THE AGE OF HUMAN EMBRYOS 87 



called a. fetus (stage w). The external metamorphosis is due principally: (1) to 

 changes in the flexures of the 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 wliich have been obtained and described can- 

 not be determined with certainty, because fertihzation does not necessarily fol- 

 low directly after coitus. It has been shown also that ovulation does not always 

 coincide with menstruation so that the menstrual period cannot be taken as the 

 starting point of pregnancy. In 1868, Reichert, from studying the corpus luteum 

 in ovaries obtained during menstruation, concluded that ovulation takes place as 

 a rule just before menstruation and that if the ovum is fertihzed 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 Reichert's views and 

 since then the ages of embryos have largely been estimated on this basis. Ac- 

 cording to this view, Peter's ovum, obtained thirty days after the last period, is 

 only three or four days old. This does not agree at all with what is known of the 

 age of other mammahan embryos. 



From the observations of Mall and obstetricians of the present day, we must 

 conclude that ovulation does not immediately precede menstruation, but that 

 most pregnancies take place during the first or second week after the menstrual 

 period. It is therefore more correct to compute the age of the embryo from the end 

 of the last menstruation, or, according to Grosser, from the tenth to the twelfth day 

 before the first missed menstrual period. Peter's embryo then would be about 

 fifteen days old. To compare an embryo with one of known age, the crown-rump 

 length (i. e., from vertex to breech) is usually taken. Embryos of the same age 

 vary greatly in size so that their structure must be taken into account. At the 

 present time the exact relation of ovulation to menstruation is not known, nor the 

 exact time required for the fertihzed ovum to reach the uterus. The computed 

 age of the embryo thus can be only approximate. 



The period of gestation of the human fetus is usually computed from the 

 beginning of the last menstrual period. Forty weeks or two hundred and eighty 

 days is the time usually allowed. As some women menstruate once or more 

 after becoming pregnant this is not a certain basis for computation. 



