344 EDGAR ALLEN 
of the follicles is coincident with the beginning of the metoes- 
trum. This indicates that the growth and congestion of the 
pro-oestrous and oestrous stages are caused by maturing ova in 
large normal follicles, and the degeneration and removal by 
leucocytosis of much of the uterine and vaginal epithelium re- 
sults after the extrusion of the ovum from, or its atrophy in the 
follicle. 
That ovulation is the dividing line and that mice of different 
strains have distinct modes of cycle length seems to point to the 
maturing ova themselves as the ultimate cause of growth changes 
and the absence of them from the follicle as the cause of degenera- 
tive phenomena of the oestrous cycle. 
9. SUMMARY 
1. External signs are unreliable criteria of oestrus in mice. 
The presence of cornified cells in the vaginal smear is a much 
more accurate indication. When these cells appear in masses, 
ovulation has usually occurred. 
2. The chief changes in the vaginal epithelium are its rapid 
growth, the formation of a stratum corneum, and (after ovula- 
tion) its degeneration and removal by leucocytosis. The stra- 
tum germinativum is also partly destroyed. It may grow from 
four to six to twelve or thirteen layers in thickness in one day. 
3. There is considerable degeneration and leucocytosis in 
the uterine epithelium which is, however, seldom removed from 
the stroma. Bleeding rarely occurs in the mouse, but a heavy 
leucocytic infiltration takes place during the metoestrum. 
4, Periodic degenerative changes in the oviduct parallel those 
in the rest of the genital tract. They are evidenced in extrusion 
of nuclei from the ciliated epithelium. 
5. Ovulation is the dividing line between the anabolic and 
catabolic phases of the oestrous cycle, maturing ova in large 
follicles always being present during the pro-oestrum and oestrum, 
while newly forming corpora lutea or large atretic follicles re- 
place them in the metoestrum. 
