256 THE EVOLUTION OF MAN. 



siderable (Fig. 243, g), and the retina disproportionally 

 thick. As the former expands, the latter becomes much 

 thinner, till at last the retina appears only as a very delicate 



{ h FIG. 243. Horizontal transverse 



section through the eye of a human 

 embryo of four weeks ; 100 times 

 enlarged (after Koelliker) : t, lens 

 (the dark wall of which is equal to 

 the diameter of the central cavity) ; 

 g, vitreous body (connected with the 

 leather-plate by a stalk, g') ; v, vas- 

 cular loop (penetrating through the 

 stalk (g') into the vitreous body be- 

 hind the lens); r, retina (inner, 

 thicker, inverted lamella of the 

 primary eye-vesicle) ; a, pigment membrane (outer, thinner, uninverted 

 lamella of the same) ; h, intermediate space between the retina and the 

 pigment membrane (remnant of the cavity of the primary eye-vesicle). 



coat of the thick, almost globular vitreous body, which fills 

 the greater part of the secondary eye-vesicle. The outer 

 layer of the vitreous body changes into a highly vascular 

 capsule, the vessels of which afterwards disappear. 



The slit-like passage through which the rudiment of the 

 vitreous body grows from below in between the lens and 

 the retina, of course causes a break in the retina and the 

 pigment-membrane. This break, which appears on the inner 

 surface of the vascular membrane as a colourless streak, has 

 been inaptly called the choroidal cleft, though the true 

 vascular membrane is not cleft at all at this point (Fig. 

 234, sp, 235, sp, p. 243). A thin process of the vitreous body 

 passes inward on the under surface of the optic nerve, which 

 it inverts in the same way as the primary eye-vesicle was 

 inverted. The hollow cylindrical optic nerve (the stalk of 



