CESTROUS CYCLE IN THE GUINEA-PIG. 243 



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 cestrus. 



4. Copulation takes place in the guinea-pig during stage one 

 of the cestrous 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 leuco- 

 cytes 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 cestrus. 



The core or center of the vaginal plug is composed chiefly 

 of coagulated seminal fluid. This is enclosed within an en- 

 velope of epithelial cells, being simply the slufTed-off mucosa 

 from the wall of the uterus and vagina. This epithelial cover 

 which is thrown off at every cestrous period is loosened or dis- 

 solved 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. 



6. LITERATURE CITED. 



Beard, J. 



'97 The Span of Gestation and the Cause of Birth. A Study of the Critical 



Period and its Effects upon Mammals. Fischer, Jena. 

 Bischoff, Th. L. W. 



'52 Entwickelungsgeschichte des Meerschweinchens. Giessen. 

 Camus, L., and Gley, E. 



'96 Action coagulante du liquide prostatique sur le contenu des vesicules 

 seminales. Soc. de Biologic, Juillet, pp. 787-788. 



