and many of the skull bones are simply plates and bars of cartilage;
the future membrane bones, however are planned out in connective
tissue. The development of the latter is simple, the connective
tissue corpuscles functioning by a simple change of product as
osteoblast. The development of the cartilage bones, however, is more
complicated. Figure XVII., represents, in a diagrammatic way, the
stages in the conversion of a cartilaginous bar to bone. To begin with,
the previously sporadically-arranged (scattered anyhow) corpuscles
(u.c.c.) are gathered into groups in single file, or in other words, into
"columnar" groups (as at c.c.). The matrix becomes clouded with
inorganic salts of lime, and it is then said to be calcified. This
calcified cartilage then undergoes absorption-- it must not be
imagined for a moment that bone is calcified cartilage. Simultaneous
with the formation of the cavities (s.) due to this absorption,
connective tissue (p.c.i.) from the surrounding perichondrium (p.c.)
grows into the ossifying* bar. It is from this connective tissue that the
osteoblasts (o.b.) arise, and bone is built up. Throughout life a bone is
continually being absorbed and reformed by the activity of the
osteoblasts. An osteoblast engaged in the absorption instead of the
formation of bone is called an osteoclast.
* The formation of bone is called ossification. To ossify is to become
bony.