62 



THE VERTEBRATE RETINA 



Double Rods — Still another kind of visual cell is the double rod. These 

 were long known in geckoes (a family of nocturnal lizards) and have 

 recently been found in snakes. The gecko double rod (Fig. 25) was 



Fig. 25^Double rods in lizards, and their derivation, x 1000. 



a, the two cell-types of the pure-cone retina of the (diurnal) collared lizard, Crotaphytus 

 collaris; parts as in Figs. 22 and 24. The outer segments are tiny and the oil-droplet is 

 yellow in life. 



b, cell-types of Rivers' night lizard, Xantusia riversiana. The outer segments have become 

 rod-like but contain no rhodopsin, and the oil-droplets are large and colorless. Morpholog- 

 ically, these elements are intermediate between cones and rods; physiologically, they are 

 low-threshold. 



c, the cell-types (single and double rods) of the banded gecko, Coleonyx variegatus. The 

 massive outer segments contain rhodopsin, and the oil-droplets have disappeared. 



certainly not derived from a bifurcated single rod, but directly from a 

 double cone. It is thus closely homologous with the ordinary type 

 of double cone since it is the latter which occurs in diurnal lizards. The 

 double rods in certain snakes (Fig. 26) were just as certainly derived 

 from the peculiar ophidian type of double cone, for they have exactly 



