The Vaginal Closure Membrane, Copulation, and the Vaginal Plug in the Guinea-Pig, With Further Considerations of the Œstrous Rhythm

The œstrous cycle in the guinea-pig is very definitely limited in length. Ovulations follow one another every fifteen to seventeen days in younger individuals, while in old females the period is slightly lengthened, in exceptional cases to eighteen days. 2. These individual variations are readily controlled by the method of vaginal examination described in this and a former article, so that the actual moment of ovulation in any given female may be determined to within almost an hour of the rupture of the Graafian follicles. 3. During the period of sexual inactivity, the dioestrum, as well as during pregnancy, the orifice of the vagina is completely closed by an overgrowth of epithelium which we have termed the "vaginal closure membrane." This membrane ruptures just before or during the first stage of the œstrus in non-pregnant females and before parturition in the pregnant. It always reforms to close the vagina shortly after the "heat period" has passed. The presence of this closure membrane is therefore positive evidence that the time of a new ovulation has not been reached. When the membrane is ruptured and the vagina open, the ovarian condition may then be determined by examination of the fluid content of the vagina as described. A knowledge of this closure membrane greatly facilitates the examination of females in locating the beginning of the œstrus. 4. Copulation takes place in the guinea-pig during stage one of the œstrous period and ovulation occurs, as we previously showed, at the end of the second stage or the beginning of the third. Long ('19) finds exactly the same to be true for the rat. 5. At the time copulation occurs, the vagina is filled with a clear foamy mucus, and this is the only time at which no leucocytes are present in the fluid. Both the nature of the vaginal fluid at this time and the absence of leucocytes contribute to the success of copulation and fertilization. 6. A vaginal plug is formed a few minutes after copulation, during the first stage, and remains in the vagina for only a short time, being expelled during the fourth stage of the œstrus. The core or center of the vaginal plug is composed chiefly of coagulated seminal fluid. This is enclosed within an envelope of epithelial cells, being simply the sluffed-off mucosa from the wall of the uterus and vagina. This epithelial cover which is thrown off at every œstrous period is loosened or dissolved away from the uterine and vaginal wall by an enormous invasion of leucocytes which progresses from the, first to the third stage. The breaking away from the vaginal wall of this enveloping epithelium causes the plug to fall out of the vagina during the fourth stage. After the expulsion of the vaginal plug, the mouth of the vagina is closed by the growth of the vaginal closure membrane.