THE EMBRYOLOGY OF THE EYE. 59 



nerve, showing the outer layer folded. It will 

 be noticed that the pigment of the outer layer 

 is wanting in the region of the fold. At a 

 slightly later period these folds are obliterated 

 and the pigmentation is uniform in the outer 

 layer ; C Is the optic stalk near the vesicle, 

 with its Inferior cupping ; D Is the stalk a 

 short distance from the vesicle ; and E is the 

 round stalk with a central lumen some distance 

 from the vesicle. x\t this period the optic 

 stalk is entirely cellular and shows no trace of 

 nerve-fibres. 



As w^e have seen, the cells of the retina are 

 at first closely packed, but just before the 

 appearance of the nerve-fibres the cells of the 

 layer at the inner surface of the retina become 

 branched, and the branches form a network 

 and the cells lie farther apart. At the same 

 time a similar change takes place In the cells 

 of the cerebral vesicles. The walls of the 

 cerebral vesicles Increase In thickness until the 

 cavity Is almost obliterated. The central cells 

 are closely packed, while those on the surface 



