We now know that the nucleus, or rather the chromatin that it contains, plays a remarkable and interesting rôle in the life of the cell. To this we shall return presently. The remainder of the protoplasm, outside of the nucleus, shows the greatest possible variety of form, according to the kind of cell at which we happen to be looking. In some of the simpler types this part of the protoplasm seems to be merely a nearly uniform mass, perhaps with tiny particles scattered through it. In other types the protoplasm is drawn out into long slender threads, and these threads may have many branches; or the protoplasm may be distorted into a thin shell inclosing a mass of fat; or it may be subdivided into dense and thin portions with sharp lines of division between them. These various forms are related to the special functions which the cells have, and we shall learn more about them as we take up the different functions in order. On the whole, study of cell structure shows clearly that the protoplasm outside the nucleus carries on the greater part of the metabolism or power development, and is correspondingly important as the seat of the special functions shown by the cell. If it is a muscle cell, this is the part that does the moving; if a gland cell, this is the part that secretes. Nevertheless, the nucleus is a vital part of the cell. It has been definitely proven that a cell from which the nucleus is lost cannot survive more than a brief time. To gain some idea of the actual part played by the nucleus, we shall have to return to it in some detail. [Illustration: DIAGRAM SHOWING CELL SUBDIVISION _A_, a cell; _B_ to _F_, successive stages in its subdivision; _a_, cell-sac; _b_, cell contents; _c_, nucleus. (From Martin’s “Human Body”)]