The Development of the Frog 37 



the brain region, dividing it into three more or 

 less distinctly marked portions, the fore-brain, 

 mid-brain, and hind-brain (Fig. 12.) The de- 

 velopment of these regions of the brain will be 

 taken up, briefly, in turn. 



The hind-brain forms the medulla and cere- 

 bellum of the adult, its cavity remaining as the 

 fourth ventricle. The floor and sides of the 

 hind-brain become thickened, while the roof 

 becomes very thin, except at the part next to 

 the mid-brain, where the cerebellum is devel- 

 oped (Fig. 1 6). 



The floor of the mid-brain thickens to form 

 the crura cerebri, while from the roof grow 

 out two hollow, ovoid bodies, the optic lobes. 

 The cavity of the mid-brain persists as the 

 Sylvian aqueduct or tier. 



The walls of the fore-brain (thalamenceph- 

 alon of the adult) thicken to form the optic 

 thalami and its cavity, the third ventricle, is 

 thus reduced to a narrow, vertical slit. From 

 the floor of the fore-brain the infundibulum is 

 formed as a hollow pouch, pushed out in a 

 ventro-posterior direction (Figs. 14, IN, 1 7, and 

 1 8, /). Since the pituitary body is so closely 

 associated with the infundibulum, its origin 

 will be spoken of at this time. 



