THE LATER DEVELOPMENT OF THE CHICK 317 



obliterates the blastoccel. The result is the formation of a two- 

 layered optic cup; its inner layer, which may now be distin- 

 guished as the retinal layer is already quite thickened, while its 



ir 



FIG. 127. Diagrams illustrating the development of the eye in the chick. 

 After Froriep (slightly modified). A. Transverse section through part of the 

 fore-brain and optic vesicles of a chick, late the second day of incubation. On 

 the left the section passes through the optic stalk, on the right to one side of it. 

 B. Transverse section through the optic vesicle and associated structures at the 

 end of the second day. C. Same, slightly later. D. Section through the pupil- 

 lary region of the eye, the latter part of the fifth day. a, Anterior chamber 

 of eye; ec, ectoderm of head (epidermis); eck, ectoderm of cornea; F, cavity (or 

 wall, in B) of fore-brain; i, inner or retinal layer of optic cup; ir, mesodermal 

 rudiment of iris; I, lens; m, mesoderm; mk, mesoderm of cornea; o, outer or pig- 

 ment layer of optic cup; t, optic stalk; v, cavity of primary optic vesicle (in C and 

 D nearly obliterated by the invagination of the optic vesicle to form the optic 

 cup). 



outer layer has become quite thin (Figs. 127, 128). The new 

 cavity of the optic cup is the rudiment of the large posterior 

 chamber of the eye. The cavity of the cup is at first widely 

 open toward the lens and ectoderm, but its margin rapidly 



