116 THE GENESIS OF THE VERTEBRATE EYE 



connected with them, become the arteries and veins of the chorioid coat. 

 The mesoderm around the optic cup condenses to form the connective- 

 tissue substrate of the chorioid and sclera, at first one mass but later 

 separated by the formation of the epichorioidal lymph-spaces. Other 

 early mesodermal condensations develop into the extra-ocular muscles 

 and other orbital contents. 



The anterior chamber is formed quite early as a cleft in the mesoderm 

 between the lens and the surface ectoderm, separating this mesoderm 

 into that of the iris and that of the cornea (Fig. 40, ac; Fig. 44). The 

 two layers may become neat and regular even before their separation. 

 The corneal mesoderm differentiates into the substantia propria, and the 

 overlying surface ectoderm contributes the corneal epithelium. The 

 lining cells of the embryonic anterior chamber become the latter's meso- 

 thelium, secreting (on the inner side of the cornea) Descemet's mem- 

 brane as their basement membrane. 



The iris is thus at first wholly mesodermal, and there is no aperture 

 in it, the future pupil being filled in by a mesodermal 'pupillary mem- 

 brane' which must later atrophy. The two ectodermal layers of epi- 

 thelium on the posterior surface of the iris are laid down by the optic 

 cup in the following way : 



The thick rim of the optic cup, the future ora terminalis of the sensory 

 retina, suddenly resumes proliferation, and a bud-like prolongation of 

 it creeps out under the mesoderm of the iris, between that mesoderm 

 and the lens, forming a thin double epithelium whose two layers are 

 respective continuations of the pigment epithelium and the retina proper. 

 This is actually new growth, for the optic cup proper does not expand 

 to accomplish it, as is evidenced by the fact that the original thick rim 

 'stays put'. The first structure laid down by this thin anterior continu- 

 ation of the cup lip is the future corona ciliaris; but when this has been 

 produced the growing lip does not stop, but cuts through the vitreous 

 which is joined to the root of the iris and continues out under the iridic 

 mesoderm as far as the site of the future pupil margin, leaving the 

 pupillary membrane devoid of an ectodermal backing. 



The outer layer of this epithelial fold is pigmented like the retinal 

 pigment epithelium of which it is a continuation, and during its growth 

 the pigmentation begins also to involve the inner layer of cells, creeping 

 backward from the growing lip as far as the root of the iris, where it 

 stops. This leaves the innermost of the two layers of epithelium which 

 cover the ciliary body forever free of pigment granules, forming the 



