, however, the first propounders of
the cell theory (Schleiden and Schwann) had no clear or accurate idea of
the origin of cells, or of their essential characters and structure. As
to origin, they supposed that cells arose by a sort of crystallization
from a mother liquor; and as to structure, they looked upon the
cell-wall as the really important part, the fluid contents being quite
subordinate. Hugo von Mohl (1846) applied to the fluid contents of the
cell the term "protoplasm," and Max Schultze (1861) showed that this
protoplasm is really identical in all organisms, plants and animals,
also that the cell-wall is frequently absent in many animal tissues and
in many unicellular forms, indicating that the protoplasm is the really
important substance. By this time also it had become known that cells
never arise _de novo_, as had been supposed by the earlier
investigators, but that cells arise only by division of preexisting
cells; or as Rudolf Virchow (1858) expressed it, "_omnis cellula e
cellul[=a]._"

It was, however, many years before the details of the growth and
reproduction of the cells (cell-division) became well understood. Not
until the last quarter of the nineteenth century was it settled that the
nucleus of the cell is also a supremely important part; but finally in
1882 Flemming was able to extend Virchow's aphorism to the nucleus also:
_omnis nucleus e nucleo_.

Since these discoveries our knowledge of the methods of cell-division
has much increased; and in the light of our modern knowledge of these
matters there is nothing in all nature more marvellous than the regular
orderly way in which cells reproduce themselves according to fixed laws.
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 m