PLATE 7 



EXPLANATION OF FIGURES 



Microphotographs showing various stages in the development of the eye. 

 Al and PI, anterior and posterior portions of the lens; Lc, lens; OpSt, optic 

 stalk; Opv, optic vesicle; P, pigment layer; R, retinal layer. 



43 Section through the optic vesicles, Opv, and the optic stalks; OpSt, of the 

 chick about forty hours old. The thickening of the epithelium, L, which later 

 becomes the lens, is very marked. X SO. 



44 Section through the head of a fifty- to fifty-six-hour chick, showing the 

 invagination of the ectoderm in the formation of the lens vesicle, Lc, and the 

 pushing in of the optic vesicle to form the optic cup. X 80. 



45 Section through the head of a sixty-four-hour chick. The lens vesicle, 

 Lc, is completely formed and separated from the ectoderm. The retinal, R, and 

 pigment, P, portions of the optic cup can be distinguished. X 80. 



46 Section through the head of a sixty-eight-hour chick. The lens vesicle, 

 Lc, shows an increase in the posterior wall, PI, over the anterior wall, Al. The 

 optic cup with its retinal, R, and pigment, P, layers separated by a small por- 

 tion of the primary optic vesicle, Opv, is seen. X 80. 



47 Section through the head of a 90- to 100-hour chick, showing the rapid 

 growth of the cells of the posterior portion of the lens almost filling the lens 

 vesicle, Lc. Portions of the optic stalk, OpSt, can also be seen. X 80. 



48 Section through the eye of a sparrow embryo at the age of two days, 

 showing the optic stalk, OpSt, the corneal ectoderm, c, and the anterior, Al, and 

 posterior, PI, layers of the lens. The lens cavity has entirely disappeared. 



49 Section through the eyes of a sparrow embryo at the age of two days 

 passing through the center of the left eye. The ectoderm, c, the retinal, R, 

 and the pigment, P, portions of the retina and a part of the optic stalk, OpSt, 

 are seen. 



336 



