Ovulation in the Domestic Fowl 149 



subcutaneously, 1 mg/hen, at about 4.00 p.m. for effect on the Cj follicle. 

 Lesions varying in size and position were placed at known times thereafter. 

 Progesterone-induced ovulation was regularly prevented only by lesions 

 placed in the anterior region of the ventro-mcdian hypothalamus — the region 

 shown to be involved in normal ovulation — or along fiber tracts originating 

 in this region and directed caudally toward the median eminence (Fig. 3). 

 Ovulation was not regularly prevented by lesions placed elsewhere. In view 

 of these observations, it was suggested that certain neurones of the para- 

 ventricular nucleus may be the site of progesterone "excitation", although 

 the possibility that these neurones and their associated fibers are only elements 

 in a structural complex was not excluded. 



Participation of the preoptic hypothalamus in the normal processes of 

 ovulation of the Ci follicle was found not to be essential beyond about 6 hr 

 before the event, since lesions placed at less than about 6 hr before expected 

 ovulation did not prevent its occurrence (48). Following the systemic injection 

 of progesterone, the integrity of the preoptic hypothalamus must be main- 

 tained for about 2 hr, that is, to within about 6 hr before the time of expected 

 ovulation (52). Observations based on the effects of lesions cannot define 

 the time of onset of hypothalamic activity in normally timed processes. But 

 since the same highly essential hypothalamic region becomes dispensable at 

 about the same hour before normal and progesterone-induced ovulation, 

 onset of activity may be inferred to occur by the same interval prior to normal 

 ovulation as does onset of activity associated with progesterone-induced 

 ovulation. As has been noted elsewhere (16, 17), ovulation of the Q follicle 

 follows systemic administration of progesterone by not more than 8 hr, and 

 thus "activation" of the hypothalamic region cannot occur at a greater interval 

 prior to ovulation. In these terms, the minimal duration of hypothalamic 

 participation in the ovulatory process, normally incident or progesterone- 

 induced, is of the order of 2 hr. 



According to Rothchild and Fraps (58), the anterior pituitary must 

 remain in situ until some 4 to 6 hr before expected normal ovulation if 

 ovulation is to occur. The same authors (59) observed that removal of the 

 pituitary within 2 hr following progesterone injection prevented all expected 

 ovulations, and that the gland must remain in situ for 4 hr to assure maximal 

 incidence of progesterone-induced ovulation. The duration of pituitary 

 participation in the processes of ovulation, again either normal or induced 

 ovulation, would thus appear to vary between 2 and 4 hr. The lesser estimate 

 of pituitary participation, 2 hr, is clearly in good agreement with estimated 

 duration of hypothalamic participation in ovulatory processes, and may 

 indicate concurrent "excitation" and OIH release, as was surmised by van 

 Tienhoven (64) on other grounds. In some hens, however, ovulation 

 apparently required an intact pituitary over somewhat longer intervals from 

 the assumed onset of OIH release. It is difficult to say just how much of 



