DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 123 



occurred 16 days prior to abortion. The fertilization age was computed to be 

 about 14 days. An embryo described by Eternod was aborted 21 days after 

 coitus, and the fertilization age was reckoned at about 19 days. Other cases 

 of a similar nature have been studied and the actual age of the embryo in 

 each case was regarded as two days less than the copulation age. Such reck- 

 oning could be done only in those cases where the time of coitus and of abortion 

 are both known. 



In the majority of cases the only datum available is the time that the 

 last menstrual flow occurred. In these cases the age of the embryo is reck- 

 oned from the beginning of the last menstrual period, and is known as the 

 menstrual age. This at best can be only an approximate age because even if 

 the date of coitus is known the date of ovulation is undetermined and there- 

 fore the time that the sperm and ovum meet is unknown. It has been shown 

 that in perhaps the majority of cases ovulation occurs in the intermenstrual 

 period, on the average about the middle of its duration, and therefore 

 fertilization would not bear a direct time relation to menstruation. The 

 condition of the corpus luteum, if it could be determined, would be of great 

 value in reckoning the age of the embryo resulting from the development of 

 the ovum that escaped from that particular follicle. The method of reck : 

 oning from the first day of the last menstruation i? used by obstetricians for 

 determining the probable time of birth. 



In statistics collected by Issmer the average duration of pregnancy in 

 1220 cases was found to be 280 days, when estimated from the first day of 

 the last menstrual period; and in 628 cases it was found to be 269 days from 

 the date of fruitful cohabitation. It would seem therefore, on the basis of 

 these statistics, that deducting about 10 days from the menstrual age would 

 give the approximate copulation age which is not far removed from the actual 

 age. 



The length of an embryo can be determined by direct measurement. 

 It is the general practice to take the greatest length of the embryo in a 

 straight line in its natural attitude, not including the extremities. In em- 

 bryos between 4 and 14 mm., when the body is much curved, one point of 

 measurement lies on the apex of the cervical flexure and the other on the 

 apex of the sacral flexure. (See Figs. 87 and 88.) This is known as the neck- 

 rump length. In later stages where the curvature is reduced and the body 

 is more nearly straight, the measurement falls between the apex of the 

 cephalic flexure and the apex of the sacral flexure. (See Figs. 90 and 91.) 

 This is known as the crown-rump length. The relation of length to age has 

 been the subject of much study, but no formula for deducing the age from 

 the length, which can be determined, has been wholly satisfactory. The 

 formula given by Mall some years ago was later abandoned by him as un- 



