THE EYE. 



277 



vesicle, which thus becomes doubled on itself to form the optic 

 cup (Fig. 121, oc). 



The lens. The pitting-in of the epiblast, to form the lens, 

 rapidly deepens ; the lips of the pit close in, and unite, so as to 

 convert the pit into a closed sac, the lens vesicle, which separates 

 completely from the external epiblast during the third day. 

 After this separation, the outer wall of the lens vesicle remains 

 thin, and is formed of a single layer of flattened epithelial cells ; 

 the inner wall thickens rapidly, by elongation of its component 



EF 



OL 



FIG. 122. Transverse section across the fore-brain and eye of a Chick Embryo 

 at the sixtieth hour of incubation. On the right side the section passes 

 through the optic 'stalk ; on the left side it passes just behind the stalk. 

 x45. 



BF, cavity of fore-brain. BS, cavity of commencing cerebral hemisphere. MX, 

 maxillary arch. OC, inner wall of optic cup. OD, outer wall of optic cup. OL, 

 lens. OS, optic stalk. 



cells (Fig. 122, OL) ; and by the fourth day it comes in contact 

 with the outer wall, so as to obliterate the cavity of the vesicle 

 entirely. From the epithelial cells of this thickened inner wall 

 the whole of the substance of the adult lens is derived. The 

 outer, thin wall of the lens vesicle becomes the epithelial lining 

 of the lens capsule ; while the lens capsule itself is apparently a 

 cuticular membrane excreted by the epithelial cells of the lens 

 vesicle. 



The optic cup. In the optic cup important changes occur. 

 The two layers of the cup soon come in contact with each other 



