458 Ossification Centers in Human Embryos 
ance of the metatarsal between the 9th and 10th weeks. Unfortunately 
Hasselwander does not give the crown-rump measurements of his speci- 
mens, making it difficult for me to estimate their age. However, I am in- 
clined to think that he has overestimated the ages of all of his embryos. 
Phalanges, 1—The center for the first bone of this row is present in 
an embryo 83 days old and all of them are present in another embryo of 
the same age. On the 85th day a specimen shows but the first and sec- 
ond bones. In the earliest stages the bones often appear as double centers, 
one on the dorsal side and one on the volar side of the bone. Soon two 
delicate half rings unite the primary centers to form the shaft which 
in the older embryos of my list is but half a millimeter long. Hassel- 
wander places the time for the appearance of this row all the way from 
11 weeks to 4 months, the centers not being constant until the latter part 
of the 4th month.—The second row of phalanges is not ossified in any of 
my preparations. They appear shortly after the 110th day, according to 
Hasselwander, although the older French anatomists place the time much 
too early, Rambaud and Renault on the 45th day! 
Phalanges, I11.—The first terminal phalanx appears on the 58th day 
and is therefore one of the first bones of the foot to ossify, as was cor- 
rectly stated by Meckel nearly a century ago. After this time, in all the 
embryos studied, the first four of the terminal phalanges are present, 
while the fifth is not constant until the 90th day, although it is present 
in three younger embryos. Rambaud and Renault fix the time of ossifica- 
tion of the first four terminal phalanges in the 4th month, and the fifth 
after birth. 
It may be noted again that early ossification centers are best seen in 
Schultze specimens viewed in direct sunlight with a purple background, 
with a large lense which magnifies two or three diameters, in order that 
both eyes may be used. ‘The very earliest deposit of bone cannot be seen 
with certainty in ordinary serial sections stained with haematoxylin and 
eosin or with carmine. 
Much of the trouble in determining the time of ossification is due to 
the uncertainty regarding the age of the embryos studied. In my speci- 
mens the age is estimated in days by multiplying the square root of the 
crown-rump length in millimeters by 10. This measurement was made 
with great care, and is given each time with the age. If the calculations 
of embryologists are correct, my estimations of the age of the embryos 
cannot be out of the way more than a few days. Estimations of the age 
from the last menstrual period alone may be fully a month in error and 
are nearly always in need of correction. 
