Page 353 - My FlipBook

TEETH OF THE VEBTEBRATA. 363

ance, while in the remainder of the bulb numerous other cells, identical
with those of the tooth-pulp, are developed. It also becomes highly
.vascular. Very soon the odontoblasts nearest the surface undergo
metamorphosis into a gelatinous matrix, and their nuclei disappear;
thev are next calcified from the summit downward, and we soon recognize
a thin dentine cap over the entire bulb, which gradually increases as
development proceeds. The central portions of the odontoblasts remain
uncalcified and form the dentinal fibrils, while the lateral ])rocesses occa-
sion the numerous anastomoses of the dentinal tubuli and fibrils seen in
the adult tooth. The dentine mass is gradually thickened by successive
increments from within by a repetition of the process above described,
so that it will thus be readily seen that the configuration of the dentine
body, and consequently the entire tooth, is established as soon as calcifi-
cation has fairly set in.
Returning to the enamel organ, we can now briefly follow its devel-
opment to completion. We have already seen that it consists of an
outer layer of columnar epithelium covering the convex portion, and is
connected by a slender cord with the INIalpighian layer above. It con-
sists also in part of an internal stellate reticulum which passes by means
of a layer of rounded cells (stratum intermedium) into the enlarged,
greatly-elongated prismatic cells lining the concave lower surface, which
invests the dentine organ like a cap. Before the enamel is completed
the external epithelium, the stellate reticulum, and stratum interme-
dium disappear altogether, but before this atrophy takes place the neck
or epithelial cord of the enamel organ gives rise to the tooth-germ of the
permanent tooth as a diverticulum which is developed in the same way
as the germ of the first or deciduous tooth just described.
The essential part of the enamel organ, or rather that which ulti-
mately results in the formation of enamel, consists of enamel-cells.
These, as we have said, become greatly elongated and assume the form
of regular hexagonal prisms, which agree in shape with the calcified
enamel-prisms of the complete tooth. Just as in the odontoblaf^ts of the
dentine, they are transformed into a gelatinous matrix, the nucleus dis-
appears, and calcification begins from above, the only difference being
that the enamel-prisms calcify completely, and are therefore not tubular,
Avhile in the corresponding structures of the dentine dentinal tubuli are
left. Different views have been advanced in regard to the exact desti-
nation as well as the function of the several parts of the enamel organ
spoken of above as disappearing by atrophy. As to the fate of the
external epithelium, Waldeyer holds that after the disappearance of
the stellate pulp it becomes applied to the outer surface of the enamel
as the membrane of Nasmyth, which Avould certainly seem to be its
most natural fate ; but Kolliker, Magitot, and Legros claim, on the
other hand, that it disapjiears altogether. Most authors believe that
the enamel organ is devoid of vascularity, but Beal asserts that there is
a vascular network in the stratum intermedium. If it be non-vascular,
then it is more than probable that the pulp represents stored-up pabulum
from which the requisite formative energy is derived. If vascular, it
then probably subserves a mechanical purpose only, as some authorities
believe.