60 THE EMBRYOLOGY OF THE EYE. 



of the vesicle in places become branched like 

 the retinal cells. From these branched cells 

 near the surface of the wall of the third vesicle 

 arise the nerve-fibres. These nerve-fibres run 

 anteriorly in a compact mass on the surface of 

 the vesicle, thus forming the optic tract, and 

 then, passing into the optic stalk, run without 

 interruption to the retina. 



In the optic tract, as in the wall of the cere- 

 bral vesicle, there are a number of capillaries 

 which pass in from the surrounding mesoblast. 

 Apart from the capillaries there are no connec- 

 tive-tissue elements, and the only cells in the 

 tracts are a few branched neural-epiblastic 

 cells, mostly lying among the peripheric fibres, 

 and identical with the cells of the vesicle- 

 wall. 



At the point of origin of the optic nerve 

 (Fig. I 7, A) there is a considerable accumula- 

 tion of these cells among the nerve-fibres, and 

 passing up throughout the length of the optic 

 nerve are the same cells arranged in short 

 longitudinal rows which separate the fibres 



