50 



VERTEBRATE PHOTORECEPTORS 



In the third stage the innermost cells of the inner neuro- 

 blastic layer begin to show characteristics of ganglion cells, 

 sending out their axones centripetally into the optic stalk 

 (Figure 33, C). The remainder of these cells become the 



A B C D E 



Fig. 33. The development of the human retina. (Modified from Duke- 

 Elder, 1939, C. V. Mosby Co., St. Louis.) Sections through the inner layer 

 of the developing optic cup of A, a 12 mm. embryo; B, a 17 mm. embryo; 

 C, a 21 mm. embryo; D, a 48 mm. embiyo; and E, a 170 mm. foetus, a, mar- 

 ginallayer; b, primitive neuroepithelium; c, outer neuroblastic layer; d, inner 

 neuroblastic layer; e, transitory layer of Chievitz; f, basement membrane; 

 m, fibers of Mliller. 1, rods and cones; 2, external limiting membrane; 3, nuclei 

 of rods and cones; 4, outer molecular layer; 5, inner granular layer; 6, inner 

 molecular layer; 7, ganglion cells; 8, nerve fiber layer; 9, internal limiting 

 membrane. 



amacrine cells and the Mlillerian cells, of which the latter 

 send their processes to the inner surface, spread out, and 

 unite with each other to form the internal limiting membrane. 

 The cells of the outer neuroblastic layer differentiate later 

 than do those of the inner layer. The innermost ones become 

 the bipolar cells, those more external, the horizontal cells, 

 and the outermost become the nuclei of the rods and cones. 



