116 VERTEBRATE PHOTORECEPTORS 



of the rat eye at birth is followed by a hypoplastic develop- 

 ment of the corresponding external geniculate body. The 

 effect was not discernible until about one month after the 

 enucleation. 



WooUard (1927), who described the rod retina of the owl 

 monkey, Nyctipithecus, the only true monkey of strictly 

 nocturnal habits, says: ''Nyctipithecus is no exception to 

 the rule that a true macula and fovea are only found when 

 cones form the chief constituent of the sensory epithelium 

 of the retina. These observations add support to the con- 

 ception that Nyctipithecus is the most primitive of the 

 Platyrrhines." The first part of this statement is unques- 

 tionably true, but it is also true that the lack of cones and 

 a fovea can in no manner be used as an argument in support 

 of the thesis that Nyctipithecus is the most primitive of the 

 Platyrrhines. Some authorities (personal interviews) regard 

 the marmoset as a more primitive form than Nyctipithecus, 

 yet its retina not only predominates in cones, but it possesses 

 a typical anthropoid fovea (Figure 64). Neither the presence 

 nor absence of a macula and fovea can be used to pigeon- 

 hole any animal in the evolutionary scale. Eyes with pure 

 rod retinas are specializations and are correlated with 

 nocturnal life. Those with pure cone retinas are adaptations 

 for diurnal vision at high intensities of light. Foveas con- 

 stitute mechanisms for increased visual acuity, be they 

 present in fishes, lizards, birds, monkeys, or man. Pure rod 

 retinas are adaptations for nocturnal vision whether they 

 occur in lizards, bats, lemurs, or monkeys. 



With regard to the functional significance of the fovea, 

 a stimulating theory has been advanced by Walls (1937). 

 It has been generally regarded that the attenuation or dis- 

 placement of the retinal layers at the fovea presents a condi- 

 tion whereby light may reach the foveal cones unimpeded, 

 i.e. without passing through the ordinarily present nuclear 

 layers of the retina. This idea has become quite firmly fixed 

 and is stressed in textbooks of ophthalmology. It necessi- 

 tates the assumption, of course, that the transparency of 



