VAGINAL EPITHELIUM OF GUINEA-PIG 441 
not be suspected by a mere study of the smears. . Consequently, 
the method of obtaining material for histological study at this 
critical time involved a departure from the ordinary procedure. 
In the ordinary procedure it was only necessary to keep records 
of smears made from the animals under operation and to kill the 
pig at the desired period in order to correlate the smear with the 
condition in the vagina as shown in sections. In the above ex- 
ception, the one killed for the high epithelium shown in figures 2 
and 9, the method used was to study the length of consecutive 
cycles in all the animals and then to select the animal with the 
most regular recurring cycles and to kill it twenty-four hours be- 
fore the calculated cornified stage. 
In this connection the question arises, may not frequent 
douching of the vagina with warm saline solution affect the length 
of the cycle, and also may not the operation for hysterectomy 
affect the following cycles in the animal. By referring to table 
2, the following facts will be made clear. The average length 
of the one hundred eleven cycles observed in twenty-four guinea- 
pigs was found to be 15.87 days. This corresponds to the length 
of the cycle as found by Stockard and Papanicolaou who found 
the average length of the cycle to be 15.73 days for sixty-seven 
cycles. There was no perceptible difference in the length of 
eycle in the normal and hysterectomized animals (table 2). 
SUMMARY 
1. The syringe method for taking vaginal samples is more 
satisfactory than any other method yet described for the guinea-. 
pig. 
2. The length of the oestrous cycle of the guinea-pig was found 
to be 15.87 days, the same as determined by Stockard and 
Papanicolaou (15.73). ; 
3. The oestrous cycle has four well-defined periods or stages 
besides the interval. 
4, Stage 1. The epithelium is at its greatest height at the be- 
ginning of this stage, ten to twelve cells. Cornification has been 
going on beneath the superficial layers. Vaginal smears contain 
large, vaculated, granular, odd-shaped, epithelial cells. 
