Evolution and death of metazoa.—Possible rejuvenescence of the differentiated cells by the conditions of the medium.—Conditions of the medium for immortal cells.—The immortal elements of metazoa.—The element in accidental and remediable death.—Somatic cells and sexual cells. _Evolution and Death of Metazoa._—We have seen that the infusoria are no longer animals in which material exchanges take place with sufficient perfection, and in which cellular division, the consequence of growth, is produced with sufficient precision and equality for life to be carried on indefinitely in a perfect equilibrium in the appropriate medium without alteration or check. _A fortiori_ we no longer find the perfect regularity of nutritive exchange in the classes above them. In a word, starting from this inferior group, there are no animated beings in the state of existence which Le Dantec calls “condition Iº of manifested life?” Living matter, instead of being continually kept identical in conditions of identical media, is modified in the course of existence. It becomes dependent on time. It describes a declining trajectory; it experiences evolution, decay, and death. Thus the fundamental condition of invariable youth and of immortality fails in all metazoa. The vital wastes accumulate in all through the insufficiency or the imperfection of nutritive absorption or of excretion. Life decays; the organism progressively alters, and thus is constituted that state of decrepitude by atrophy or chemical modification which we call senescence, and which ends in death. To sum up, old age and death may be attributed to cellular differentiation. _Possible Alimentary Rejuvenescence of the Differentiated Cells—Conditions of Medium._—We must add, however—as the teaching of experiments in general and in particular as the teaching of the experiments of Loeb and of Calkins—that a slight change of the environment, made at the right time, is capable of re-establishing equilibrium and of completely rejuvenating the infusorian. Senescence has not in this case a definitive any more than an intrinsic character; a modification in the composition of the alimentary medium will successfully resist it. If we are allowed to generalize this result, it may be said that senescence, the declining trajectory, the evolution step by step down to death, are not for the cells considered in isolation an inevitable and essentially inherent in the organism, and a rigorous consequence of life itself. They preserve an accidental character. In senescence and death there is no really natural, internal cause, inexorable, and irremediable, as was claimed in the past by J. Müller, and more recently by Cohnheim in Germany and Sedgwick Minot in America.