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528 DENTAL EMBRYOLOGY AND HISTOLOGY. '

this condition so as to adapt them for assimilation by itself. The cell
does not merely take in and give out material ; it acts ' catalytically
on its environment. This is proof at least that the cell possesses a high
degree of spontaneity—that it has the power of making more available
for its own sustenance the various forms of nutriment that come in its way.
" It is also of great interest to note that the cell is ultimately limited in
its formative activity by its own products. When the amount of nutri-
ment present is abundant, the activity of the cell comes to an end, not
through the exhaustion of the supply, but through its contamination
with certain products of cell-metabolism.
" Many of the substances engendered in fermenting liquids by the
action of fungi tend to check the growth and nndtiplication of the
fungi themselves ; \\hen present in quantity they may put a stop to
multiplication altogether.
The alcoholic lermentation, and the multiplication of the yeast-plant
''
which produces it, come to an end when a certain proportion of alcohol
has been generated in the fermenting liquid. In septic putrefaction the
bacteria generate compounds, such as carbolic acid, which are destructive
to themselves. If we may apply these facts of fungus physiology to
the cell physiology of higher organisms, M'e find that they illustrate,
first of all, this principle : that the quantity and quality of the nutritive
material at the disposal of the cell have a profound influence upon its
behavior ; and, secondly, this other : that the cell has nevertheless an
intrinsic power of utilizing this material, and of appropriating what is
suitable to itself out of various combinations. Lastly, the limits im-
posed on the midtiplication of fungi by the products of their o\\n\
activity may help us to understand how the formative activity of the
cells of complex organism may be temporarily checked.
" We cannot, indeed, regard the intercellular substance of the connec-
tive tissues as equivalent in significance to the products of the chemical
changes induced by the bacteria. Yet the comparison may at least
enable us to conceive how^ cell-growth may tend to limit and to check
itself without the interposition of extensive resistance. In the connec-
tive tissues the formation of the intercellular substance is the limiting
factor ; in the epithelia it is the cohesion or cementation of the individ-
ual cells into a firm and single whole, just as in yeast fermentation it
is the formation of alcohol. When the alcohol is withdrawn in the
latter case, the nudti])lication of the yeast-fungus goes on again. So,
likewise, if the intercellular substance be dissolved away from a connec-
tive tissue, or if the continuity of the epithelial mosaic be loosened or
interrupted, the faculty of multiplication is again awakened in the con-
stituent cells ; or if (as in the epithelia) it has never been dormant, it is
at once intensified."
Cellular activity, as found in normal development, depends upon some
force which cannot be explained by chemistry and physics or ^vithout
calling in the aid of the hypothesis of vita/ fimcHons. Cells stimulated
by the indwelling vital power proliferate until the typal demands of the
tissues are reached. In many tissues this growth is so adjusted as to be
self-limiting, as in the Plaversian system of bones, in which the devel-
opment is centripetally arranged, thus lessening the calibre of the enclosed