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]