THE URODELE RETINA 603 



The Retina — The retina of a salamander differs from that of a frog 

 chiefly in the larger size and smaller number of its elements, and in the 

 total absence of vitreal or hyaloid vessels. The latter are presumably 

 dispensable owing to the smaller size of the eye and — probably — lower 

 metabolic rate of the retina (owing to the paucity of cones) . Large-eyed 

 forms tend to have thin retinae with extensive summation, the whole eye 

 being thus devoted to sensitivity. Small-eyed forms have thicker retinae, 

 in which no great pains have been taken to promote sensitivity through 

 summation or otherwise. But salamanders in general have much higher 

 visual-to-ganglion cell ratios than do the frogs. Whereas Rana pipiens 

 has about three visual cells per optic nerve fiber (within the area 

 centralis) , Burkhardt computed the following numbers of visual cells per 

 opticus fiber in American salamanders: Amby stoma maculatiim, 11; 

 A. jejfersonianum, 8; Triturus viridescens, 7; Eurycea bislineata, 22; 

 Desmognathus fuscus, 19; Plethodon glutinosus, 12; and Hemidactyl- 

 ium scutatum, 19. 



The visual cells are of the same morphological types as those of anuran 

 amphibians (Fig, 174, p. 599). Both rods and cones are present in all 

 species excepting the four cave forms, whose visual cells are mere nub- 

 bins and all alike, reduced by degeneracy to a common denominator. 

 But not all salamanders have green rods (they are definitely stated by 

 European investigators to be lacking in Salamandra, though present in 

 Triton [-Triturus]) ; and cones — particularly the double ones — may be 

 sparse in strongly light-shunning forms {e.g., Megalobatrachus) . Both 

 the rods and the cones tend to be shorter and stouter than those of frogs 

 and toads, and indeed the red rod of Necttirus, two and a half times 

 the diameter of that of a frog, is the thickest known to science.* 



The absence of cone oil-droplets is in adaptation to the habits of the 

 animals, as in non-ranid anurans, and does not prejudice against the 

 presence of colored oil-droplets in the ancestral Stegocephali (see Plate 

 I). In its own way, the urodele rod gives evidence of cone-ancestry: 

 though it does not have an oil-droplet when immature (cf. Rana) , it can 

 and does sometimes exhibit another cone-organelle, the paraboloid (e.g., 

 in Necturus). 



*It is for Necturus that the only counts of the retinal elements of an entire eye have ever 

 been made for any vertebrate. Palmer, in 1922, found about 53,000 rods, 42,000 single 

 cones, and 15,000 double cones in an average-sized retina, along with 176,000 inner-nuclear 

 elements (26,734 of these being Miiller fibers) and 30,464 ganglion-layer cell-bodies (most 

 of them glial — the optic nerve, near the chiasma, showed only 962 fibers). 



