VISION. 



529 



comes more contracted at the base, and the optic stalk remains 

 as the optic nerve. 



At an early stage of development (second or third day in the 

 chick) the outer portion of the optic vesicle is pushed inward, 



Fig. 379. 



Fig. 378. 



Fig. 378.— Section through head of chick on third day. showing origin of eye (after 

 Yeo). 0, epiblast undergoing thickening to form lens; o, optic vesicle; Vj, first 

 cerebral vesicle; V3, posterior cerebral vesicle. It will be observed that the retina 

 is already distinctly indicated. 



Fig. 379. — Later stages in development of eye (after Cardiat). a, epiblast; c, develop- 

 ing lens; 0, optic vesicle, 



SO that the cavity is almost obliterated ; the anterior portion, 

 becoming thickened, ultimately forms the retina proper, while 

 the posterior is represented by the tesselated pigment layer of 

 the choroid. 



As this retinal portion breaks away from the superficial 

 epithelium, the latter forms an elliptical mass of cells, the future 

 lens, the changes of which in the formation of the cells peculiar 

 to the lens illustrate to how great lengths differentiation in 

 structue is carried in the development of a single organ. It 

 will thus be seen that the most essential parts of the eye, the 

 optic nerve, the retina, and the crystalline lens, are, according 

 to a general law, the earliest marked out. The cornea, the iris, 

 the choroid, the vascular supply, the sclerotic, etc., are all sec- 

 ondary in importance and in formation to these, and are derived 

 from the mesoblast, while the essential structures are traceable, 

 like the nervous system itself, to the epiblastic layer. 



Any act of perfect vision in a mammal may be shown to 

 consist of the following: (1) The focusing of rays of light from 

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