178 Vertebrate Embryology 



tinuous with the cells of the inner wall, are be- 

 coming flatter and flatter, until they form a 

 mere membrane : this membrane forms the 

 epithelial lining of the lens capsule. The lens 

 capsule is probably a cuticular membrane se- 

 creted by the epithelial cells of the lens vesicle, 

 though it has been held, by some, to be of 

 mesoblastic origin. 



As the ectoblast thickens to form the lens 

 vesicle, the optic vesicle becomes invaginated, 

 from the side next to the lens vesicle, just as a 

 hollow rubber ball might be pushed in on one 

 side with the finger (Figs. 59 and 5o). The 

 invaginated optic vesicle is now known as the 

 optic cup, o c, and consists, naturally, of two 

 walls : of these walls, the inner soon becomes 

 the thicker, and this inequality in the thickness 

 of the two walls becomes greater as develop- 

 ment proceeds. The two walls of the optic 

 cup gradually approach each other until they 

 meet and thus obliterate the cavity of the 

 original optic vesicle (Fig. 60). 



The lips of the optic cup lie close to the 

 circumference of the lens, and by their growth 

 the depth of the cup is constantly increased. 

 This growth of the lips of the optic cup takes 

 place at all points except one : at a point near 



