128 THE NERVOUS SYSTEM. 



canal, is spoken of as the neurenteric canal : it is only present 

 for a short time, and closes up before the tadpole hatches. 



The neural tube, formed in this way, soon separates from the 

 surface epiblast, and by thickening of its walls and other 

 changes becomes converted into the central nervous system ; 

 the anterior part forming the brain, and the posterior part the 

 spinal cord. The lumen or cavity of the tube persists as the 

 central canal of the spinal cord and the ventricles of the brain. 



The Brain. At the time of its first appearance the brain is 

 bent at right angles about the middle of its length ; the axis of 

 the anterior portion being vertical, and that of the posterior 

 portion horizontal. (Fig. 28.) The posterior portion, or hind- 

 brain, BH, is wide from side to side, and has moderately thick 

 sides and floor, but a thin roof : it is continuous behind with 

 the spinal cord. 



The anterior or vertical portion has walls of nearly uniform 

 thickness in all parts. It is divided by a slight constriction, 

 best marked at the sides, into an upper or posterior part, the 

 mid-brain, BM, which forms the angle of the bend and lies 

 opposite the anterior end of the notochord ; and a lower and 

 larger portion, the fore-brain, BF, which is produced laterally 

 into a pair of hollow outgrowths, the optic vesicles. 



The further development of the brain is illustrated by Figs. 

 29 and 30. It will be seen that the rectangular bending of the 

 brain, which is known as cranial flexure, and which was so 

 prominent a feature in the earlier stage, is no longer obvious : 

 a closer comparison of the figures will show, however, that this 

 straightening of the brain, or rectification of the cranial flexure, 

 is apparent rather than real, and is brought about partly by the 

 development of the cerebral hemispheres, which grow upwards 

 and forwards from the fore-brain, and still more largely by the 

 formation of the mouth and the growth forwards of the face and 

 lips, which cause the brain to take a much less prominent share 

 in determining the shape of the head. 



The hind-brain, BH, has undergone but little change in Fig. 

 29, except an increase in thickness of its floor and sides. At the- 

 stage represented in Fig. 30 it is separated from the mid-brain 

 on the dorsal surface by a well-marked groove, immediately 

 behind which the roof of the hind-brain is thickened transversely 

 to form the cerebellum, CB. The cavity of the hind-brain 

 remains as the fourth ventricle, the roof of which is very thin 



