436 Ossification Centers in Human Embryos 
which extracts the water from the gelatin and makes it very firm. The 
ossification centers show best when viewed with a large lense over a dark 
background in direct sunlight. 
I have made numerous attempts to study the ossification centers in 
serial sections stained in carmin and in hematoxylin, but find that they 
are by no means as satisfactory for this purpose as are the Schultze speci- 
mens. An embryo 20 mm. long (No. 22), which we have studied with 
great care shows no ossification centers in the sections, while embryos 
15 mm. long show. them when cleared.” The model made by Ziegler, 
which is from Hertwig’s reconstruction of an embryo 80 mm. long does 
not show the bones of the skull developed to as great an extent as they 
are in cleared specimens. So for the present the Schultze method en- 
ables us to locate the first bones with much greater certainty than do sec- 
tions, with the possible exception of those colored with Mallory’s con- 
nective-tissue stain. 
OSSIFICATION CENTERS IN THE SECOND MONTH. 
All anatomists agree that the clavicle is the first bone to ossify and 
that it appears about the middle of the second month or during the sixth 
_week. Béclard states that he found it in an embryo 30 days old, but as 
E. H. Weber remarks in Hildebrandt’s Anatomy, he has probably under- 
estimated the age of the embryo. Judging by the number of ossification 
centers present Béclard’s embryo must have been 44 days old, for it corre- 
sponds with my specimens 263, C and A. Meckel must also have under- 
estimated the age of an embryo in which the clavicle was found to be 
three lines long, for he places it in the middle of the second month. Ac- 
cording to my reckoning, his specimen is fully 56 days old, for it corre- 
sponds with my specimen 263, b. 2, Table II. Similar differences of 
opinion will be found regarding the time of ossification of all sharply 
defined bones due, no doubt, to the difficulty in determining the age of 
embryos 100 years ago. Therefore, little is to be gained in reviewing 
the literature upon this subject for such specimens, since the results are 
not satisfactory when compared with a good series of Schultze specimens. 
Especially is this true regarding the literature of obscure bones which are 
said to arise from more than one center. In my description I shall, 
therefore, confine myself closely to my own specimens. 
°The same is shown in Bardeen’s studies on the development of bones 
(Anat. Anz., XXV & Amer. Jour. Anat., IV). The earliest stage of different 
centers he describes are always a little later than those in which they may 
be seen in embryos cleared in potash. 
