Many of the vital phenomena of cells may, as Martin Fischer,[82] Lumière,[83] and others point out, be interpreted in terms of the behaviour of simple hydrophilic colloids. Bechhold[84] has shown that, like a simple colloidal jelly, the cells of the human body, which are colloidal masses, lose their affinity for water progressively with age and become less elastic; the water content of the fetus being 94 per cent, as compared with 70 per cent at birth and 58 per cent in adult life, thus recalling the old idea of the dryness of elderly bodies. According to Marinesco[85] dehydration of the colloids in the cells is an inherent progressive change in evolution and leads to senescence and death. It has also been thought that as the result of differentiation and metabolism the cell protoplasm may become overladen with products inimical to its vitality, which it cannot, as can the protozoa, utilize for rejuvenation, and which hinder metabolism. The micellae, or aggregations of albumin constituting the colloids of the cells, alter in structure and become more stable until the colloid state disappears and the cell accordingly dies (Lumière, Danysz). Such changes must obviously be greatly influenced, as H. Campbell[86] and others have insisted, by their environment; the individual cells are in some degree of material continuity by “protoplasmic bridges” which probably serve not only for nutritional purposes but also for the transmission of physiological impulses. It also appears that the life cycle of the cell is determined not so much by time as by the interaction of the tissues and the plasma. By cultivating connective tissue in the plasma of chickens of different ages Carrel[87] showed that its growth was more abundant in the plasma of younger chickens than in that of older ones. Working on these lines Loeb and Northrop[88] concluded that the duration of life was determined either by a substance leading to old age or by the destruction of a substance which normally prevents old age and natural death. More recently Carrel and Ebeling[89] found that in these cultures of connective tissue the rate of multiplication of fibroblasts and the duration of life _in vitro_ varied in inverse ratio to the age of the animal from which the plasma was taken, and that this depended on the presence of an inhibitory body in the plasma of older animals. These experiments recall the practice centuries old of transfusion of young persons’ blood into the old which, however, led to disastrous results on account of a technique now known to be dangerous. Whether with modern methods success will be obtained remains to be proved, but probably repeated transfusions would be required. But to return to the subject: Carrel, Champy, and Grandcourt have shown that, with frequent washings to remove waste products, tissue cells can be cultivated indefinitely for years _in vitro_, and in themselves have an unlimited capacity for multiplication; it therefore appears that