When a cell divides the fission or splitting of the cell is preceded by peculiar changes in the nucleus. There is a material in the nucleus of every cell—of those which are simple animalcules, as well as of those which are germ-cells and sperm-cells, and of those which form, heaped up in enormous numbers, the living substance of larger animals and plants—a material which is an elaborated sort of proteid (see p. 185) and stains strongly with carmine, logwood, and such dyes, and is called “chromatin.” It exists often in the shape of minute granules and filaments (Fig. 42_a_), but always takes on, sooner or later, the form of an irregularly undulated thread or threads. When the cell is about to divide into two—as all growing and active cells do—the thread arranges itself like a zigzagging girdle around the equator of the globular nucleus (Fig. 42_b_). The margin of the nucleus then seems to melt away into the general protoplasm, and the zigzag bits of the stainable thread break from each other, forming a ring-like group of V-shaped pieces (Fig. 42_c_). There is a remarkable fact as to the number of these V-shaped pieces. They are identical in number in all the cells of one species or kind of animal or plant, but may be of a different number in allied species. The salamander has twenty-four of them; some worms have only two, some insects thirty-six, some plants eight, others twelve, and so on. When the V-shaped pieces have thus taken up their position in the dividing cell, each splits longitudinally, so as to form two V-shaped pieces lying one over the other (Fig. 42_d_). Then the halves separate and travel away from each other. In this way two circlets, each made up by the correct number of V-shaped pieces, come into place at opposite sides of the cell (Fig. 42_e_). After this the protoplasm becomes nipped in between the two circlets so as to separate the cell into two halves, each with its circlet of exactly the correct number of V-shaped pieces of “chromatin” formed by the splitting of those of the parent cell (Fig. 42_f_). It is in this way that the nuclei of the new cells are accurately provided with not merely half of the nuclear chromatin of the mother cell, but with half taken from all parts of it, owing to the thread-like form of the chromatin and the longitudinal splitting of the thread.