RETINAL STRUCTURE AND ANIMAL HABITS 71 



relation which much more approaches the conditions in 

 Nycticebus than in the dhirnal monkeys (marmoset and 

 rhesus). 



The retina of Nyctipithecus is composed of fine filamentous 

 rods which are very similar in shape, size and number to 

 those of the nocturnal lemu- 



m^ 



#■■ 



roids (cf. Figures 47 a, b, 

 and 54). There is no evi- 

 dence of cones, or of any 

 macular specialization such 

 as a fovea or an area cen- 

 tralis retinae. Despite this 

 similarity with the retina of 

 the lemurs, there is one 

 notable difference, viz., in 

 the pigment epithelium. 

 This layer in Nyctipithecus 

 is pigmented throughout, 

 whereas in Galago and Nyc- 

 ticebus it is entirely devoid 

 of pigment in the fundus. 

 Furthermore, there is no 

 evidence in this form of a 

 choroidal tapetum such as 

 occurs in the lemur eyes. 

 Woollard (op. cit.), who described the retina of this form 

 as a pure rod retina, without a fovea, uses his observations 

 to support the contention that Nyctipithecus is the most 

 primitive of the Platyrrhines. It may be pointed out that 

 the marmoset is regarded by some authorities as a more 

 primitive form than Nyctipithecus, yet its retina is rich in 

 cones and it possesses a typical anthropoid fovea. It is 

 apparent that neither the presence nor absence of a fovea 

 and cones can be used to determine the evolutionary position 

 of an animal. Especially is this true of the fovea, as will be 

 discussed later. 



The presence or absence of rods or cones, as well as their 



¥ 



Fig. 54. Photomicrograph of the 

 retina of the owl monkey (Nyctipi- 

 thecus). X 485. 



