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582 DENTAL EMBRYOLOGY AND HISTOLOGY.

formed by fellow-workmen on either side. As deposition progresses
the osteoblast beeomes encircled by a shell of lime. As the trabecula
widens by enclosing the osteoblasts which lie upon its sides new layers
of osteoblasts are found arranging themselves on the walls, which in
turn become enclosed in a layer of bone. Thus, by the accumulation
of successive layers of calcospherules, the broadening of the bands of
bone-tissue is accomplished. As the osteoblasts build themselves into
the wall their places are taken by fresh recruits. AVhen each osteoblast,
by seci'cting its calcospherule, completes its life-work as a bone-
builder, it becomes a bone-ail, and from that time on occupies the
house it has builded. (See Fig. 322.)
The maxillary bones, including the alveolar processes, are thus pre-
formed in provisional bone. The fo'tal jaw is as truly the antetype of
the mature jaw as the fcetal
Fig. 322. head of the femur, wliich is
preformed in cartilage, is the
j)attern or matrix-former of
the mature femur. This is
so even in the very early
stages of its development.
The alveolar walls surround
the microscopic follicle and
C.SD present the same ragged ap-
pearance when the overly-
ing mucous membrane is re-
moved that we see later in
mature tissue. The maxil-
lary bones cannot be said to
have any special poinU of
Human Foetus, 2 months (y 250) : o, ostef)t)lasts (the dark
lines which come to the edge of the fi^'uie at h represent ossification, as do other
l)an an osleolilast ; ct, embryonal connective tissue.
eral. The fact that the al-
veolar processes have not theii" analogue in form in any other portion
of the body, strongly |)oints to the correctness of my views regarding
their special manner of development.
The first-form(>d bone is removed by internal resorption, which is
concomitant with the external growth by regular methods, which we
sliall now describe.
After the formation of the pcriosti'um two other classes of bone-
development occur—viz. iiiir tial point of ditference between the two is found in the location of the
embryonal j^lates of i)one. Itdramembranous development, as such per
fie, belongs entirely to fetal life, and is found in its most typical form in
the development of the skull-caj) ; here it has reference only to the first-
formed i)one, wliich wc have befijre said is provisional in character, the
growth which takes place after birth being subjx'riosteal in its nature.
And so it is in regard to i)ifersfif!(i/ development : the cortical substance
of the mature jaw is developed underneath the periosteum. Indeed, we
may say that all bone-fi)rmation is provisional until such time as the
bones have nearly reached the typal demands of nature, for of those