corpora lutea in _Echidna_. In _Ornithorhynchus_ the eggs are hatched in a
nest and there is no pouch.

On this view that the corpora lutea are the result, not the cause, of
intra-uterine gestation, it would no longer be possible to maintain the
theory that the corpus luteum in the human species is the cause by its
internal secretion of the phenomenon of menstruation. This was the theory
of Born and Fraenkel. [Footnote: See Biedl, _Internal Secretory Organs_
(Eng. trans.), 1912, p. 404.] Biedl's conclusion is that the periodic
development and disintegration of the uterine mucous membrane in the
menstrual cycle is due to the hormone of the interstitial cells of the
ovary. Leopold and Ravana found that ovulation as a rule coincides with
menstruation, but may take place at any time. Here, again, the problem
must be considered from the point of view of evolution. It can scarcely be
doubted that the thickening and growth of the mucous membrane in the
menstrual cycle is of the same nature as that which takes place in
pregnancy. When the ovum or ova are not fertilised the development comes
to an end after a certain time, differing in different species of Mammals,
and the membrane sloughs, returns to its original, state, and then begins
the same process of development again.

Menstruation, then, must be interpreted as an abortive parturition, both
in woman and lower Mammals, though in the latter it is not usually
accompanied by hemorrhage, and is called pro-oestrus. The question then to
be considered is, what determines parturition and menstruation? The
presence of the fertilised ovum must have been the original cause of the
hypertrophy of the uterine mucous membrane, and in its congenital or
hereditary development the chemical substances diffusing from the ova in
the uterus or even in the Fallopian tube may well be the stimulus starting
the hypertrophy. But what determines the end of the pregnancy? Is it