576 



THE ORGANS 



result of proliferation of cctodermic cells at this point is pushed inward 

 (invaginated), forming the optic cup. The invagination of the optic vesicle 

 extends also to the stalk, the sulcus in the latter being known as the chorioid 



Fore-brain 



Lens invagination- 



Optic vesicle 



Lens invagination 



Optic vesicle 

 Fig. 382. — Section through head of chick of three days' incubation. (Duval.) 



fissure. The latter serves for the introduction of mesenchyme, and the develop- 

 ment of the hyaloid retinal artery. Three distinct parts may now be distin- 

 guished in the developing eye, which at this stage is known as the secondary 



Fore-brain 



Lens vesicle 



Optic cup. 



Fig. 383. — Showing somewhat later stage in development of optic cup and lens 

 than is shown in Fig. 382. (Duval.) 



optic vesicle: (a) The proliferating epidermis which is to form the lens; {b) the 

 more superficial of the invaginated layers which is to become the retina; and 

 (c) the surrounding mesodermic tissue from which the outer coats of the eye 

 are to develop. 



TECHNIC 



(i) For the study of the general structures of the eyeball the eye of some large 

 animal, such as an ox, is most suitable. Fix the eye for about a week in ten- 



