VAGINAL EPITHELIUM OF GUINEA-PIG 435 
At about the middle of the interval the epithelium was found to 
be thinner than at any other time, being only one or two layers 
of cells high at the thinnest places. At the deep ends of the down- 
ward projections of the epithelium were numerous mitoses—a 
condition which apparently substantiated Stockard’s and Papani- 
colaou’s statement that the vaginal epithelium was regenerated 
from the base of the infoldings. 
Towards the end of the interval the epithelium rapidly in- 
creased in height until it reached a maximum of ten to twelve 
cells. The uppermost of these cells which bordered the lumen 
had become enlarged and vacuolated, with their nuclei usually 
at their bases. The cells lining the inner surface of the vagina 
were the most vacuolated, while the layers beneath were smaller 
and gradually approached the normal size of the epithelial cells 
at about three or four layers below the surface (figs. 2 and 9). 
Stage 1. Large irregular epithelial cells only 
(No corresponding stage described by Stockard and Papanicolaou) 
Stage 1 marks the beginning of the oestrous cycle. A smear 
made early in this stage exhibited, in marked contrast to that 
of the interval, first, numerous, large, round or odd-shaped, epi- 
thelial cells containing many vacuoles, and, secondly, very few 
leucocytes. The latter undoubtedly remained over from the 
interval which, as just described, was characterized by the pres- 
ence of many leucocytes and a very few epithelial cells. These 
epithelial cells appeared in the smear singly or in groups of three 
or more. 
A section of the mucosa early in stage 1 showed the epithelium 
to be practically as high as at the end of the interval, i.e., about 
ten to twelve cells; also that a characteristic transformation had 
already made its appearance. ‘This transformation was exhibited 
in cornification of certain of the epithelial cells as shown in 
figures 3 and 10. It will be observed that the most superficial 
cells were enlarged, were odd-shaped, and contained vacuoles; 
that the next deeper cells were somewhat smaller and more 
spherical, and that the next deeper levels showed the beginning 
