168 
J. P. HILL AND CHAS. H. o’dONOGHUE. 
account the close similarity which is apparent between the 
cyclical changes in the pregnant and non-pregnant marsupial, 
it can hardly be doubted that the oestral cycle in this group 
is not only simpler but much more primitive than that of 
Eutheria, so far as known. If this be admitted, then it 
follows that, in the Eutheria, the inclusion in the pro-oestral 
period, of the degenerative uterine changes which condition 
menstruation is a purely secondary phenomenon, the result 
of a thrusting forward of these events to a much earlier 
period in the oestrous cycle as compared with the mar- 
supial. 
So far, then, as concerns the uterine changes during pseudo- 
pregnancy, we conclude that these are represented in the 
Eutlieria by the alterations which occur in the mucosa during 
the latter part of pro-oestrus, and which, in some forms, cul- 
minate in menstruation.^ 
What induced this remarkable dislocation of events in tbe 
Eutherian oestrous cycle is a problem by no means easy of solu- 
tion, but we venture to suggest that it may perhaps be 
brought into relation with the omission or marked shortening 
in the Eutheria, of the post-oestral period whereby ovulation is 
directly transferred to oestrus. 
It is legitimate to assume that the shortening of the cycle 
in this way may have induced an increased growth of the 
mucosa during the pro-oestral period, and that this growth in 
its turn may have directly conditioned the earlier occurrence 
of the degenerative and regenerative changes, with the result 
that these latter now came into operation before instead of 
after the ovulation to which they were primitively related. 
Whether or not there be anything in these suggestions, there 
can be little doubt that the precocious onset of the degene- 
‘ The histological changes in the premenstrual nterns of the human 
subject have been fully described by Hitschmann and Adler in their 
important paper (15) published in 1908. These observers demonstrate 
that the changes in the premenstrual uterus are identical with those 
seen in the early pregnant uterus. To this paper, which contains a 
very full bibliography, the reader is referred for further details. 
