§ 1 The Cellular Theory. First Period: Division of the Organism Into Cells
Cellular Theory. First Period.—Morphological unity results from the existence of a universal anatomical basis, the cell. The cellular theory sums up the teaching of general anatomy or histology.
At the beginning of the nineteenth century anatomy was following a routine dating from ancient times. It divided animal and vegetable machines into units in descending order, first into different forms of apparatus (circulatory, respiratory, digestive, etc.); then the apparatus into organs examined one by one, figuring and describing each of them from every point of view with scrupulous accuracy and untiring patience. If we think of the duration of these researches—the Iliad, as Malgaigne says, already containing the elements of a very fine regional anatomy—and especially of the powerful impulse they received in the seventeenth and eighteenth centuries, we shall understand the illusion of those who, in the days of X. Bichat, could fancy that the task of anatomy was almost ended.
As a matter of fact this task was barely begun, for nothing was known of the intimate structure of the organs. X. Bichat accomplished a revolution when he decomposed the living body into tissues. His successors, advancing a step in the analysis, dissociated the tissues into elements. These elements, which one would have thought were infinitely varied, were reduced in their turn to one common prototype, the cell.
The living body, disaggregated by the histologist, resolves under the microscope into a dust, every grain of which is a cell. A cell is an anatomical element the constitution of which is the same from one part to the other of the same being, and from one being to another; and its dimensions, which are sensibly constant throughout the whole of the living world, have an average diameter of several thousandths of a millimetre—i.e., of several microns. This element, the cell, is a real organ. It is smaller, no doubt, than those described by the ancient anatomists, but it is not less complex. Its complexity is only revealed later. It is an organic unit. Its form varies from one element to another. Its substance is a semi-fluid mass, a mixture of different albuminoids. In the mean value of its dimensions, so carefully measured—exceptis excipiendis—we have a condition the significance of which has not yet been discovered, but which may be of great value in the explanation of its peculiar activities.
Such is the result to which have converged the researches of the biologists who have examined plants or the lower animals, as well as of the anatomists who have been more especially occupied with the vertebrates and with man. All their researches have brought them to the same conclusion—the cellular theory. Either living beings are composed of a single cell—as is the case with the microscopic animals called protozoa, and the microscopic vegetables called protophytes—or, they are cellular complexes, metazoa or metaphytes—that is to say, associations of these microscopic organic units which are called cells.
The Law of the Composition of Organisms.—The law of the composition of organisms was discovered in 1838 by Schleiden and Schwann. From that time up to 1875 it may be said that micrographers have spent their time in examining every organ and every tissue, muscular, glandular, conjunctive, nervous, etc., and in showing that in spite of their varieties of aspect and form, of the complexity of structures due to cohesion and fusion, they all resolve into the common element, the cell. Contemporary anatomists, Koelliker, Max Schultze, and Ranvier, have thus established the generality of the cellular constitution, while zoologists and botanists confirm the same law for all animals and vegetables, and exhibit them all as either unicellular or multicellular.
The Cellular Origin of Complex Beings.—At the same time embryogenic researches showed that all beings spring from a corpuscle of the same type. Going back in the history of their development to the most remote period, we find a cell of very constant constitution—namely, the ovule. This truth may be expressed by changing a word in Harvey’s celebrated aphorism—omne vivum ex ovo; we now say omne vivum e cellula. The myriads of differentiated anatomical elements whose association forms complex beings are the posterity of a cell, of the primordial ovule, unless they are the posterity of another equivalent cell. The second task of histology in the latter half of the nineteenth century consisted in following up the filiation of each anatomical element from the cell-egg to its state of complete development.
The whole cellular theory is contained in the two following statements, which establish the morphological unity of living beings:—Everything is a cell, everything comes from an initial cell; the cell being defined as a mass of substance, protoplasm or protoplasms, of an average diameter of a few microns.