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BONES. 45 ;

ordinary red blood-corpuscles. Those holding this view claim that
the marrow of the bones is one of the blood-producing tissues of the
body.
Osteoblasts (described p. 42) are found princi])ally and in large
numbers in red marrow along the osseous trabeculi, especially during
the development of bone.
Red cells, having every appearance of red blood-corpuscles, are
numerous. Their presence has been explained by the fact that the
walls of the capillary blood-vessels are very thin and delicate, permit-
ting the escape of the corpuscles into the surrounding tissue.

Development of Bone.
The central column of the body, consisting of the vertebrae and the
base of the skull, is seen in rudimentary outline at a very early period of
embryonal life. This outline, formed from the mesoblastic layer of the
blastoderm, and composed of tissue not unlike the primordial structure
by which it is surrounded, soon becomes cartilage ; and in man, about
the fortieth day of embryonal life, bone-development commences in the
clavicle and inferior maxilla. This early ossification of the lower jaw,
before the up])er, may be explained as due to the same law of develop-
ment that brings about the eruption of the inferior teeth before the
superior. Some of the lower animals, however, have more teeth in the
lower .than the upper maxilla, the former being more important, as in
mastication it has active movement, while the upper jaw is passive.
Nearly all the bones of the body have their commencement in hyaline
cartilage. Those of the face, with the two exceptions of a portion of
the inferior maxilla and the inferior turbinated, and those covering the
brain, excepting a part of the occipital, are developed within memliranes.
In addition to these two modes of formation, the cartilaginous and mem-
branous, bone seems to be principally developed from the osteogenetic
layer of the periosteum. From this it will be seen that it is best to con-
sider the development in three divisions—viz. : I. Intracartilaginous
II. Subperiosteal ; III. Intramembranous.
I. The Intracartilaginous (endochondral) Bones are those hav-
ing their origin or first formation in hyaline cartilage, whicli usually
presents, in minature, a general outline of the future bone. The trans-
formation from cartilage to bone is gradual, and commences at one or
more points, called centres of ossification. In the bones of the higher
animals, including man, the number of ossific centres varies, being
dependent on the degree of complexity in the formation of the bones
and the number of vessels and nerves which pass through them. If
there is but one point, it is usually situated near the middle and upon
the surface, else next to the perichondrium or future periosteum. For
convenience of description and facility of study the development of
endochondral bone may be divided into four stages :
1st. The first observable change is at one or more of the ossific points,
at which the cartilage-cells (for description see p. 138) immediately under
the perichondrium enlarge and multiply within their capsules, the matrix-
substance becoming partly absorbed.