THE EMBRYOLOGY OF THE EYE. 1 3 



II. — Histological. 



I. Primary optic vesicle. — The wall of the 

 primary optic vesicle is the direct continuation 

 of the neural epiblastic layer forming the wall 

 of the cerebral vesicles (Fig. i, A). For a 

 considerable time its histological structure 

 does not change. It consists of cells, some- 

 what similar to ordinary epithelial cells, in a 

 layer from three to five cells deep, more or 

 less regularly superimposed in a radial direc- 

 tion, the limits of the cell-bodies being mostly 

 indistinct. The external and the internal 

 margins of the layer are sharp and regular. 



Outside the cerebral vesicles lies the meso- 

 blast composed of branched cells not closely 

 packed, and containing many thin-walled blood- 

 vessels, which lie for the most part near the 

 wall of the vesicles. Covering the whole is 

 the external epiblast, a homogeneous layer of 

 protoplasm with a single row of granular nuclei. 



In birds the primary optic vesicle reaches 

 the external epiblast, there being no meso- 

 blastic cells between the two. In mammals, 



