the words of one of the greatest of modern authorities, "We still do not
know why a certain cell becomes a gland-cell, another a gangleon-cell;
why one cell gives rise to smooth muscle-fiber, while a neighbor forms
voluntary muscle.... It is daily becoming more apparent that epigenesis
with the three layers of the germ furnishes no explanation of
developmental phenomena."[11]

[Footnote 11: _Nature,_ May 23, 1901.]

In accordance with the general principle of a division of labor, certain
cells become early set apart to particular functions, and in accordance
with the varying demands of these functions the developing cells may
become greatly changed in form and in vital characteristics. That is,
one cell specializes, let us say, in secretion, another in
contractility, another in receiving and carrying stimuli, etc. In this
way we will have the gland-cell, the muscle-cell, and the nerve-cell,
each cell destined to produce one of these organs developing others
"after its kind," the result being that it is soon surrounded with
numerous companions doing a similar work, making up in this way a
particular tissue or organ--gland, muscle, or nerve--which in the
aggregate has for its function the work of the particular cells
composing it.

But the important thing for us to remember in this connection is that
when cells once become thus differentiated off and dedicated to any
particular function, _they can never grow or develop into any distinctly
different type of cell with other and different functions_. It is true
that through pathologic degeneration the form and even the function of
cells may become greatly changed; but never does it amount to a complete
metamorphosis or complete transformation into another distinctly