THE OESTROUS CYCLE IN THE MOUSE 305 
that here again external signs as criteria of a condition of oes- 
trus are not reliable. That such signs are due primarily to con- 
gestion and consequent poor tonus of the muscle layers of the 
vagina is indicated by their rapid disappearance coincident with 
the relaxation of the urethral sphincter following death. How- 
ever, to settle the question as to their value in diagnosis, the ex- 
ternal signs were always noted when taking smears, and as they 
were typical in 53.5 per cent of cases, a description is desirable. 
Heape’s terminology is followed in describing the phases of the 
eycle, with emphasis on the following points: 1) that growth and 
congestion continue through oestrus; 2) that the metoestrum 
may be further subdivided into two periods. 
To eliminate a too frequent use of ‘oestrum’ in its various forms, 
the stage will be designated as shown below: 
D, dioestrum, period of relative quiescence. 
P, prooestrum, period of augmented growth and con- 
gestion. 
O, oestrum, period of sexual excitement, the climax of 
prooestrous conditions. 
M, | metoestrum, period of return to the dioestrous 
M, J condition. 
Stage D. In the mouse during the dioestrous interval the 
vulva is very inconspicuous and the orifice of the vagina is usually 
tightly closed. There is a little fluid which is viscous and stringy. 
The smear shows epithelial cells, usually only a few, in various 
stages of nuclear degeneration and cytoplasmic shrinkage, and 
always some polymorphonuclear leucocytes. 
Stage P. During the prooestrum, if external signs are marked, 
the vulva is pink or red and swollen, the vagina gapes open, the 
fluid in its lumen is serous, and the smear shows only nucleated 
epithelial cells (fig. 2). 
Stage O. During the oestrous period, the vulva may still 
be swollen, the orifice of the vagina open and a dull white in color, 
the vaginal mucosa almost dry, and the smear shows only corni- 
fied, non-nucleated, red eosin-staining cell remains (fig. 3). In 
animals where external signs are absent or slight, the finding of 
only cornified cells in the smear makes a diagnosis of ‘heat’ 
possible. 
