One of the most important _reproductive_ activities of a cell is _mitosis_ (see below). Mitosis essentially consists of a series of processes by which each _nuclear thread_ of the nucleus _splits longitudinally into two equal parts_, and then these equivalent parts separate from each other, so that from the one nucleus we get two smaller nuclei. Then each of these smaller nuclei appropriates its share of the enveloping protoplasm, finally splitting it into two parts. Thus from the larger cell (nucleated piece of protoplasm) we get two smaller cells (two smaller nucleated pieces of protoplasm). In technical language, we say that the larger cell is the _mother cell_, and the two smaller cells that it has divided into are the _daughter cells_. In consequence of the method of mitosis, the two daughter cells very frequently are exactly like the mother cell, except in size. But by the absorption of nutriment, and through digestion and assimilation, they grow and finally become exactly like the mother cell. This is the simplest illustration of _heredity_. The reproductive process may be repeated very many times, so that from one cell we may get millions of cells.[1] It is necessary to assume that the nutritive and reproductive activities of _cells_ are based upon and controlled by the nutritive and reproductive activities of the _physiological units_, inasmuch as these are the ultimate living units. In the activities of a cell the nucleus and protoplasm are intimately correlated with one another. The nucleus is looked upon by the majority of cytologists as the _formative center_ of the cell in a chemical, and also, consequently, in a morphological, sense. Active exchanges of material take place between the nucleus and the protoplasm during the nutritive processes of the cell. Possibly this may be altogether a chemical process, or possibly it may be due, as Hertwig suggests, to the migrations of the _physiological units_ as carriers and elaborators. In these exchanges, and in the upbuilding chemical activities (anabolism) of the cell, the nucleic acid plays a leading part. Here the nucleic acid in the physiological units of the nuclear threads, combines with albumins from the protoplasm, forming nuclein. Much of this nuclein, undergoing further elaboration, is passed into the protoplasm as one of its finished products (metaplasm). The more purely nutritive the activity of a cell, the more nuclein its nuclear threads contain; on the other hand, when the cell is in the phase of reproductive activity, the nucleus contains little nuclein, and is almost entirely composed of pure nucleic acid. [Illustration: FIG. 2.—Stylonychia: c, an entire animal, showing planes of section; the middle piece of c contains two nuclei and can regenerate a perfect animal; a, and b, contain no nuclei,—they live and swim about for a while and then die.]