In contrast with this very elaborate and very strictly differentiated psychic cell (Figure 1.9), we have our ovum (Figures 1.1 and 1.2), which has hardly any structure at all. But even in the case of the ovum we must infer from its properties that its protoplasmic body has a very complicated chemical composition and a fine molecular structure which escapes our observation. This presumed molecular structure of the plasm is now generally admitted; but it has never been seen, and, indeed, lies far beyond the range of microscopic vision. It must not be confused—as is often done—with the structure of the plasm (the fibrous network, groups of granules, honey-comb, etc.) which does come within the range of the microscope.
But when we speak of the cells as the elementary organisms, or structural units, or "ultimate individualities," we must bear in mind a certain restriction of the phrases. I mean, that the cells are not, as is often supposed, the very lowest stage of organic individuality. There are yet more elementary organisms to which I must refer occasionally. These are what we call the "cytodes" (cytos = cell), certain living, independent beings, consisting only of a particle of plasson—an albuminoid substance, which is not yet differentiated into caryoplasm and cytoplasm, but combines the properties of both. Those remarkable beings called the monera—especially the chromacea and bacteria—are specimens of these simple cytodes. (Compare Chapter 2.19.) To be quite accurate, then, we must say: the elementary organism, or the ultimate individual, is found in two different stages. The first and lower stage is the cytode, which consists merely of a particle of plasson, or quite simple plasm. The second and higher stage is the cell, which is already divided or differentiated into nuclear matter and cellular matter. We comprise both kinds—the cytodes and the cells—under the name of plastids ("formative particles"), because they are the real builders of the organism. However, these cytodes are not found, as a rule, in the higher animals and plants; here we have only real cells with a nucleus. Hence, in these tissue-forming organisms (both plant and animal) the organic unit always consists of two chemically and anatomically different parts—the outer cell-body and the inner nucleus.
In order to convince oneself that this cell is really an independent organism, we have only to observe the development and vital phenomena of one of them. We see then that it performs all the essential functions of life—both vegetal and animal—which we find in the entire organism. Each of these tiny beings grows and nourishes itself independently. It takes its food from the surrounding fluid; sometimes, even, the naked cells take in solid particles at certain points of their surface—in other words, "eat" them—without needing any special mouth and stomach for the purpose (cf. Figure 1.19).
Further, each cell is able to reproduce itself. This multiplication, in most cases, takes the form of a simple cleavage, sometimes direct, sometimes indirect; the simple direct (or "amitotic") division is less common, and is found, for instance, in the blood cells (Figure 1.10). In these the nucleus first divides into two equal parts by constriction. The indirect (or "mitotic") cleavage is much more frequent; in this the caryoplasm of the nucleus and the cytoplasm of the cell-body act upon each other in a peculiar way, with a partial dissolution (caryolysis), the formation of knots and loops (mitosis), and a movement of the halved plasma-particles towards two mutually repulsive poles of attraction (caryokinesis, Figure 1.11.)
(FIGURE 1.10. Blood-cells, multiplying by direct division, from the blood of the embryo of a stag. Originally, each blood-cell has a nucleus and is round (a). When it is going to multiply, the nucleus divides into two (b, c, d). Then the protoplasmic body is constricted between the two nuclei, and these move away from each other (e). Finally, the constriction is complete, and the cell splits into two daughter-cells (f). (From Frey.))
FIGURE 1.11. Indirect or mitotic cell-division (with caryolysis and caryokinesis) from the skin of the larva of a salamander. (From Rabl.). A. Mother-cell (Knot, spirema), with Nuclear threads (chromosomata) (coloured nuclear matter, chromatin), Cytosoma, Nuclear membrane, Protoplasm of the cell-body and Nuclear sap. B. Mother-star, the loops beginning to split lengthways (nuclear membrane gone), with Star-like appearance in cytoplasm, Centrosoma (sphere of attraction), Nuclear spindle (achromin, colourless matter) and Nuclear loops (chromatin, coloured matter). C. The two daughter-stars, produced by the breaking of the loops of the mother-star (moving away), with Upper daughter-crown, Connecting threads of the two crowns (achromin), Lower daughter-crown and Double-star (amphiaster). D. The two daughter-cells, produced by the complete division of the two nuclear halves (cytosomata still connected at the equator) (Double-knot, Dispirema), with Upper daughter-nucleus, Equatorial constriction of the cell-body and Lower daughter-nucleus.)
The intricate physiological processes which accompany this "mitosis" have been very closely studied of late years. The inquiry has led to the detection of certain laws of evolution which are of extreme importance in connection with heredity. As a rule, two very different parts of the nucleus play an important part in these changes. They are: the chromatin, or coloured nuclear substance, which has a peculiar property of tingeing itself deeply with certain colouring matters (carmine, haematoxylin, etc.), and the achromin (or linin, or achromatin), a colourless nuclear substance that lacks this property. The latter generally forms in the dividing cell a sort of spindle, at the poles of which there is a very small particle, also colourless, called the "central body" (centrosoma). This acts as the centre or focus in a "sphere of attraction" for the granules of protoplasm in the surrounding cell-body, and assumes a star-like appearance (the cell-star, or monaster). The two central bodies, standing opposed to each other at the poles of the nuclear spindle, form "the double-star" (or amphiaster, Figure 1.11, BC). The chromatin often forms a long, irregularly-wound thread—"the coil" (spirema, Figure A). At the commencement of the cleavage it gathers at the equator of the cell, between the stellar poles, and forms a crown of U-shaped loops (generally four or eight, or some other definite number). The loops split lengthwise into two halves (B), and these back away from each other towards the poles of the spindle (C). Here each group forms a crown once more, and this, with the corresponding half of the divided spindle, forms a fresh nucleus (D). Then the protoplasm of the cell-body begins to contract in the middle, and gather about the new daughter-nuclei, and at last the two daughter-cells become independent beings.
Between this common mitosis, or indirect cell-division—which is the normal cleavage-process in most cells of the higher animals and plants—and the simple direct division (Figure 1.10) we find every grade of segmentation; in some circumstances even one kind of division may be converted into another.
The plastid is also endowed with the functions of movement and sensation. The single cell can move and creep about, when it has space for free movement and is not prevented by a hard envelope; it then thrusts out at its surface processes like fingers, and quickly withdraws them again, and thus changes its shape (Figure 1.12). Finally, the young cell is sensitive, or more or less responsive to stimuli; it makes certain movements on the application of chemical and mechanical irritation. Hence we can ascribe to the individual cell all the chief functions which we comprehend under the general heading of "life"—sensation, movement, nutrition, and reproduction. All these properties of the multicellular and highly developed animal are also found in the single animal-cell, at least in its younger stages. There is no longer any doubt about this, and so we may regard it as a solid and important base of our physiological conception of the elementary organism.
Without going any further here into these very interesting phenomena of the life of the cell, we will pass on to consider the application of the cell theory to the ovum. Here comparative research yields the important result that EVERY OVUM IS AT FIRST A SIMPLE CELL. I say this is very important, because our whole science of embryology now resolves itself into the problem: "How does the multicellular organism arise from the unicellular?" Every organic individual is at first a simple cell, and as such an elementary organism, or a unit of individuality. This cell produces a cluster of cells by segmentation, and from these develops the multicellular organism, or individual of higher rank.