Development of the Mammal 323 



takes place at about the time of menstruation ; 

 and, this process of menstruation being the pre- 

 paration of the uterus for the implantation of 

 the fertilized ovum, it is likely that pregnancy 

 can occur only when fertilization takes place 

 at the beginning of the menstrual period. 

 The occurrence of pregnancy stops the men- 

 strual flow, hence it is customary to calculate 

 the age of an embryo from the date of the 

 first menstrual period which has lapsed. Some- 

 times conception may occur without interrupt- 

 ing the menstrual flow of that particular month, 

 the interruption not occurring until the follow- 

 ing period ; in this case the difference of twenty- 

 eight days between the supposed and real time 

 of conception would be so great that it would 

 be apparent and would not cause error in the 

 calculation of the age of the embryo. 



It frequently happens, in the study of human 

 embryos, that information as to the time of con- 

 ception cannot be had. In such cases it is 

 necessary to calculate the age of the embryo 

 chiefly from its size, though there is consider- 

 able variation in this. 



There are several methods of measuring the 

 size of a human embryo. Perhaps the best 

 single measurement is that known as the 



