134 EMBEYOLOGT OF THE LOWEE VERTEBRATES ch. 

 invaginated to form a cup-like structure— the optic cup (Fig. 74, 



thai. 



eel 



Fig. '74. — Development of the eye as seen in transverse sections of Fowl embryos. 



A, latter half of second day of incuhation ; B, end of second day ; C, 2\ days ; T), 3 days ; E, latter 

 half of fifth day. ect, external ectoderm; I, lens; o.r, rudiment of eye; o.s, optic stalk; p.l, pigment 

 layer of retina ; r, retina; thai, wall of thalamencephalon. 



B and 0). The cup-like lens-rudiment becomes gradually con- 



