THE ANURAN RETINA 599 



double cones, ordinary ('red') and 'green' rods (Figs. 64b, 174a). The 

 foot-pieces of all four types are dendritic, and their nuclei also 'cone- 

 like' (as to chromatin distribution). The red rods have their nuclei in 

 contact with the external limiting membrane, a position usually reserved 

 in other retinae for the cone nuclei (since cones ordinarily have plump 

 bases) ; the unusual plumpness of the amphibian red rod accounts for 

 the location of its nucleus. 



Fig. 174 — Visual cells of anurans and urodeles. xlOOO. 



a, single cone, double cone, red rod, and green rod of leopard frog, Rana pipiens (in other 

 families of anurans, the cones lack oil-droplets). 



b, single cone, double cone, red rod, and green rod of tiger salamander, Ambystoma tigrinum. 



The cones of ranid frogs possess oil-droplets, some at least of which 

 are yellow in life; and the double elements are built on the standard 

 plan (see Fig. 24a, b, c, p. 59). Bufonids and hylids, probably also other 

 nocturnal forms (e.g., pelobatids, brachycephalids,* microhylids, poly- 



*Some of these however {e.g., Atelopus, Dendrobates) ate reported by the late G. K. 

 Noble (personal communication) to be strongly diurnal. Such anurans may prove to have 

 colored oil-droplets, or some substitute for them such as a yellow lens or cornea. 



