452 Ossification Centers in Human Embryos 
and the eighth or ninth, or both, in the upper thoracic region. This ap- 
pearance indicates that the arch first to appear is the largest for a con- 
siderable time after other arches appear, a conclusion which the earlier | 
anatomists deduced in studying the ossification centers of the ribs, arches 
and bodies of the vertebre. 
The bodies.—The bodies are present in large number in an embryo 58 
days old, although none are present in another embryo of the same age 
as well as in a few excellent specimens a little younger. In this specimen 
(272) the bodies extend from the 10th to the 25th vertebra being very 
small above and below, the 19th, 20th and 21st being the largest. A 
specimen of the 65th day shows much the same appearance. They now 
extend more rapidly towards the head than into the sacrum, fluctuating 
in number on both ends of the spinal column. Until the 90th day the 
bodies on the 20th and 21st vertebra are the largest, indicating that these 
two bones were the first to ossify. At no time were accessory ossification 
centers seen nor were the bodies found to arise from two centers. Meckel 
made the same observation in 1815, but several writers since his time 
have spoken in favor of double ossification centers in the bodies of the 
vertebra, an idea much in vogue about the time of Haller. 
OSSIFICATION OF THE BONES OF THE ARM. 
The clavicle-—The ossification center for the clavicle is present in 
two, and uncertain in one, out of five embryos of the 39th day. In these 
specimens, as well as in older ones, the clavicle is clearly made up of two 
centers, a large one about .5 mm. in diameter near the median line and 
a smaller one (.2 mm. in diameter, and .5 mm. long) reaching something 
like a handle from the first, towards the shoulder joint. Together they 
are about a millimeter long. A few days later (No. 42) these two bones 
measure nearly 2 mm. together, the inner one, however, is much the 
larger, and fully separate from the outer one. Towards the 45th day the 
two centers blend, and a recent series of sections of an embryo 20 mm. 
long (No 240), which had been stained in iron hemotoxylin, shows 
them fairly well united. This specimen also shows the anlage of the 
clavicle as composed of a peculiar cartilage with a deposit of granules 
between the cells. The appearance is unlike that seen in the mandible 
or in the humerus. By the 49th day the two centers are fully united, and 
appear in a single bone 2 mm. long as is shown in an excellent specimen 
(No. 333). By the 55th day it is 3 mm. long; the 58th day, 5 mm.; the 
v5th day, 9 mm., and the 85th day, 12 mm. 
The humerus.—This bone appears on the 42d day as a very small cylin- 
