CELLULAR THEORY. — With the discovery of the cell, the idea matured that all animals and plants are constituted of cells.
However, it was only in 1838 that Schleiden, for plants, and in 1839 Schwann, for animals, formulated this conception, which was defined as the cell theory or doctrine, namely: metazoans and metaphytes must be considered to be constituted by a colony of simple organisms, the cells, and the life of the whole is given by the sum of the elementary lives. Protozoans and proto phytes represent organisms formed by a single cell. In 1850 Milne-Edwards introduced the concept of the division of labor, whereby the cells of the various tissues specialize in a specific task, assuming the form most suited to its accomplishment (histological differentiation). With the discovery of cell division, made in 1850 by Mohl in filamentous algae and subsequently confirmed by Naegeli in higher plants and by Vogt, Kölliker, Remak, etc., in animals, it was understood that the colony originated through the successive division of an initial progenitor cell, namely the egg. Virchow, the great German pathologist, formulated the well-known aphorism: «Omnis cellula a cellula» and regarded diseases as cellular alterations that would manifest themselves during division.
The cellular theory, as it was formulated, found in Haeckel a fervent supporter, since it accorded very well with the transformist evolutionary idea, placing the protozoan and man on the same constitutive level.
The colonial conception of higher forms was, however, soon criticized by such great minds of biology as Wolfgang Goethe and later Driesch. Indeed, the life of the whole that constitutes a higher individual is something more than the sum of the lives of its elementary components. There is a higher-order life that is not constructed by the parts but rather is superimposed upon them, dominates them, and regulates them. De Bary, among botanists, said that it is not the cells that make the trees, but the trees that make the cells, signifying precisely the dominance of the higher unity over the activities of the individual cells composing II. But the cellular theory, toward the end of the nineteenth century, was also attacked on a purely structural level. The various cellular organelles first began to be criticized, being regarded as artifacts of histological technique, and thus it was demonstrated that the structure of the cytoplasm (granular, filamentous, alveolar) assumed various aspects depending on the fixative used (Hardy and Fischer, 1899). Finally, the unity of the cells themselves was attacked, though this led to excesses. Thus Rhode, at the beginning of this century, produced a histology without ever speaking of cells. Plasmodesmata (protoplasmic bridges between cell and cell), plasmodial tissues (in which there is division of the nucleus without division of the cytoplasm), syncytial tissues (formed by the fusion of cells into undivided protoplasmic masses), and tissues in which the intercellular parts predominate (such as connective tissues) have been regarded as evidence against the cell doctrine.
Organisms not subdivided into cells have also been described, such as certain algae (Caulerpa), and acellular organisms have been produced experimentally, such as Lillie’s larvae of Caetopterus.
Thus protozoans came to be regarded not as a single cell but as a much more complex, noncellular organization. Other acellular conditions among living beings, such as bacteria and viruses, further limit the universal concept of this doctrine. This extremist viewpoint against the cellular theory must, however, today be regarded as superseded by the evidence of other facts.
If there is no longer any doubt that a higher life exists which is far more than the sum of the lives of the cells, and which is not formed by them but rather dominates and governs them, there is likewise no doubt that the cells composing the organism have a life of their own, independent, which manifests itself especially when their relations with the higher unity cease.
The first demonstrations in this regard may be considered those given in 1897 by Tirelli, who studied the survival of tissues in the corpse. After the death of the whole, after the extinction of the higher life, the various cells continue to live for a certain time and to varying degrees, depending on the tissues. They ultimately die as well, because the cessation of collective organic functions no longer permits the exchanges at the level of the individual cells that are necessary for their life.
Still more demonstrative, however, is the method of tissue culture, which consists in isolating a fragment of tissue from an organism in vitro and cultivating it, nourishing and washing it in order to remove the products of metabolism. This method, perfected by Carrel, made it possible to keep the cells of a chicken embryo alive and in continuous reproduction for approximately twenty years, that is, for a period much longer than they would have lived had they remained part of the chicken itself. The extinction of the strain was due to purely accidental causes. Thus it became possible to cultivate still-living cells taken from a corpse and make them multiply actively.
It was thereby seen that the proliferative activity of cells is much greater in culture, isolated from the higher organism, than in the organism itself. This confirms the regulating action of the higher individuality upon the activity of the individual elements. Theoretically, the life of cells is indefinite: it is their reciprocal bond within the soma of the higher individuality that condemns the cells to die with II. The cellular theory is therefore not dead, but must be modified in its formulation.
It must be admitted that organisms with a noncellular organization exist, such as bacteria, viruses, and certain other inferior forms (blue-green algae). Protozoans, although they have a cellular organization, are something more than a cell, and this is related to their independent life. Metazoans and metaphytes are multicellular insofar as they arise from the successive division of an initial cell (egg and oosphere). In the earliest embryonic stages one may perhaps speak of a true colony of cells, but as development proceeds, a higher individuality is established that dominates the reproduction and activity of the components. With development, the various cells, through the division of labor, differentiate into the different tissues. In this process, the cells may also lose their anatomical individuality and unite into more extensive masses, as, for example, in striated muscle fibers, if functionality requires it, or produce extracellular structures that eventually come to predominate over the cells themselves, as in connective tissues; but these are formations always dependent upon the cellular condition.
The multicellular organism is therefore not a colony of cells but rather an association. One may conclude with Cotronci (on p. 9 of the op. cit. in the bibliography): «L'organismo vivente è dunque una associazione di elementi dotati di relativa autonomia che costituiscono una entità di ordine superiore che domina e dirige la vita dei singoli elementi. Questa dipendenza della vita dei singoli elementi dalla vita dell'organismo, possiamo ritenere sia tanto più elevata quanto più elevato è il grado di organizzazione».