DOUBLE CONES 



59 



osity. Perhaps if they occurred in the human retina we would before 

 now have gained some clue to their role in visual processes; but their 

 functional significance, their exact mode of formation in the developing 

 retina, and the probable time and manner of their evolutionary origin 

 have yet to be determined. Next to the amacrine cells, the double cones 



Fig. 24 — Double and twin cones, x 1000. 



a, double cone of a holostean fish, the bowfin, Amia calva. b, double cone of leopard 

 frog, Rana pipiens; dark-adapted {i.e., with myoid of chief cone elongated), c, double 

 cone of western painted turtle, Chrysemys picta marginata. d, double cone of European 

 grass snake, Matrix natrix. e, twin cone of a teleost fish, the bluegill, Lepomis m. 

 macrochirus; light-adapted (i.e., with fused myoids contraaed). f, conjugate element (of 

 Fundulus heteroditus; after Butcher) characteristic of some teleosts; perhaps intermediate 

 between a and e, perhaps instead a derivative of e. 



c- 'clear mass'; d- oil-droplet; e- ellipsoid of chief cone; e'- ellipsoid of accessory cone; 



/- foot-piece; g- 'granular mass'; /- external limiting membrane; m- myoid; n- nucleus of 



chief; n- nucleus of accessory; o- outer segment of chief; o'- outer segment of accessory; 



p- paraboloid. 



are physiologically the most obscure elements in any and all retinae. 

 They have unfortunately not greatly interested visual physiologists, 

 since the latter have their attention focused upon the human retina, in 

 which double cones are lacking. 



Double cones appear phylogenetically first in the holostean fishes 

 (Fig. 24a). They occur in amphibians, reptiles, birds, one monotreme 

 (Ornithorhynchus) and marsupials, but not in any known placental 



