Ovulation in the Domestic Fowl 155 



the last of the sequence would thus become a factor in determination of time 

 of the succeeding excitation and therefore of the extent of lag. The last OIH 

 release, by delaying onset of low thresholds to or beyond the time of day at 

 which the period of lapse intervened, would thereby terminate the sequence. 

 Estrogen levels presumably would be low at the hour of excitation and 

 OIH release for the first follicle of the succeeding cycle or sequence, and excita- 

 tion would occur in response to the appropriate phase of photoperiod at the 

 onset of the "open" period, which by definition is the case. 



This schematic statement of presumed periodicity in estrogen levels and 

 effects is no doubt overly simple. We do not know, for example, when 

 inhibitory levels may be attained, although gonadotropin injected as much 

 as 13 hr before estimated hour of expected OIH release for ovulation of C2 

 follicles effectively blocked the release (25). The considerable interval (24 to 

 29 hr) between a given OIH release and an effect on the succeeding OIH 

 release thus would seem to present no problem. 



Progesterone 



Since progesterone has been shown to effect OIH release over the same 

 hypothalamic structures involved in the normal release and, in addition, in 

 similar relationships insofar as these have been determined, it has been 

 suggested that progesterone (or a progestagen) might be the natural ovarian 

 hormone eliciting nervous "excitation". If this were so, we should be able to 

 adduce some evidence that progesterone is produced by the hen's ovary and 

 is found in the blood stream. 



Using the bioassay of Hooker and Forbes (34), progestogenic activity was 

 demonstrated in the blood of actively ovulating hens (27). In other tests, the 

 same authors found blood levels of what then was believed to be progesterone 

 to exceed 5 jixg/ml (unpublished results). Although the Hooker-Forbes test 

 is now known not to be specific for progesterone, the inference based on this 

 test apparently was confirmed when the substance was identified on chromato- 

 grams of extracts of ovaries of regularly laying hens (39). Progesterone was 

 found in extracts of maturing as well as of ruptured follicles, but these authors 

 failed to detect the substance in the blood of ovulating hens. This was 

 accomplished, hovv'ever, by Lytic and Lorenz (41) in extracts of samples 

 consisting largely of blood from the ovary "prior to contact with the liver or 

 any capillary bed". Their extracts were estimated to contain an average of 

 about 0.05 jLtg/ml blood. In comparison with values yielded by the Hooker- 

 Forbes test, this may seem a very low concentration, but physicochemical 

 determinations of progesterone in the systemic blood of pregnant women 

 have likewise failed to yield values comparable with those indicated by the 

 bioassay of Hooker and Forbes (68). Other naturally occurring compounds, 

 metabolites of progesterone, now are known to exhibit progestational activity 

 by the Hooker-Forbes and Clauberg tests and are therefore considered to be 



