776 REPRODUCTION AND MIGRATION IN BIRDS 



(1956). It will be recalled, however, that Warren and Scott (1936) 

 did not alter the random distribution of eggs laid over the 24 hr by 

 limiting the availability of feed to a daily 12-hr period. Possibly other 

 stimuli were more effective than feeding, or activity associated with 

 feeding, in maintaining the individual hen's pattern of exposure to 

 light, but the nature of such stimuli is not clear. The restriction of lay 

 to the period of daylight under continuous artificial light plus day- 

 light, or under supplementary all-night lighting, clearly involves a 

 definite pattern of diurnal activity, set largely by daylight. Presumably 

 the period of diurnal activity is associated with a greater exposure to 

 light than can occur during the period of inactivity, even though the 

 hens were under light continuously. 



Bastian and Zarrow (1955) have reported that enforced "wakeful- 

 ness" plus light, but neither alone, on the night preceding ovulation of 

 the first follicle of the hen's sequence delays its ovulation by approxi- 

 mately the extent of the added period of wakefulness plus light. The 

 authors suggested "that the normal restriction of ovulation to a given 

 period each day ... is the result of physiological inhibition of the 

 follicle release mechanism by the daily period of light and activity." 

 Since hens may lay in complete darkness (Rider, 1938; Wilson and 

 Woodward, p. 787), it would be of interest to know the relationship 

 between patterns of lay and activity under such conditions. 



THE ROLE OF PHOTOPERIOD IN TIMING OF OVULATION 



The lay of an Qgg is of course the terminal event in a series of 

 processes and antecedent events which include growth and maturation 

 of the ovarian follicle, nervous activation of the anterior pituitary 

 gland to effect release of ovulation-inducing hormone (OIH), and 

 ovulation. Basically, the processes culminating in ovulation appear to 

 be much the same in birds and mammals. Ovulation in the hen, how- 

 ever, like oviposition, occurs at progressively later hours until the 

 sequence is completed. We must account therefore not merely for 

 ovulation as such, but also for the retardation in time of day at which 

 successive ovulations take place and finally, for the conditions under 

 which ovulation fails to occur on the single day separating sequences. 

 These relationships have been described elsewhere in some detail 

 (Fraps, 1954, 1955a,b). 



