236 CHARLES R. STOCKARD AND GEORGE N. PAPANICOLAOU. 



The most valuable and extensive investigations of the repro- 

 ductive act'.vities of the guinea-pig were those made by Leo Loeb 

 ('n, '14). But here the data were derived almost entirely from 

 examinations of the uterus and ovaries after their removal from 

 the body of the female. While such studies did give a means of 

 comparing the conditions found among different individuals at 

 different times, and made it possible to estimate approximately 

 the length of the sexual periods, yet this estimate could not be 

 transferred with certainty to any one living individual. We 

 further objected to Loeb's method of study since it failed to 

 permit an investigation of the recurring cestrous periods in a 

 number of unoperated females. The results of such an investiga- 

 tion w r ould be most important in determining the influence of 

 any unusual or experimental conditions introduced with intent 

 to modify the intervals between ovulations or other periods of 

 the sexual cycle. These are just such problems as Loeb had 

 under consideration. 



The entire literature showed that any such thing as a regular 

 cestrous flow was completely undiscovered for the guinea-pig. 

 It became necessary, however, for our studies to have an accurate 

 knowledge of ovulation times, and to determine this, extensive 

 investigations of the sexual cycle in the guinea-pig were under- 

 taken. A simple method of examining the vagina of the living 

 animal proved to be of the greatest value. Virgin females were 

 selected and the fluid present in the vaginae was taken daily by 

 means of a small nasal speculum and cotton swab. This fluid 

 was smeared on slides, stained and studied microscopically. 

 The method is fully described in the former paper. 



It very soon became evident that the vagina generally con- 

 tained little or no fluid, but that periodically a great accumulation 

 of mucus and cells was to be found. This excessive amount of 

 mucus and cells is to be recognized as a typical cestrous flow. 

 The constituent elements of the fluid change in their relative 

 abundance in a definite manner from the beginning to the 

 cessation of the flow. Four clearly marked stages, as mentioned 

 above, could be separated by microscopic examination of the 

 fluid smears. 



These changes in the composition of the vaginal fluid were 



