5<d Diseases of the Genital Organs 



occurs immediately following parturition. Ovulation fol- 

 lows about twelve hours later — an interval sufficiently great 

 for the migration of spermatozoa from the vagina to the 

 ovary. In the cow, estrum persists ordinarily for but twelve 

 to twenty-four hours, and ovulation occurs at about the 

 close of this period. If copulation occurs early in estrum, 

 better opportunity is afforded for the arrival of spermatozoa 

 at the pavilion of the oviduct prior to the rupture of the 

 ovisac and discharge of the ovum. In swine, carnivora, and 

 to some degree in solipeds, the estrual period is prolonged, 

 extending over several days. In these there are no accurate 

 data of the epoch of ovulation. Probably ovulation occurs 

 at a time corresponding to that observed in the cow — that is, 

 at about the close of estrum. 



Fig. 25 — Diagram of ovary of cow showing- comparative 

 sizes of the follicles shown in Figs. 22, 2b, 24 



The number of ovisacs rupturing at a given estrual period 

 corresponds as a rule with the maximum number of possible 

 fetuses. It is said that rarely two ova are contained in one 

 ovisac. I have been unable to verify this statement, and 

 have in all cases of twins in the cow observed two corpora 

 lutea, sometimes both in one ovary but most frequently one 

 in each. Sometimes a single fertilized ovum divides to con- 

 stitute two embryos, which form identical twins in man, 

 but this is not known to occur in domestic animals, though 

 it probably does so. In multipara, it is assumed that there 

 are at least as many ova discharged, from as many ovisacs, 

 as there are embryos. 



