THE LATER DEVELOPMENT OF THE FROG 145 



quite to the surface of the head. The cavities of these struc- 

 tures are continuous with the brain cavity (third ventricle), 

 so that the relation of the optic cells to the optic vesicle is the 

 same as to the original cavity of the brain, and the free surfaces 

 of these cells are now turned away from the surface of the 

 head ; the true optic cells form the most distal part of the optic 

 vesicle. 



The sensory, or recipient, and the nervous elements of the 

 eye (retina and optic nerve fibers), originally on the surface of 

 the head, are the essential parts of the eye; they now form the 

 optic vesicles, and are to be distinguished from the various 

 accessory parts (the choroid and sclerotic coats, the aqueous 



I. 



FIG. 48. Frontal section through the fore-brain and optic vesicle of an em- 

 bryo of R. fusca, in which the tail is just growing out. From Von Kupffer 

 (Hertwig's Handbuch, etc.). a, Optic vesicle; as, opening of optic stalk out of 

 fore-brain; 7, posterior wall of infundibulum; I, rudiment of lens (placode); 

 P, wall of prosencephalon; r, rudiment of olfactory organ. 



and vitreous humors, and the cornea), which are derived 

 chiefly from the mesoderm (mesenchyme) of the region, and 

 from the ectoderm outside of the optic vesicle (lens and cornea, 

 in part). 



Upon reaching the surface of the head the optic vesicle is 

 converted into the optic cup. The peripheral part of the ves- 

 icle becomes flat and then folds into the proximal part, forming 

 a roughly hemispherical, two-layered cup, obliterating the 

 original cavity of the optic vesicle and establishing a new 

 cavity (Figs. 49, 50). The extent of the optic cup is increased 



