A discussion of this matter will be found in Wilson,[50] but the many experiments made in the study of this subject show conclusively that the cells, singly, grouped, and in mass, are a morphological unit, not an aggregation of distinct individuals. They are not, of course, absolutely homogeneous, because such a body could not have organs. The substantial form, therefore, is not confined to the first cell. [50] _The Cell in Development and Inheritance_, pp. 413 _et seq._ New York, 1906. The cell-mass, then, has a unity sufficient to be the receptacle of a human vital principle; again, the basic vital operation of the human body at any age is metabolism, and this is actually carried on in the first somatic cell of the embryo as in the cells of the adult man. In the development of the human body in the embryonal stage the energy of cell-division is most intense in the early cleavage stage, and this diminishes as the limit of growth approaches because further division is not needed. When that limit is attained a more or less definite equilibrium is established. Some of the cells in the fully formed body cease to divide, the nerve-cells, for example; others divide under special conditions, as the blood-cells, the connective-tissue cells, gland-cells, epithelial and muscle cells; others continue to divide throughout life and thus replace worn-out cells of the same tissue, as the Malpighian layer of the skin. Cells grow, divide, function, reproduce themselves, and so on, all through their vital activity, sustained by the material brought to them by the blood. Weismann[51] and other biologists think that the vital processes of the higher animals are accompanied by a renewal of the morphological elements in most tissues. The material is carried to the fetus in the womb by various agents, but mostly by the maternal blood after the embryo uses up the yolk; and when the fetal circulation has been established the nutritive material is taken from the maternal blood into the fetal circulation through the placenta, and then carried to the cells by the fetal circulation itself. After the child has been born the stomach and intestines take in the food. The stomach does very little with it except in a preparatory manner; the intestines further prepare it, pass it into the body, where it is again modified by other organs, and finally it is carried by the blood to the cells. The cells really use it; the other organs are the farmers, grocers, railways, and the like; the cells are the consumers. So far as the essential processes are concerned, the embryological cells act as do the adult cells. [51] _The Duration of Life._