relation they bear to each other, and what the significance of the
whole process is, are problems that have been solved with the aid of
the microscope. In this way the mucous membrane lining the womb has
been studied both at the time of the periods and in the interval
between them, and we have learned that it is constantly undergoing
changes intended to facilitate the reception and the maintenance of
an embryo. Anticipating these duties the mucous membrane receives a
more abundant supply of blood; it also increases in thickness and all
the structures which enter into its composition become more active.
Unless conception takes place these preparations, which represent the
most important phase in the menstrual process, are without value; and
therefore failure to conceive means that the mucous membrane will
return to the same condition as existed before the preparations were
begun. The congestion is relieved by rupture of the smallest blood
vessels, and there follow other retrogressive steps which completely
restore the various structures to their former state. Then there is a
pause, though it is not long, until preparatory changes are again
initiated, or, as we say, another Menstrual Cycle is begun. Each
cycle lasts twenty-eight days, and includes four stages, namely, a
stage of preparation, of bleeding, of restoration, and of rest.

Although pregnancy may become established at any time during the
interval between the periods of bleeding, it is more likely to be
established just before a period is expected or shortly after it has
ceased. Furthermore, whenever conception does take place, the
preliminary preparations for the reception of the embryo are followed
by much more elaborate arrangements for its protection and nutrition.
Under these circumstances the hemorrhagic discharge does not appear.

Were there no other condition to bring about the cessation of