CELLULAR THEORY. — With the discovery of the cell, the idea of the cellular constitution of all animals and plants matured. However, it was only in 1838 that Schleiden formulated this conception for plants, and in 1839 that Schwann did so for animals; it was defined as the cell theory or doctrine, namely: metazoans and metaphytes must be considered as constituted by a colony of simple organizations, the cells, and the life of the whole is given by the sum of the elementary lives. Protozoans and protophytes represent organizations formed by a single cell.
In 1850, Milne-Edwards introduced the concept of the division of labor, whereby the cells of various tissues specialize for a particular function, assuming the most suitable form for its execution (histological differentiation). With the discovery of cell division, made in 1850 by Mohl in filamentous algae and then confirmed by Naegeli in higher plants and by Vogt, Kölliker, Remak, etc. in animals, it was understood that the colony originated from the successive division of an initial ancestral cell, such as the egg. The German pathologist Virchow formulated the famous aphorism: «Omnis cellula a cellula» and considered diseases as cellular alterations that manifest themselves during divisions.
The cell theory, as formulated, found a fervent supporter in Haeckel, since it accorded very well with the evolutionary idea, linking protozoans and humans on the same constitutive plane.
However, the colonial conception of higher forms was soon criticized by great minds in biology, such as Wolfgang Goethe and later Driesch. Indeed, the life of the whole, which constitutes a higher individuality, is something more than the sum of the lives of the elementary components. There is a life of a higher order that is not constructed from the parts but rather superimposes itself upon them, dominating and regulating them. Among botanists, De Bary stated that it is not the cells that make trees, but trees that make cells, precisely signifying the dominance of the higher unity over the activities of the individual cells that compose II.
But toward the end of the 19th century, the cell theory was also attacked on a purely structural level. Criticism began to be directed at the various cell organelles, considered artifacts of histological technique, and it was shown that the structure of the cytoplasm (granular, filamentous, alveolar) assumed various aspects depending on the fixative used (Hardy and Fischer, 1889). Finally, the very unity of cells was attacked, though this led to excesses. Thus, at the beginning of this century, Rhode produced a histology without ever mentioning cells. Plasmodesmata (protoplasmic bridges between cells), plasmodial tissues (in which nuclear division occurs without cytoplasmic division), syncytial tissues (formed by the fusion of cells into undivided protoplasmic masses), and tissues in which intercellular parts prevail (such as connective tissues) were considered as evidence against the cell doctrine.
Moreover, organisms not divided into cells were described, such as some algae (Caulerpa), and acellular organisms were produced experimentally, such as the larvae of Caetopterus by Lillie. Protozoans were thus considered not as a single cell but as a much more complex, non-cellular organization. Other acellular conditions of living beings, such as bacteria and viruses, further limited the universal concept of this doctrine. This extreme point of view against the cell theory must, however, today be considered outdated in light of other evidence.
While there is no longer any doubt that a higher life exists that is far more than the sum of the lives of the cells and is not formed by them but rather dominates and governs them, there is likewise no doubt that the cells composing the organism have an independent life of their own, which manifests itself especially when their relations with the higher unity cease.
Early demonstrations in this regard can be considered those given in 1897 by Tirelli, who studied the survival of tissues in the cadaver. After the death of the whole organism, after the extinction of higher life, the various cells continue to live for a certain time and to varying degrees depending on the tissues. Eventually, they too die because the cessation of collective organic functions no longer permits the exchanges at the level of individual cells necessary for their life.
But even more demonstrative is the method of tissue culture, which consists of isolating a fragment of tissue from an organism in vitro and cultivating it, nourishing and washing it to remove metabolic waste products. This method, perfected by Carrel, made it possible to keep cells from a chicken embryo alive and continuously reproducing for about twenty years, a period much longer than they would have lived had they remained part of the chicken itself. The extinction of the strain occurred due to purely accidental causes. Thus, cells taken from a cadaver were also cultivated and made to multiply actively.
It was thus observed that the proliferative activity of cells is much greater in culture, isolated from the higher organism, than within the organism itself. This confirms the regulatory action of the higher individuality on the activity of individual elements. Theoretically, the life of cells is indefinite: it is their reciprocal bond within the body of the higher individuality that condemns the cells to die with II. The cell theory is therefore not dead but must be modified in its formulation.
It must be admitted that there are organisms with non-cellular organization, such as bacteria, viruses, and some other lower forms (blue-green algae). Protozoans, although they have a cellular organization, are something more than a single cell, and this is related to their independent life. Metazoans and metaphytes are multicellular in that they arise from the successive division of an initial cell (egg and oosphere). In the early embryonic stages, one might 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. As development progresses, the various cells, through the division of labor, differentiate into different tissues. In this process, cells may even 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 formations are always dependent on the cellular condition.
The multicellular organism is therefore not a colony of cells but rather an association. We can conclude with Cotroni (p. 9 of the work cited in the bibliography): «The living organism is thus an association of elements endowed with relative autonomy that constitute an entity of higher order which dominates and directs the life of the individual elements. This dependence of the life of the individual elements on the life of the organism can be considered all the more elevated the higher the degree of organization.»