All animal and vegetable structure arises from cellular tissue, and in fact is either cellular tissue or, as in the case of bones, scales, etc., the mineral deposit formed by the action of cells. The simplest living forms are composed of single cells, and the most complex and huge of them were each once nothing more than a single cell, possessed of the powers of development and growth. In multicellular organisms, this single originating cell is usually formed by the fusion of two imperfect cells by what is indifferently called conjugation, sexual reproduction, or ‘amphimixis.’ All cells, whether they are the product of conjugation or not, grow, when they do grow, fundamentally in the same way, and this way must now be described.
The contents of the typical cell are broadly differentiated into (1) a more or less hardened envelope containing (2) a substance called cytoplasm (Gk. κύτος, a cell), and (3) a small, rounded, dark-coloured body called the nucleus. Until recently nothing more than this was known of the structure of the cell, and nothing at all of the functions of the nucleus. Now, keener microscopic research and better instruments have thrown a flood of light on cell-organization, and the nucleus is revealed as a powerful factor in the vital processes of the cell and the bearer of its hereditary substance[36]—that which makes it a cell of some particular organism, plant or animal, and of no other. This hereditary substance, divined by the botanist Nägeli, and since observed by Weismann and others, is called ‘chromatin’ (from the fact that it is observed by means of the stain it takes from the addition of an aniline dye), or ‘idioplasm’ (Nägeli’s appellation), which might be rendered the ‘selfhood substance’ of the cell.
Cellular structure begins, as has long been known, by the division of a cell into two, each of the parts then proceeding to grow by the assimilative power of protoplasm and in due time to divide in its turn. A mass of these cells is called ‘cellular tissue.’ The so-called ‘budding’ of a small cell from the side of the parent is, of course, simply a form of division. The process of division and redivision goes on, accompanied by a differentiation in the shape and function of the different cells or groups of cells which are formed, until the structure of the plant or animal is completed. In these operations the nucleus plays the principal part. The division of the cell is essentially the division of the nucleus. A detached portion of a cell which contains nothing of the nucleus can reproduce itself no more; it perishes.
Fig. I.
This illustration, which (by permission of The Macmillan Co.) I take from Wilson’s work, The Cell, is one of remarkable interest, for in it the microscope has caught, in a piece of actual tissue from the skin of the salamander, Amblystoma, three nuclei in different stages of mitotic division. Most of the nuclei, which are seen as large, roundish objects in their respective cells, show the chromatin in its ‘resting’ condition interspersed through the nucleus. The nucleus under a shows the chromatin gathered into chromosomes. At b the centrosomes with their astral figures (which can barely be detected) have been formed, the chromosomes have carried out their longitudinal division, and are being attracted half towards one centrosome and half towards the other. A little above this the process has been carried further, and the sides of the cell are beginning to contract, preparatory to forming two new ones. In Fig. 2 will be found a clear representation of the astral figures.
To face p. 40.
