In the busy hive of cells which constitutes what we call the animal body, there is thus ceaseless activity. During periods of apparent rest the protogen filaments of the cell-net are engaged in constructive work, building up fresh supplies of complex and unstable materials, which, during periods of apparent activity, break up into simpler and more stable substances, some of which are useful to the organism while others must be got rid of as soon as possible. From another point of view, the cells during apparent rest are storing up energy which is utilized by the organism during its periods of activity. The storing up of available energy may be likened to the winding up of a watch or clock; it is during apparent rest that the cell is winding itself up; and thus we have the apparent paradox that the cell is most active and doing most work when it is at rest. During the repose of an organ, in fact, the cells are busily working in preparation for the manifestation of energetic action that is to follow. Just as the brilliant display of intellectual activity in a great orator is the result of the silent work of a lifetime, so is the physical manifestation of muscular power the result of the silent preparatory work of the muscle-cells.[E] One point to be specially noted is the varied activity of the cells. While they are all working for the general good of the organism, they are divided into companies, each with a distinct and definite kind of work. This is known as the physiological division of labour. It is accompanied by a morphological differentiation of structure. By the form of a cell, therefore, we can generally recognize the kind of work it has to perform. The unstable compounds produced by the various cells must also be different, though not much is known at present on this subject. The unstable compound which forms bone and that which forms the salivary ferment, the unstable matter elaborated by nerve-cells and that built up by muscle-cells, are in all probability different in their chemical nature. Whether the formative plasmogen from which these different substances originate is in all cases the same or in different cases different, we do not know.