NERVOUS SYSTEM OF THE VERTEBRATA. 



429 



The optic vesicles become gradually constricted off from the 

 fore-brain in a direction obliquely backwards and downwards. 

 They remain, however, attached to it at the anterior extremity 

 of the base of the fore-brain (fig. 251, op.v.). While the above 

 changes are taking place in the optic vesicles the anterior part 



opy 



FIG. 251. SECTION THROUGH THE 

 FRONT PART OF THE HEAD OF A LEPIDOS- 

 TEUS EMBRYO ON THE SEVENTH DAY AFTER 

 IMPREGNATION. 



al. alimentary tract ; fb. thalamencepha- 

 lon ; /. lens of eye ; op.v. optic vesicle. The 

 mesoblast is not represented. 



FIG. 252. LONGITUDINAL 

 SECTION THROUGH THE BRAIN 

 OF A YOUNG PRISTIURUS EM- 

 BRYO. 



cer. commencement of cerebral 

 hemisphere; pn. pineal gland; In. 

 infundibulum ; pt. ingrowth of 

 mouth to form the pituitary body ; 

 mb. mid-brain ; cb. cerebellum ; ch. 

 notochord; al. alimentary tract; 

 laa. artery of mandibular arch. 



of the fore-brain becomes prolonged, and at the same time 

 somewhat dilated. At first there is no sharp boundary between 

 the primitive fore-brain and its anterior prolongation, but there 

 shortly appears a constriction which passes from above obliquely 

 forwards and downwards. This constriction is shallow at first, 

 but soon becomes much deeper, leaving however the cavities of 

 the two divisions of the fore-brain united ventrally by a some- 

 what wide canal (fig. 252). 



Of these two divisions the posterior becomes the thalamen- 

 cephalon, while the anterior and larger division (cer) forms the 

 rudiment of the cerebral hemispheres and olfactory lobes. For 

 a considerable period this rudiment remains perfectly simple, and 

 exhibits no signs, either externally or internally, of a longitudinal 

 constriction dividing it into two lobes. 



From the above description it may be concluded that the 



