ESTIMATION OF AGE OF EMBRYOS. 469 



to convert the retrogressive changes of the uterus into pro- 

 gressive ones. 



(iv) The rule is not in accord with the well-established fact 

 that, in order to insure pregnancy, the most favourable time for 

 intercourse is shortly, or immediately, after the conclusion of a 

 menstrual period. This is intelligible enough if the ovum to be 

 fertilised is the one discharged at that period ; but is hard to 

 understand if, as the rule requires, these spermatozoa have to 

 wait for a period of three weeks or more, until the next discharge 

 of ova. 



These objections are serious ones, and Professor His, in the 

 second part of his work, recognises that the rule as originally 

 formulated cannot apply to all cases. He quotes instances in 

 which the dates were accurately recorded, and in which the 

 fertilised ovum must have belonged to the last occurring period, 

 and not to the first omitted one ; he is of opinion, however, that 

 the rule as stated above will still apply to the majority of 

 cases. 



This more recent view may be expressed graphically, thus. 

 If I. is the first day of the last actually occurring menstrual 

 period, and II. is the first day of the first omitted period ; then 

 the possible days of fertilisation are as follows : 



L, 2, 3, 4, 5, 6, 7 26, 27, 28, II. 



That is, an ovum discharged during an actually occurring period 

 remains capable of fertilisation for a certain number of days, ex- 

 pressed in the formula as a week, commencing with I., and 

 ending at 7. During this time it may be fertilised, either by 

 spermatozoa received after the period is over, or received before 

 the period and retained in the oviduct during it. In the case 

 of these embryos the age should be calculated from I., the first 

 day of the last actually occurring period. 



On the other hand, Professor His, and others, maintain that 

 there are possibilities of fertilisation at the other end of the 

 series ; and that an ovum, discharged from the ovary a day or two 

 before the next period, II., is due, may, if fertilised, stop that 

 period from occurring; and in such cases, if they really happen, 

 the age of the embryo should be calculated from the first omitted 

 period, and not from the last occurring one. It is not yet 

 certain which of these two possibilities is the normal mode of 



