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). 
Fic. 94. — Section through ~~ Fia. 95. — Section through the 
the primordium of the eye primordium of the eye of a 
of a chick embryo of 21 s. chick embryo at the end of 
(After Froriep.) the second day of incubation. 
d., Distal wall of optic (After Froriep.) 
vesicle. p., Proximal wall 
of optic vesicle. 
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 wall of the 
primary optic vesicle gradually brings this wall into contact 
with the inner wall and obliterates the primary cavity. Thus 
