648 PHOTOPERIODISM IN VERTEBRATES 



Stimulus to gonad growth may be the result of an initial stimulus to 

 only one exteroceptor, but for full gonad maturation adequate stimuli 

 from a variety of exteroceptors may be essential. 



4. In the absence of the appropriate initial external stimulus, some 

 region of the brain, probably the hypothalamus, commonly seems to 

 possess the power to exert the necessary "pressure" spontaneously at 

 approximately the correct season. In the case of migrating birds that 

 winter in the other hemisphere, it may well be that the earliest stirrings 

 of gonad activity are normally induced by this internal rhythm, the 

 external stimulus only beginning to exert its effect after the bird has 

 returned across the equator into the new spring. 



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