THE EMBRYOLOGY OF THE EYE. 



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Fig. E. 



The cupping of the primary optic vesicle, 

 however, is not confined to its distal wall, but 

 also takes place in its inferior (ventral) wall, 

 so that the secondary optic vesicle has a 

 circular opening distally, occupied by the lens, 

 and a cleft inferiorly, which extends back into 

 the pedicle connecting the optic with the cere- 

 bral vesicle. This pedicle, or optic stalk, is 

 the rudiment of the optic nerve (Fig. F). 



Fig. F. 



Later the margins of this cleft unite, and 

 the latter is obliterated. In the mammalia, 

 however, before the cleft closes, mesoblastic 



