THE OESTROUS CYCLE IN THE MOUSE 321 
set of corpora lutea are in their third and two in their second day 
of development. There are two older sets of red corpora lutea 
in these ovaries corresponding to the oestrous periods six to 
seven and eleven days previous to killing. The large number 
of these in the right ovary as contrasted with the few in the left 
would point to a hyperfunction of the former, at least during 
the last three oestrous periods. The unusual number of normal, 
moderately large follicles in the left ovary indicates the shifting 
of the major function for the next oestrus. 
There are a few interstitial cells in these ovaries. Leucocytes 
do not appear in significant numbers, although leucocytosis is 
at its height in the vagina and uterus. 
6. SUMMARY OF STAGES BY ORGANS 
1. External signs 
Although certain external signs may occur during the prooes- 
trous and oestrous periods in the mouse, these periods may be 
present without them or the signs may continue after ‘heat’ 
has passed. When present, the external signs consist of a swell- 
ing and coloration of the vulva due to congestion and the gaping 
open of the vaginal orifice. 
2. Vaginal content 
Changes in the cells and the fluidity of the vaginal contents are 
a much more reliable criterion of the condition of oestrus. Dur- 
ing the prooestrum and oestrum there are no leucocytes present 
in the vaginal contents. At all other times they are found 
there in varying numbers. During the dioestrous interval 
(D) epithelial cells (chiefly nucleated) and leucocytes are pres- 
ent; in the prooestrum (P) only light staining cells with pycnotic 
nuclei; during oestrus (O) single, non-nucleated, eosin-staining 
cornified elements constitute the smear; these are clumped in 
masses in the early metoestrum (M,), and invaded by leucocytes 
as this period progresses until most of the cornified elements dis- 
appear and a dioestrous condition again prevails (figs.2 to 5). In 
stage D the vaginal content is viscous or stringy; in P it is 
