58 THE VERTEBRATE RETINA 



22f, 23g). A long filament runs axially or peripherally in the outer seg- 

 ment of (again, presumably) every visual cell and, just within the inner 

 segment, is connected with a pair of granules from which a second, 

 much shorter, filament proceeds down the inner segment for a way 

 (Fig. 23 a). This filament-and-granule apparatus, collectively, is the cen- 

 trosome of the cell, whose function in visual physiology, if any, is not 

 known. Rods may contain paraboloids, or even oil-droplets (Figs. 23b, 

 25b, c) , though only when the rods have had a peculiar history (Chapter 

 7, section D). The rod foot-piece may be just like a cone-foot; but in 

 animals whose rods are very slender and numerous (teleosts, mammals, 

 and nocturnal birds) it is a slender filament terminating in a highly 

 specialized, unbranched 'rod end-knob' — apparently to make more com- 

 pact the connections of many rods to single bipolars (Fig. 19, p. 43). 



It is also in such animals that the rod and cone nuclei are most sharply 

 differentiated as to size, shape, and chromatin distribution. In forms 

 with fewer, more bulky rods (lampreys, amphibians, many reptiles) the 

 rod and cone nuclei are indistinguishable on any basis other than pos- 

 ition, and the foot-pieces may be nearly or quite identical. In connection 

 with the question whether the rod or the cone is the more primitive cell, 

 it is significant that when the nuclei and foot-pieces are alike in a retina, 

 they both resemble the cone structures of retinae in which they differ — 

 and, cone-type nuclei are more like nuclei in general than are rod-type 

 nuclei. The heavy, dendritic cone-foot would also appear to be a more 

 primitive sort of connecting process than the peculiar rod-fiber and its 

 end-knob. Where they are markedly differentiated, the differences be- 

 tween rod and cone nuclei have no relationship to physiological differ- 

 ences which we are able to discern at present. 



Green Rods — There is a type of so-called rod, restricted to the am- 

 phibians, whose very long stalk is but slightly contractile (Fig. 23 e). 

 It lacks rhodopsin and this, together with the shortness of its outer 

 segment, would necessarily make it have a relatively high threshold. 

 Functionally, this 'green rod' (of Schwalbe) is probably more cone-like 

 than rod-like — its nucleus even lies in the inner part of the outer nuclear 

 layer, alongside the cone nuclei; but its origin is quite unknown. 



Double Cones — Even more mysterious are the 'double cones' — and 

 the puzzle they present is particularly irritating to the curious inves- 

 tigator because they are so very widespread among vertebrates. If they 

 occurred in only one or two animals, we might dismiss them as a curi- 



