The Cessation of Est rum 115 



it ceases to recur after the third or fourth month of preg- 

 nancy. Sometimes the recurrence of estrum is regular, re- 

 peating the cycle prevailing in the individual prior to con- 

 ception, but usually the cycle is irregular. So far as I have 

 observed, the estrum of pregnancy is not followed by men- 

 struation. In fact, menstruation is not ordinarily possible, 

 owing to the uterine seal blocking the cervical canal and 

 preventing exit. 



Estrum not infrequently fails to recur after copulation 

 although conception did not follow. Sometimes the failure 

 of estrum to recur is due to retained corpus luteum, some- 

 times pyometra (without discharge) follows estrum and 

 coitus, inhibits estrum and misleads the caretaker. Poverty 

 serves to prevent alike ovulation and estrum. The failure 

 of estrum to recur in non-pregnant, and the recurrence of 

 estrum in pregnant animals render it clear that the pres- 

 ence or absence of estrum does not furnish secure ground 

 for diagnosis relative to pregnancy. The evidence must be 

 supported by other facts in order to be reliable. 



The estrum of pregnancy has been little studied. In one 

 case which I had the good fortune to examine while the ani- 

 mal was in estrum, the corpus luteum of pregnancy had 

 atrophied and a ripe ovisac was present on the non-gravid 

 side. I think it more than possible that the estrum of preg- 

 nancy is generally due to this cause. The corpus luteum of 

 pregnancy, for some unknown reason, atrophies, loses its 

 inhibitory power upon the maturation of ovisacs, and es- 

 trum, so far as the ovaries are concerned, appears in the or- 

 dinary manner. 



It is of scientific interest to note here also that the estrum 

 of pregnancy, with the atrophy of the corpus luteum and the 

 ripening of an ovisac upon the opposite ovary, is the possible 

 foundation for numerous errors regarding the migration of 

 the ovum. It has been claimed that finding a fetus in one 

 horn and the corpus luteum in the alternate ovary is proof 

 of the migration of the ovum, and that the fertilized ovum 

 has dropped into the peritoneal cavity, been transported to 

 the pavilion of the opposite oviduct and penetrated it, or that 



