CESTROUS CYCLE IN THE GUINEA-PIG. 237 



found to be associated with comparable changes in the structure 

 of the epithelial walls of the uterus and vagina. And not only 

 was this the case, but the changes in the vaginal fluid proved to 

 be most reliable indices of definite processes taking place in the 

 ovaries in connection with the rupture of the Graafian follicles 

 and the expulsion of the ova. It is, therefore, evident that by an 

 examination from time to time of this fluid, one may know the 

 exact condition of the ripening follicles in the ovary and very 

 nearly the exact moment of ovulation. 



The oestrous cycles in a group of guinea-pigs were followed for 

 a number of months in order to establish the normal periodicity 

 or rhythm. The amount of variation that might exist in the 

 length of the cycles in a given female was studied as well as the 

 variations in cycle lengths among different individuals. An at- 

 tempt was further made to discover any seasonal variations that 

 might exist. 



Only slight time variations were found in the periodic rhythm 

 of a given female. For example, in one animal the record of six 

 consecutive periods shows the oestrous flow to begin on the 

 sixteenth day five times and on the fifteenth day once. In 

 another case of seven consecutive periods the flow began on the 

 sixteenth day six times and on the seventeenth day once. For 

 further cases the reader is referred to the table given in our 

 former paper. 



There is only a limited variation in the length of the oestrous 

 cycles among different individuals, ranging between fifteen and 

 seventeen days in younger animals. In exceptional cases the 

 period is slightly lengthened in older multipart, sometimes reach- 

 ing eighteen days. These limits of fifteen and eighteen days for 

 the lengths of the cestrous cycles have never been violated under 

 normal conditions during the several hundred observations which 

 we have now recorded. The method of examining the vagina 

 for the closure membrane above described, and, in the case of 

 its rupture, for the composition of the fluid contained within 

 the lumen, renders these individual variations of no consequence 

 in determining the exact "heat period" and time of ovulation. 



Slight, if any, seasonal variations are shown by our animals. 

 This may be due, however, to the uniformly warm temperature 

 maintained in the breeding rooms during the winter months. 



