RODS; HOMOLOGY WITH CONES 57 



Rods — One rod would do about as well as another to illustrate rod 

 structure, for rods do not differ from retina to retina nearly so much 

 as do cones. The rod (Fig. 23) has the same principal parts as the cone 

 — outer and inner segments, nucleus, and foot-piece. The outer segment 

 is almost without exception a perfect cylinder and the inner segment is 

 often more slender — sometimes, as in bony fishes, much more so. 



The rod in man and other mammals is not contractile; so, the term 

 'myoid' for the undifferentiated part of the inner segment would be a 

 misnomer. A structure corresponding in microchemical behavior to the 

 cone ellipsoid is present, though it is probably optically functionless. 

 Rod nuclei tend to be smaller, more nearly spherical, and with much 

 larger and fewer masses of chromatin than cone nuclei. The latter hav- 

 ing preempted positions against the limitans (the cones being the first 

 visual cells to differentiate in embryonic retinae), the rod nuclei per- 

 force contact the limitans only between cone nuclei and for the most 

 part are forced to pile up below it to form the thick outer nuclear layer. 



Cones ordinarily vary considerably in different retinal regions, being 

 more slender and more numerous toward the fundus. Rods are uniform 

 in concentration everywhere except as this is influenced by the cones — 

 it is as though the cones had been distributed in the retina first, and 

 then the spaces between them neatly filled in with as many rods as 

 would conveniently fit. Rods are ordinarily uniform in diameter through- 

 out a retina, but their length tends to increase slightly and slowly from 

 ora to fundus. The center of concentration of cones, or of rods when 

 they have such a center, does not necessarily lie anywhere near the optic 

 axis of the eye. Seen 'on the flat', the rod and cone mosaic exhibits a 

 pattern which in different animals may have the hexagon, the square, 

 or some other geometrical figure as its unit. These patterns have not 

 yet been sufficiently studied for them to yield up any ulterior meaning 

 which they may have. 



Homology of Rods and Cones — Cone and rod are homologous part 

 for part and have many points in common. The outer segments of both 

 have thin sheaths filled up with a lipid ground-substance in which one 

 or more closely wound spiral filaments of another lipid material, derived 

 from mitochondria, are embedded (Fig. 23a). These show faintly or 

 clearly in large rod outer segments (Figs. 25 and 26), rarely also in 

 cones; but they are presumably always present. When too heavily 

 stained, they commonly give an appearance of transverse discs (Figs. 



