430 RAYMOND M. SELLE 
about twenty-one days. Similar investigations on the incidence 
of ovulation, ete., in rats and mice were being carried on in this 
laboratory by Doctor Long and several students. 
No further work of importance on the vaginal cycle of the 
guinea-pig was recorded until 1917, when Stockard and Papani- 
colaou published their paper on “The existence of a typical oes- 
trous cycle in the guinea-pig with a study of its histological and 
physiological changes.”’ They studied the vaginal cycle by tak- 
ing samples of the contents of the vagina at regular intervals. 
They obtained these samples by introducing a small nasal specu- 
lum into the vagina, the arms of which were held apart by means 
of a thumb-screw, allowing them to observe the entire lumen of 
the vagina. They were thus able to recognize an oestrous rhy- 
thm consisting of four stages, each having a characteristic vaginal 
fluid which contained cells from the walls of the vagina. 
The vaginal fluid during the first period contained a great 
amount of mucous secretion, many desquamated epithelial cells, 
and non-nucleated, or cornified cells, which appeared toward the 
end of the stage. The second stage could be recognized by the 
thick, cheese-like contents of the vagina due to the accumulation 
of the desquamated epithelial cells. During the third stage the 
vaginal fluid became thinner because of the solvent action of 
leucocytes which invaded the epithelium and entered the lumen. 
The fourth stage, during which a small amount of blood was found 
in the vagina, was of short duration and did not always occur. 
These stages were followed by the dioestrum, or intermenstrual 
period, during which the vaginal fluid contained mucus, atypical 
squamous cells, and many leucocytes. They then correlated 
these stages with the conditions of the uterus and ovary at cor- 
responding periods. 
A second paper by Stockard and Papanicolaou appeared in 
1919 in which they described the vaginal closure membrane and 
the time of copulation in relation to the oestrous cycle. They 
found that there was formed regularly a very delicate epithelial 
membrane which closed the orifice of the vagina soon after the 
‘heat period’ and remained closed until the next cycle or until 
parturition in case of pregnancy. 
