62 THE EMBRYOLOGY OF THE EYE. 



nerve are directly continuous with the epiblas- 

 tic cells of the cerebral vesicles and of the 

 retina, and in my opinion are to be considered 

 the epiblastic cells of the optic stalk. 



At a somewhat later stage in the pig I have 

 found the optic nerve much larger, and con- 

 sisting only partially of fibres. The remainder 

 consists of closely packed cells continuous 

 with the cells of the cerebral vesicle and of 

 the retina, and identical with the closely 

 packed cells found in these localities (Fig lo). 

 These cell-masses in the nerve have a sharp, 

 smooth peripheric margin bordered by the 

 mesoblastic cells of the nerve-sheath. These 

 can be nothing but proliferated cells of the 

 primitive optic stalk. 



A little later the optic nerve, which so far 

 has had no direct blood-supply, becomes per- 

 vaded with capillaries which push in from the 

 sheath, and entering the nerve run for the 

 most part in a longitudinal direction (Fig. 12). 



The nerve-fibres have now begun to take on 

 their medullary sheath and lose their sharp 



