Cells.--_A cell may be defined as a tiny mass of living matter containing a nucleus, either living alone or forming a unit of the building material of a living thing._ The living matter of which all cells are formed is known as _protoplasm_ (formed from two Greek words meaning _first form_). If we examine under a compound microscope a small bit of the water plant _Elodea_, we see a number of structures resembling bricks in a wall. Each "brick," however, is really a plant cell bounded by a thin wall. If we look carefully, we can see that the material inside of this wall is slowly moving and is carrying around in its substance a number of little green bodies. This moving substance is living matter, the protoplasm of the cell. The green bodies (the _chlorophyll_ bodies) we shall learn more about later; they are found only in plant cells. All plant and animal cells appear to be alike in the fact that every living cell possesses a structure known as the _nucleus_ (pl. _nuclei_), which is found within the body of the cell. This nucleus is not easy to find in the cells of _Elodea_. Within the nucleus of all cells are found certain bodies called _chromosomes_. These chromosomes in a given plant or animal are always constant in number. These chromosomes are supposed to be the bearers of the qualities which we believe can be handed down from plant to plant and from animal to animal, in other words, the inheritable qualities which make the offspring like its parents. How Cells form Others.--Cells grow to a certain size and then split into two new cells. In this process, which is of very great importance in the growth of both plants and animals, the nucleus divides first. The chromosomes also divide, each splitting lengthwise and the parts going in equal numbers to each of the two cells formed from the old cell. In this way the matter in the chromosomes is divided equally between the two new cells. Then the rest of the protoplasm separates, and two new cells are formed. This process is known as _fission_. It is the usual method of growth found in the tissues of plants and animals. [Illustration: Stages in the division of one cell to form two. Which part of the cell divides first? What seems to become of the chromosomes?]