432 RAYMOND M. SELLE 
With these results in mind, it seemed worth while to study the 
changes in the vaginal epithelium of the guinea-pig in order to 
ascertain whether or not a layer of epithelial cells occurred super- 
ficial to the stratum cornium as observed by Retterer; to supple- 
ment the work of Stockard and Papanicolaou, and to correlate 
the rat and guinea-pig more exactly. The problem was under- 
taken at the suggestion of Prof. J. A. Long and was carried out 
under his general direction in the Zoélogical Laboratories. 
METHODS 
The guinea-pigs chosen for these experiments were common 
white, brown, and mixed colored virgin females about a year old. 
They were kept at a relatively constant temperature averaging 
68° to 72°F. and were fed a constant, daily ration of green grass or 
alfalfa hay, carrots, and rolled barley, because there is reason to 
believe that a lower or a higher temperature as well as variation 
in the ration tends to affect the span of the oestrous cycle. 
Since the determination of the state of progress of the cycle, 
as shown by the previous work cited on the guinea-pig and rat, 
is dependent on our knowledge of the character of the contents of 
the vagina, the method of sampling the contents becomes a mat- 
ter of some importance. Several methods of taking samples were 
employed. For reasons explained later, the method employed 
by Stockard and Papanicolaou was discarded, and because of 
the extreme length of the vagina in the guinea-pig the spatula 
method as used by Long and Evans on the rat was not of much 
value. 
The pipette method, suggested by Doctor Long and developed 
by Miss E. Fisher in this laboratory for making vaginal smears 
from mice, was tried, and after several improvements a satisfac- 
tory method for taking samples was perfected. The instrument 
used was made by inserting into a 25-ec. oval, rubber bulb a piece 
of thin-walled, glass tubing 14 cm. long and 1 cm. in diameter, 
about 5 em. of which was drawn down to a diameter of 5mm. 
When a sample was to be taken, the animal was held ventral 
side up in the left hand allowing it to lie on its back along the 
left forearm with its head toward the elbow. The left thumb 
ii i 
