THE OESTRUS CYCLE 
61 
subtracting the age of the ovum in each case. As a definite figure was 
required the higher figure was arbitrarily selected in each instance, 
allowance being made for the various circumstances under which the 
ova were obtained. The date of imbedding was fixed by assuming that 
seven days elapse between fertilization and implantation of the ovum 
in the endometrium. 
The second table is the complement of the first, and it shows that, 
if the ages of the ova be correct, fertilization may occur at any time 
during the intermenstrual interval, and that imbedding may take place 
either in the period of quiescence or in the period during which, without 
the occurrence of pregnancy, the premenstrual and menstrual changes 
would have been progressing. 
This result of our seriation of these early cases is not consistent 
with the older views regarding menstruation and its relations to 
imbedding, for it carries with it the conclusions that the menstrual 
decidua is not a preparation for the reception of an ovum (in the old 
sense of the words); that menstruation is not an abortion of an un¬ 
fertilized ovum ; and that ovulation does not necessarily coincide with 
menstruation. The conclusion is not inconsistent, however, with the 
recent views on menstruation as developed by Heape, and supported by a 
considerable number of comparative researches. It is this consideration 
which gives weight and interest to our argument. 
It is now generally admitted that the menstrual cycle in man and 
monkeys is homologous with the oestrus cycle of the lower mammals. 
The oestrus cycle is divided by Heape into pro-oestrum, oestrus and 
dioestrum, and this division has been confirmed for many mammals by 
his own researches and those of F. H. A. Marshall. During pro-oestrum 
the generative organs of the female show signs of special activity, such 
as swelling of the vulva, coloration or flushing of the surroundings, and 
a discharge of blood or mucus from the vagina. This is immediately 
followed by the “oestrus,” or “period of desire,” during which alone the 
female is capable of impregnation and will receive the male. If pregnancy 
does not occur, oestrus, after a brief space in which desire subsides 
(metoestrum), is succeeded by a period of quiescence or dioestrum, which 
