62 
AN EARLY HUMAN OVUM 
lasts till pro-oestrum again sets in. In polyoestrous mammals several 
cycles of this kind may follow one another. Menstruation in the human 
female is homologous with pro-oestrum, as first pointed out by Heape. 
Though there is no fixed “period of desire’’ there is an indication that a 
vestige of this persists, in the fact that a phase of more pronounced 
oestrus commonly succeeds the cessation of menstruation. 
The accompanying table, which is borrowed from Heape’s papers, 
but has been slightly modified and adapted to our present purpose, will 
explain at a glance the general relation of the oestrus cycle of a poly¬ 
oestrous mammal to the menstrual cycle. 
It is quite clear from the results of the comparative method that 
the significance of menstruation does not lie in the mere periodic growth 
and subsequent destruction of the mucous membrane, but in the cycle as 
a whole. The essence of the process is not the preparation of a menstrual 
decidua, but the formation of a new endometrium ; in other words, the 
growth and swelling which precedes “ discharge,” does not represent a 
preparation for the ovum; it is merely a phase in that reconstitution 
of the new endometrium which is the real preparation for the reception of 
the ovum. In the case of most lower mammals, the generative organs 
lie dormant through a large part of the year, and when the breeding 
season approaches, the endometrium undergoes in the pro-oestrum a 
species of regeneration resulting in the development of a new surface on 
which the ovum may implant itself. In the case of the human female 
there is no longer this regular breeding period, but desire and the 
possibility of impregnation occur at irregular periods all the year round. 
Instead therefore of an annual (or in some mammals a seasonal) 
renewal of the endometrium, and the preparation of a new surface on 
which the ovum may be engrafted, we have in the human subject a 
monthly regeneration and preparation of the endometrium for the same 
purpose. 
Much of the difficulty regarding the decidua disappeared when 
it was shown that it had no part in the formation of the placenta. The 
services performed by the decidua in the imbedding of the ovum require 
that its connective tissue elements shall be in a condition of special 
