576 



TEXT-BOOK OF EMBRYOLOGY. 



a thin layer of mesoderm grows in between the lens vesicle and the surface 

 ectoderm, completely separating them (Fig. 501). The ingrowth of the lens 

 vesicle against the outgrowing optic vesicle has the effect as though a small hard 

 ball (the lens vesicle) had been pressed into a larger soft ball (the optic vesicle) 



Fore-brain , 



Lens vesicle - 



Optic cup ' 



FIG. 500. Showing somewhat later stage in development of optic cup and lens 

 than is shown in Fig. 499. DuvaJ,. 



' 



(Fig. 502). The lens vesicle pushes the outer wall of the optic vesicle in against 

 the inner wall, the optic vesicle thus becoming transformed into the two-layered 

 optic cup (Figs. 500, 501). Bonnet calls attention to the fact that the two proc- 

 esses, lens formation and the invagination of the optic vesicle to form the optic 



Conjunctival epithelium -* 



Vitreous * 



Lens vesicle 



Retina (inner layer 

 of optic cup) ' 



Optic stalk 



Pigmented layer of retina 



(outer layer of optic cup) 



FIG. 501. Diagram of developing lens and optic cup. Duval. 



The cells of the inner wall of the lens vesicle have begun lo elongate to form lens fibers. The epi- 

 thelium over the lens is the anlage of the corneal epithelium. The mesodermal tissue between 

 the latter and the anterior wall of the lens vesicle is the anlage of the substantia propria 



cup, are more or less independent and that it is not correct to describe the lens as 

 actually pushing in the outer wall of the vesicle. As evidence of this is noted 

 the fact that typical optic cup formation may occur in cases where no lens is 

 developed. The optic cup when first formed is not a complete cup, for the 



