Living creatures which consist of a single cell may present, nevertheless, a considerable complication of structure. Thus, an organism as simple as the amœba primitiva, before noticed, may have the power of forming, or, as it is technically called, secreting, from its own substance and its surrounding medium a most complex supporting skeleton of calcareous or silicious nature. It may have its outer envelope so markedly differentiated from its inner as to require a distinct designation as exosarc, while it may give rise in its interior not only to a nucleus and nucleolus, but to two regularly formed cavities with the power of rythmical pulsation, and one definite portion of its external wall may be perforated to form a permanent mouth instead of as in such forms as Amœba, any part serving indifferently as a mouth and every portion having similar functions without differentiation. All these and other complications of structure may arise by direct growth and transubstantiation of the single cell into the various physically and chemically different parts.
Again, a living creature which is fixed may so extend itself as to simulate stem, roots, and branches, and yet remain essentially simple, consisting merely of one greatly enlarged and complicated cell.
Thus, a unicellular plant may take on a great complexity of form while still remaining purely unicellular. It may assume the form of a stem with roots and leaves. An example of such we may see in the genus Caulerpa,[90] which, although unicellular, simulates in its outline the fern called Blechnum.
The next grade of structural complication in living creatures is produced by the lowly plants, such as Protococcus, which multiply by spontaneous self-division or fission. This process may take place repeatedly and at the same time incompletely, in this way producing an apparently compound organism. Thus, we have the second grade of structural complication in living creatures—namely, the aggregation of cells into a loosely joined mass.
Other simple forms are those presented by the minute organisms Diatoms and Desmids, the former enclosed in silicious cases, and some presenting the only exception to the general law that organic bodies are bounded by curved lines and surfaces.
Wonderful is the minute ornamentation presented by the surfaces of these microscopic plants. Some of them cohere by imperfect division in the second grade of structural complication just described; they may form longitudinal series of cells, or they may be arranged round a common centre.
One of the best examples of this secondary grade of complication is presented by the spherically aggregated cells of Volvox.[91] These present us with a good example of the way in which the shape of the individual cells may spontaneously alter, to suit the mode of their aggregation. Originally spherical, the adjacent sides of these cells become flattened, and thus the cells acquire a polygonal figure.
Other instances of the coherence of the cells of unicellular organisms into indefinite and inconstant aggregations is presented by some radiolarians, individuals which cohere into what are called colonies.
From such incomplete aggregation, the next step is to definite and stable aggregations, in which the life of the constituent parts is more or less plainly subservient to, and dominated by, the life of the whole. Such we find in all but the lowest Fungi,[92] and Algæ, in sponges,[93] and Hydræ, and also in all higher organisms. In such permanent aggregations, the dominant life of the whole is shown partly in greater constancy of external form and partly in the setting apart of separate portions of the whole, either for the nourishment of the entire creature or for the reproduction of fresh individuals, or for effecting gaseous interchange, or (in animals) for ministering to feeling and locomotion.
Thus, the overwhelming majority of living creatures are, as has been said, formed of aggregation of cells, which cohere or fuse together in various ways—and not only of aggregation of cells but of aggregation of aggregations of cells or “tissues.” Each tissue is a structure formed by the aggregation, or by aggregation and metamorphoses, of certain sets of cells. Thus, every higher plant or animal is made of an inconceivable multitude of cells, together with tissues which are not cellular, but which have originated by metamorphosis of cells, and every such higher plant or animal at first consists entirely of an aggregate of plainly distinct cells; and, first of all, of one single cell only, whence its whole structure, however complex, has originally sprung, though generally not until it has had at least a portion of another cell mixed with it.