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.
Fig. 2.
The above illustration from Wilson’s The Cell shows in more or less diagrammatic form the stage of nuclear division in which the chromosomes, as yet undivided, have arranged themselves in the centre of the nucleus. The centrosomes with their astral figures have been formed, and have taken their places near each pole of the nucleus. The next stage is represented at b in Fig. 1.
When a cell is about to divide, an organ of recent discovery, termed the ‘centrosome,’ comes into play. This appears as the core of a sort of rayed or star-like figure, and it takes up its position beside the nucleus. When the cell is resting, the chromatin is dispersed through the nucleus in a mass of broken lines, forming a kind of network. When division begins, this broken-up substance forms itself into a series of small threads, sometimes straight, sometimes looped or curved. These are called ‘chromosomes.’ There are always a definite and invariable number of chromosomes for every species of plant or animal—the cell of a man has so many,37 of a grasshopper so many, of a lily so many. The chromosomes range themselves in a belt across the centre of the nucleus, and the centrosome breaks into two parts, which take up a position one at each end of the nucleus. Regarding the nucleus as a tiny globe, we may say that the chromosomes lie in the equatorial plane, while the two parts of the centrosome move towards the North and South Poles respectively.
The centrosomes, at the two poles of the nucleus, are surrounded each with a halo of ray-like processes (the centrosphere), and on the sides next each other these rays penetrate the nucleus and join, forming a spindle-shaped figure with a centrosphere at each end. This spindle figure appears to be the organ by which the division is accomplished, for each of the chromosomes now splits itself in two longitudinally, as one cleaves a log of wood, and one half passes over to each centrosphere, thus making an exact division of the whole chromatin or hereditary substance. An indentation now appears in the outer wall of the cell and also in the nucleus—it deepens and deepens, and finally two cells appear instead of one, each with a nucleus, a centrosome, and a supply of chromatin, the latter now breaking up into its original condition of diffusion through the nucleus. In multicellular organisms the two new cells, of course, do not separate, but a wall is formed between them. Some plant-cells contain several nuclei; in this case division of the nucleus is not necessarily followed by that of the cell.38
Throughout the processes of cell division it is apparent that the utmost care is taken to ensure an exact partition of the chromatin between the two new cells. This partition has to be qualitative as well as quantitative; for one chromosome may, and no doubt does, differ in function and influence from another, and has various elements within itself. The longitudinal division of each chromosome, in which the elements are arranged like beads on a rosary, ensures that the different elements of the whole hereditary substance shall appear in each new cell in exactly the same relative proportion as in the parent cell; just as if two persons had to divide between them a dozen apples of different varieties, and secured perfect equality, not by taking six apples each, but by dividing every apple in two. This is the fundamental cause of the fixity of species, which means the production of offspring having the same specific characteristics as their parents. How, under these conditions, the mutability of species is brought about must be discussed later. It is first of all necessary to inquire more closely into the composition of the chromatin, and to study the special phenomena of cell-growth in connexion with conjugation, where new and extraordinary features come to light.
A chromosome is not, or is not usually, a simple body. In all but the very lowest organisms it is composed, as we have said, of a number of elements. Each of these elements is styled a ‘determinant,’ and it controls the form, colour, and function of some definite part of the future plant or animal. Weismann believes the determinants to be grouped into complex bodies called ‘ids,’ each id containing all the determinants necessary for a whole being, and each chromosome being composed of a number of ids. These ids are microscopically visible; they form the beads on the rosary already referred to; but their exact composition and potency are largely conjectural at present. How far the subdivision of determinants may go, it is, of course, impossible to ascertain. We cannot say, for instance, whether there is a determinant for every hair of the head, or one for the hirsute covering in general, or one for each of the different sections of the scalp. But the division is very minute. Each of the ids may be a very complex body, as we see by the manner in which, in some families, small physical signs like a patch of hair differing from the colour of the rest, or a tiny pit or mole on the skin of a certain part of the body, may be handed down, in that precise position, for generations. There may be, and, in fact, in the higher plants and animals there must be, a number of determinants for each part of the structure, and the final characteristics of that part must be the resultant of a blend of all these determinants, the more powerful predominating in proportion to their vitality and force. The whole body of the chromosomes may therefore be said to represent one or more complete beings in diagrammatic form, each part of the complete animal or plant being represented by some part of a chromosome, though of course not physically resembling it. And we thus strike on the very curious and startling fact that, as far as we can see, every cell in every organism throughout the world of life contains all the elements of the whole being to which it belongs, and is, potentially, that being.39 All the higher organisms possess two kinds of cells—reproductive cells which have the faculty of fusing together to reproduce their kind, and ‘somatic’ or body cells, which, although they all originate in a reproductive cell, multiply only by division, and have the function of forming the various parts of the bodily structure. Of the nucleus of a germ cell “we cannot say that it differs in any essential or definite way from the nucleus of any other cell.”40 All possess the chromatin or hereditary substance of the organism, though, according to Boveri, the germ cells alone receive all the chromatin of the parent cell, the derived somatic cells having to part with some of it.41 There may be some distinction, though on what it may be based it is at present impossible to say, between cells that are capable of developing into a complete organized creature and those that are not.
Every somatic cell is doomed to perish, but every reproductive cell now upon the globe is united, not metaphorically, not by a chain of successive originations or impulses, but by actual identity
