106 VERTEBRATE PHOTORECEPTORS 



rods which are not nearly as numerous per unit area as in 

 the cat. The contrast is marked in the lemurs and is greatest 

 in the owl monkey (a strictly nocturnal form) where the 

 external nuclear layer is between three and four times as 

 thick as the internal (Figure 16). If the greater thickness 

 of the internal layer over the external means anything in 

 terms of enhanced visual acuity, and one can only be inclined 

 to believe that it does (as judged from the extremes of condi- 

 tions in diurnal birds as compared with nocturnal lemurs), 

 then the marmoset and the rhesus monkey may have a 

 slightly more efficient visual apparatus from the standpoint 

 of conduction than the human retina. In connection with 

 the suggested possibilities for increased visual acuity on 

 such a basis, it would be interesting to compare the visual 

 acuity of the owl and gecko with other forms. In the owl, 

 which is regarded primarily as a nocturnal form, the wddth 

 of the internal nuclear layer is relatively almost as thick as 

 in the diurnal birds. The owl also has long closely packed 

 rods which reach more than 80 /x in length. The gecko, 

 which is nocturnal in its habits, has a broad internal nuclear 

 layer as compared with the external, and the conditions 

 approach that found in the diurnal turtle and the horned 

 toad. The gecko has large rods which are relatively widely 

 spaced as compared with those in nocturnal mammals — 

 which would indicate a lower visual acuity from the stand- 

 point of the fineness and distance apart of the receiving ele- 

 ments. Might this be compensated for, in any way, by a 

 more efficient conducting and association mechanism? If the 

 individual rods in the gecko should be connected with more 

 bipolar and association cells than they are in nocturnal 

 mammals, then the visual apparatus of this form might be 

 expected to be more efficient — just as it seems more efficient 

 in birds on this basis. 



There is no absolute proof at hand to argue for a greater 

 or less efficient visual mechanism on the basis of the width 

 of the internal nuclear layer. It would seem, however, on 

 a priori grounds that this must be considered as a factor. 



