THE CORPUS LUTEUM 729 



season, so that only one heat period or oestrus occurs instead of 

 a succession of such periods as in the higher mammals ; these 

 animals are termed moncestrous. Moncestrous mammals have 

 a long quiescent period, in some cases lasting about eleven 

 months, and the corpora lutea have disappeared months before 

 another set of follicles starts to mature, and so the inhibitory 

 influence throughout this period cannot be regarded as due to 

 the corpus luteum. As then follicles can mature, while corpora 

 lutea are present and in the absence of such bodies the follicles 

 in some cases degenerate and in others do not mature, this 

 theory of Beard is also unsatisfactory. 



Loeb (12) investigated this point in guinea-pigs. He first 

 ascertained the time elapsing between two successive ovulations, 

 the first following coition and the second spontaneous, and 

 found that in no case was it less than fifteen days and in the 

 majority it was more than nineteen. A second series, in which 

 the corpora lutea were extirpated during the first day of their 

 growth, was examined, and it was found that after sixteen days 

 all those in which the extirpation had been complete had 

 ovulated. On the other hand, in a certain number of cases in 

 which the extirpation was incomplete and some of the corpus 

 luteum tissue was left, ovulation had not occurred eighteen or 

 twenty days after. It has been claimed that these experiments 

 support Beard's hypothesis that the function of the corpus 

 luteum is to prevent ovulation. What they do show, however, 

 is that the removal of these structures tends to hasten the next 

 ovulation. The criticisms advanced above apply also in this 

 case and in the moncestrous mammals a suppression of ovulation 

 takes place without the intervention of corpora lutea. So that 

 although it may be said that the presence of a corpus luteum 

 tends to retard ovulation it cannot be claimed that this is its 

 main function. 



Two hypotheses now remain, which, in the light of our 

 present knowledge, appear more satisfactory, and it is proposed 

 to deal with them separately. 



Uterine Changes in Early Pregnancy 



The first concerns the changes in the uterus during the first 

 stages of pregnancy, and owes its origin to Gustav Born. It is 

 that the corpus luteum provides an internal secretion which 

 assists in the attachment of the embryo to the lining of the 



