The first cell has contractility, protoplasmic motion; it can absorb perfectly all food-stuffs necessary for it from the deutoplasm of the ovum, and the water that passes in from without to the ovum. In a few days the embryonic cells have used up the deutoplasm and are taking up food from the maternal blood as perfectly as any adult cell does, and are exercising their function of building up and sustaining whatever part of the body they are destined for; and this with all the complicated metabolism of the adult cell. Cell metabolism is the fundamental, chief, organic act of any human body at any age. That the embryo does this impelled by the _virtus formativa_ transmitted from the parents is a mere gratuitous assumption to fit the theory that the embryonic cell lacks organic power. The fundamental organ that conserves the body in its very existence under the government of the soul is the apparatus which effects metabolism. Incessant chemico-vital change is a characteristic of all living substances, from the single cell up to the adult man; and in all cases this activity has to do with a transformation of the complex molecules which build up the protoplasm or are associated with its operations. The totality of the chemical changes, or exchanges, in living cells, the transformation of unorganized food materials so that these may be assimilated, and the chemical processes in the tissues themselves, all are metabolism. Growth and repair (anabolism) occur side by side with the destruction of elementary tissue substance (katabolism), and the duration of life rests on these processes; and all are mere cell activities. Food-stuffs (water, inorganic salts, proteids, albuminoids, carbohydrates, and fats) undergo more or less combustion or oxidation. Oxygen unites with carbon to form carbon dioxide, and with hydrogen to form water; the nitrogen of the highly complex proteid substances reappears in combination with carbon, hydrogen, and oxygen as urea, uric acid, and other compounds; and other ingesta are thus transformed through oxidation. All maintain the temperature of the body, replace outworn parts, and accomplish the body's work. Oxidation occurs to a slight extent in the blood, but the specific reactions are intracellular. Even when nothing exists but the cells and the blood, as in the beginning embryo, the cells really do the work, and they do the work as they do in the adult. The cells also from the very beginning are the organs that make the animal heat necessary for life. Rubner[52] proved that the source of at least 90 per cent. of the animal heat in the body is a result of the chemical changes--oxidation--in the food ingested: the other 10 per cent. is caused by muscular contractions, the flow of blood, the friction of joints, and like motions. This oxidation is more active in young animals than in adults, and in each it is, of course, a cellular process. [52] _Zeitschrift f. Biologie_, 1893, bd. 30, p. 73.