64 VERTEBRATE PHOTORECEPTORS 



With regard to the owl, Katz and Revesz (1914) state that 

 the rods of nocturnal birds (owls), in bright light, are similar 

 in function to cones. This seems to be without anatomical 

 foundation for the reason that the retinae of such birds 

 contain numerous cones (Garten, 1907; Hess, 1913). 

 Whereas the owl retinae in my own collection possess nu- 

 merous characteristically long rods (80-90 ji) and look like 

 pure rod retinas, very critical examination shows the pres- 

 ence of small cones. 



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Fig. 45 (Left). Photomicrograph of the chicken retina showing rods and 

 cones. Note large oil globules in the cones. X 455. 



Fig. 46 (Right). Photomicrograph of the adult rat retina. X 455. 



Among mammals, where the majority of forms possess 

 both rods and cones, great variations exist with respect to 

 the relative proportions of these two elements. Here too we 

 find a general preponderance of rods in nocturnal forms, and 

 an entire exclusion of cones in some cases. We are all aware 

 from personal observations that our domestic neighbors, 

 the rats and mice, are much more active at night than in 

 daytime. Although it is claimed by some (Menner, 1930) 

 that these rodents have cones, I have never been able to 

 recognize their presence with certainty in my own numerous 

 histological preparations of these forms. A photomicrograph 

 of the adult rat retina is shown in Figure 46. 



