CONTROL OF ANNUAL GONADAL CYCLES 733 



indefinitely with long daily photoperiods (Burger, 1947; Miller, 1951, 

 1954; Wolfson 1952b; Vaugien, 1952a,b). Caution must be exercised 

 in generalizing, however, since Benoit, Assenmacher, and Brard 

 (1956) have shown that drakes kept in continuous light will pass 

 through several testicular cycles with gradually decreasing amplitude 

 but with greater and more irregular frequency. 



400 



200 



100 

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 60 



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s 

 < 

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D 20 



S 



z 10 



UJ 



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8 



O Adult Brrd 



• First-yeor Bird ^ 



° O 



I 



400 



200 



100 



80 



60 



40 

 20 



10 

 8 

 6 



I - 

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I7s«pt- 27Sept- II Oct- 250ct- 8Nov- I Jan- 



220ct 31 Oct 14 Nov 28N0V 1 2 Dec * ^eb 



PERIOD OF TREATMENT 



Fig. 5. The natural termination of the refractory period in Zonotrichia 

 leucophrys gambelii. Open circles represent first-year birds. Closed circles 

 represent adults. Birds were tested for refractoriness by subjecting them 

 to 15-hr daily photoperiods of supramaximal Hght for the periods indi- 

 cated. (From Farner and Mewaldt, 1955.) 



The actual site of refractoriness is still incompletely known. Since 

 the gonads of refractory birds, of at least several species, can be 

 caused to develop by the injection of gonadotropic hormones (Riley 

 and Witschi, 1938; Schildmacher, 1939; Miller, 1949; Vaugien, 

 1954b, 1955, 1956a; Benoit et al, 1950), it seems evident that the 

 site is not gonadal. In domestic drakes the gonadotropin content of 

 the adenohypophysis is relatively high during the refractory period 



