While the descent of cells from preëxisting cells by division of the
nucleus during the development of the egg, in the embryos of plants and
animals, and in adult vegetable and animal tissues, both in healthy and
diseased conditions, had now become generally recognized, the mechanism
of the process by which the cleavage of the nucleus took place was
for a long time unknown. The discovery had to be deferred until the
optician had been able to construct lenses of a higher penetrative
power, and the microscopist had learned the use of coloring agents
capable of dyeing the finest elements of the tissues. There was reason
to believe that in some cases a direct cleavage of the nucleus, to
be followed by a corresponding division of the cell into two parts,
did occur. In the period between 1870 and 1880 observations were made
by Schneider, Strasburger, Bütschli, Fol, van Beneden and Flemming,
which showed that the division of the nucleus and the cell was due to
a series of very remarkable changes, now known as indirect nuclear and
cell division, or karyokinesis. The changes within the nucleus are of
so complex a character that it is impossible to follow them in detail
without the use of appropriate illustrations. I shall have to content
myself, therefore, with an elementary sketch of the process.
I have previously stated that the nucleus in its passive or resting
stage contains a very delicate network of threads or fibers. The first
stage in the process of nuclear division consists in the threads
arranging themselves in loops and forming a compact coil within the
nucleus. The coil then becomes looser, the loops of threads shorten and
thicken, and somewhat later each looped thread splits longitudinally
into two portions. As the threads stain when coloring agents are
applied to them, they are called chromatin fibers, and the loose coil
is the chromosome (Waldeyer).
As the process continues, the investing membrane of the nucleus
disappears, and the loops of threads arrange themselves within the
nucleus so that the closed ends of the loops are directed to a common
center, from which the loops radiate outwards and produce a starlike
figure (aster). At the same time clusters of extremely delicate lines
appear both in the nucleoplasm and in the body of the cell, named the
achromatic figure, which has a spindle-like form with two opposite
poles, and stains much more feebly than the chromatic fibers. The loops
of the chromatic star then arrange themselves in the equatorial plane
of the spindle, and bending round turn their closed ends towards the
periphery of the nucleus and the cell.
The next stage marks an important step in the process of division of
the nucleus. The two longitudinal portions, into which each looped
thread had previously split, now separate from each other, and whilst
one part migrates to one pole of the spindle, the other moves to the
opposite pole, and the free ends of each loop are directed toward its
equator (metakinesis). By this division of the chromatin fibers, and
their separation from each other to opposite poles of the spindle, two
starlike chromatin figures are produced (dyaster).
Each group of fibers thickens, shortens, becomes surrounded by a
membrane, and forms a new or daughter nucleus (dispirem). Two nuclei
therefore have arisen within the cell by the division of that which had
previously existed, and the expression formulated by Flemming--_omnis
nucleus e nucleo_--is justified. Whilst this stage is in course of
being completed, the body of the cell becomes constricted in the
equatorial plane of the spindle, and, as the constriction deepens, it
separates into two parts, each containing a daughter nucleus, so that
two nucleated cells have arisen out of a preëxisting cell.
A repetition of the process in each of these cells leads to the
formation of other cells, and, although modifications in details are
found in different species of plants and animals, the multiplication of
cells in the egg and in the tissues generally on similar lines is now a
thoroughly established fact in biological science.
In the study of karyokinesis, importance has been attached to the
number of chromosomes in the nucleus of the cell. Flemming had seen
in the Salamander twenty-four chromosome fibers, which seems to be a
constant number in the cells of epithelium and connective tissues.
In other cells, again, especially in the ova of certain animals, the
number is smaller, and fourteen, twelve, four and even two only have
been described. The theory formulated by Boveri that the number of
chromosomes is constant for each species, and that in the karyokinetic
figures corresponding numbers are found in homologous cells, seems to
be not improbable.
In the preceding description I have incidentally referred to the
appearance in the proliferating cell of an achromatic spindle-like
figure. Although this was recognized by Fol in 1873, it is only during
the last ten or twelve years that attention has been paid to its more
minute arrangements and possible signification in cell-division.
The pole at each end of the spindle lies in the cell plasm which
surrounds the nucleus. In the center of each pole is a somewhat opaque
spot (central body) surrounded by a clear space, which, along with the
spot, constitutes the centrosome of the sphere of attraction. From
each centrosome extremely delicate lines may be seen to radiate in two
directions. One set extends towards the pole at the opposite end of the
spindle, and, meeting or coming into close proximity with radiations
from it, constitutes the body of the spindle, which, like a perforated
mantle, forms an imperfect envelope around the nucleus during the
process of division. The other set of radiations is called the polar
and extends in the region of the pole towards the periphery of the cell.
The question has been much discussed whether any constituent part of
the achromatic figure, or the entire figure, exists in the cell as a
permanent structure in its resting phase; or if it is only present
during the process of karyokinesis. During the development of the
egg the formation of young cells, by division of the segmentation
nucleus, is so rapid and continuous that the achromatic figure, with
the centrosome in the pole of the spindle, is a readily recognizable
object in each cell. The polar and spindle-like radiations are in
evidence during karyokinesis, and have apparently a temporary endurance
and function. On the other hand, van Beneden and Boveri were of
opinion that the central body of the centrosome did not disappear
when the division of the nucleus came to an end, but that it remained
as a constituent part of a cell lying in the cell plasm, near to
the nucleus. Flemming has seen the central body with its sphere in
leucocytes, as well as in epithelial cells and those of other tissues.
Subsequently Heidenhain and other histologists have recorded similar
observations. It would seem, therefore, as if there were reason to
regard the centrosome, like the nucleus, as a permanent constituent
of a cell. This view, however, is not universally entertained. If not
always capable of demonstration in the resting stage of a cell, it is
doubtless to be regarded as potentially present, and ready to assume,
along with the radiations, a characteristic appearance when the process
of nuclear division is about to begin.
One can scarcely regard the presence of so remarkable an appearance
as the achromatic figure without associating with it an important
function in the economy of the cell. As from the centrosome at the pole
of the spindle both sets of radiations diverge, it is not unlikely
that it acts as a center or sphere of energy and attraction. By
some observers the radiations are regarded as substantive fibrillar
structures, elastic or even contractile in their properties. Others,
again, look upon them as morphological expressions of chemical and
dynamical energy in the protoplasm of the cell body. On either theory
we may assume that they indicate an influence, emanating, it may be,
from the centrosome and capable of being exercised both on the cell
plasm and on the nucleus contained in it. On the contractile theory,
the radiations which form the body of the spindle, either by actual
traction of the supposed fibrillæ or by their pressure on the nucleus
which they surround, might impel during karyokinesis the dividing
chromosome elements toward the poles of the spindle, to form there the
daughter nuclei. On the dynamical theory, the chemical and physical
energy in the centrosome might influence the cell plasm and the nucleus
and attract the chromosome elements of the nucleus to the poles of the
spindle. The radiated appearance would therefore be consequent and
attendant on the physico-chemical activity of the centrosome. One or
other of these theories may also be applied to the interpretation of
the significance of the polar radiations.