142 THE THIRD DAY. [CHAP. 



between the two walls of the optic cup. By the end 

 of the third day the obhteration 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 contrast 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. 51). 



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

 hp of the cup, anterior to a line which afterwards be- 

 comes the ora serrata, both layers not only cease to 

 take part in the increased thickening, accompanied by 

 peculiar histological changes, which the rest of the cup 

 is undergoing, but also completely coalesce together. 

 Thus a hind portion or true retina is marked off from a 

 front portion. 



The front portion, accompanied by the choroid 

 which immediately overlays it, is, behind the lens, 

 thrown into folds, the cihary ridges ; whUe further for- 

 ward it bends in between the lens and the cornea to 

 form the iris. The original wide opening of the optic 

 cup is thus narrowed to a smaller orifice, the pupil; 

 and the lens, which before lay in the open mouth, is 

 now inclosed in the cavity of the cup. While in the 

 hind portion of the cup, or retina proper, no deposit of 

 black pigment takes place in the layer formed out of 

 the inner or anterior wall of the vesicle, in the front 

 portion we are speaking of, pigment is largely deposited 

 throughout both layers, so that eventually this portion 

 seems to become nothing more than a forward pro- 

 longation of the pigment-epithelium of the choroid. 



