But we must proceed to examine the behavior of the various kinds of
cells of which the various multicellular organisms are composed.
Plants were known to be composed of cells, and their cells were studied
and described some years before it was understood that animals also are
composed of cells as units. Even then, 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 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
different type.
This is a very important principle, and it contains so many lessons for
us bearing on the philosophy of life in general that it may be allowable
to establish this fact by several somewhat lengthy quotations from
standard authorities.
The first will be from one of the highest authorities on embryology,
Charles Sedgwick Minot, of Harvard:
"In accordance with this law [of differentiation] we encounter no
instances, _either in normal or pathological development_, of the
transformation of a cell of one kind of tissue into a cell of another
kind of tissue; and further we encounter no instances of a
differentiated cell being transformed back into an undifferentiated cell
of the embryonic type with varied potentialities."[12]
Again, we have the following from one of the foremost pathologists, as
to the strict and rather narrow limits of even pathologic change:
"Epithelium and gland cells ... never become converted into bone or
cartilage, or vice versa; while, again, it may be laid down that among
epiblastic and hypoblastic tissues, on the one hand, and mesoblastic
tissues on the other, there is no new development or _metaplasia_ of the
most highly specialized tissues from less specialized tissues; a simple
epithelium cannot in the vertebrate give rise to more complex glandular
tissue, or to nerve cells; in regeneration of epithelium there is no new
formation of hair roots or cutaneous glands. The cells of white fibrous
connective tissue have not been seen to form striated or even
non-striated muscle."[13]
[Footnote 12: _Science_, March 29, 1901, p. 490.]
[Footnote 13: J.G. Adami, "Principles of Pathology," pp. 641-642.]
As implied by these quotations, a constant and progressive
differentiation of cells prevails in the developing embryo; and when
complete, certain groups of cells act as specialists in doing only
certain kinds of work for the body. These cells maintain their specific
characters in a very remarkable degree under normal conditions. Under
various abnormal conditions, however, these cells may become modified as
to functions, so that cells or tissues of one type may assume more or
less completely the characters of another type. "But," as a very high
authority declares, "the limitations in this change in type are strictly
drawn, so that one type can assume only the characters of another which
is closely related to it. This change of one form of closely related
tissue into another is called _metaplasia_....
"When differentiation has advanced so that such distinct types of tissue
have been formed as connective tissue, epithelium, muscle, nerve, _these
do not again merge through metaplasia. There is no evidence that
mesoblastic tissues can be converted into those of the epiblastic or
hypoblastic type, or vice versa_."[14]
[Footnote 14: Delafield and Prudden, "Text-Book of Pathology," pp. 62,
63.]
This modification of function among the cells which sometimes goes on in
the developing embryo, or under pathologic conditions, is very closely
analogous to the variation which goes on among species of animals and
plants. But, as we shall see later, there is a well marked limit to this
variation among species, just as we see there is in the variations among
the cells. Practically the same general laws hold good in each case.
If cells did not maintain their ancestral characters in a very
remarkable way, what would be the use of grafting a good kind of fruit
onto a stock of poorer quality? The very permanency of the grafts thus
produced is proof of the persistency with which cells reproduce only
"after their kind."