CESTROUS CYCLE IN THE GUINEA-PIG. 235 



It is clear, therefore, that what was termed by Lataste the 

 "enveloppe vaginale" is the layer of epithelium separated from 

 the underlying connective tissue by the dissolving action of the 

 leucocytes which invade the walls of the uterus and vagina at 

 this time. It is also readily understood how the plug, after its 

 short sojourn in the vagina and cervix of the uterus, is finally 

 separated from its adhesion or tight connection with the wall 

 and expelled as a mass from the vagina. 



A possible function or effect of the vaginal plug in addition to 

 those before mentioned has recently been suggested by Long 

 ('19). He states that a stimulation of the cervix of the uterus in 

 rats, by merely inserting a glass rod during stage one of the 

 oestrus, prolongs the next cycle, and suggests that the vaginal 

 plug may also act in this mechanical way. We have not tested 

 the prolongation of the cycle in guinea-pigs following copulation 

 without conception as compared with its length in virgin animals. 



4. THE (EsxROUS RHYTHM. 



In our earlier review of literature it was pointed out that the 

 knowledge of the actual time of ovulation in the guinea-pig was 

 decidedly inexact. Nothing scarcely was known of the periodic 

 recurrence of the oestrus stages in a given female. In short the 

 moment of ovulation in the guinea-pig was not available for 

 accurate experimental purposes and no definite criterion or 

 method had been devised for detecting the cestrous condition. 

 And this was true in spite of a very long list of studies pertaining 

 to the reproductive activities of these animals. 



Reichert, as long ago as 1861, had found that the Graafian 

 follicles rupture about nine to ten hours after copulation. This, 

 in general, approaches correctness, but in cases where copulation 

 has not taken place, or failed to be observed, such knowledge is 

 of little consequence. Rubasckhin ('05) had more recently 

 claimed that the vagina was open and the vulva somewhat 

 inflamed ten to twelve days after parturition, but this is certainly 

 too short an interval to indicate an actual return of heat. It 

 must be remembered that the female guinea-pig goes into "heat" 

 and accepts the male almost immediately after the delivery of 

 her litter. This fact makes the length of Rubaschkin's interval 

 still more improbable. 



