PHOTOPERIODISM IN FEMALE DOMESTIC FOWL 783 



Oviposition occurs only during lighted hours under natural or 

 equivalent artificial photoperiods; reversal of photoperiod reverses the 

 restricted hours of lay after 50 to 70 hr. Under uniform continuous 

 light oviposition may occur at any hour of the 24, or Only within 

 restricted hours of the twenty-four in association with feeding or 

 maintenance practices. Patterns of activity may impose patterns of 

 varying exposure under uniform illumination and thus restrict the 

 period of lay. The restriction of oviposition to daylight hours when 

 daylight is supplemented with either continuous or all-night lighting 

 suggests response to intensity differentials, possibly with the interven- 

 tion of "psychological" factors. 



One role of photoperiodicity in the hen's ovulation cycle is to time 

 the appearance of a period of low thresholds to ovarian hormones in 

 the neural component of the mechanism controlling the release of 

 ovulation-inducing hormone (OIH) from the pituitary. The period of 

 low thresholds (or high sensitivity) has a duration of not over 8 or 9 

 hr, and its onset is thought to follow rather closely (1 to 3 hr) upon 

 the onset of darkness. With appropriate assumptions regarding rela- 

 tionships between concentrations of ovarian hormones and neural 

 thresholds, most of the characteristics of the ovulation cycle (and thus 

 of the oviposition cycle) can be accounted for, however formally. At 

 the least, the restriction of oviposition within lighted hours is seen to 

 have a basis in the association between photoperiod and a similarly 

 restricted phase of neural control over pituitary activity. 



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Barott, H. G., L. G. Schoenleber, and L. E. Campbell. 195L The effect of 

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Bastian, J. W., and M. X. Zarrow. 1952. Failure of nembutal to block ovula- 

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Benoit, J., and L. Ott. 1944. External and internal factors in sexual activity. 

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