The life of the cell has two phases--an early brief one during which the young material is produced and the later and prolonged one in which the process of differentiation goes on; and that which was young, through a prolonged senescence becomes old. I believe these are the alternating phases of life, and that as we define senescence as an increase and differentiation of the protoplasm, so we must define rejuvenation as an increase of the nuclear material. The alternation of phases is due to the alternation in the proportions of nucleus and protoplasm. In adults, and even in the old, there are always young cells in reserve, often grouped in certain foci, for example, the marrow of the bones, which can in emergencies come forward, take up the function of growth, regenerate lost tissues or, in lower animals, even lost organs. At and even after the death of the aged there are always cells and even parts that are relatively young and growing. There are also, of course, the cells and their matrix, which are very early set apart for the purpose of reproduction, and these, of course, are least of all differentiated. Most cells of the body, however, follow the law of genetic restriction. This means that as differentiation proceeds, the possible directions in which cells can develop become more and more limited till finally they cannot divide at all and lose even the power of nourishing themselves, and so die. The cell and all of it represents life, and Minot has no use for any of the smaller metamicroscopic vital units, gemules, plastidules, plasomes, ideosomes, granules, etc., but thinks that if we wish to accept any kind of ultimate elements of this sort, Weismann’s scheme of them is perhaps, on the whole, the best. As to the practical questions, how we can help rejuvenation and delay senescence, he states that he has nothing to suggest, although he believes it possible that some time in the future a means may be found of increasing the activities and volume of the nucleus and restricting the growth and differentiation of the protoplasm, which would mean a prolongation of youth.