434 Ossification Centers in Human Embryos 
time giving their age in days or weeks, and usually omitting their length. 
A similar criticism may be made regarding Meckel’s great paper from 
which is derived our main information regarding the development of the 
spinal column and skull.’ 
The older illustrations of primary bone centers are not especially good, 
for in them the finer details are obscure and the enlarging glass did not 
aid to make them sharper. However, some of Meckel’s pictures are still 
used in the anatomies, but the small dried arms pictured in Bell’s Anat- 
omy * and in Rambaud & Renault’s Atlas* have not been copied exten- 
sively. Semidiagrammatic illustrations, 1. e., older bones with the earlier 
centers marked in them have gradually taken their place. And it is only 
in very recent years that suitable illustrations from X-ray pictures,’ from 
transparent embryos or from reconstructions‘ are taking their place. 
The newer methods enable us to recognize and to follow the early ossi- 
fication centers with much greater precision than was possible in 
Meckel’s time. Instead of a dissected and dried specimen we now have 
sections and reconstructions, and what is still better transparent specimens 
made according to the method of Schultze, which enable us to detect the 
minutest bone (0.1 mm. in diameter) and to study it in relation to the 
rest of the skeleton without destroying the embryo. And last, but not 
least, we have a standard of measurement (the crown-rump line of His) 
from which we can determine the age of the embryo with an error of but 
a few days. It naturally follows that all that is now required is a 
good collection of transparent embryos to determine the time of appear- 
ance and order of development of the bone centers. 
During the past half dozen years I have cleared from time to time 
human embryos which were shrivelled or otherwise made unfit to cut 
into serial sections. Gradually the number increased so that now I have 
some 60 transparent specimens in my collection with crown-rump meas- 
urements ranging from 10 to 110 mm. in length. Most of these speci- 
mens are used in this study. 
1 Béclard, Meckel’s Archiv, 1820. 
*Meckel, Meckel’s Archiv, 1815. 
3’ Bell’s Anatomy, New York, 1834, p. 166. 
*Rambaud & Renault, Origin et Dével. d. Os., Paris, 1864. 
5Lambertz, Entwickl. d. Mensch. Knochengertistes wahrend d. foetelen 
Lebens, Hamburg; Bade, Arch. f. Mik. Anat., 55; Cunningham’s Anatomy, 
2d Edition, 1905. 
® Schultze, Grundriss d. Entwickl. d. Menschens, 1897. 
7Gaupp, Hertwig’s Handbuch d. Entwickelungslehre, Jena, 1905; Bardeen, 
Amer. Journal of Anatomy, IV, 1905. 
