The phenomena of cell-growth, reproduction, and heredity are those which lie at the basis of all organized protoplasmic life, and in all the forms of that life, vegetable as well as animal, they are extraordinarily similar; there is, in fact, nothing which all the species of living things have so much in common. One of the most wonderful and fascinating chapters in the whole range of science is that which contains the account of these processes, and it is only within the last few years that it has been possible to write it. Weismann, in a certain section of his Evolution Theory, has brought the facts together in a manner which, for its lucidity and mastery of the subject-matter, deserves to be called a classic example of scientific exposition.[34] To understand the basis of the higher manifestations of life, these processes, as we have said, must first be understood, and an account of them, based on Weismann, and accepting his germ-plasm theory so far as it seems to accord with established facts, will be given, of course only in the broadest outlines.[35] At the same time it will be attempted, here and there, to throw some light on the _rationale_ of the processes described. 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.