V.] THE OPTIC CUP. 97 



involuted epiblast. This cup, in order to distinguish its 

 cavity from that of the original optic vesicle, is generally 

 called the secondary optic vesicle. We may, for the sake of 

 brevity, speak of it as the optic cup ; in reality it never is a 

 vesicle, since it always remains widely open in front. Of its 

 double walls the inner or anterior (Fig. 27 B, r) is formed 

 from the front portion, the outer or posterior (Fig. 27 

 B, u) from the hind portion of the wall of the primary optic 

 vesicle. The inner or anterior (r), which very speedily 

 becomes thicker than the other, is converted into the retina ; 

 in the outer or posterior (u), which remains thin, pigment is 

 eventually deposited, and it ultimately becomes the tesselated 

 pigment-layer of the choroid. 



FJG. 27. 

 A. B. 



DIAGRAMMATIC SECTIONS ILLUSTRATING THE FORMATION OP THE EYE. 

 (After Eemak.) 



In A, the thin superficial epiblast h is seen to be thickened at x, in front of 

 the optic vesicle, and involuted so as to form a pit o, the mouth of which 

 has already begun to close in. Owing to this involution, which forms the 

 rudiment of the lens, the optic vesicle is doubled in, its front portion r being 

 pushed against the back portion u, and the original cavity of the vesicle thus 

 reduced in size. The stalk of the vesicle is shewn as still broad. 



In B, the optic vesicle is still further doubled in so as to form a cup with a 

 posterior wall u and an anteiior wall r. In the hollow of this cup lies the lens /, 

 now completely detached from the superficial epiblast x, h. Its cavity is still 

 shewn. The cavity of the stalk of the optic vesicle is already much narrowed. 



By the closure of its mouth the pit of involuted epiblast 

 becomes a completely closed sac with thick walls and a small 

 central cavity. (Fig. 27 B, I). At the same time it breaks 

 away from the external epiblast, which forms a continuous 

 layer in front of it, all traces of the original opening being 

 lost. There is thus left lying in the cup of the secondary 

 optic vesicle, an isolated elliptical mass of epiblast. This is 

 E. 7 



