THE DURATION OF PREGNANCY. 467 



no certain knowledge, but it is commonly held that the intervals 

 during which there are no ova capable of being fertilised are at 

 least as long as the periods in which there are such ova. In 

 other words, assuming that the ova discharged at a given men- 

 strual period retain their vitality for from ten to fourteen days 

 — a pure assumption — there would be an interval of about two 

 weeks before the next menstrual period, i.e. before the next dis- 

 charge of ova, and during this interval there would be no ferti- 

 lisable ova in the oviduct, and fertilisation could not take place. 

 Any spermatozoa received during this interval would have to 

 wait until its close, at the next period of ovulation, before they 

 had a chance of meeting with ova capable of being fertilised. 



There seems to be a general consensus of opinion that the 

 first day or days after the cessation of the menstrual period are 

 the most favourable time for fertilisation to take place. This is 

 in complete accordance with what has been said above, both 

 with regard to the ovum and the decidua, for the ovum will be 

 lying within the Fallopian tube in a healthy fertilisable condi- 

 tion and easily accessible to the spermatozoa ; while if the ovum 

 takes another week or so to travel down the tube to the uterus it 

 will enter this latter while it is in the quiescent state, which, it has 

 been shown above, there is reason for regarding as the most 

 favourable one for the reception of the ovum. 



8. Estimation of the Age of Human Embryos. 



It follows from what has been said above, that there is no 

 means of determining with certainty the age of a human 

 embryo prematurely discharged from the uterus; for develop- 

 ment dates, not from the discharge of the ovum from the ovary, 

 but from the moment of fertilisation ; and this latter cannot be 

 determined. 



Ovulation is a process easily overlooked, but the fact that it 

 occurs simultaneously with the menstrual periods renders its 

 date readily determinable, but within certain limits only. The 

 connection between the two processes is a loose one, and it is 

 probable that ovulation may occur either from two to three days 

 before a menstrual period, or during the period ; giving a pos- 

 sible error of about a week in estimating the age of an embryo 

 from the date of menstruation. 



