STRUCTURE OF RETINA — PHRYNOSOMA 355 



of American turtles could not be demonstrated (Detwiler '16). 

 Rochon-Duvigneaud ('17) describes it in the eye of Chameleon 

 and of the Gecko as a cone-shaped structure. In Alligator mis- 

 sissippiensis (Laurens and Detwiler '20) it was found to be onh^ 

 slightly developed — being present in the form of a lightly pig- 

 mented cap covering the orbital pole of the optic nerve. 



Pigment epithelium. The epithelial layer offers no special pecu- 

 liarities. With the exception of AUigator mississippiensis (Lau- 

 rens and Detwiler '20) it is not unlike that m other reptiles 

 previously investigated (Detwiler '16). The pigment is ex- 

 tremely abundant and, in addition to occupymg finger-like pro- 

 cesses embracing the visual elements, is so compact in the cell 

 body as to occlude from view the epithelial nuclei (fig. 5). It is 

 present in the form of delicate brownish needle-like granules 

 which in the cell body proper, are so thick as to form a black 

 pigment mass appearing ahnost homogeneous. In the vitreal 

 processes, however, it thins out slightly and the separate pigment 

 granules are more clearly discernable. 



Under normal conditions of illumination (diffused dayhght) 

 the pigment extends down over the visual cells almost as far as 

 the paraboloid, and in some cases to the refractive disc (fig. 5), 

 covering thereby not only the outer segment, but a considerable 

 portion of the inner segment as well. At the apex of the foveal 

 depression the pigment does not extend quite as close to the 

 external Imiiting membrane as it does more peripheralward 

 (fig. 3), and the entire inner segment is uncovered. 



The position of the pigment has not been studied under differ- 

 ent conditions of illummation. It is the aim, however, of the 

 authors to investigate this as a sequence to the present paper. 



SUMMARY 



1. The general structure of the retina of Phrynosoma bears a 

 close resemblance to that of Chameleon. 



2. There is a prominently developed area centralis in the form 

 of a circular convexity above the point of entrance of the optic 

 nerve (fig. 1), with a maximally developed fovea at its center 

 (figs. 1 and 3). 



