FROM TWELVE TO THIRTY-SIX SOMITES 



165 



and this constitutes the primordium of the lens (Fig 94). The 

 thickening of the external wall of the optic vesicle and of the 

 lens primordium now proceed rapidly, and soon an invagination 

 is formed in each (Fig. 95). 



Fig. 94. — Section through 



the primordium of the eye 



of a chick embryo of 21 s. 



(After Froriep.) 



d., Distal wall of optic 

 vesicle, p., Proximal wall 

 of optic vesicle. 



Fig. 95. — Section through the 

 primordium of the eye of a 

 chick embryo at the end of 

 the second day of incubation. 

 (After Froriep.) 



It is probable that a stimulus is exerted by the optic vesicle on the 

 ectoderm with which it is in contact, causing it to thicken and become 

 the primordium of the lens. This has been demonstrated experimentally 

 to be the case in the embryo of the frog, and the morphological rela- 

 tions are the same in the chick. The" invagination of the primary optic 

 vesicle to form the secondary optic vesicle is not mechanically produced 

 by the growth of the lens, as some have supposed, for it has-been shown 

 (see Fol and Warynsky) that the secondary optic vesicle is formed in 

 the absence of the lens. 



We may now consider the formation of the optic cup and of 

 the lens separately. 



The Optic Cup. The invagination of the outer w^all of the 

 primary optic vesicle gradually brings this wall into contact 

 with the inner wall and obliterates the primary cavity. Thus 



