THE AGE OF THE OVUM AND RELATION OF IMBEDDING 
TO MENSTRUATION 
The following chapter is of the nature of an appendix to the descriptive 
part of our memoir. It deals with the age of our ovum and certain 
theoretical considerations regarding the decidua which arise in connection 
with that problem. In attempting to place our specimen, in point of 
age, among other recorded cases we were met with so many difficulties and 
contradictions that it seemed worth while to ascertain whether, by applying 
the precise data of our own case to the other records, we could arrive at 
a more consistent chronological sequence than afforded by His’ rule. We 
have therefore reviewed the literature from this point of view, and shall 
now give the result of our enquiries. 
In estimating the age of the human ovuni or embryo, it is customary 
to make use of the convention of His, and to reckon, as the approximate 
age of the ovum, the interval between the date on which the first 
omitted period should have commenced and the termination of the 
pregnancy. This is fairly satisfactory from the embryological point of 
view when dealing with embryos of the sizes commonly found, but it 
conflicts not only with some facts of clinical experience, but also with 
the data of comparative embryology, and it fails altogether when applied 
to many of the very young ova which are on record. 
For the calculation of the age of the present ovum we have quite 
precise data regarding coition. The coitus of 2nd October may be 
definitely rejected as a possible factor in the case. The data provided 
by several of the cases which are summarised in this section make it 
certain that fertilization must have taken place after, and was presumably 
effected by, the coitus of 19th October. Any other conclusion would 
