Surrounding the nucleus is the _main body_ of the cell, sometimes referred to as the "protoplasm." Since the protoplasm forms all parts of the cell, this substance is more properly called the _cytoplasm_, or cell plasm. Surrounding and inclosing the cytoplasm, in many cells, is a thin outer layer, or membrane, which affords more or less protection to the contents of the cell. This is usually referred to as the _cell-wall_. A fourth part of the cell is also described, being called the _attraction sphere_. This is a small body lying near the nucleus and coöperating with that body in the formation of new cells. Food particles, wastes, and other substances may also be present in the cytoplasm. The parts of a typical cell are shown in Fig. 4. *Importance of the Cells.*—The cells must be regarded as the living, working parts of the body. They are the active agents in all of the tissues, enabling them to serve their various purposes. Working through the tissues, they build up the body and carry on its different activities. They are recognized on this account as _the units of structure and of function_, and are the "individuals" in the body organization. Among the most important and interesting of the activities of the cells are those by which they build up the body, or cause it to grow. *How the Cells enable the Body to Grow.*—Every cell is able to take new material into itself and to add this to the protoplasm. This tends to increase the amount of the protoplasm, thereby causing the cells to increase in size. A general increase in the size of the cells has the effect of increasing the size of the entire body, and this is one way by which they cause it to grow. There is, however, a fixed limit, varying with different cells, to the size which they attain, and this is quite low. (The largest cells are scarcely visible to the naked eye.) Any marked increase in the size of the body must, therefore, be brought about by other means. Such a means is found in the formation of new cells, or _cell reproduction_. The new cells are always formed _by_ and _from_ the old cells, the essential process being known as _cell-division_. [Fig. 5] Fig. 5—Steps in cell-division (after Wilson). Note that the process begins with the division of the attraction sphere, then involves the nucleus, and finally separates the main body. *Cell-Division.*—By dividing, a single cell will, on attaining its growth, separate into two or more new cells. The process is quite complex and is imperfectly understood. It is known, however, that the act of separation is preceded by a series of changes in which the attraction sphere and the nucleus actively participate, and that, as a result of these changes, the contents of the old cell are rearranged to form the new cells. Some of the different stages in the process, as they have been studied under the microscope, are indicated in Fig. 5.