100 Discussion 



and steroids, i.e. they all elevate the EEC afterreaction threshold. These findings 

 fit many of the known facts and can be used to explain many of our most puzzling 

 experimental results. They are particularly impressive, for example, when applied to 

 our results of experiments on the efTccts of progesterone on ovulation in the bovine. 

 Small doses of progesterone injected within 2 hours after the beginning of estrus 

 hasten ovulation, presumably by lowering the threshold for the release of pituitary 

 ovulating hormone, so that this event occurs at an earlier than normal time. On the 

 other hand, daily injections of larger amounts of progesterone from the 15th day of 

 the cycle onward delay both estrus and ovulation until 4 to 5 days after cessation of 

 the injections, presumably by maintaining elevated arousal and afterreaction 

 thresholds. 



Evidence has been accumulating to suggest that the neurohumors involved in 

 anterior pituitary gonadotropin secretion are of hypothalamic origin, and that the 

 blocking drugs exert their eflfects at more remote sites in the central nervous system. 

 Perhaps the best of this evidence is Everett's demonstration of ovulation in the 

 atropinized rat in response to electrical stimulation of the preoptic region. These 

 results all suggest that the next major advance in our knowledge of the mechanism 

 of ovulation is very likely to come about as a result of preparing hypothalamic extracts 

 and injecting these into suitably prepared experimental animals, as Harris has already 

 suggested. 



