THE SHAPES OF ANIMAL CELLS.
§ 260. Among animals as among plants, the laws of morphological
differentiation must be conformed to by the morphological units, as
well as by the larger parts and by the wholes formed of them. It
remains here to point out that the conformity is traceable where the
conditions are simple.
[Illustration: Fig. 294.]
In the shapes assumed by those rapidly-multiplying cells out of which
each animal is developed, there is a conspicuous subordination to
the surrounding actions. Fig. 294 represents the cellular embryonic
mass that arises by repeated spontaneous fissions. In it we see how
the cells, originally spherical, are changed by pressure against one
another and against the limiting membrane; and how their likenesses
and unlikenesses are determined by the likenesses and unlikenesses
of the forces to which they are exposed. This fact may be thought
scarcely worth pointing out. But it is worth pointing out, because what
is here so obvious a consequence of mechanical actions, is in other
cases a consequence of actions composite in their kinds and involved
in their distribution. Just as the equalities and inequalities of
dimensions among aggregated cells, are here caused by the equalities
and inequalities among their mutual pressures in different directions;
so, though less manifestly, the equalities and inequalities of
dimensions among other aggregated cells, are caused by the equalities
and inequalities of the osmotic, chemical, thermal, and other forces
besides the mechanical, to which their different positions subject them.
§ 261. This we shall readily see on observing the ordinary structures
of limiting membranes, internal and external. In Fig. 295, is shown
a much-magnified section of a papilla from the gum. The cells of
which it is composed originate in its deeper part; and are at first
approximately spherical. Those of them which, as they develop, are
thrust outwards by the new cells that continually take their places,
have their shapes gradually changed. As they grow and successively
advance to replace the superficial cells, when these exfoliate, they
become exposed to forces which are more and more different in the
direction of the surface from what they are in lateral directions; and
their dimensions gradually assume corresponding differences.
[Illustration: Fig. 295.]
[Illustration: Fig. 296.]
Another species of limiting membrane, called cylinder-epithelium,
is represented in Fig. 296. Though its mode of development is
such as to render the shapes of its cells quite unlike those of
pavement-epithelium, as the above-described kind is sometimes called,
its cells equally exemplify the same general truth. For the chief
contrast which each of them presents, is the contrast between its
dimension at right angles to the surface of the membrane, and its
dimension parallel to that surface.
It is needless for our present purpose to examine further the evidence
furnished by Histology; nor, indeed, would further examination of this
evidence be likely to yield definite results. In the cases given above
we have marked differences among the incident forces; and therefore
have a chance of finding, as we do find, relations between these and
differences of form. But the cells composing masses of tissue are
severally subject to forces which are indeterminate; and therefore
the interpretation of their shapes is impracticable. It must suffice
to observe that so far as the facts go they are congruous with the
hypothesis.