateral ventricles, and the cavity of the part of the fore-brain (or first secondary vesicle) from which they spring, forms the anterior extremity of the third ventricle. The optic thalamus is formed by a thickening of the lateral wall of the second vesicle, the cavity of which comes to be the main part of the third ventricle; the corpora quadrigemina are thickenings in the roof, and the crura cerebri thickenings of the sides and floor of the third vesicle, which becomes the aqueduct of Sylvius; the cerebellum and pons are respectively thickenings of the roof and floor and the crura cerebelli of the sides of the fourth vesicle (anterior part of hind-brain), the cavity of which becomes the anterior (superior) part of the fourth ventricle; and finally, the medulla oblongata is developed as a thickening of the wall of the fifth vesicle, the cavity of which expands from the central canal of the spinal cord to form the calamus scriptorius of the fourth ventricle.

On the other hand, certain parts of the walls of the vesicles become thin and greatly expanded, and even eventually project into the cavities as folds of epithelium

h

pin
bg

cbl

amv

ch

1-3

14

CHICK OF TEN DAYS (after Mihalkovics). h, cerebral hemisphere; olf, olfactory lobe and nerve; st, corpus striatum; lv, lateral ventricle: ac, anterior commissure; lt, lamina terminalis; opc, optic commissure; pit, pituitary gland; inf, infundibulum; cai, internal carotid artery; 3, third ventricle; ch3, choroid plexus of third ventricle; pin, pineal gland; bg, corpora bigemina; amv, anterior medullary velum; below which two last references are the aqueduct of Sylvius and crura cerebri; cbl, cerebellum; 4, fourth ventricle; ba, basilar artery: ps, pons Varolii; ch1, choroid plexus of the fourth ventricle; obl, medulla oblongata; r, roof of fourth ventricle.

opc

Sinf ba ps
Ocai

obl

covering ramified vascular expansions of pia mater (choroid plexuses). These vascular expansions occur along the lower border of the mesial surface of each hemisphere-vesicle (choroid plexuses of lateral ventricles); along the roof of the second vesicle (choroid plexus of third ventricle), and in the roof of the fifth vesicle (choroid plexus of fourth ventricle).

While these changes are going on in its walls the embryonic brain does not remain straight as at first, with its axis in a line with that of the spinal cord, but undergoes certain flexures (fig. 70), the general result of which is to bend the anterior end towards the ventral surface. The first of these flexures to make its appearance is a sharp bend opposite the base of the mid-brain and around the anterior end of the notochord. The result of this flexure, which produces a complete doubling round of the anterior part of the brain, is that the mid-brain is for a time the most prominent part of the encephalon. Later, the growth of the cerebral vesicles, and of the thalamencephalon, brings these parts again into prominence, and tends to obscure the flexure, which is, however, never actually obliterated. The second cerebral flexure, which is also very sharp and well marked, occurs in the region of the hind-brain (pons Varolii). It is in the opposite direction to the first one, its concavity being directed towards the dorsum of the embryo, and it produces the appearance of a deep depression at the part of the brain where it occurs. The third flexure is a more gradual one. It occurs at the junction of the hind-brain with the cord, the embryonic medulla oblongata being bent ventralwards from the line of direction of the medulla spinalis.

The result of these flexures is that the axis of the embryonic brain takes a crookshape, passing from the end of the spinal axis at first ventral, then dorsal, and then again ventral, finally bending sharply backwards towards its termination at the foramen of Monro.

The second and third flexures become eventually almost entirely obliterated with the further growth of the brain.

FURTHER DETAILS REGARDING THE DEVELOPMENT OF SPECIAL PARTS OF THE BRAIN.

The fifth cerebral vesicle: bulbar vesicle, or metencephalon.—This part of the embryonic brain, afterwards to become the medulla oblongata, often shows at its first appearance-especially in the chick—a series of slight constrictions (fig. 68), which have by some been taken to indicate a segmentation of the neural tube. But even where they occur they are quite temporary, and the fifth vesicle soon becomes a well marked dilatation opening out from the anterior end of the embryonic spinal cord. Its wall, like that of all the other cerebral vesicles, is composed of cells similar to those of the rest of the neural tube, and the histogenetic changes which occur to form the nervous tissue are also entirely similar.

Sections across this part of the neural tube are of a compressed oval outline in the lower part (fig. 71, A, B), but in the upper part, which afterwards becomes the lower part of the fourth ventricle, the thinning out and lateral expansion of the dorsal wall of the tube gives to sections of this and the next (fourth) vesicle the shape of an irregular triangle, or shield, the base of the triangle being directed towards the dorsum (roof) and the sides bent more or less sharply inwards about their middle to unite with one another ventrally at the apex of the triangle (figs. 72, 73). This bend serves to mark a division of each side of the tube into two parts, a dorso-lateral and a ventro-lateral, which correspond, both in their situation and in their relationship to afferent and efferent nerves, with the alar and basal lamine of the embryonic cord (p. 60), with which they are in fact continuous. The thinning out and lateral expansion of the roof in the region. of the fourth ventricle tends to open up the angle which the ventral laminæ form with one another, and to throw the dorsal laminæ more to the side, so that what were previously the lateral boundaries of the neural tube come to occupy the so-called floor of the fourth ventricle, and since in this region the roof becomes

Fig. 70.-PROFILE VIEWS OF THE BRAIN OF HUMAN EMBRYOS AT THREE SEVERAL STAGES,
RECONSTRUCTED FROM SECTIONS (His).

