VERTEBRATA. 409 
vesicle connected with the fore-brain by a narrow stalk, 
called the optic stack. 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 
the pigment-dayer, and the inner becomes the sensory-dayer, 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. 
Vitreous 
Humor. 
Conjunctiva. 
Cornea. 
Aqueous 
Humor.’ 
Tris. 
Blind Spot. 
Sheath of Optic 
erve, 
Retina. i 
Sclerotic. Choroid. Optic Nerve. 
vesicle, which becomes the Zens 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 
