In the multicellular body of the tissue-forming organisms we must distinguish between the partial death of the various cells and the total death of the whole organism, or cell-state, which they make up. In many of the lower tissue-plants and tissue-animals the communal link is very loose and the centralization slight. Odd cells or groups of cells may be set loose, without any danger to the life of the whole histon, and grow into new individuals. In many of the algæ and liverworts (even in the _bryophyllum_, closely related to the stone-crop, or _sedum_)--as well as in the common fresh-water polyp, hydra, and other polyps--every bit that is cut off is capable of growing into a complete individual. But the higher the organization is developed and the closer the correlation of the parts and their co-operation in the life of the centralized stock or person, the slighter we find the regenerative faculty of the several organs. Even then, however, many used-up cells may be removed and replaced by regenerated new cells. In our own human organism, as in that of the higher animals, thousands of cells die every day, and are replaced by new cells of the same kind, as, for instance, epidermic cells at the surface of the skin, the cells of the salivary glands or the mucous lining of the stomach, the blood-cells, and so on. On the other hand, there are tissues that have little or nothing of this repairing power, such as many of the nerve-cells, sense-cells, muscle-cells, etc. In these cases a number of constant cell-individuals remain with their nucleus throughout life, although a used-up portion of their cell-body may be replaced by regeneration from the cytoplasm. Thus our human body, like that of all the higher animals and plants, is a "cell-state" in another sense. Every day, nay, every hour, thousands of its citizens, the tissue-cells, pass away, and are replaced by others that have arisen by cleavage of similar cells. Nevertheless, this uninterrupted change of our personality is never complete or general. There is always a solid groundwork of conservative cells, the descendants of which secure the further regeneration.