DIFFERENTIATION OF RETINAL ELEMENTS 55 



the materials in their outer segments (Figure 36, D). It was 

 shown also (Detwiler, 1923&) that in Amblystoma larvae 

 just prior to the feeding stages (Harrison's stage 44), the 

 two categories of conical-shaped elements could be distin- 

 guished on physiological grounds as well. When such larvae 



C D 



Fig. 36. Stages in the early development of the visual cells in Amblystoma 

 punctatum. A, retina of an embryo 11 days after tail-bud stage; B, 13 days 

 after tail-bud stage; C, 15 days after tail-bud stage; D, 17 days after tail-bud 

 stage. Arrow indicates double cone. X 920. (From Detwiler and Laurens, 

 1921, /. Co7np. Neur., v. 33.) 



were light-adapted, the myoids of the true cones contracted, 

 and when dark-adapted they elongated. The ^potential' 

 conical-shaped rods, however, showed no phototropic 

 response — a condition characteristic of the adult urodele 

 rod (Figure 38). We concluded, therefore, on structural 

 as well as on functional grounds that rods do not represent 

 later stages in the formation of cones, neither is there any 

 evidence to suggest that cones represent specialized rods, 

 but that both are formed by divergent differentiation of a 

 cell of low morphological specialization. These conclusions 

 are in agreement with those of Levi (1901, 1914) who studied 

 the development of the retina in Salamandra. 



