Every Cell from a Cell. The early formulations of
the Cell Theory, especially in the classic form stated
by Matthias Schleiden and Theodor Schwann in 1839,
were not helpful to the development of the concept
of genetic continuity. At the turn of the century (1802)
K. Sprengel had thought that cells originate inside of
other cells in the form of granules or vesicles. He
probably mistook starch grains for newly forming cells.
Nevertheless, and in spite of criticism by others, this
mistaken idea was adopted by others and was accepted
even by Schleiden himself as late as 1849. As for
Schwann, he seems to have gotten his ideas of the
formation of new cells from the notions of Christian
Friedrich Wolff almost a century earlier (1759; 1768).
Schwann, in brief, thought that new cells might form
outside existing cells in the midst of a ground substance
supposed to exist between the cells, or alternatively
that they might form inside of the older cells by a kind
of crystallization from the mother liquor. Better ideas
of the genetic continuity of cells were to be based upon
the discovery made by Robert Brown, in 1831, that
a nucleus is a regular feature of each cell in a flowering
plant. (It is not true, though often so stated, that Robert
Brown discovered the nuclei of cells. They had been
seen many times before. What he actually did was to
develop a general concept of the essentiality of the
nucleus for the cell.) Schleiden and Schwann recog-
nized the importance of this concept, and Schwann's
work on animal cells, such as the cells of the notochord
and developing cartilage in embryos, made it possible
to extend the concept to the cells of animals as well
as of plants.
Cell division had already been observed carefully
and critically by a number of workers: by J. P. F.
Turpin (1826) and B. C. Dumortier (1823) in filamen-
tous algae, by Hugo von Mohl (1835-39) in filamentous
algae and in the club moss Anthoceros; by J. Meyen
(1830) in green algae, the mycelia of molds, and the
terminal buds and root tips of flowering plants; and
by C. G. Ehrenberg (1833) in the fission of various
protozoans. It was especially Meyen and von Mohl who
most vigorously opposed the views of cell formation
put forward by Schleiden and Schwann and who
maintained that on the contrary cells arise by self-
division. Over the two decades from 1840 to 1860,
these views were supported on the botanical side by
F. Unger and Carl Nägeli, and on the zoological side
by A. Kölliker, R. Remak, and Rudolf Virchow. These
men first succeeded in obtaining an admission that cells
do arise by division, and ultimately that they arise only
in that manner. Virchow's aphorism, so often quoted
—
Omnis cellula e cellula—merely put a period to the
long dispute. It is very significant that both Remak and
Virchow opposed Schleiden's and Schwann's idea of
free cell formation because they regarded it as equiva-
lent to spontaneous generation.
Great changes in point of view rarely occur abruptly.
Although Virchow's and Remak's views eventually
carried the day and laid the foundation for the concept
of cellular continuity that is a basic corollary of overall
genetic continuity, the arguments continued for some
time after 1855. There were still many biologists who
believed that while cells might arise by division of
preexisting cells, they could also arise by free cell
formation. But slowly the increasing weight of evi-
dence and scientific opinion prevailed.
One of the most important early observations made
on the nature of cell divison was Nägeli's observation
that the nuclei of the two daughter cells are derived
from the division of the parent nucleus. (He saw this
in the stamen hairs of the spiderwort Tradescantia, still
a classic material for demonstrations of mitosis to biol-
ogy students of all ages.) Nägeli, however, thought that
division of the nucleus was exceptional. By laborious
and careful work Wilhelm Hofmeister (in 1848-49),
using the same material, detected the breakdown of
the nuclear membrane prior to divison of the cell, and
with remarkable clarity he figured the presence of a
cluster of what were later to be called chromosomes.
According to his observations these separated into two
groups, each of which became reconstituted into one
of the daughter nuclei. Considering that all of this was
observed without the benefit of staining and with the
imperfect microscopes of the time, it was a truly re-
markable achievement. But the fact that others were
unable to see nearly as much left them unconvinced
that Hofmeister was correct.
It was the zoologists, who were working largely with
separate dividing cells, such as blood cells in the chick
embryo or the dividing cells of newly fertilized eggs
of marine invertebrates, who seem first to have become
convinced that nuclear division is invariably a part of
cell division. Remak saw the chick's blood cells in late
stages of division, when connected by a narrow stalk,
and he observed that a fine thread connected the
daughter nuclei. He also figured the star-shaped asters
in some dividing cells. Some of the animal cytologists
became convinced that the original nucleus becomes
dissolved in the course of each cell division, and that
the daughter nuclei are reconstituted within each
daughter cell; but by 1852 Remak concluded that the
nuclear material does in fact persist from one cell
generation to the next. A most remarkable failure of
interpretation at this time was that of E. G. Balbiani,
who in 1861 was one of the very first biologists to apply
a fixative and then a stain, carmine, to produce a degree
of selective staining of different parts of the cell.
Observing ciliate protozoans during their conjugation,
he was misled into thinking of them as animals with
organ systems analogous to those of multicellular ani-
mals. Thus he interpreted the micronuclei as the
“testes” of the protozoan and completely missed the
significance of the beautiful examples of mitosis which
he actually saw and figured.
To sum up, by 1870 it was generally believed that
cells arise only from parent cells, but the origin of the
daughter nuclei from a parent nucleus remained in
some doubt because of the dissolution of the parent
nucleus at the commencement of cell division. What
was needed was a clear and unmistakable sign that the
principal bodies within the nucleus, namely, the chro-
mosomes, possess their own genetic continuity.
