1 82 Vertebrate Embryology 



formed outside of it, and, by the inward growth 

 of what we may now call the iris, the opening 

 of the optic cup is reduced to a small circular 

 opening, \hepupil. 



FB 



FIG. 60. TRANSVERSE SECTION THROUGH THE FORE- 

 BRAIN OF A CHICK OF 50 TO 60 HOURS' INCUBATION. To 

 illustrate the formation of the optic cup, lens vesicle, etc. 



A, the amnion. 7>, superficial ectoblast, FB, cavity of the 

 fore-brain. Z,, Lens. L V, lens vesicle. M, mesoblast. OC, cavity 

 of the optic cup. OS, stalk connecting the optic cup with the fore- 

 brain. OV, remains of the cavity of the original optic vesicle. 



The pecten originates as a vascular tuft of 

 mesoblast which grows into the cavity of the 

 optic cup through the choroid fissure, near the 

 origin of the optic stalk. It is first seen at 

 about the fifth day, and by the tenth day it 



