132 VERTEBRATE PHOTORECEPTORS 



together and stained on the same shde, this difference indi- 

 cated that the droplets were related in some way to the 

 rod outer segment rather than to the pigment epithelial cells. 

 This view was later adopted by Kolmer (1925) who then 

 claimed to have demonstrated their presence among the 

 foveal cones of the chimpanzee. The rod origin of the drop- 

 lets was substantiated by Keeler (1927) who was unable 

 to demonstrate their presence in the retinae of mice exhibit- 

 ing an hereditary absence of rods, although the pigment 

 epithelial layer was present and normal in appearance. 

 Further proof of the rod origin of these structures was pro- 

 duced by Johnson (1935) who demonstrated their presence 

 in the grafted retinae of Amblystoma punctatum which 

 lacked completely an epithelial pigment layer. 



In the developing rat retina (Detwiler, 1932) it was shown 

 that stained droplets in fixed retinae were not demonstrable 

 until after visual purple was definitely visible in the fresh 

 dark-adapted retina. The fresh retina could be recognized 

 as pink at twelve days after birth, at which time the rods 

 in fixed eyes exhibited granular material within the outer 

 segments, but no droplets outside the rod were seen in fixed 

 preparations until fourteen to eighteen days (v. Detwiler, 

 op. cit., p. 487). 



In 1939 Johnson reported that the droplets are not present 

 in the retinae of rats suffering from severe avitaminosis A. 

 Their absence under these conditions added further indirect 

 evidence that these structures bear some close relationship 

 to visual purple or some phase of its cycle, since Tansley 

 (1933a) has shown that visual purple is also incapable of 

 being formed in the retinae of rats suffering from a lack of 

 vitamin A. 



Except for the work of Walls (19346, 1939a), who regards 

 the droplets as artifacts, bearing no relation to the visual 

 process, all of the aforementioned observations seemed to 

 point to some relationship of the droplets to the visual cycle, 

 but offered no evidence as to the nature of the droplets nor 

 of the relationship between them and rhodopsin. Evidence 



