28 BAUR, ON OSSIFICATION OF CARTILAGE. 
with a layer of soft, streaked interstitial substance, which 
lines the interior of the cartilage-cavities. Of true bone- 
substance, no trace was, up to this stage, to be perceived. 
This is not formed until this time by the direct ossification 
of this blastema, that is to say, by the transformation of its 
cells into bone-corpuscles, and of its intercellular substance 
into a homogeneous, not granular, osseous matrix. Whence 
it is evident that the first bone-substance must make its 
appearance in the form of a tube imclosing these calcified 
cartilage-cavities, and which in a transverse section presents 
the appearance of a ring beset with a single series of bone- 
cells. This osseous cylinder now becomes thickened from 
within outwards, by the successive ossification of new layers 
of blastema, deposited m a similar manner to the first, so 
that each cartilage-canal is gradually more or less completely 
filled up with a system of concentric osseous lamelle. The 
uniform concentrically lamellated structure of the long bones, 
which ever exists, is thus explained by the circumstance, that 
in the interior of each medullary canaliculus, a successive 
formation of ossifying lamin takes place from the centre 
outwardly, exactly in the same way as in the formation of 
the cortical substance from the periosteum, it takes place 
from the periphery. 
From what has been said, it is apparent that the individual 
tubuli of newly formed bone-substance must at first be sur- 
rounded by calcified cartilage-substance, and be separated 
from each other. It is now generally admitted, as in fact 
appearances render probable, that the calcified matrix of car- 
tilage itself is gradually transformed into homogeneous bone- 
substance, either by the coalescence of the separate calca- 
reous particles into a homogeneous substance, or that these 
particles, being only a provisional calcareous deposit, are 
previously absorbed. Upon this point it may now be re- 
marked, that the granular earthy deposit at any rate dis- 
appears, but with it also the organic substance to which it 
belonged ; and that thus the already commenced resorption 
of the cartilaginous matrix continues also after the com- 
mencement of the deposition of calcareous matter, in order 
to make way for the new bone-substance. This may be 
proved by direct observation. If a portion of cartilage 
undergoing ossification be treated with dilute hydrochloric 
acid, and transverse sections be made at the proper place, 
parallel with the border of the ossification, on using a due mag- 
nifying power, the ring, consisting of a single layer, of yellow- 
ish, strongly refractive bone-cartilage, beset with opaque 
bone-cells, will be seen, sharply defined, and in strong con- 
