2o6 INTERNAL SECRETIONS OF THE OVARY 



It is clear, therefore, that the dedine of the corpus luteum and 

 the reassertion of the oestrus-producing stimulus both tend to 

 increase the sensitivity of the uterus to oxytocin. It is difficult 

 to estimate at the moment just how important this effect may 

 be in the causation of parturition, but it would seem that it must 

 be a highly important, if not crucial, factor. 



(d) RELATION OF PARTURITION TO EFFECTS OF 

 OVARIECTOMY DURING PREGNANCY 



It is interesting to consider the effects of double ovariectomy 

 and removal of corpora lutea during pregnancy in connection 

 with the theories of the mechanism of parturition. The fact 

 that double ovariectomy leads to abortion means that either this 

 effect is not comparable to true parturition, or else that parturi- 

 tion is not a positive ovarian action. In other words, if the 

 experimental abortion is comparable to parturition it is difficult 

 to explain how the latter, as on Dixon and Marshall's results, 

 can be due to some internal secretion of the ovary. If, on the 

 other hand, Knaus's work is accepted as the basis of a theory 

 of the mechanism of parturition, it is clearly possible to compare 

 the abortion which follows ovariectomy or removal of corpora 

 lutea during pregnancy with true parturition. Assuming that 

 the corpora lutea have a de-sensitizing effect on the uterus, their 

 removal by ovariectomy would permit the reappearance of 

 uterine sensitivity to oxytocin; if the corpora lutea only were 

 removed the sensitivity would be increased by the action of the 

 oestrus-producing hormone of the ovarian stroma. On the whole, 

 therefore, it seems probable that the termination of pregnancy 

 which follows removal of the corpora lutea or double ovariec- 

 tomy can be compared to some extent with true parturition. 

 This, of course, only applies to the later stages of pregnancy. 

 The interruption of pregnancy in the early stages, either by 

 ovariectomy, removal of corpora lutea, or injection of 

 oestrin, is probably due to adverse uterine changes. 



