446 Ossification Centers in Human Embryos 
Sphenoid bone.—The first center of the sphenoid to appear is that of 
the pterygoid which may be seen just behind the palate in an embryo 57 
days old. It is a very small bone about one millimeter long with a small 
T-shaped handle above, which possibly represents the vaginal process. A 
day later (263, b, 1) the alisphenoid is present as a small rectangular 
bone measuring about a millimeter on each side. Both centers grow 
slowly until the 75th day when the alisphenoid measures 4 mm. in iength. 
At this time the main body of the pterygoid is not much larger than it 
Fic. 4. Mesial view of the head of embryo No. 284 (54 mm. long), enlarged 
214 diameters. The centers of the occipital bone surround the foramen 
magnum. Behind the maxilla may be seen the palate, the pterygoid, and the 
alisphenoid in order. 
was at first, but the cross piece of the T is nearly a millimeter long. On 
the 83d day (M) the orbitosphenoid is present as a rectangular bone about 
a millimeter long. In the same embryo (M) the alisphenoid measures 
3 x 6 mm. and the pterygoid is fully two millimeters long. A little later 
(N) the basisphenoid appears as two fairly large granules of bone, one 
on either side. In an older embryo (300) the basisphenoid granules are 
