The Development of the Frog 43 



rounding mesoblast enters the optic cup and 

 forms the vitreous humor. The outer layers of 

 the eye are formed from the mesoblast. As the 

 optic vesicles approach the superficial ectoblast, 

 the inner layer of the latter becomes thickened, 

 and eventually separates from the outer layer 

 and lies at the opening of the optic cup as a 

 hollow spherical, or ovoid body, the lens vesicle 

 (Figs. 19, OL, and 20 OL). By the growth of 

 its walls, chiefly the inner, the cavity of the 

 lens vesicle is obliterated and the adult lens is 

 formed. 



THE EAR 



The ear begins as a thickening, followed by 

 an invagination of the inner or nervous layer 

 of the ectoblast. This invagination begins very 

 early and, in the frog, never opens to the ex- 

 terior. It is, almost from the first, connected 

 with the brain by the auditory nerve. The 

 invagination gradually closes to form a com- 

 paratively thin-walled cavity, lying in the re- 

 gion of the hind-brain, known as the auditory 

 vesicle. This vesicle, whose walls are com- 

 posed of a single layer of cells (Fig. 21, E), 

 forms the lining of the inner ear. A more 



o 



complete description of the development of the 

 ear will be given in connection with the chick. 



