Gradually, through the formation of new cells and by the growth of these cells after they have been formed, the body attains its full size. When growth is complete, cell reproduction is supposed to cease except where the tissues are injured, as in the breaking of a bone, or where cells, like those at the surface of the skin, are subject to wear. Then new material continues to be added to the protoplasm throughout life, but in amount only sufficient to replace that lost from the protoplasm as waste. [Fig. 6] Fig. 6—A tumbler partly filled with marbles covered with water, suggesting the relations of the cells to the lymph. *Cell Surroundings.*—All cells are said to be _aquatic_. This means simply that they require water for carrying on their various activities. The cells, in order to live, must take in and give out materials, and water is necessary to both processes. It is also an essential part of the protoplasm. Deprived of water, cells become inactive and usually die. Aquatic surroundings are provided for the cells of the body through a liquid known as the _lymph_, which is distributed throughout the intercellular material (Fig. 6). This consists of water containing oxygen and food substances in solution. Besides supplying these to the cells, the lymph also receives their wastes. Through the lymph the necessary conditions for cell life are provided in the body. *The General Work of Cells.*—In handling the materials derived from the lymph, the cells carry on three well-defined processes, known as absorption, assimilation, and excretion. _Absorption_ is the process of taking water, food, and oxygen into the cells. _Assimilation_ is a complex process which results in the addition of the absorbed materials to the protoplasm. Through assimilation the protoplasm is built up or renewed. _Excretion_ is the throwing off of such waste materials as have been formed in the cells. These are passed into the lymph and thence to the surface of the body. Absorption, assimilation, excretion, and also reproduction are performed by all classes of cells. They are, on this account, referred to as the _general work of cells_. *The Special Work of Cells.*—In addition to the general work which all cells do in common, each class of cells in the body is able to do some particular kind of work—a work which the others cannot do or which they can do only to a limited extent. This is spoken of as the _special work of cells_. Examples of the special work of cells are found in the production of motion by muscle cells and in the secretion of liquids by gland cells. It may be noted that while the general work of cells benefits them individually, their special work benefits the body as a whole. Another example of the special work of cells is found in the [Fig. 7] Fig. 7—Cartilage cells, surrounded by the intercellular material which they have deposited.