CONTROL OF ANNUAL GONADAL CYCLES 727 



sistent with all the data from a wide variety of photoperiodic treat- 

 ments of Zonotrichia leucophrys gambelii. It appears to be consistent 

 with the data from the above-cited experiments of other investigators 

 with interrupted light as well as with the results of similar investiga- 

 tions with egg production in the domestic fowl (Dobie, Carver, and 

 Roberts, 1946; Staffe, 1950, 1951; Weber, 1951; Wilson and Abpla- 

 nalp, 1956). 



It is to be emphasized that our hypothesis assigns no positive role 

 to the dark periods either in experimental patterns involving inter- 

 rupted light or to night under natural conditions. To us it appears at 

 this time unnecessary to assume that there is a critically important 

 dark-dependent phase (Jenner and Engels, 1952), particularly in 

 view of the fact that no pattern of interrupted light gives a greater 

 response than continuous light over the same period. Likewise, al- 

 though some semantic difficulties may be involved, it appears un- 

 necessary to assign an inhibitory role to darkness (Kirkpatrick and 

 Leopold, 1952) despite the attractiveness of the analogy with long- 

 day plants. Rather it would seem more in keeping with the Law of 

 Parsimony simply to regard the dark period as one in which there is 

 no active light effect and that the photoperiodic effect is a function 

 only of the light periods. 



MECHANISM OF THE PHOTOPERIODIC TESTICULAR RESPONSE 



Rowan (1928, 1929) reached the conclusion, from experiments 

 with Junco hyemalis, that testicular development is caused by exercise 

 resulting from the increased light rather than by light per se. How- 

 ever, experiments with several other species, including Sturnus vulgaris 

 (Bissonnette, 1932a; Burger, Bissonnette, and Doolittle, 1942; Burger, 

 1943, 1949), Passer domesticus (Riley, 1940; Kendeigh, 1941), 

 Zonotrichia leucophrys gambelii ( Farner and Mewaldt, 1953, 1955a), 

 and domestic ducks (Benoit, 1935a; Benoit and Ott, 1944) fail to 

 sustain this hypothesis. The suggestions of wakefulness caused by 

 light (Rowan, 1937, 1938a,b; Wolfson, 1941). rather than light per 

 se, as the cause of testicular development present rather formidable 

 semantic difficulties so that further discussion of them appears fruit- 

 less at present. 



