150 THE THIRD DAY. [CHAP. 



During their multiplication they become arranged 

 in the manner characteristic of the adult lens. 



The lens capsule is probably formed as a cuticular 

 membrane deposited by the epithelial cells of the lens. 

 But it should be stated that many embryologists regard 

 it as a product of the mesoblast. 



The vitreous humour. The vitreous humour is a 

 mesoblastic product, entering the cavity of the optic 

 cup by the choroid slit just spoken of. It is nourished by 

 the vascular ingrowths through the choroid slit. Its 

 exact nature has been much disputed. It arises as a 

 kind of transudation, but frequently however contains 

 blood-corpuscles and embryonic mesoblastic cells. It 

 is therefore intermediate in its character between or- 

 dinary intercellular substance, and the fluids contained 

 in serous cavities. 



The integral parts of the eye in front of the lens are 

 the cornea, the aqueous humour, and the iris. The 

 development of the latter has already been sufficiently 

 described in connection with the retina, and there re- 

 main to be dealt with the cornea, and the cavity con- 

 taining the aqueous humour. 



The cornea. The cornea is formed by the coales- 

 cence of two structures, viz. the epithelium of the 

 cornea and the cornea proper. The former is directly 

 derived from the external epiblast, which covers the 

 eye after the invagination of the lens. The latter is 

 formed in a somewhat remarkable manner, first clearly 

 made out by Kessler. 



When the lens is completely separated from the epi- 

 dermis the central part of its outer wall remains directly 



