THE OESTROUS CYCLE IN THE MOUSE 319 
The lumen of the vagina is collapsed and the walls much 
folded. It is crowded with polymorphs, among which lie big 
fragments of delaminated cornified elements. They are very 
thin in section. The epithelium is without any vestige of a base- 
ment membrane, pointed shoots of the stroma penetrating into 
the lower layer of the germinativum. There remain only four 
to seven layers of healthy epithelial cells beneath a broad region 
which has been reduced almost to the appearance of a fine retic- 
ulum by the infiltration of enormous numbers of leucocytes 
which lie in its meshes. In regions less affected, small clear 
lacunae containing several leucocytes are distributed throughout 
the epithelium. No signs of a granular layer exist, and the 
cornified layer has been freed from its attachments in most 
places. This action is just as marked in the crypts of the mu- 
cosa as on the ridges. Leucocytosis is at its maximum in this 
animal. In spite of this extended destruction of the epithelium 
an occasional mitosis may be found. 
The sagittal section shows that the process of leucocytosis 
does not extend far through the vaginal orifice, for as it approaches 
the vulva the epithelium still appears normal and its granular 
and horny layers are superficial but intact. As these layers are 
traced back into the lumen, the eleidin granules begin to disappear 
and the cells gradually to stain pink and then red until they 
merge into a fully formed cornified layer. 
The lumen of the uterus is small in diameter and contains an 
occasional small mass of free cells. The epithelium in most 
regions 1s quite completely degenerated. There is no basement 
membrane, but in its place a broad, blurred red or pinkish band 
involving the lower part of the epithelial cells and the adjacent 
stroma. The epithelium consists for the most part of several 
layers of nuclei, which might lead one to classify it _as pseudo- 
stratified. But cell outlines are lacking or very indistinct, so 
that it represents the appearance of unorganized masses of 
nuclei in poorly staining cytoplasm. In a very few restricted 
areas occasional mitoses are evident, but for the greater part no 
signs of growth activity are apparent. The epithelium and, to 
a greater extent, the subepithelial zone are heavily infiltrated 
with leucocytes (fig. 16). 
