DEVELOPMENT OF SPECIAL PARTS OF THE BRAIN. 63 



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 ante- 

 rior 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 Yarolii). 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 crook- 

 shape, 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 OP 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. C8), 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 

 che 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 laminae 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 laminae 

 form with one another, and to throw the dorsal laminae 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 



