488 SECONDARY OPTIC CUP. 



along the line of the fissure, at the same time that the lens is being thrust in 

 in front. In Mammalia, the doubling up involves the optic stalk, which 

 becomes flattened (whereby its original cavity is obliterated) and then folded 

 in on itself, so as to embrace a new central cavity continuous with the cavity 

 of the vitreous humour. And in other forms a partial phenomenon of the 

 same kind is usually observable, as is more particularly described in the 

 sequel. 



Before describing the development of the cornea, aqueous 

 humour, etc. we may consider the further .growth of the parts, 

 whose first development has just been described, commencing 

 with the optic cup. 



During the above changes the mesoblast surrounding the 

 optic cup assumes the character of a distinct investment, whereby 

 the outline of the eye-ball is definitely formed. The internal 

 portions of this investment, nearest to the retina, become the 

 choroid (i.e. the chorio-capillaris, and the lamina fusca; the 

 pigment epithelium, as we have seen, being derived from the 

 epiblastic optic cup), and pigment is subsequently deposited in 

 it. The remaining external portion of the investment forms the 

 sclerotic. 



The complete differentiation of these two coats of the eye 

 does not however take place till a late period. 



The cavity of the original optic vesicle was left as a nearly 

 obliterated space between the two walls of the optic cup. By 

 the end of the third day the obliteration is complete, and the two 

 walls are in immediate contact. 



The inner or anterior wall is, from the first, thicker than the 

 outer or posterior ; and over the greater part of the cup this con- 

 trast increases with the growth of the eye, the anterior wall 

 becoming markedly thicker and undergoing changes of which we 

 shall have to speak directly (fig. 289). 



In the front portion however, along, so to speak, the lip of 

 the cup, anterior to a line which afterwards becomes the ora 

 serrata, both layers cease to take part in the increased thickening, 

 accompanied by peculiar histological changes, which the rest of 

 the cup is undergoing. Thus a hind portion or true retina is 

 marked off from a front portion. 



The front portion, accompanied by the mesoblast which 

 immediately overlies it, is behind the lens thrown into folds, the 



