Certain cells in the developing embryo, for example, are early set apart
for a particular function or for building certain structures, and
thereafter are never diverted from this duty so as to do a different
work or produce a different kind of structure. In the young embryo
certain structures arise at certain predestined times in particular
places, and only there and out of these cells alone. As to _why_ it
should be so, we cannot tell, save as the result of deliberate design
and as an expression of the order-loving mind of the God of nature. In
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.
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