A first book in organic evolution

by D. Kerfoot Shute

One of the most important reproductive activities of a cell is mitosis (see below). Mitosis essentially consists of a series of processes by which each nuclear thread of the nucleus splits longitudinally into two equal parts, and then these equivalent parts separate from each other, so that from the one nucleus we get two smaller nuclei. Then each of these smaller nuclei appropriates its share of the enveloping protoplasm, finally splitting it into two parts. Thus from the larger cell (nucleated piece of protoplasm) we get two smaller cells (two smaller nucleated pieces of protoplasm). In technical language, we say that the larger cell is the mother cell, and the two smaller cells that it has divided into are the daughter cells. In consequence of the method of mitosis, the two daughter cells very frequently are exactly like the mother cell, except in size. But by the absorption of nutriment, and through digestion and assimilation, they grow and finally become exactly like the mother cell. This is the simplest illustration of heredity. The reproductive process may be repeated very many times, so that from one cell we may get millions of cells.[1]

It is necessary to assume that the nutritive and reproductive activities of cells are based upon and controlled by the nutritive and reproductive activities of the physiological units, inasmuch as these are the ultimate living units.

In the activities of a cell the nucleus and protoplasm are intimately correlated with one another.

The nucleus is looked upon by the majority of cytologists as the formative center of the cell in a chemical, and also, consequently, in a morphological, sense. Active exchanges of material take place between the nucleus and the protoplasm during the nutritive processes of the cell. Possibly this may be altogether a chemical process, or possibly it may be due, as Hertwig suggests, to the migrations of the physiological units as carriers and elaborators.

In these exchanges, and in the upbuilding chemical activities (anabolism) of the cell, the nucleic acid plays a leading part. Here the nucleic acid in the physiological units of the nuclear threads, combines with albumins from the protoplasm, forming nuclein. Much of this nuclein, undergoing further elaboration, is passed into the protoplasm as one of its finished products (metaplasm). The more purely nutritive the activity of a cell, the more nuclein its nuclear threads contain; on the other hand, when the cell is in the phase of reproductive activity, the nucleus contains little nuclein, and is almost entirely composed of pure nucleic acid.

Fig. 2.—Stylonychia: c, an entire animal, showing planes of section; the middle piece of c contains two nuclei and can regenerate a perfect animal; a, and b, contain no nuclei,—they live and swim about for a while and then die.

That the nucleus is the formative center of the cell is indicated by the following, among many facts: If a unicellular animal, such as Stylonychia ([Fig. 2]), for instance, be broken up into several fragments, it will be observed that some of the fragments are nucleated and others non-nucleated. The nucleated fragments have the power of quickly healing the wounds on them, regenerating the missing parts, and thus restoring the mutilated fragments to perfect individuals. These nucleated fragments have the power to perform all the activities of the perfect animal. The non-nucleated portions, on the other hand, cannot undergo regeneration. They cannot digest food, or grow or secrete substances as the nucleated fragments can. They can simply live for awhile, responding to stimuli and moving about. They finally perish.

Having mentioned in a general way some of the wonderful powers of cells, it will be well now to describe briefly a few of the unicellular plants and animals that can be so easily obtained and studied in warm weather, and which may thus serve as illustrations of the powers of nucleated pieces of protoplasm or cells. Many unicellular plants and animals can be obtained in summer from the superficial ooze on the bottom of slow-running streams and also on the under surfaces of the leaves of water plants, a study of which will be of the greatest value and interest.

Amœba proteus ([Fig. 3]). This little unicellular animal, which belongs to the Rhizopod type, is very common in ponds and streams in warm weather. In the resting state it is spherical in form, but when active its form is as changeable as the fabled Proteus, hence its name, Amœba proteus. This little creature is a naked piece of protoplasm, with its outer layer differentiated into a firmer and pellucid part called the ectoplasm ([Fig. 3], ec); its interior, the endoplasm (en), is quite granular and much more fluid, the granular particles moving quite freely upon one another when the animal changes its shape. The superficial portion of the endoplasm is firmer than its more central parts, and graduates insensibly into the more consistent ectoplasm.