The reason why the body can only last a certain time, in spite of the many quacks with recipes for immortality—recipes including such items as the avoidance of all trouble, worry, or work—must remain a secret until we know the chemical basis of life. It seems to lie in the cell. If a unicellular organism, as described above, be placed in a vessel of sterilized water and left to live alone under otherwise ideal conditions, it will start dividing and multiplying, as though it meant to reproduce itself indefinitely. After a time, however, the shoal begins to deteriorate; each successive generation is feebler than the last, and eventually all die. If, however, before this happens one of the effete cells be placed in another vessel with a similar individual derived from a different ancestor, the two will fuse and form a single fresh animal with entirely restored vigour, ready to multiply to the same extent as either of its original ancestors. A few individuals of a different stock will in this way revivify the whole brood. There is, therefore, evidently something in the cell which wears out after it has divided a certain number of times—something which must be restored by blending with cells of another strain. What this is we do not know, and perhaps never shall. The most we know is that it seems to be something inherent in the nucleus, not the main body of protoplasm of the cell, for some unicellular animals do not fuse, under the circumstances related above, but exchange only pieces of their nuclei, and yet derive the advantages of mutually increased vitality. But if we apply the fact to our conception of the body as a vast colony of cells with a common origin, we find that it has an important bearing upon the duration of its life. The single eggcell which gave rise to our schematic embryo in Diagram 3, Fig. 1, was formed by the fusion of two cells shed by two distinct animals. How this one cell grows and multiplies by division is roughly shown in the diagram and those immediately following it; but though the cells do not separate, but hang together and form a body, it is obvious that the colony only amounts to a shoal of unicellular organisms, like that described above as growing weaker with each successive division unless blended with individuals of a different stock. This cannot be done in the body; what would become of our individuality even if such a thing were possible? The body can help to give rise to new bodies, but its own tissues must wear out, and when the colony of cells is exhausted it must die. Careful diet and regular habits, the minimum of wear and tear, may enable the body to run its full term; but they cannot lengthen the lease of life.