324 Vertebrate Embryology 



" crown-rump " or " vertex-breech " measure- 

 ment ; it is the distance, in a straight line, 

 between the point immediately over the mid- 

 brain and the lowest point of the rump (Fig. 

 r 1 8, a-b). When this measurement has been 

 carefully made, the age of an embryo, up to 

 TOO mm., may be calculated, with more or 

 less accuracy, by using the following formula : 

 a = ]//" X 10, in which a is the age in days, and 

 /is the crown-rump measurement in millimeters. 



FIG. 1 1 8. FIGURE TO ILLUSTRATE THE 

 " VERTEX-BREECH" METHOD OF MEASURING 

 HUMAN EMBRYOS. (Altered from Kollmann.) 



-, vertex-breech length of the embryo. 



