THE HORMONE OF PREPARATION 



nightly injections of estrogenic hormone will cause it to 

 undergo the changes characteristic of estrus. The ovaries 

 being absent, there will be no follicles to ripen and hence 

 no corpus luteum. If the experimenter wants to make his 

 artificial cycle complete, he will have to follow up his estrogen 

 with injections of the corpus luteum hormone — but that is 

 next chapter's story. 



I have chosen the guinea pig as my example, but I might 

 well have spoken of other animals. In the spayed female dog, 

 for example, all the physical signs of "heat" characteristic 

 of that species are brought on by estrogenic hormones, in- 

 cluding the swelling of the external genital region and the 

 sanguineous discharge normally seen at estrus or shortly 

 before. In short, we can say that the physical changes of 

 uterus and vagina that go with the ripening follicle of the 

 ovary, are caused by the estrogenic hormone. In the life 

 of the normal animal, however, these changes are accom- 

 panied also, during the estrous period, by the psychological 

 urge to mate. Is this also produced by the estrogenic hormone 

 of the ovary? Not all the necessary evidence on this im- 

 portant question is at hand. Psychic reactions are notori- 

 ously difficult to study. It is not easy to observe, test, and 

 measure the sex behavior of animals in the laboratory. We 

 know of course that if the ovaries are removed, the female 

 is no longer receptive sexually, but even this statement re- 

 quires reservations, especially in the case of our own unpre- 

 dictable species. In many animals that have been studied 

 (e.g. rat, mouse, dog, cat, sheep) there is evidence that the 

 administration of estrogenic hormone to castrated females, 

 or to intact females outside of the mating season, will awaken 

 sexual receptivity. Josephine Ball found the same result 

 in the Rhesus monkeys in the colony of the Carnegie Embryo- 

 logical Laboratory. In the guinea pig, recent work by Wil- 

 liam Young and collaborators at Brown University and 

 Yale Medical School strongly suggests that not only the 



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