170 EMBRYOLOGY 



The optic cup itself has two layers: a thick inner layer, the nervous or 

 sensory portion of the retina; and a thin outer layer, the pigmented layer of 

 the retina. Within the nervous layer the rods and cones develop from cells 

 farthest from the cavity of the optic cup. The neuroblasts nearest the cavity 

 of the optic cup differentiate into neurons which send fibers along the inner 

 surface of the cup to the optic stalk. The fibers grow through the optic stalk 

 into the brain, and the cells of the optic stalk become converted into support- 

 ing tissue. Thus it is seen that light coming into the optic cup first falls not 

 upon the sensory cells but on the nerve fibers. The light passes through these 

 nerve fibers and the cell bodies of the nerves before reaching the light- 

 sensitive rods and cones. The so-called inverted structure of the retina in the 

 adult is thus caused by the fact that the neuroblasts differentiate from the 

 inner layer of the cells lining the optic cup. 



