56 THE VERTEBRATE RETINA 



Not all single cones are built like those of the frog. The oil-droplet 

 is lacking in the cones of nearly all living forms lower than the frogs; 

 but even so there are reasons for thinking the oil-droplet to be a very 

 primitive visual-cell feature. Such droplets occur in pigment epithelial 

 cells, which are homologous with the visual cells, and apparently also 

 (in salamanders) in the type of brain-cell from which the rods and cones 

 originated. The ellipsoid, which appears to be a light-concentrating 

 device, is sometimes supplemented by a second dioptric organelle, the 

 paraboloid, lying proximal to it in the myoid. The paraboloid may have 

 some very important function other than its incidental optical one. 

 While the ellipsoid always stains heavily with acid fuchsin, an out- 

 standing peculiarity of the paraboloid is its usual refusal to take any 

 stains at all. It is quite likely that some paraboloids are fluid vacuoles — 

 perhaps sometimes artificial spaces (Figs. 22a, 23b, 24a and b) ; but 

 many are solid or semisolid (Figs. 2 2d, 25) and keep their shape when 

 expressed from the living cell. 



The cone outer segment may actually be cylindrical when it is so very 

 slender that it could hardly be expected to taper, as in many lizards and 

 birds, and even sometimes when there is plenty of room for a more 

 bulky, conical structure (Fig. 22). The myoid may be quite non-con- 

 tractile and thus undeserving of that name, as in man; and it may be 

 permanently greatly elongated, marooning the body of the cone opposite 

 or even beyond the tips of the rods (flying squirrels, some lampreys and 

 snakes — see Fig. 69a, p. 167). The nucleus of the frog cone is typical 

 structurally, but not as regards its position, for cone nuclei almost always 

 lie in contact with the limitans or even (some fishes) beyond it, on its 

 scleral surface (Fig. 22a and b). 



One of the most noteworthy peculiarities which cones may have is 

 that presented by the cones of the greater portion of the human retina, 

 and also by some other placental mammals, the dog and cat for exam- 

 ple : the cone outer segment is a cylinder enclosed by a tubular process 

 of the pigment epithelial cell opposite to it, and apparently (though this 

 is not yet certain) fused at its tip with the pigment cell, actual proto- 

 plasmic continuity existing between the two (Figs. 19, 20b; pp. 43, 44). 

 No such arrangement is ever seen in rods, and its obvious advantages for 

 the facilitation of the nutrition of the cone constitute important evidence 

 for the cone's having a faster metabolism than the rod — something 

 which has long been suspected on other grounds. 



