DEVELOPMENT OF THE EYE. 255 



(inner) part (Fig. 242, r), and the black pigment membrane 

 (w) from the latter (the outer, uninverted part). The hollow 

 stalk of the primary eye-vesicle changes into the optic 

 nerve. 



The lens (I) which enacts so important a part in this 

 inverting process of the primary eye-vesicle, lies at first 

 directly upon its inverted part, that is, on the retina (r). 

 Very soon, however, the two separate, a new body, the 

 vitreous body {corpus vitreum, gl), coming in between them. 

 While the lens-sac is detaching itself, and the primary eye- 

 vesicle is being inverted from without, another inversion 

 simultaneously proceeds from beneath — from the superficial 

 portion of the skin-fibrous layer, i.e., from the leather-plate 

 of the head. At the back of the lens and below it, a ledge-like 

 process of the leather-plate arises (Fig. 243, g), which inverts 

 the primary eye-vesicle (now shaped like a cup) from below, 

 and presses in between the lens (I) and the retina (r) 

 Thus the primary eye-vesicle assumes the form of a hood. 

 The opening of this hood, answering to the face, is covered 

 by the lens ; but the opening, through which the neck 

 would pass, answers to the indentation through which the 

 leather-skin passes in between the lens and the retina (the 

 inner wall of the hood). The space within this secondary 

 eye-vesicle is almost filled by the vitreous body, which 

 answers to the head wrapped in this hood. The liood itself 

 is, properly speaking, double : the inner hood itself is the 

 retina, and the outer one, directly surrounding the former, 

 is the pigment membrane. The comparison with a hood 

 renders this process of inversion, which is sometimes hard 

 to explain, more clearly understood. The rudiment of the 

 vitreous body {corpus vitreum) is at first very incon- 

 50 



