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76 EMBRYOLOGY OF THE DENTAL TISSUES.

The odontoblasts are modified connective-tissue cells that superin-
tend the deposition of the caleific material which is to form the calcified
matrix. The tliiekeniiiii; of the dentin is by successive deposits of this
material in the form of layers which calcify. I'^ibrils from the odonto-
blasts remain within the formed and forminii; dentin as the persistent
organic contents of the canals. This forming of the dentin is at the
expense of the dentin germ, which is thus gradually reduced until it
becomes, Avhen the tooth is fully formed, its pulp. Thus it is seen that
dentin is a secretion in the form of calcific material coming from the
abundant blood supply in the pulp tissue near the odontoblasts. The
material is given out from the cells in a globular form (calco-spherites)
into a protoplasmic fluid, or semifluid, found everywhere against the
calcifying dentin. In this substance is the scaffolding of fine con-
nective-tissue fibers spoken of by Mr. Mummery, of London. The
calco-spherites meeting against the formed dentin coalesce into a
layer of calco-globulin, and this, becoming fully calcified, forms an
additional layer of dentin, and the process continues until the tooth
is formed.
By the deposition of calcium salts into the protoplasmic layer calco-
globulin is formed, and by its calcification the dentin tissue becomes a
homogeneous mass, penetrated by many parallel canals filled with the
persistent dentinal fibrils. Beside these parallel canals with their
fibrillar contents many lateral canals are seen branching off from the
main canals and anastomosing with neighboring canals.
Exceptions may be taken to many of the statements of histologists
in this field ; many or most mistakes are traceable to faulty methods
of technique. Processes which involve the securing of specimens
while they are yet warm are greatly preferable. These are placed in
a quarter of one per cent, to one-half of one per cent, solution of
chromic acid, wdiieh is changed several times a day, for three or four
davs. At the end of this time the eda;es of the dentin which were
calcified are found to be sufficiently softened to make a number of
sections. The teeth are then taken from the acid solution, washed in
distilled water, placed in a solution of gum arabic for several hours, and
next transferred to a solution of alcohol to abstract the water. Paraffin
and lard are melted together and poured into a convenient mould.
When this clouds in the process of cooling, the tooth, which has had its
outer surface dried as much as possible with bibulous paper, is placed in
it and the whole allowed to cool. The microtome for this purpose should
permit the immersion of both tissue and knife when the sections are
cut. These sections float off in the fluid, and remain there until used.
Sections are cut until the calcified tissue is reached. The sections
are placed in distilled water for a few minutes to dissolve out the