454 Ossification Centers in Human Embryos 
drical center, but half a millimeter in length. It grows quite rapidly, 
being 5 mm. long on the 58th day; 9 mm. on the 75th day; and 14 mm. 
on the 85th day. 
The radius and ulna.—The radius and tibia both arise on the 42d 
day, and the ulna appears a few days later. They are hollow cylinders, 
a millimeter long on the 56th day, and 3 mm. on the 58th day. From 
now on the ulna is always a little longer than the radius; on the 85th 
day they are 12 mm. long. 
Fic. 5. Hand of embryo No. 300 (73 mm. long), enlarged 3 diameters. 
Scapula.—tThe first center appears about the middle of the region of 
the spine as a small granule on the 55th day. By the 58th day it is 24 
mm. in diameter, and on the 85th day it is 9 mm. long. 
Metacarpal.—The second and third metacarpal bones begin at the same 
time, for they are equally large in an embryo of the 57th day. On the 
next day (263, b, 1) small fourth and fifth metacarpal bones are also 
present, and in an embryo of the same age (272) the first one is added 
as a small crescent-shaped bone with the opening of the ring turned 
