Claire Haller prismx at
Fri Nov 24 23:04:33 EST 2000

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SW REPORTS November 20, 2000

     The term "cell theory" refers to the proposition that all
forms of life (on Earth) are composed of cells, and that cells
are the simplest units to exhibit the functions characteristic of
living systems. Cell theory is a "theory" in the same sense that
atomic theory is a theory in the physical sciences -- the
proposition is universally accepted by biologists, and the
extensions flowing from the theory embrace all of modern biology
with widely diverse and ramifying implications. (A similar
statement can be made concerning so-called evolutionary
"theory".) Cell theory is a unifying concept that has provided
direction for the analysis of fundamental biological problems
such as reproduction, sexuality, development, heredity,
evolution, metabolism, coordination, growth, and numerous equally
basic biological phenomena.
     Although cursory treatments of cell theory often focus on
cell theory as discussed in the 19th century, in actuality the
theory originated in the 17th century, when researchers first
noted the existence of cells, and the theory passed through
various stages of understanding during the following 300 years.
The various forms of cell theory have included an elementary
conception of basic microscopic units in the mid-17th century, a
more fully articulated cell theory in the early 19th century,
which held that cells are the basic building blocks of living
organisms, a later 19th century conviction that the actions of
cellular material actually bring about organic development and
differentiation, and the idea that cells hold the key to
evolutionary development as well. Controversies and disagreements
about detail have occurred, but some version of cell theory and
the fundamental role of cells in development has persisted, and
the general idea of cells as the basic functional units of living
systems remains the foundation of modern biology.
... ... Paul Nurse (Imperial Cancer Research Fund, UK) presents a
review of the history of cell theory, the author making the
following points:
     1) The discovery of biological cells followed quickly after
the invention of the microscope in the 17th century. Robert Hooke
(1635-1703) published drawings of sections of cork in 1665, first
calling the observed walled cavities "cells". Within a few years,
Nehemiah Grew (1641-1712) and Marcello Malpighi (1628-1694)
published studies that led to the view that plant tissues are
mostly composed of aggregates of cells, and later in the 17th
century, Malpighi, Anton van Leeuwenhoek (1632-1723), and Jan
Swammerdam (1637-1680) first recognized cells in animals. These
microscopists described corpuscles in blood -- no one at that
time proposed that solid animal tissues were also made of cells:
that proposition was not made until the 19th century, and in
particular by Theodor Schwann (1810-1882) and Matthias Schleiden
     2) Although Schleiden and Schwann correctly articulated the
cell theory, their ideas concerning the formation of cells were
wrong: they believed cells arose by processes similar to
precipitation or crystallization. Others, particularly Robert
Remak (1815-1865), recognized that cells arose from preexisting
cells by a process of binary fission. This view was championed by
Rudolf Virchow (1821-1902), who popularized the phrase "all cells
come from cells."
     3) Most cells contain a single nucleus that reproduces
during mitosis and cell division. Elongated chromosomal threads,
described by Walther Flemming (1843-1905) and Eduard Strasburger
(1844-1912) in the 1880s, were observed to split lengthwise
before shortening and thickening as mitosis proceeds. The
longitudinal halves then separate into the two daughter nuclei.
Approximately the same time as these observations were made,
Edouard van Beneden (1846-1910) demonstrated that the chromosomes
in a fertilized roundworm (nematode) egg are derived in equal
numbers from the egg and the sperm. That led August Weismann
(1834-1914) to propose that the hereditary system is based on the
     4) The author concludes: "The cell is the simplest unit to
exhibit life's functions. We now have both the molecular tools
and the conceptual frameworks to undertake a concerted program to
understand how cells operate. The genome projects will anchor
that foundation by identifying all the genes required for a cell
to function, yet researchers will still have to work out how the
relevant gene products act and interact to generate cellular
organization... The coming years will be exciting ones during
which new ideas and theories will help us fully understand cells
and thereby life itself."
Paul Nurse: The incredible life and times of biological cells.
(Science 8 Sep 00 289:1711)
QY: Paul Nurse, Imperial Cancer Research Fund, London, UK.
Summary by SCIENCE-WEEK 6Oct00
For more information:
Related Background:
"When organic nature, animals and plants, is regarded as a
Whole, in contradistinction to the inorganic kingdom, we do not
find that all organisms and all their separate organs are compact
masses, but that they are composed of innumerable small particles
of a definite form. These elementary particles, however, are
subject to the most extraordinary diversity of figure, especially
in animals; in plants they are, for the most part exclusively,
cells. This variety in the elementary parts seems to hold some
relation to their more diversified physiological function in
animals, so that it might be established as a principle that
every diversity in the physiological signification of an organ
requires a difference in its elementary particles; and, on the
contrary, the similarity of two elementary particles seems to
justify the conclusion that they are physiologically similar...
The greater the number of physiologically different elementary
parts, which, so far as can be known, originate in a similar
manner, and the greater the difference of these parts in form and
physiological signification, while they agree in the perceptible
phenomena of their mode of formation, the more safely we may
assume that all elementary parts have one and the same
fundamental principle of development... The elementary parts of
all tissues are formed of cells in an analogous, though very
diversified manner, so that it may be asserted, that there is one
universal principle of development for the elementary parts of
organisms, however different, and that this principle is the
formation of cells." [*Note #1]
Theodor Schwann: _Microscopical Researches into the Accordance in
the Structure and Growth of Animals and Plants_.
(Sydenham Society, London 1847, transl. Henry Smith, from the
original German published in 1839)
Text Notes:
... ... *Note #1: The "cell theory" is probably the most
important biological generalization of the first half of the 19th
century, a generalization that has grown in importance and which
serves as a unifying principle in the continued development of
modern biology. A number of biologists had been writing about the
cellular organization of animals and plants, but it was Theodor
Schwann (1810-1882) and Matthias Schleiden (1804-1881) who most
clearly stated and summarized the case for the cell theory,
Schwann for animals and Schleiden for plants. Although a major
weakness of the theory was its proposition that the formation of
cells involved the appearance of a nucleus first and the
remainder of the cell afterward, the general idea of cell theory,
that of individual physiological entities ("cells") as the
fundamental units of biological systems, was a correct and
profound conceptual contribution. Schwann also apparently coined
the term "metabolism" to represent the overall chemical changes
occurring in living systems. He also did important work on
digestion, fermentation, and histology. He identified yeast as
consisting of tiny plant-like organisms, and he was one of the
first to propose that fermentation of sugar and starch was the
result of a life process, a proposition that provoked so much
scientific criticism in Germany that Schwann left Germany and
moved to Belgium. He became professor of anatomy at Louvain in
1838 and at Liege in 1847. It is ironic that in the last 40 years
of his life he devoted most of his energies to mysticism and
religious meditation, doing nothing to match his earlier
intellectual and scientific accomplishments in the one decade of
the 1830s.
Notes by SCIENCE-WEEK 17Sep99
For more information:


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