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OSSIFICATION. 579

which first of all becomes calcified, and thi.s is invaded, and for the
most part removed, bv an embryonic tissue which deposits bony matter
in the interior of the cartilage, whilst at the same time lavers of bone
are being formed outside, under-
Fig. 319.
neath the periosteum. This is in-
fercartilaginous or enchondral ossi-
fication. Sometimes the bone is
not preceded by cartilage, and then
the only process which occurs is one
corresponding to subperio.'^teal ossi-
fication of the former variety. The
ossification is then known as intra-
memhrcmous." From the above it
is .seen that this author makes in-
tramembranous and subpcrio.steal
bone-formation analogous except
as regards position.
Usteoblasts Irom the Parietal Bone of a Human
Gray makes two main divis- Embryo thirteen weeks old: «, bony septa, with
the cells of the lacunte, or bone-corpuscles:
ion.s—intracartilaginous and in- layers of osteoblasts; c, the latter in transition 6,
to bone-corpuscles (very high power).
tramembranous—and places sub-
periosteal as a subdivision of the second. As an example of the
"
first, he cites the " long bones ;" of the second, the " cranial bones —
viz. the occipital as far as it enters into the formation of the vault
of the skull, the parietal and frontal bones, the squamous portion of
the temporal with the tympanic ring, the Wormian bones, the nasal,
lachrymal, malar, palate, upper and lower maxillary, and vomer ; also,
apparently, the internal pterygoid and the sphenoidal turbinated
bones. " The intramembranous o.ssification," he further .'savs when
discussing that division, " is that by which the bones of the vortex
of the skull are entirely formed. In the bones which are so devel-
oped no cartilaginous mould precedes the appearance of the bone-tis.sue.
The process, though pointed out originally by Dr. Nesbitt in the vear
178(>, was fir,st accurately described by Dr. Sharpey, and it does not
appear that subsequent observers have been able to add anvthing essen-
tial to his description. This is substantially as follows : In the mem-
brane, which occupies the place of the future bone a little network of
bony spiculse is at first noticed, radiating from the point of ossification.
When these rays of growing bone are examined by the microscope,
there is found a network of fine clean fibres (osteogenetic fibres), which
become dark and granular from calcification, and as thev calcify thev
are found to enclo.se in their interior large granular corpuscles, or osteo-
blasts. These corpuscles at first lie upon the o.steogenetic fibres, so that
the corpuscles must be removed by bru.shing the specimen with a hair
pencil in order to render the fibres clear, but they graduallv sink into
the areolae developed among the fibres. The areolae appear to be the
rudiments of the lacunae, the pa,ssages between the fibres form the
canaliculi, and the osteoblasts are the rudiments of the bone-cells."
This, with slight modification, is a very good description of intramem-
branous ossification as .'^een in the parietal bones of the skull-cap, but
does not an.swer for the maxillae, as we will see later on.