148 



ADAPTATIONS TO ARHYTHMIC ACTIVITY 



the rod myoid. The two movements are not perfectly synchronized, how- 

 ever, for the visual cells usually complete their migrations much more 

 rapidly than does the retinal pigment, though always consuming from 

 several minutes to an hour or more in the process, in different species. 

 There may be both indefinite and very definite differences within a single 



Fig. 64 — Photomechanical changes of the leopard frog, Rana pipiens. x 500. 



a, ventral periphery of light-adapted retina. The expanded pigment obscures the visual cells, 

 but a cone and a rod have been emphasized to show their positions. 



b, same region, dark-adapted. The outlines of the visual cells have been reinforced. Note 

 that the cone myoids are greatly lengthened, the rod myoids somewhat shortened, as com- 

 pared with a. Toward the right is a double cone, whose chief member has migrated but 

 whose accessory member never leaves the limitans (c/. Figs. 22c, 23d, 24b, pp. 54-59). 



retina, for the cones may be either uniform or very ragged in their re- 

 sponses, and both pigment and cones may respond less in particular 

 retinal areas than in others. In fishes the single and twin cones migrate 

 at different rates to different extents, and in other vertebrates the acces- 

 sory members of double cones never migrate whether the chief cones do 

 or not (see Fig. 24, p. 59, and Fig. 64b) . 



