so much division of labour is found, and to such an extent, that a large number of cells are entirely dependent upon the functions of others. Were such cells to cease to work this would result, as has already been mentioned, in the sickness and finally in the death of many other cells. In this direction there still lies an extensive field of research before us. The "whys" and the "wherefores" in this case extend indefinitely.
The possibility of breeding single cells and pieces of tissues in the blood plasma outside the organism, and keep them alive for a certain time, opens out a prospect of answering many problems by experimental means. We shall see in due course why some of the cells lose their normal functions when the secretion of certain organs is lacking. The number of possibilities is almost unlimited. For example, some substances, such as grape sugar, can only be dissociated by the cells into final products—carbon dioxide and water—after they have been prepared in a certain manner. A gradual dissociation takes place. The cell is equipped with appliances for the alteration of a given substance, but they are not at first in a condition suitable for use. A second agent must first of all make them capable of their respective functions—just as a hammer without a handle, or a screw without a screwdriver, are only useful when the missing parts are at our disposal.
