Fig. 2—Stem-cell of one of the echinoderms (cytula, or “first segmentation-cell” = fertilised ovum), after Hertwig. k is the nucleus or caryon.
The shape or outer form of the cell is infinitely varied, in accordance with its endless power of adapting itself to the most diverse activities or environments. In its simplest form the cell is globular (Fig. 2). This normal round form is especially found in cells of the simplest construction, and those that are developed in a free fluid without any external pressure. In such cases the nucleus also is not infrequently round, and located in the centre of the cell-body (Fig. 2k). In other cases, the cells have no definite shape; they are constantly changing their form owing to their automatic movements. This is the case with the amœbæ (Fig. 15 and 16) and the amœboid travelling cells (Fig. 11), and also with very young ova (Fig. 13).However, as a rule, the cell assumes a definite form in the course of its career. In the tissues of the multicellular organism, in which a number of similar cells are bound together in virtue of certain laws of heredity, the shape is determined partly by the form of their connection and partly by their special functions. Thus, for instance, we find in the mucous lining of our tongue very thin and delicate flat cells of roundish shape (Fig. 3). In the outer skin we find similar, but harder, covering cells, joined together by saw-like edges (Fig. 4). In the liver and other glands there are thicker and softer cells, linked together in rows (Fig. 5).
The last-named tissues (Figs. 3–5) belong to the simplest and most primitive type, the group of the “covering-tissues,” or epithelia. In these “primary tissues” (to which the germinal layers belong) simple cells of the same kind are arranged in layers. The arrangement and shape are more complicated in the “secondary tissues,” which are gradually developed out of the primary, as in the tissues of the muscles, nerves, bones, etc. In the bones, for instance, which belong to the group of supporting or connecting organs, the cells (Fig. 6) are star-shaped, and are joined together by numbers of net-like interlacing processes; so, also, in the tissues of the teeth (Fig. 7), and in other forms of supporting-tissue, in which a soft or hard substance (intercellular matter, or base) is inserted between the cells.
Fig. 3—Three epithelial cells from the mucous lining of the tongue.
Fig. 4—Five spiny or grooved cells, with edges joined, from the outer skin (epidermis): one of them (b) is isolated.
Fig. 5—Ten liver-cells: one of them (b) has two nuclei.
The cells also differ very much in size. The great majority of them are invisible to the naked eye, and can be seen only through the microscope (being as a rule between 1/2500 and 1/250 inch in diameter). There are many of the smaller plastids—such as the famous bacteria—which only come into view with a very high magnifying power. On the other hand, many cells attain a considerable size, and run occasionally to several inches in diameter, as do certain kinds of rhizopods among the unicellular protists (such as the radiolaria and thalamophora). Among the tissue-cells of the animal body many of the muscular fibres and nerve fibres are more than four inches, and sometimes more than a yard, in length. Among the largest cells are the yelk-filled ova; as, for instance, the yellow “yolk” in the hen’s egg, which we shall describe later (Fig. 15).
Cells also vary considerably in structure. In this connection we must first distinguish between the active and passive components of the cell. It is only the former, or active parts of the cell, that really live, and effect that marvellous world of phenomena to which we give the name of “organic life.” The first of these is the inner nucleus (caryoplasm), and the second the body of the cell (cytoplasm). The passive portions come third; these are subsequently formed from the others, and I have given them the name of “plasma-products.” They are partly external (cell-membranes and intercellular matter) and partly internal (cell-sap and cell-contents).