AGE OF HUMAN EMBRYOS 



421 



based are not altogether satisfactory, but if the six cases given 

 in the table are recorded, the average date is one day later; that 

 is, the 14th. The figures of these cases are as follows: 



TABLE 6 



Days between 

 menstruation 

 and copula- 

 tion 



No. of cases.. . . 



2122 

 1 2 



23 



2425 



l| 



2830 

 1 1 



In this group just half of the cases date from the first two 

 weeks of the menstrual cycle, and the other half from the second 

 two weeks. It is recalled that in over 1000 cases of full term 

 births the mean as computed from the time of menstration 

 and of copulation duration of pregnancy differed exactly 1 1 days, 

 and that in my curve a difference of 10 days was taken. The 

 theory of Bryce and Teacher and of Triepel is that fertilization 

 takes place two days after copulation, and, therefore, they 

 figure two days less than I did as the mean ovulation or fertiliza- 

 tion age. Between these two theories I presume that we are 

 within two or three days of the real age. 



It appears to me probable that fertilization takes place nearly 

 always within the uterine tubes — very rarely upon the surface 

 of the ovary, as ovarian pregnancy is extremely rare; and prob- 

 ably also because the spermatozoa have lost their fertilizing power 

 by the time they have passed the tube. No doubt this power is 

 greatest in the tubes, as in these narrow channels the sperm would 



