66 
AN EARLY HUMAN OVUM 
similar in structure to menstrual decidua; in fact many authorities deny 
the possibility of distinguishing between them, and doubtless the processes 
concerned in their formation are similar. But although the processes are 
similar, it does not follow that their purpose is identical. As already 
pointed out, it is apparent from the tables that the ovum can imbed 
itself during the period of quiescence of the endometrium, and the case 
of ovarian pregnane } 7 described in the second part of this memoir 
demonstrates that the presence of decidua is not necessary to imbedding. 
In the mouse and guinea-pig there is no change in the endometrium 
until it is attacked by the ovum. In the human subject, no doubt, in 
certain instances, menstruation is arrested and degeneration is checked before 
imbedding takes place; but it is not essential that actual development 
of the endometrium into decidua should precede its invasion by the ovum. 
Indeed, the analogy of the guinea-pig and other mammals makes it probable 
that the development rapidly takes place immediately after the invasion, 
and that it may in the first instance be of local occurrence only. This 
apparently is what occurred in the case of Peters, where a definite decidual 
reaction is said to be limited to the immediate neighbourhood of the 
ovum. On the other hand, should an ovum be fertilized after the pre¬ 
menstrual changes have set in, there is no reason to suppose that they 
would offer any obstacle to its implantation; indeed, as the early changes 
in the formation of the menstrual swelling are identical with those of 
decidua formation, it might be favourable to the process. We have no 
evidence, however, to show whether fertilization may not completely arrest 
the whole menstrual changes. 
Menstruation on the premisses detailed above is thus a cyclical pro¬ 
cess which provides for the maintenance of the endometrium in a suitable 
condition for producing the decidua of pregnancy, but the ovum is not 
dependent upon the development of a menstrual decidua for a suitable 
nidus in which to become imbedded. The uterus is capable of developing 
a decidua of conservation whenever its integrity is threatened by the 
growing ovum. 
O o 
