v.] THE OPTIC CUP. 103 



a nearly obliterated space between the two walls of the optic 

 cup. By the end of the third day the obliteration is com- 

 plete, 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. 31). 



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 not only cease to take part in 

 the increased thickening, accompanied by peculiar histo- 

 logical 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 ciliary ridges ; while further forward 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 of, 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 prolongation of the pigment- 

 epithelium of the choroid. 



Thus while the hind moiety of the optic cup becomes the 

 retina proper, including the choroid-pigment in which the 

 rods and cones are imbedded, the front moiety is converted 

 into the ciliary portion of the retina, covering the ciliary 

 processes, and into the uvea of the iris ; the bodies of the 

 ciliary processes and the substance of the iris, their vessels, 

 muscles, connective tissue and ramified pigment, being derived 

 from the mesoblastic choroid. The margin of the pupil marks 

 the extreme lip of the optic vesicle, where the outer or poste- 

 rior wall turns round to join the inner or anterior. 



We have still to speak of the choroidal fissure. In 

 mammals the slit remains open for a short time only. After 



