Although von Mohl, the botanist, seems to have been the first to
recognize (1835) in plants a multiplication of cells by division,
it was not until attention was given to the study of the egg in
various animals and to the changes which take place in it, attendant
on fertilization, that in the course of time a much more correct
conception of the origin of the nucleus and of the part which it
plays in the formation of new cells was obtained. Before Schwann had
published his classical memoir in 1839, von Baer and other observers
had recognized within the animal ovum the germinal vesicle, which
obviously bore to the ovum the relation of a nucleus to a cell. As the
methods of observation improved, it was recognized that, within the
developing egg, two vesicles appeared where one only had previously
existed, to be followed by four vesicles, then eight, and so on in
multiple progression until the ovum contained a multitude of vesicles,
each of which possessed a nucleus. The vesicles were obviously cells
which had arisen within the original germ-cell or ovum. These changes
were systematically described by Martin Barry so long ago as 1839
and 1840 in two memoirs communicated to the Royal Society of London,
and the appearance produced, on account of the irregularities of the
surface occasioned by the production of new vesicles, was named by him
the mulberry-like structure. He further pointed out that the vesicles
arranged themselves as a layer within the envelope of the egg or zona
pellucida, and that the whole embryo was composed of cells filled
with the foundations of other cells. He recognized that the new cells
were derived from the germinal vesicle or nucleus of the ovum, the
contents of which entered into the formation of the first two cells,
each of which had its nucleus, which in its turn resolved itself
into other cells, and by a repetition of the process into a greater
number. The endogenous origin of new cells within a preëxisting cell
and the process which we now term the segmentation of the yolk were
successfully demonstrated. In a third memoir, published in 1841, Barry
definitely stated that young cells originated through division of
the nucleus of the parent cell, instead of arising, as a product of
crystallization, in the fluid cytoblastema of the parent cell or in a
blastema situated external to the cell.
In a memoir published in 1842, John Goodsir advocated the view that
the nucleus is the reproductive organ of the cell, and that from it,
as from a germinal spot, new cells were formed. In a paper, published
three years later, on nutritive centers, he described cells, the nuclei
of which were the permanent source of successive broods of young cells,
which from time to time occupied the cavity of the parent cell. He
extended also his observations on the endogenous formation of cells
to the cartilage cells in the process of inflammation and to other
tissues undergoing pathological changes. Corroborative observations on
endogenous formation were also given by his brother, Harry Goodsir,
in 1845. These observations on the part which the nucleus plays by
cleavage in the formation of young cells by endogenous development from
a parent center--that an organic continuity existed between a mother
cell and its descendants through the nucleus--constituted a great
step in advance of the views entertained by Schleiden and Schwann,
and showed that Barry and the Goodsirs had a deeper insight into the
nature and functions of cells than was possessed by most of their
contemporaries, and are of the highest importance when viewed in the
light of recent observations.
In 1841 Robert Remak published an account of the presence of two nuclei
in the blood corpuscles of the chick and the pig, which he regarded as
evidence of the production of new corpuscles by division of the nucleus
within a parent cell; but it was not until some years afterwards (1850
to 1855) that he recorded additional observations and recognized that
division of the nucleus was the starting-point for the multiplication
of cells in the ovum and in the tissues generally. Remak’s view was
that the process of cell division began with the cleavage of the
nucleolus, followed by that of the nucleus, and that again by cleavage
of the body of the cell and its membrane. Kölliker had previously, in
1843, described the multiplication of nuclei in the ova of parasitic
worms, and drew the inference that in the formation of young cells
within the egg the nucleus underwent cleavage, and that each of
its divisions entered into the formation of a new cell. By these
observations, and by others subsequently made, it became obvious that
the multiplication of animal cells, either by division of the nucleus
within the cell, or by the budding off of a part of the protoplasm
of the cell, was to be regarded as a widely spread and probably a
universal process, and that each new cell arose from a parent cell.
Pathological observers were, however, for the most part inclined
to consider free cell-formation in a blastema or exudation by an
aggregation of molecules, in accordance with the views of Henle,
as a common phenomenon. This proposition was attacked with great
energy by Virchow in a series of memoirs published in his ‘Archiv,’
commencing in Vol. 1, 1847, and finally received its death-blow in his
published lectures on Cellular Pathology, 1858. He maintained that
in pathological structures there was no instance of cell development
_de novo_; where a cell existed, there one must have been before.
Cell-formation was a continuous development by descent, which he
formulated in the expression _omnis cellula e cellulâ_.