134 THE THIRD DAY. [CHAP. 



Towards the end of the second day the external 

 or superficial epiblast which covers, and is in all but 

 immediate contact with, the most projecting portion of 

 the optic vesicle, becomes thickened. This thickened 

 portion is then driven inwards in the form of a shallow 

 open pit with thick walls (Fig. 47 A, o), carrying before 

 it the front wall (r) of the optic vesicle. To such an 

 extent does this involution of the superficial epiblast 

 take place, that the front wall of the optic vesicle is 

 pushed close up to the hind wall, and the cavity of the 

 vesicle becomes almost obliterated (Fig. 47, B). 



The bulb of the optic vesicle is thus converted into 

 a cup with double walls, containing in its cavity the 

 portion of 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. 47 B, r) is formed 

 from the front portion, the outer or posterior (Fig. 47 

 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. 



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. 47 B, I). At 

 the same time it breaks away from the external epi- 



