dtd Ossification Centers in Human Embryos 
middle line, and lateral to them two small new centers are seen. On the 
next day all four centers begin to unite to form a single bone, the supra- 
occipital, measuring 2 mm. in height and 6 mm. in width. In the same 
embryo (263, b, 2) two very small exoccipitals one-quarter of a milli- 
meter in diameter are seen. On the 57th day (266) the swpraoccipital 
centers are well blended and the interparietals make their appearance 
as two reticular membrane bones, which unite during the next day 
(263, b, 1 and 272). On the 65th day the bastoccipital makes its appear- 
ance and on the 75th day all the bones above the foramen magnum are 
united into the single tabular part of the occipital bone. The accompany- 
ing diagram shows the general arrangement of the centers with the date 
of their appearance marked upon them and their time of union marked 
between them. 
Although there are over 20 perfect specimens in the cleared embryos 
of the proper ages, in none of them is there any sign of more than two 
centers for each interparietal as is asserted by Meckel, Ranke” and 
Bolk.* The supraoccipital is 6 mm. long, crossing the middle line in 
an embryo 56 days old, and above it lie the interparietals which together 
are a little narrower and nearly as long. At this time the exoccipital is 
about a millimeter long. They all grow quite rapidly and on the 73d 
day the supraoccipital and the interparietal begin to unite. Now the 
former measures 4 x 10 mm. and the latter 2 x 7 mm. The common 
squamous portion of the occipital bone measures 6 x 10 mm. on the 
75th day, 12 x 16 on the 85th day, and 18 x 30 on the 105th day. The 
erowth of the exoccipital and basioccipital is not so rapid, the former 
is 3 mm. long on the 73d day and 7 mm. on the 105th day; during the 
same time the basioccipital grows to measure 2 x 6 millimeters. 
The zygomatic bone-——The malar bone appears as a small three- 
cornered center just beneath and to the lateral side of the eye on the 
56th day. On the 58th day it is four-cornered and nearly two millimeters 
long. Two of the horns, that is two of the corners, encircle the orbit and 
one of the remaining two corners points towards the maxilla and the other 
towards the temporal bone. The zygomatic grows larger and gradually 
becomes hour-glass shaped, the constriction or narrow stem con- 
necting the orbital side with the temporal and maxillary processes. By 
the 75th day the bone has an area of 10 square mm., and the orbital end 
has given rise to the orbital surface which now grows more rapidly than 
the rest of the bone. On the 105th day the orbital surface is fully twice 
1 Ranke, Abhandl. d. K. Bayer. Akademie u. Wiss., XX. 
2 Bolk, Petrus Camper, II. 
