ii6 Veterinary Obstetrics 



lying in the space between the vitelline membrane and the egg. 

 The exact process of fertilization has not been fully determined 

 in our higher animals. It is generally held that a single sperma- 

 tozoon fuses with the female pronucleus. 



The Relation between Estrum, Ovulation, Fertilization 

 and Menstruation. 



The relationship existing between the phenomena of estrum, 

 ovulation, fertilization and menstruation has not been clearly 

 determined. Owing to the very brief duration of estrum in the 

 cow, she offers a specially favorable opportunity for study. As 

 already related in our personal experience in the spaying of 

 cows, it has been found that estrum is the first of these phenom- 

 ena to present itself and that, if the ovaries be examined dur- 

 ing the estrual period, it is found that there is an enlarged, 

 mature Graafian follicle ready to rupture and that the walls give 

 way under ordinary manipulation allowing the follicular con- 

 tents to escape. 



If the ovaries be examined during menstruation, it is found 

 that the ovisac has ruptured and its contents have escaped. It 

 seems, therefore, that, in the cow at least, the chronologic order 

 of these phenomena is estrum, ovulation and menstruation, pro- 

 vided that this cycle is not interrupted by copulation and ferti- 

 lization, when the order would be estrum, copulation, ovulation, 

 fertilization. If successful copulation ensues early in estrum, 

 it is the general observation that menstruation does not occur. 

 Thus, its advent would seem to indicate the death and expulsion 

 of the ovum along with some of the epithelium of the uterus, 

 accompanied by hemorrhage in a manner to correspond in a way 

 to the lochial discharge of parturition. 



It would appear that, in our higher animals, the phenomenon 

 of menstruation is analogous in a way to parturition itself. In 

 animals having a duration of pregnancy greater than the in- 

 terval between two estrual periods, estrum becomes interrupted 

 during gestation, except in rare pathologic cases. The follow- 

 ing table by Simons (Graduation Thesis, I,ibrary of New 

 York State Veterinary College, 1903) shows some interesting 

 observations in reference to the relationship existing between 

 this group of phenomenona and brings out especially the influ- 

 ence of impregnation upon the occurrence of estrum. In those 



