THE AGE OF Hl'MAN EMBRYOS 89 



embryo; (2) to the development of the face; (3) to the development of 

 the external structure of the sense organs (nose, eye, and ear) ; (4) to the 

 development of the extremities and disappearance of the tail. The more 

 important of these changes will be dealt with in later chapters. 



THE AGE OF HUMAN EMBRYOS 



The ages of the human embryos which have been obtained and de- 

 scribed cannot be determined with certainty, because too little is known 

 of the time relations between ovulation, coitus, and fertilization. Further- 

 more, ovulation need not bear a definite relation to menstruation (p. n). 

 This lack of a reliable basis makes any computation only approximate. 



In 1868, Reichert, from studying the corpus luteum in ovaries ob- 

 tained during menstruation, concluded that ovulation takes place as 

 a rule just before menstruation, and that if the ovum is fertilized the 

 approaching menstruation does not occur. Reichert then decided that a 

 human embryo of 5.5 mm., which he had obtained from a woman two weeks 

 after menstruation failed to occur, must be two weeks, not six weeks, old. 

 His accepted Reich ert's views and the ages of embryos were for a long time 

 estimated on this basis. According to this method, Peters' ovum, ob- 

 tained thirty days after the last period, is only three or four days old. 

 This, however, does not agree at all with the known ages of other mam- 

 malian embryos equally developed. 



From numerous clinical observations we must conclude that ovula- 

 tion does not immediately precede menstruation, but that most pregnan- 

 cies follow a coitus within a week or ten days after the menses cease. It 

 is therefore more correct to compute the age of an embryo from the tenth day 

 after the onset of the last menstruation. To compare an embryo with one 

 of known age, the crown-rump length (that is, from vertex to breech) is 

 usually taken. Young embryos vary greatly in size so their structure must 

 be taken into account as well. 



Of practical interest is the determination of the date of delivery of a 

 pregnant woman. Most labors occur ten lunar months, or 280 days, from 

 the first day of the last menstrual period. The month and day of this 

 date are easily found by counting back three months from the first day of 

 the last period, and then adding six days. As some women menstruate 

 once or more after becoming pregnant this computation is not infallible. 



