OPTIC NERVE; RODS VS. CONES 53 



derived from a cone-type in an ancestor of different habits. In an at- 

 tempt to resolve the confusion resulting from an overemphasis of shape- 

 differences — which has even led some to deny any distinction between 

 rods and cones! — the writer several years ago proposed the names 

 'photocyte' and 'scotocyte' for the two physiological types of visual 

 cells contrasted in the Duplicity Theory (see next Section). But it is 

 perhaps too late to bring about any such revolution in the terminology. 



Of the two types, there can be no doubt that the cone is the older and 

 more primitive. This statement however — which is quite contradictory 

 to any the reader will find in other books — is not to be taken to mean 

 that cones entirely like those of man were the original vertebrate visual 

 cells. It is certain, for instance, that the ancestral cell lacked any means 

 of analyzing colors. It is equally certain that the common ancestor of 

 present-day rods and cones lacked any such ingenious sensitizing sub- 

 stance as rhodopsin (see Chapter 4) . With a slender, pointed, stimulable 

 organelle, the outer segment, derived from a formerly vibratile flagellum 

 (see Chapter 5, section B) and connecting directly to a simple afferent 

 neuron, the pro-vertebrate visual cell could not but have been a high- 

 threshold receptor, which limited the excursions of its owner to the 

 brightly lighted surface waters. 



Rods came later, as a means of extending the period of activity over 

 a greater portion of the twenty-four hours. They were derived quite 

 simply from cones by the enlargement of the outer segment and by an 

 increase in the number of visual cells connected to each nerve cell. It 

 was not desirable for all of the visual cells to make these changes, for 

 unless two types were preserved side by side in a nice balance, sensitivity 

 to dim light could not be increased without too great a sacrifice of re- 

 solving power. The needs of the animal — whether greater for sensitivity, 

 or for visual acuity — then determined the proportion of small un-sum- 

 mated and larger, summated visual cells which would give him optimal 

 visual capacity. With the invention of the powerfully sensitizing rhod- 

 opsin by the rod on the one hand, and the differentiation of a photo- 

 chemical basis for hue-discrimination in the cones on the other hand, 

 the widely useful duplex retina as we know it today came into being. 



Single Cones — Because of the antiquity and priority of the high-thres- 

 hold cell, we will consider first the cytology of a typical single cone such 

 as that of the frog (Fig. 22c). The elaborate cytoplasmic portion of this 

 complex cell protrudes through a lacuna of the external limiting mem- 



