782 REPRODUCTION AND MIGRATION IN BIRDS 



termination of the sequence of releases, might possibly be associated 

 with certain phases of photoperiodicity. 



Bastian and Zarrow (1955) have proposed a somewhat different 

 hypothesis to account for "the asynchronous ovulatory cycle of the 

 domestic hen." They recognize "two separate and independent cycles" 

 which interact to produce the ovulatory cycle: (i) The release of LH 

 (OIH) is assumed to be continuous over a relatively long period (e.g., 

 8 hr) during the same hours every night, "even when a follicle is not 

 ovulated the next day." (ii) The maturation of ovarian follicles is as- 

 sumed to be gradually completed at more or less regular intervals. 

 Ovulation occurs if a sufficiently mature follicle is subject to LH. 

 Ovulation does not occur if a follicle of insufficient sensitivity for 

 response to LH passes through the hours of LH release. During the 

 same hours of the following night this follicle is highly sensitive to LH 

 and is therefore ovulated as the first of a new cycle. 



However much the views of Bastian and Zarrow (1955) and this 

 author may differ in some respects, we are in agreement in assigning 

 to photoperiodicity the role of timing the appearance of a period of 

 high sensitivity in the neural component of the OIH (or LH) release 

 mechanism. We are clearly not yet able to formulate time relationships 

 between photoperiod and neural behavior in the hen within anything 

 like the close limits demonstrated by Everett and Sawyer (1950) and 

 Everett (1956) to exist in the rat. This, however, should not prove to 

 be an impossible task. 



SUMMARY AND CONCLUSIONS 



The response to seasonal photoperiods by the common domestic 

 fowl differs from that of most wild birds in the absence of a strictly 

 limited "breeding season," exhibiting instead variation in rates of Qgo 

 production. Under natural photoperiods, maximum production may 

 be advanced to late fall or winter months by artificial light regimens 

 similar to those which so advance the breeding season in many wild 

 species. Activity or time available for feeding plays no part in the 

 stimulatory effects of photoperiod on production; the stimulatory 

 effects are transmitted by the central nervous system to the pituitary 

 body, thence by gonadotropins to the ovary. 



