98 VERTEBRATE PHOTORECEPTORS 



the human foveal cones. So deep is the foveal depression that 

 all of the nuclear layers are displaced laterally (Figure 68). 

 In the anthropoid and human fovea, the depression is not 

 nearly so deep, the external nuclear layer is continuous over 

 the foveal pit, and the cones are not as slender nor as closely 

 packed as in the chameleon fovea. The comparisons are seen 

 in Figures 63-66. When one compares the highly developed 

 fovea of the chameleon and other diurnal lacertilians with 

 that of the human, one can only be led to believe that in 

 the former, the conditions for high visual acuity are more 

 nearly met than in man. 



In the alligator, Chievitz (1889) first described a tapetum 

 extending through the entire upper half of the retina in the 

 form of a bright band. In this band he found a fovea in the 

 form of a very superficial narrow furrow with thickened 

 edges, and running horizontally across the entire tapetum 

 about 1 mm. from its lower edge. In my own studies on the 

 alligator retina, I described the tapetum and relative dis- 

 tribution of rods and cones, but did not observe the striped 

 fovea. My sections, however, were cut vertically and prob- 

 ably outside the foveal area. 



In some birds a similar situation is encountered. Casey 

 Wood (1917) describes a band-like area in the retina with 

 which is associated a well-defined fovea or macula. To this 

 form of avian retinal differentiation he gave the term infula- 

 macular fundus — (infula, meaning band or ribbon) . This 

 type is exemplified by the Greater Yellow-legs (Totanus 

 melanoleucus) . In this class the band encloses in some part 

 of its course a single fovea which is found on the nasal side 

 of the fundus (Figure 70). In some forms this band-like 

 area contains two macular regions which may be joined. 

 This type he has called the injulabimacular fundus. An 

 ocular specialization of this kind is illustrated by the common 

 Old World flamingo (Figure 71). It is well known that the 

 majority of birds possess a central (nasal) fovea. The diurnal 

 rapacious birds and the swallow possess both a central and 

 a temporal fovea. The latter in some instances is said to 



