348 LESLIE B. AREY 



PROCEDURE 



Essential to success in a determination of this kind is the 

 choice of appropriate experimental animals. Previous experi- 

 ence with a variety of forms led to the selection of two fishes and 

 the frog. The cones of the golden shiner, Abramis crysoleucas, 

 have large, conspicuous ellipsoids and are so highly mobile that 

 the light-adapted myoid can shorten to one-tenth its maximum 

 length in darkness ('16). The cones of the common grass frog, 

 Rana pipiens, are easy to observe, but show a narrower range of 

 movement, the limits of extensibility being as one is to four. 

 The rods of the horned pout, Ameiurus nebulosus, not only are 

 of exceptional size in comparison with the usual diminutive ele- 

 ments of fishes, but they also undergo changes in length in the 

 ratio of one to ten ('16) ; the value of Ameiurus for experimenta- 

 tion of this sort cannot be overestimated. There is a further 

 inherent advantage in the animals chosen, inasmuch as their 

 visual cells remain at a uniform level during the characteristic 

 positional changes. The retinal pigment of all three animals 

 exhibits extensive movements: in darkness it is confined to a 

 narrow zone at the bases of the pigment cells, whereas in the 

 light it migrates nearly to the external limiting membrane. 



Temperature is an additional factor which must be carefully 

 controlled, although the necessity of this has been recognized 

 only recently ('16). In dark-adapted fishes a temperature 

 near the freezing point brings about a maximal contraction of 

 the cone myoid, such as is characteristically associated with the 

 action of light, while raising the temperature to the limit which 

 is compatible with life proportionately elongates the myoid. 

 Light, however, is so much more effective than temperature that 

 the latter factor does not enter as a complication in the light 

 adaptation of cones. The relatively slight quantitative effects 

 of temperature acting in darkness on the position of the rods 

 and on the distribution of pigment may also be disregarded. 

 In the frog, on the contrary, high and low temperatures evoke a 

 maximal expansion from the pigment during dark adaptation, 

 complete contraction being obtainable only at an intermediate 

 grade of about 15°C.; the cones, however, are shortened at the 



