PHOTOPERIODISM IN FEMALE DOMESTIC FOWL 773 



Time of Lay 



As a rule, the domestic fowl in regular production lays an egg a 

 day on two or more successive days, does not lay on one day, and then 

 again lays on two or more successive days. The eggs laid on consecu- 

 tive days have long been and continue to be known as a clutch (Jull, 

 1952). It has been pointed out that the clutch in this sense is some- 

 thing quite different from the clutch of the ornithologist, a nest com- 

 plement or brood (Romanoff and Romanoff, 1949; Hutt, 1949). 

 More recently, the eggs laid on successive days have been referred to 

 as a sequence (Fraps, 1954), a term equally applicable to the daily 

 succession of ovipositions and other events. 



Normal Photoperiods. Under natural lighting the hen lays only 

 during daylight hours. The eggs constituting a sequence are not laid, 

 however, at the same time of day on consecutive days. In a typical 

 sequence of moderate length (e.g., 3 to 6 or 7 eggs) the first egg is 

 laid during early morning hours and successive eggs are laid later on 

 consecutive days until, with lay of an egg in mid- or late afternoon, 

 the sequence is completed. The eggs of a sequence are thus laid not 

 only during daylight, but within restricted hours of dayhght. 



The timing of lay under controlled photoperiods is similar in all 

 respects to timing of lay under equivalent natural photoperiods. By 

 reversing a 12-hr photoperiod, Warren and Scott (1936) demon- 

 strated the dependence of time of lay on the prevailing alternation of 

 fight and darkness. Some 50 to 70 hr were required for restoration of 

 characteristic sequence relationships with the reversed day. Eggs were 

 laid in darkness as well as under fights during the period of adjust- 

 ment. Warren and Scott concluded that the influence of photo- 

 periodicity in regulation of oviposition occurred sometime before 

 ovulation. 



Exceptional Photoperiods. There have been few investigations 

 directly of the effects of unusual photoperiods on the timing of ovi- 

 position, and many reports make no mention of the subject. 



(a) Twenty-six-hour days. In experiments referred to earlier, 

 Byerly and Moore (1941) found that sequence lengths were much 

 greater for hens under 14 hr light-12 hr dark (26-hr day) than for 

 hens under 14 hr light-10 hr dark (24-hr day). Hens under the 26- 



