108 THE GENESIS OF THE VERTEBRATE EYE 



Differentiation of the Retina — The optic cup now has two layers 

 of tissue in its wall whereas the optic vesicle had but one (Fig. 38c', il, 

 ol) . The outermost of these layers remains forever one cell thick and its 

 cells shortly develop pigment granules, the whole layer becoming even- 

 tually the pigment epithelium of the retina. The cells of the inner layer 

 of the optic cup rapidly proliferate, forming many layers from which 

 will be derived the various layers of the adult sensory retina (Fig, 39). 

 Since it is the outermost of these cells (toward the pigment epithelium) 

 which are multiplying, their daughter cells are pushed ever inward 

 toward the cavity of the optic cup. It follows that these innermost cells 

 are the oldest at any one time and they are naturally the first to differ- 

 entiate. They lie in the position of the ganglion cells of the adult retina; 

 and it is into these that they develop, soon protruding their axon fibers 

 which grow along the inner surface of the optic cup. These fibers all 

 aim for the cup end of the optic stalk — the site of the future disc — and 

 here turn outward and grow down through the tissue of the stalk {not 

 through the groove on its under side) to make their connections in the 

 wall of the diencephalon. They form the optic nerve fibers; and a few 

 cells of the stalk tissue, which escape destruction by them, proliferate 

 the neuroglial cells which help to form the system of interfascicular 

 septa in the adult nerve. 



The further differentiation of the cells of the inner layer of the optic 

 cup proceeds in a two-fold manner: from the inner surface toward the 

 outer (next the pigment epithelium) and from the posterior pole of the 

 cup forward along all meridians toward the rim. At the posterior pole, 

 the future amacrine cells can be recognized soon after the ganglion cells 

 and Miiller fibers have differentiated. The bipolars and horizontal cells 

 next become distinguishable; and, when proliferation finally ceases, the 

 cells nearest the pigment epithelium (which have been doing the pro- 

 liferating) are finally free to differentiate into the rods and cones — the 

 last elements in the retina to mature though they are the most ancient 

 cells in the eye and are its whole reason for being. 



At any one time, these changes are further advanced at the posterior 

 pole than they are out toward the rim of the cup, where cell-division 

 may still be seen long after it has ceased in the fundus of the retina. 

 The optic cup thus grows at its lip, and rapidly increases manyfold in 

 diameter and in surface area as the embryo enlarges. A convenient con- 

 sequence of this is that it is possible to study the whole process of retinal 

 differentiation in a single favorable section of a single embryonic eye, 



