PHOTOPERIODISM IN FEMALE DOMESTIC FOWL 781 



sent terminal excitation hormone concentrations associated with matu- 

 ration of follicles likewise designated at ovulation. The levels at which 

 successive concentration curves intercept the curve describing diurnal 

 rhythmicity in thresholds of response determine times of successive 

 excitations Ei, E2, E3 • • • Eg. The timing of successive excitations 

 thus depends not merely on the displacement of excitation hormone 

 curves, but also on the time of day at which threshold values are at- 

 tained. 



The Ci' curve is displaced by the extent of lag at Ee. It fails to 

 reach threshold levels at or around hour 8 of day 7 because of this 

 displacement in relation to increasing threshold requirements for 

 excitation. With the recurrence of decreasing threshold requirements, 

 excitation Ei' occurs at hour 24, which is hour of the first day of a 

 new cycle. Excitation Ei of Fig. 3 is understood to have been similarly 

 determined. 



The possibility that the excitation hormone may be progesterone or 

 a progestin has been discussed elsewhere (Fraps, 1955a,b). But estro- 

 gen has been shown to delay ovulation under circumstances which 

 suggest that it acts by increasing excitation threshold requirements 

 (Fraps, 1954; Fraps and Fevold, 1955). If estrogen levels vary with 

 successive releases of OIH (or LH), such variations might well be 

 important in the normal cycle. Answers to these and other questions 

 will be required for a clearer formulation of the excitation cycle. 



Huston and Nalbandov (1953) discovered that irritation of the 

 magnum of the hen's oviduct by a contained object or a surgical 

 thread prevented ovulation as long as 20 to 30 days without, how- 

 ever, interrupting the secretion of hormones required for maintenance 

 of the ovary, oviduct, and comb. These findings led to two generaliza- 

 tions (Nalbandov, 1953): (i) Luteinizing hormone (LH), like 

 follicle-stimulating hormone, is secreted at a continuous base level, 

 over and above which the pituitary periodically releases greater quan- 

 tities to induce ovulation, (ii) The presence of an object, or of an 

 egg, in the magnum suppresses, over neurogenic pathways, the peri- 

 odic release of LH for ovulation. When the egg clears the magnum, 

 neurogenic suppression ceases and LH is released for ovulation. Nal- 

 bandov (1953) notes that this theory does not account for timing of 

 the first LH release of the sequence. This first release, as well as 