A. Brain of an embryo of about 15 days (the embryo itself is shown in fig. 117) magnified 35 diameters.

B. Brain of an embryo about three and a half weeks old. The optic vesicle has been cut away. C. Brain of an embryo about seven and a half weeks old. The optic stalk is cut through. A, optic vesicle; H, vesicle of cerebral hemisphere, first secondary vesicle; Z, thalamencephalon, second secondary vesicle; M, mid-brain; J, isthmus between mid- and hind-brain; Hh, fourth secondary vesicle; N, fifth secondary vesicle; Gb, otic vesicle; Rf, fourth ventricle; Nk, neck curvature; Br, pons curvature; Pm, mammilary process; Tr, infundibulum; Hp (in B), outline of hypophysis-fold of buccal epiblast; Rl, olfactory lobe. In C the basilar artery is represented along its whole course.

reduced to a thin layer of flattened epithelium, the substance of this part of the medulla oblongata is wholly formed by a thickening of the shifted lateral boundaries." In these, the bend marking the distinction between the ventral and dorsal laminanow by change of position mesial and external-continues to be evident, and is in fact recognizable even in sections of the fully-developed brain.

Of the longitudinal columns of the medulla oblongata the restiform bodies first become prominent (third month in the human embryo). The (anterior) pyramids are obvious in the fifth month, and the olivary tubercle about the sixth. But before any of these, and indeed with

Fig. 71.-SECTIONS ACROSS THE REGION OF THE CALAMUS SCRIPTORIUS OF THE BRAIN REPRESENTED IN FIG. 70, A. (His.)

A, region of the glossopharyngeal ganglion.

B, of the auditory-facial ganglion.

A, section taken through the lower part.

B, across the widest part (trigeminus region).

C, through upper part (cerebellar region).

r, roof of neural canal: al, alar lamina; bl, basal lamina; v, ventral border.

Fig. 73.-SECTIONS ACROSS THE LOWER HALF OF THE FOURTH VENTRICLE OF A STILL OLDER EMBRYO,

SHOWING GRADUAL OPENING OUT OF THE NEURAL CANAL AND THE COMMENCING FOLDING OVER OF THE ALAR LAMINA (at ƒ).

v, ventral border; t, tænia; ot, otic vesicle ; rl, recessus labyrinthi.

In the succeeding stage (not here represented) the angle at v has almost disappeared, the fold ƒ has extended over the alar lamina, and the two thickened halves are in the same horizontal plane, covered by a greatly expanded and thinned out roof.

VOL. I.

F

the earliest appearance of the nerve roots, the white bundles-not yet medullated, however-which are known as the ascending root of the fifth, and the ascending root of the vagus and glossopharyngeal (solitary bundle) begin to make their appearance, both being at first on the surface of the medulla. They gradually, however, become covered in by a folding over of the dorsal part of the alar lamina, and thus come later to lie imbedded in the substance of each lateral half of the medulla. This fold is shown in its commencement in fig. 73, A and B, f. According to His the bundles grow downwards towards the spinal cord from the places of entrance of the corresponding nerve roots, emerging from the ganglia, as in the case of the posterior spinal roots; and after entering the medulla grow gradually along the course of the future so-called ascending roots, so that the latter are at first visible only in sections taken near the places of entrance of the nerve roots into the medulla.

The fourth cerebral vesicle: cerebellar vesicle, or epencephalon.-The constriction, which is at first obvious between this and the fifth vesicle, does not long persist, so that the two together form a long boat-shaped cavity which becomes

Fig. 74.-MEDIAN SECTION THROUGH

THE BRAIN OF A TWO AND A

HALF MONTHS FETUS. (His.)
Magnified 5 diameters.

The mesial surface of the left cerebral hemisphere is seen in the upper and right hand part of the figure; the large cavity of the third ventricle is bounded above and in front by a thin lamina; below is seen the infundibulum and pituitary body. Filling the upper part of the cavity is the thalamus opticus; in front and below this is the slit-like foramen of Monro. Behind the thalamus is seen another slit-like opening which leads into the still hollow external geniculate body.

o'f, olfactory lobe; p, pituitary body; cq, corpora quadrigemina; cb, cerebellum; m.o., medulla oblongata.

the fourth ventricle. As in that part of this cavity which

has already been described with the fifth vesicle, the roof inferiorly becomes greatly thinned and expanded. Superiorly the tube becomes gradually more contracted and the roof thicker, this thickening being the rudiment of the cerebellum and of the valve of Vieussens (fig. 74). In the meanwhile a considerable thickening of the lateral boundaries, which, as in the medulla oblongata, have been thrown outwards by the roof expansion, occurs, and from this the substance of the pons is gradually formed.

The dorsal and ventral laminæ of the lateral walls are still evident in this part of the embryonic brain. With the former, the sensory fibres of the fifth nerve are immediately connected; with the latter, the motor fibres of the fifth and also the sixth and seventh nerves.

In the human embryo the cerebellum is seen as early as the second month, forming a thin plate arching over the anterior part of this vesicle (fig. 74). From this plate, which enlarges only gradually, is formed the middle lobe; later the lateral lobes grow out at the sides. The cerebellar surface is at first smooth, but a subdivision into the subordinate lobes occurs in the fifth month, and the folia appear about the sixth. In the seventh month all the parts of the organ, except the amygdala, are formed.

Of the cerebellar peduncles, the inferior appear in the third month, the middle in the fourth, and the superior in the fifth. The transverse fibres of the pons develop pari passu with the lateral lobes, appearing about the fourth month.