AGE OF HUMAN EMBRYOS 399 



between these two lines was then divided exactly, and it is this 

 line which marks the mean menstrual age of embryos throughout 

 pregnancy. In a general way it is reproduced as the line CH 

 in figure 145 in the Manual of Human Embryology (p. 200). 



1 have spoken of the age thus determined at different times as 

 the menstrual age, or more properly speaking the mean men- 

 strual age, because there is a very large probable variation. For 

 instance, a number selected at random from the table on page 199 

 of the Manual, with a mean menstrual age of 51 days, would 

 also show a probable deviation of from 40 to 62 days. Such 

 embryos have a height of 11 mm.; therefore, when we obtain 

 embryos of this length, we may expect that one-half of them have 

 a menstrual age of from 40 to 62 days; in other words, in small 

 specimens there is a probable variatioii of three weeks. Viewed 

 from another angle, one-half of the embryos with a menstrual 

 age of 51 days w^ould range from 4 to 25 mm., while the average 

 would be 11 mm.; hence, we are probably dealing with a pretty 

 large error which cannot be definitely locatecT. At present it 

 would appear that pregnancy may begin at any time during the 

 intermenstrual period, but it is difficult to determine the most 

 probable time. What I published in the Manual has received 

 careful criticism from Triepel,- but he nevertheless also accepts 

 the term menstrual age, and recommends that we use it in the 

 future. 



COPULATION AGE 



After constructing the curve and table referred to above, 

 showing the mean menstrual age, I entered as the probable or 

 true age a line in the curve and a column in the table which fall 

 in a position exactly ten days earlier than the mean menstrual 

 age. This was done for the following reasons : According to the 

 more recent statistics of Issmer, that writer found the average 

 duration of pregnancy in 1220 cases to be 280 days, when esti- 

 mated from the first day of the last menstrual period; and in 

 628 cases, 269 days when estimated from fruitful copulation. In 

 general these figures correspond with those of Ahlfeld, Hecker 



2 Triepel, A. 1915 Alterbestimmung bei menschlicheu Embryonen. Anat. 

 Anz., Bd. 46, 1914. Also Bd. 48. 



