CESTROUS CYCLE IN THE GUINEA-PIG. 225 



, 



membrane breaks. Thus the membrane has reached the break- 

 ing point or has actually broken at just about the time the female 

 is in heat and ready to copulate. 



While the presence of this membrane is a reliable index of the 

 cestrous condition, the open vagina, or its absence, is by no means 

 indicative of the cestrous state. Although the vagina is always 

 open during what we have termed the second and third stages of 

 cestrous, and, therefore, at the time of ovulation whether copula- 

 tion has taken place during the first stage or not, it is nevertheless 

 frequently open at other times. It is not permissible to assume 

 that the open vagina indicates a state of heat or the time of 

 ovulation in a guinea-pig. Only when the open vagina contains 

 fluid showing on examination the cells described as second or 

 early third stage is the ovary almost exactly in the condition of 

 ovulation. It may be stated parenthetically that after long 

 experience one is able as a rule to diagnose the stages of the 

 vaginal fluid by slight difference in color and consistency without 

 microscopic examination. 



Finally, then, when the vagina is open one may only be certain 

 of the uterine and ovarian conditions by examining the contents 

 of its lumen, but, on the other hand, if it be closed by this mem- 

 brane one may be certain that the time of the new ovulation has 

 not yet arrived. 



2. THE TIME AND MANNER OF COPULATION AND THE CONDITIONS 

 IN THE REPRODUCTIVE ORGANS OF THE FEMALE AT 



THIS MOMENT. 



It is well known that female guinea-pigs in common with 

 other animals of their class, and in fact most mammals, have a 

 definite limited time during which they accept the male, the 

 so-called "period of heat." This period, very slightly revealed 

 by external signs at the mouth of the vagina, but chiefly by the 

 act of copulation has been the starting point in all previous 

 studies on the reproductive activities of the guinea-pig. In 

 order to prevent the modifying conditions of pregnancy following 

 copulation, various operations have been resorted to, as in the 

 case of some of Loeb's experiments. Such operations might com- 

 plicate or even vitiate the results which follow. 



