228 
JOUENAL OF MAl^MALOGY 
in such a way that the uterus is prepared to receive and nourish the 
embryos which may result from the ovulation if mating occurs. It 
is likely that all the phenomena of the female reproductive cycle can 
in the end be related to these two ideas. 
With regard to the first point an important step was gained by Stock- 
ard and Papanicolaou (1917) when by extending certain old and almost 
unnoticed observations they showed that the oestrus of the guinea-pig, 
outwardly so inconspicuous, is actually marked by a series of charac- 
teristic changes in the vaginal wall, so that microscopic examination 
of a smear of the vaginal fluid enables the observer to predict the oc- 
currence of ovulation with accuracy. At intervals of about fifteen 
days there is first a desquamation of epithelial cells and then the passage 
of white blood cells in great numbers through the vaginal wall; if the 
animal is killed on the day of these changes its ovaries are found to 
contain ripe follicles with mature ova, and the uterine epithelium is 
in a characteristic state interpreted by Stockard and Papanicolaou as 
indicating active degeneration. 
Long and Evans (1920, 1921) whose preliminary notes are about to 
be extended in a comprehensive monograph, have attained such accuracy 
with similar studies on the albino rat that they can predict the time 
of ovulation within one hour, and in their hands the method is already 
serving as a tool for the investigation of many problems of the oestrous 
cycle. In the white rat ovulation is very frequent (every four to six 
days), the interval being actually less than the time required for the 
passage and implantation of the fertilized ova. It will be seen that 
this frequency would inevitably lead to superfoetation, were it not for 
a special mechanism discovered by Long and Evans, namely that the act 
of copulation itself postpones the next ovulation for a sufficient length 
of time to protect the mother from a second crop of ova, until the preg- 
nancy itself can work the usual further postponement. The mecha- 
nism by which this end is attained seems to be a reflex from the genital 
canal, for the mere insertion of a glass rod into the cervix uteri is as 
effective as a normal copulation. 
Allen (1921) has worked out a similar cycle in the albino mouse; and 
we may now look forward to the application of such studies to some of 
the supposedly special cases among rodents, such as the rabbit, in 
which it is said that ovulation cannot be completed without copulation. 
No applications of this method to mammals of other orders have as 
yet been reported, although there is good reason to expect interesting 
results in this direction. However there have been several contri- 
