4 THE EMBRYOLOGY OF THE EYE. 



The first important step to occur in the 

 development of the cerebral system is the 

 bulging out of a small portion of the lateral 

 wall of the anterior primary cerebral vesicle 

 on either side, forming a cavity called the 

 primary optic vesicle (Fig. B). This primary 



Fig. B. 



optic vesicle, which is thus formed from the 

 involuted or neural epiblast, pushes out until 

 it reaches the external epiblast. At the spot 

 where the vesicle is in contact with the exter- 

 nal epiblast, the latter becomes thickened and 

 cupped (Fig. C). This cup closes, forming a 



Fig. C. 



