32 LEO LOEB. 



between the tenth and fifteenth day after copulation. We 

 observed it as early as eight and a half and nine days after 

 copulation. Pregnancy does not prevent the early ovulation 

 after a complete extirpation of the corpora lutea. It is only 

 the persistence of the corpora lutea of pregnancy which prevents 

 the ovulation. Pregnancy however prevents, as we saw, the 

 cyclic changes in the uterus which accompany and follow ovu- 

 lation. On a former occasion I suggested that embryonic 

 structures determine the persistence of the corpora lutea during 

 pregnancy. 1 We are at the present time testing this hypothesis 

 experimentally. 2 If we extirpate the corpora lutea incompletely, 

 no new ovulation takes place. 



These experiments, as well as those mentioned above which 

 have already been partly published, and which as far as they 

 concern new problems shall be published more in detail in the 

 near future, clear up to a great extent the mechanism of the sexual 

 cycle. 



VI. THE SIGNS OF HEAT IN THE GuiNEA-Pio. 3 



(Report by Miss A. E. C. Lathrop.) 4 

 "When a number of females are kept together in a large pen, 



1 Zentralblattf. Physiologic, Bd. XXIV., No. 6. 



2 We produced recently experimentally an early stage of extrauterine pregnancy 

 in a guinea-pig. The presence of some living embryonic structures and of fetal 

 placenta did not prevent the degeneration of corpora lutea and the subsequent 

 rupture of follicles. On the other hand the corpora lutea persisted as long as 

 experimentally produced placentomata (structures of maternal origin) remained 

 alive in cases in which we made incisions into the uterus six or seven days after 

 copulation. In some of these cases fetal structures developed in these placento- 

 mata; in other cases no fetal structures could be found. It is therefore possible 

 that the maternal placenta determines directly the life of the corpora lutea and 

 that the embryonic structures act only indirectly by prolonging the life of the 

 placentomata. We are testing experimentally this theory at the present time. 



3 The condition of heat in the majority of the guinea-pigs which we used was 

 observed by the breeders, Mr. J. M. Simpson in Colwyn, Pa., and Miss A. E. C. 

 Lathrop in Granby, Mass. Dr. O. Ishii also observed the condition of heat in a 

 number of cases. I had an opportunity to control the correctness of these obser- 

 vations by the microscopic examination of ovaries and uterus. In almost all cases 

 I found the observations of the breeders confirmed by the microscopical examina- 

 tion. 



4 I asked Miss A. E. C. Lathrop to observe the signs of heat in the guinea-pig 

 and she sent a statement of her observations which, with her permission, I wish to 

 incorporate in my paper after some slight changes have been made. She writes as 

 above. 



