254 THE EVOLUTION OF MAN. 



it. Exactly as the medullary tube originally separates from 

 the outer germ-layer does this lens-sac separate from the 

 horn-plate, in which it originated. The space within this 

 sac is afterwards entirely filled by the cells of its thick wall, 

 and the solid crystalline lens is thus formed. The latter is, 

 therefore, purely a formation of the epidermis. Together with 

 the lens the small fragment of the leather-plate (corium) 

 lying below the lens separates from the outer skin-covering. 

 This small piece of the leather-skin very soon forms a highly 

 vascular sac round the lens (capsula vasculosa lentis). 

 Its anterior portion at first covers the pupillary orifice, and 

 is then known as the pupillary membrane (membrana 

 pupillaris). Its back portion of the same membrane is called 

 the "membrana capsulo-pupillaris." This "vascular lens 

 capsule, which merely serves to nourish the growing lens," 

 afterwards entirely disappears. The later, permanent lens 

 capsule contains no vessels, and is a structureless secretion 

 of the lens cells. 



As the lens thus separates from the horn-plate and 

 grows inward, it must necessarily indent the adjoining 

 primary eye-vesicles from without (Fig. 242, 1-3). This 

 process may be compared to the inversion of the geim-mem- 

 brane vesicle (blastula}, which, in the Amphioxus and in 

 many low animals gives rise to the gastrula (vol. i. p. 192). In 

 both instances the inversion of one side of the closed vesicle 

 proceeds until finally the inner, inverted portion touches the 

 outer, uninverted portion of the wall of the vesicle, so that 

 the cavity disappears. Just as in the gastrula the former 

 part changes into the intestinal layer (entoderma), and 

 the latter into the skin-layer (exoderma), so in the inverted 

 primary eye-vesicle the retina develops from the former 



