VERTEBRATA, 



409 



vesicle connected with the fore brain by a narrow stalk, 

 called the optic stalk. The outer half of the vesicle then 

 becomes pressed in, like an invaginating blastula, and the 

 rim so produced gradually constricts to a small aperture, 

 like the blastopore of a gastrula. Hence the sac is now a 

 two-layered optic cup, like a gastrula, and contains a cavity, 

 the posterior chamber of the eye. The outer layer becomes 

 ^^pigment-layer, and the inner becomes the sen scry- layer, of 

 the retina. Meanwhile, the epiblast on the lateral wall 

 of the head opposite the optic cup invaginates a small 



Fig. 296.— Diagram of the Vertebrate Eye. 



(Seen in median section.) 



Lens, 



Conjunctiva. 

 Cornea. 

 Aqueous 



Humor. 



Iris. 



Retina. 

 Sclerotic. Choroid. 



Blind Spot. 



Sheath of Optic 

 Nerve. 



Optic Nerve. 



vesicle, w^hich becomes the le7is of the eye and fills up the 

 small aperture of the optic cup. 



The sensory cells of the retina send out nervous pro- 

 cesses, which grow along the optic stalk and eventually 

 reach the brain where they end in the optic lobes. 

 These processes arise from the ends of the retinal cells 

 which are nearest the posterior chamber; and the actual 

 sensory elements, called rods and cones, arise from their 

 deeper ends towards the pigment layer. Hence the light 

 has to pass through the nervous layer to reach the sensory 



