438 Ossification Centers in Human Embryos 
tion in the frontal, zygomatic, squamo-zygomatic and parietal bones in the 
skull, the ilium in the lower extremity and the terminal phalanx of the 
thumb in the hand. 
Fig. 1. Embryo No. 333, enlarged 5 times. The outlines of the embryo 
were taken from a fresh specimen. 
The remarkable regularity of the appearance of the bones make of them 
the best index of the size and age of an embryo we now possess. The 
measurement of the crown-rump length of an embrvo is modified by the 
