25(5 CHARLES R. STOCKARD AND G. N. PAPANICOLAOU 



appearance of its secretion permits the uterine mucosa to 

 vmdorgo the degenerative changes typical of the 'heat period.' 



Therefore, we must object to their conclusion that the occur- 

 rence of heat is not dependent upon corpora lutea — and further 

 we are unable to believe that their experiments, or any other so 

 far recorded, indicate that ''the ovarian interstitial cells are 

 possibly concerned in the process." The evidence to our minds 

 does not in the least point in such a direction. 



A most ingenious attempt at an explanation of menstruation 

 and one of the first logical views regarding the function of the 

 corpus luteum was advanced twenty years ago by Beard' in his 

 monograph on the 'Span of gestation and the cause of birth.' 

 According to Beard "Menstruation is comparable to an abor- 

 tion prior to a new ovulation, and it is an abortion of a decidua 

 prepared for an egg which was given off subsequent to the 

 preceding menstrual period, and which had escaped fertihzation." 



In the earlier mammals. Beard imagines that gestation ex- 

 tended over only one ovulation period or short dioestrum of 

 Heape's terminology. Thus prior to each ovulation, a birth 

 would take place provided pregnancy had ensued after the pre- 

 vious ovulation, and if not the ovulation would be preceded by 

 an abortive birth act. In this connection it is interesting to 

 recall the well known fact that in man and otjier mammals 

 abortions occur with a far greater frequency at the times for 

 regular menstrual periods than at other times. In the human 

 the time of the first menstruation after conception is a most 

 critical period, and the time when the third menstruation 

 should occur is responsible for the great predominance of three 

 month foetuses to be seen in most collections, and so on up to 

 the tenth period when the normal birth takes place. 



In the evolution of mammals Beard calls attention to the tend- 

 ency to develop a longer gestation period and more fully devel- 

 oped offspring, but in all ca^es the length of the gestation period 

 is a multiple of the primitive ovulation periods. A reminis- 

 cence of the earlier primitive conditions still exist in all of the 

 polyoestrous mammals. The gestation period of the guinea-pig 

 extends over four oestrous cycles making it about sixty-two days 

 long. 



