THE OESTROUS CYCLE IN THE MOUSE 333 
3. Long-and Evans (’20) have again called attention to the 
formation of the cornified and granular layers of the vaginal 
epithelium in the rat and guinea-pig as a remarkable histogenetic 
process because they do not form superficially. However, it 
should be observed that the overlying layers, although nucleated, 
early lose their affinity for stain even before the appearance of 
granular layer which precedes cornification (described under 
animal 8, fig. 7). 
b. The uterus. 1. Gross changes. The attention of investi- 
gators has been directed towards the uterus from the very begin- 
nings of anatomical study, primarily because of its importance 
to the embryo during pregnancy. In the absence of pregnancy 
in the primates, its striking changes during the menstrual cycle 
have led to its designation as ‘the organ of menstruation.’ In 
the mouse the vagina, and also the oviducts to a less extent, 
share with it the typical oestrous changes. 
2. Destructive histological changes. The amount of epithe- 
lial and connective-tissue destruction during the metoestrum 
and menstruation has been under continual controversy which 
seems to have compromised on ‘‘great variability even within 
the same species.’ In menstruating animals extensive removal 
of epithelium has been reported and denied, but nearly always 
bleeding occurs. Bleeding has also been reported in many of 
the lower mammals which periodically exhibit typical oestrus. 
Lataste (’87) has recorded bleeding during ‘heat’ in several 
European rodents. Stockard and Papanicolaou (717) have 
reported an occasional slight bleeding in the guinea-pig. There 
is little denudation of the epithelium and only rarely bleeding 
during the metoestrum in the mouse. The process is restricted 
to the degeneration of the epithelium in situ (it seldom breaks 
free from the stroma) and heavy subsequent leucocytosis. This 
lack of severe destruction may be accounted for in the mouse 
by the rapidly ensuing prooestrum, or period of growth, which 
may set in three days after the metoestrum begins. 
3. Is leucocytosis primary or secondary? Loeb (11), in dis- 
cussing this problem in the guinea-pig, says: ‘“‘It is not very 
probable that the changes in the epithelium (of the uterus) 
