468 THE HUMAN EMBRYO. 



Professor His, in the first part of his monograph on the 

 development of the human embryo, laid down the following 

 rule : 



' The age of an embryo is the time that has elapsed since 

 the first day of the first omitted period.' 



Thus, supposing the commencement of a menstrual period 

 to be due on January 5, and that when this time comes, the period 

 is omitted ; but that at some subsequent time, say February 9. 

 an embryo is aborted ; then, according to Professor His' rule, 

 the age of the embryo would be the interval between January 5 

 and February 9, i.e. thirty-five days. 



In arriving at this result, Professor His argues in the follow- 

 ing manner : The ovum leaves the ovary either at, or shortly 

 before, the menstrual period ; if it is fertilised, presumably by 

 spermatozoa previously introduced, menstruation does not occur ; 

 but the changes in the uterine mucous membrane, instead of, as 

 usual, becoming retrogressive, either remain stationary or else 

 continue to be progressive ; and so prepare the uterus for the 

 reception of the ovum. Hence the first omitted menstrual 

 period corresponds in point of time with the fertilisation of the 

 ovum ; and hence the age of the embryo may be taken as the 

 time that has elapsed since the first omitted period. 



This method of calculation is, however, open to very grave 

 objections, the more important of which are as follows : 



(i) There are strong reasons, which have been fully con- 

 sidered in the previous portion of this chapter, for regarding 

 the decidua which is broken up and discharged at a menstrual 

 period to be related, not to the ovum discharged from the 

 ovary at the same period, but to the ovum discharged at the 

 preceding period. 



(ii) Professor His' rule assumes that the ovum is invariably 

 fertilised on the first day of the first omitted period. There is 

 no direct evidence in support of this ; and the loose nature of 

 the connection between ovulation and menstruation renders it 

 highly improbable. 



(iii) The rule assumes that the act of fertilisation of an ovum, 

 which in all probability will not reach the uterus for at least a 

 week, is able to arrest the degenerative changes already com- 

 menced in the decidua, to suddenly stop the menstrual flow that 

 is on the verge of taking place, or has actually commenced, and 



