54 VERTEBRATE PHOTORECEPTORS 



around the bases of the cones to form the definitive external 

 Kmiting membrane, through the pores of which the visual 

 cells protrude. The outer segment appears to be connected 

 with the diplosome and with the nucleus by a very fine 

 filament. 



The rods apparently have a similar mode of origin, but 

 from the beginning are smaller and thinner and their nuclei 

 occupy a deeper position (Figure 35). 



Apparently the human retina does not reach its full dif- 

 ferentiation until after birth. Particularly is this true of the 

 macular region. Whereas the eye is said to be light sensitive 

 at the seventh month, form perception and color discrimina- 

 tion are not acquired until after birth, and according to Mann 

 (op. cit.) the macular region (with the fovea), which begins 

 to differentiate late in foetal life (seven to eight months), 

 continues to differentiate until about sixteen weeks post 

 partem. 



Many observers have claimed that the rods and cones are, 

 from the beginning, discrete elements, and develop prac- 

 tically simultaneously (Levi, 1901, 1914; Leboucq, 1909; 

 Magitot, 1910; Seefelder, 1910; Cajal, 1911; Detwiler and 

 Laurens, 1921). Others have regarded the cones as special- 

 ized rods — viewing the rod as the more primitive element 

 (Parsons, 1915; Kerr, 1919). Still others have concluded 

 that cones represent early stages in the formation of rods 

 (Bernard, 1903; Cameron, 1905, 1911). These last investi- 

 gators came to this conclusion because they saw, in the 

 early stages of the developing amphibian retina, conical- 

 shaped elements only, whereas in later stages typical rods 

 were prevalent. It is true that in the early stages of de- 

 velopment of the amphibian retina, all the visual elements 

 are cone-like in shape. This appearance, however, is not 

 sufficient evidence to conclude that they are cones. In our 

 own observations (Detwiler and Laurens, op. cit.) we 

 showed that many of the so-called cones are really develop- 

 ing rods as evidenced by their larger size, the different 

 position of their nuclei, and the lamellar arrangement of 



