300 EDGAR ALLEN 
This is the sort of observation on which so much of the literature 
of oestrus is based. . 
Lataste has done considerable careful work on the vaginal 
plug of rodents and in this connection notes that the dioestrous 
cycle is usually about ten days. 
Loeb (’11 a) reported a sexual cycle in the guinea-pig recurring 
every twenty to twenty-five days and described the histologi- 
cal changes in the uterus and ovaries at intervals. In a later 
paper (711 b) he concludes the duration of the cycle to be from 
fifteen to twenty days. 
In 1917, Stockard and Papanicolaou described a method of 
diagnosing the stages of oestrus in animals showing only slight 
external signs of their condition by a histological examination 
of the cell contents of the vaginal fluid. The cellular content of 
this fluid changes characteristically as the cycle progresses. 
This method offers the advantage of providing a complete his- 
tory obtained from the repeated observation of reliable criteria 
upon the same living animal. It thus permits the study of 
individual variations and gives a record of the events taking 
place before killing the animal to study histologically the internal 
genital organs. By this method they obtained an oestrous cycle 
in the guinea-pig of remarkable regularity averaging 16+ days. 
This method also permitted Stockard and Papanicolaou to locate 
very exactly the moment of ovulation in a living guinea-pig. 
The rupture of the follicle occurs when the vaginal smear shows 
a definite cellular picture. 
In the rat, Heape placed the duration of the oestrous cycle at 
ten days. Long and Evans (’20), using the above-mentioned 
vaginal fluid examination method, have shown this to be from 
four to six days, or actually only one-half as long as Heape sup- 
posed. 
Of course much valuable information concerning certain phases 
of the sexual cycle in the mouse is already at hand in the work 
of Sobotta, Kirkham, Long and Mark, Smith, and others; this 
information, however, may be added to when approached from 
another viewpoint. 
