_Metchnikoff’s Theory of Senescence. Objections._—Metchnikoff has proposed a theory of the mechanism of this general senescence. The elements of the conjunctive tissue, phagocytes, macrophages, which exist everywhere around the specialized and higher anatomical elements would destroy and devour them as soon as their vitality diminishes, and would take their place. In the brain, for example, it would be the phagocytes which, attacking the nervous cellules, would disorganize the higher elements, incapable of defending themselves. This substitution of the conjunctive tissue, which only possesses vegetative properties of a low order, for the nervous tissues, which possesses very high vegetative properties, results in an evident breaking-up. The gross element of violent and energetic vitality stifles the refined and higher element. This expulsion is a very real fact. It constitutes what is called senile sclerosis. But the active _rôle_ attributed to it by Metchnikoff in the process of degeneration is not so certain. An expert observer in the microscopic study of the nervous system, M. Marinesco, does not accept this interpretation as far as the senescence of the elements of the brain is concerned. Diminution of the cell, the decrease in the number of its stainable granulations, chromatolysis, the formation of inert, pigmented substances—all these phenomena which characterize the breaking-up of the cerebral cells would be accomplished, according to this observer, without the intervention of the conjunctive elements, the phagocytes. The characteristic of extensive and progressive process presented by death necessitates in a complex organism, which is a prey to it, the existence side by side of living and dead cells. Similarly, in the organism which is growing old, there are young elements and elements of every age side by side with senile elements. As long as the disorganization of the last has not gone too far, they may be rejuvenated. All we have to do is to restore to them an appropriate ambient medium. The whole question is one of knowing and being able to realize, for this or that part which we wish to reanimate and to rejuvenate, the very special or very delicate conditions that this medium must fulfil. As we have said, success is attained in this respect as far as the heart is concerned, and this is why we are able to reanimate and to revive the heart of a dead man. It is hoped that ideas along these lines will extend with the progress of physiology. After this sketch of the conditions and of the varieties of cellular death we must return to the essential problem which is engaging the curiosity of biologists and philosophers. Is death unavoidable, inevitable? Is it the necessary consequence of life itself, the inevitable issue, the inevitable end?