242 CHARLES R. STOCKARD AND GEORGE N. PAPANICOLAOU. 



thirteen and a half to nineteen days, a range of almost six 

 days. 



On this basis it is seen to be practically impossible to state 

 within a day or so of when the next ovulation will take place 

 in a female that has just passed the "heat period." And the 

 "heat period" was very indefinitely known unless copulation 

 had been observed. Thus, as a usual practice, in order to prevent 

 pregnancy following a copulation used to prove the existence of 

 heat, the oviducts were previously tied. Such a procedure might 

 easily modify to some degree the sexual cycle and is inconvenient 

 for further study of the animal. 



Any significant experimental modification of the ovulation 

 intervals could be readily detected by a simple examination of 

 the microscopic structure of the vaginal fluids collected from a 

 guinea-pig. 



Finally, the "signs of heat" recorded from a report by Miss 

 Lathrop, an animal breeder, are, according to our experience in 

 examining and mating guinea-pigs, generally inaccurate and of 

 little value for use in experimental studies. 



5. SUMMARY. 



1. The cestrous 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 oestrus in non-pregnant 

 females and before parturition in the pregnant. It always re- 

 forms to close the vagina shortly after the "heat period" has 



