Page 514 - My FlipBook

524 DENTAL E3IBB.Y0L0GY AND HISTOLOGY.

mucous membraue of the mouth in the embryo is formed of a layer of
protoplasm, which is conspicuous in preparations stained with hsema-
toxylon and eosin in that it stains more darkly than the surrounding
tissue. In this protoplasmic basis-substance are found small spheroidal
cells (nuclei), sometimes arranged in regular layers ; in other cases the
dark-stained layer of protoplasm is wider, and several layers of sphe-
roidal cells exist—not arranged in strata, but presenting an irregular
appearance, and in some instances being four or five cells deej). These
spheroidal cells have no distinct cell-body or membrane, and the sur-
rounding protoplasm presents no characteristic feature. The youngest
cells of the Malpighian layer take the stains similarly to the embryonal
connective-tissue cells lying immediately beneath in the submucous
layer, and at this stage present the same shape, and can be seen in the
pig embryo 1— centimeter in length, and in the human embryo at the
thirty-fifth day.
As the nuclei are crowded up from this bed of protoplasm, they carry
with them a certain portion of protoplasm which surrounds the nucleus
as a cell-body, and as they approach the surface of the epithelium they
apparently develop a cell-wall ; in this state they present an imbricated
border Avliich unites them to their fellows. (For further description and
figures see section on Mucous Membrane of the Mouth, p. 611.)
The shape of cells depends to a great extent upon the reci})rocal
pressure of fellow-cells. This is specially noticeable in cells developed
from the epiblast and hypoblast : these may be round, oval, cylindrical,
columnar, prismatic, hexagonal, or tessellated in form. The cells devel-
oped from the mesoblast—viz. the connective-tissue group—vary from
round or oval to fusiform with numerous fibrillee. The size of a cell
may be -g-g-oth part of an inch in diameter ; some are larger, some
smaller ; the nucleus may be gQ^Q^ th of an inch in diameter ; the nucle-
olus 000 ^^^ ^^ ^^ "^^^^ ^^ diameter, more or less.
1
Physiological Consideration of Cells.
Dependent upon an inherent principle, the nature of which we have
never been able to divine, cells have a threefold character : the power
of self-preservation, of multiplication, and of functional activity.
In the first place, out of the common stock of cell-])abulum each cell
has the power to assimilate such constituents as are needed to prolong
its existence. That different cells require diiferent kinds of food, and
are able to convert the same into matter like themselves, is evidenced
by the fact tliat chemical reagents give manifestly different results on
the various cells. A simple demonstration is found in the action of
staining agents upon different tissues. Cells have, within a certain
limit, the power of overcoming deleterious agents or conditions. This
limit is not great as regards the cell itself, but for the tissue of which
it is a comjMnient part the range is much more extended.
Recovery after the loss of a portion of the cellular elements that com-
pose a tissue is generally very rapid, and depends upon that attribute
of cells we term muUiplicaiion. Increase of cells is accomjjlished by
segmentation, which, beginning in the nucleus, results in the division