THE EMBRXOLOGY OF THE EYE, 



5 



sac — the lens-sac, — which becomes detached 

 from the external epiblast (Fig. D). The 



Fig. D. 



anterior cells of this sac form the anterior 

 layer of epithelium of the lens, while the 

 cells of the posterior layer of the sac de- 

 velop into the nucleated fibres which ex- 

 tend forward and make up the substance of 

 the lens. 



As the external epiblastic layer becomes 

 cupped to form the lens, the distal wall of the 

 primary vesicle is first depressed, and then, 

 inasmuch as the vesicle continues to grow, it 

 is gradually involuted until it comes to lie in 

 contact with the proximal (posterior) wall. 

 The double-walled cup thus formed is called 

 the secondary optic vesicle (Fig. E). 



