414 



Comparative Animal Physiology 



spawns in fresh water. This dilemma was solved upon examination of tad- 

 poles, just entering metamorphosis (small hind legs). These tadpoles ex- 

 hibited two pigments, with porphyropsin predominating. Tadpoles further 

 along in metamorphosis (well developed front and hind legs and mouth for- 

 mation beginning) exhibited equal amounts of vitamins Ai and Ao, and 

 presumably equal concentrations of rhodopsin and porphyropsin. Newly 

 emerged frogs exhibited a preponderance of rhodopsin. Migration from fresh 

 water to land has had the same effect on the photosensitive pigment as mi- 

 gration from fresh water to sea water. 



Although the data bearing on the evolution of these photopigments are 

 vet incomplete, the information at hand indicates that the utilization of 

 vitamin A;, in a conjugated photosensitive pigment began early in the ver- 

 tebrate line. Migration from the fresh-water environment into sea water and 

 onto land has promoted the utilization of vitamin Ai. 



LOG INTENSITY 



Fig. 122. Diagram showing rod and cone curves for flicker fusion frequency plotted 

 against log intensity, and the changes produced in the cone curve by changes in tem- 

 perature (T2 is higher than Ti), and'by changes in the ratio of the light to dark (L/D) 

 periods, during a flicker cycle. The inserts indicate the type of field (left) and apparatus 

 (right) used in obtaining flicker fusion data. From Jahn.^°° 



Measurement of Visual Functions and Their Photochemical Bases. It is 

 possible to design many types of experiments which test the functioning of 

 the visual system of an animal. Such experiments may test numerous items, 

 among which are: (1) the ability of an animal to determine whether or not 

 a light is flickering; (2) the ability of an animal to distinguish varying de 

 grees of brightness; (3) the increase in the ability to detect light (i.e., sen- 

 sitivity of the eye) during a prolonged period of darkness; (4) the ability of 

 an animal to detect small objects; (5) the decrease of sensitivity during 

 either brief or prolonged illumination. 



The general types of experiment which are used to measure these abili- 

 ties are described below. 



