I think it indubitable that in senescence the most active factor is some alteration in the higher cells of the body, accompanied by a destruction of these by macrophags which gradually usurp the places of the higher elements and replace them by fibrous tissue. Such a process affects the organs of secretion (kidneys), the reproductive organs, and in a modified form the skin, the mucous membranes, and the skeleton. The testes are amongst the organs which resist invasion by macrophags. I have already given an example (“The Nature of Man,” p. 98) of an old man of 94 in whom active spermatozoa were produced. I know of a similar case, the age being 103 years. Such cases are not rare, and not only in old men, but in old animals, the testes continue to be active. Dr. Weinberg and I have investigated these organs in a dog which died at the age of 22 years after several years of pronounced senility. Many of the organs of the animal exhibited serious invasions by macrophags but the testes were extremely active, the cells being in free proliferation and producing abundant spermatozoa (Fig. 11). In harmony with this condition of the sexual organs, the sexual instincts of the animal remained normal. We have investigated another dog which died at the age of eighteen years. In this case the testes were cancerous and there was no possibility of the production of spermatozoa. None the less, this dog although markedly senile (Fig. 12) still showed sexual instincts until shortly before it died. [Illustration: FIG. 12.—An old dog, aged eighteen years.] It is manifest that the tissues do not invariably degenerate in old age, nor do all the organs that are modified in old age show destruction by phagocytes and replacement by connective tissue. Organs which produce phagocytes, such as the spleen, the spinal marrow and the lymphatic glands, certainly show traces in old age of fibrous degeneration but remain sufficiently active to produce macrophags which destroy the higher cellular elements of the body. I have frequently noticed cell division in such organs, and as an example may give the case of the bone marrow taken from a man of 81 years (Fig. 13).