60 THE VERTEBRATE RETINA 



mammals although some of the most primitive of these may prove to 

 have them when examined. So, most vertebrate groups have double 

 cones; yet we have no idea what they mean. The most that can be said 

 is that the number of double cones, relative to the total number of cones, 

 tends to be high in strongly diurnal animals and low in strongly noc- 

 turnal ones. As a maximum, double cones may about equal in number 

 the single cones of the same retina. 



The typical double cone (Fig. 24b, c ) consists of two very unlike 

 cones fused together in the lower myoid region. One member — the chief 

 cone — is always very much like the single cones in the same retina. The 

 other, or accessory cone is decidedly different. The ellipsoid is usually 

 unclear in outline proximally and its material blends with the ground 

 substance of the inner segment. There is almost never an oil-droplet, 

 but an enormous paraboloid is almost invariably present. This so dis- 

 tends the accessory myoid that the myoid of the chief cone is thinned 

 and curved around the paraboloid region so as to be almost indistin- 

 guishable proximally. There are two nuclei, and some indications that 

 the two foot-pieces connect with different bipolars. The two members 

 of a double cone seem to supplement each other — an organelle which 

 one lacks, the other possesses; but since everything that may be present 

 in the two members together may also occur in one single cone, the 

 segregation of parts in the double cone is without obvious meaning. 



Twin Cones — Quite another sort of element is the 'twin cone' (Fig. 

 24e) found in so many teleost fishes. In this receptor the two members 

 are identical and are fused throughout the length of the inner segment. 

 Thus the twinned myoid contracts and elongates as a unit during photo- 

 mechanical changes, whereas in double cones only the chief member 

 moves, the accessory having no myoid in the proper sense of the word. 

 Twin cones are strictly a teleostean monopoly. These fishes being a 

 terminal group in evolution, it is impossible to believe that ordinary 

 double cones developed from twin cones; nor is there much reason to 

 suppose that twin cones were ever double ones of the type described 

 above. But there are elements in some teleosts which for want of a third 

 possible name we shall have to call double cones (Fig. 24f ) . They seem 

 to represent twin cones in which the two ellipsoids and outer segments 

 have become unequal in size and different in staining properties and 

 hence, chemico-physical makeup; but the zone of fusion still extends the 

 whole length of the inner segment so that the two myoids contract and 



