g6 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



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 menstrua- 

 tion, concluded that ovulation takes place as a rule just before menstruation and 

 that if the ovum is fertilized the next 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 been estimated on this basis. According to this view, Peter's ovum, ob- 

 tained 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 mammalian 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 

 length 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 fertilized ovum to reach the uterus. The computed age 

 of the embryo can be thus 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 

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



The following are the estimated ages, lengths, and weights of human em- 

 bryos according to Mall, Schroeder, and Fehling: 



Length in Weight 



Age Millimeters in Grams 



Eighteen to twenty-one days 0.5 



Twenty-four to thirty days 2.5 



Thirty-one to thirty-five days 5.5 



Thirty-eight to forty-two days 11.0 



Fifty days . 20.0 



Second lunar month 30.0 



Third lunar month 70.0-100.0 20 



Fourth lunar month 180.0 120 



Fifth lunar month 250.0 285 



Sixth lunar month 315-0 635 



Seventh lunar month 370.0 1220 



Eighth lunar month 425.0 1700 



Ninth lunar month 470.0 2240 



Tenth lunar month . . 500.0 3250 



