Accepting Metchnikoff’s dictum that senility is atrophy and that toxins of intestinal origin poison and debilitate tissues so that they succumb to, if they do not actually attract, the predaceous phagocytes (though not proposing his substitute of sour milk for religion and philosophy), Minot points out that we are always throwing off dead cells. Blood corpuscles collapse and are utilized by the liver; the skin is incessantly shedding dead cells, as is the whole intestinal tract and each organ; stature declines some 13 cm.; the brain loses some 19 gms. in weight; the rate and depth of respiration sink; the heart, although growing larger and from the age of prime to senility beating some eight times per minute faster, is nevertheless obstructed in its action by rigidifying arteries; the bones grow spongy and their hard outer part becomes a thin shell; the muscle fibers decline both in size and number, exercise being able to increase only the former and not the latter; both structure and function go on to rigidity and inflexibility after sufficient firmness and size have been attained, till the part becomes too hard and inflexible to function and then is shed as the ripened leaf falls in autumn. But none of these processes are abnormal and hence death is in no sense a disease. Indeed, the power of repair and even recuperation persists far more in the old than has been generally recognized. The more specific cause of what is generally called old age he finds in the increase of the quantity and the hyperdifferentiation of the structure of the protoplasmic envelope of the nucleus. This protoplasm constitutes the body of the cell. In the earliest stages of cytomorphosis, which follow impregnation, the total amount of nuclear material increases fastest, while later and especially in the senescent cells it is the protoplasm that does so. In the early stages of their embryonic development, too, the cells differ relatively little; but those that constitute the adult body differ so greatly that any skilled observer can tell from which organ they came, whether from the brain, muscle, skin, stomach, liver, etc.; that is, they differentiate more and more as these organs mature. This differentiation is, however, all on the way to death and is never reversible; that is, old body cells never grow young. Nuclei change but it is the protoplasm that changes the most and acquires a new structure, while the composition of the nucleus not only changes less but always retains certain fundamental traits. “The increase of the protoplasm, together with its differentiation, is to be regarded as the explanation (or should we say cause?) of senescence” (p. 134). This is necrobiosis. All old cells, from whatever organ, are thus as recognizable as old faces. “Growth and differentiation of protoplasm are the cause of the loss of the power of growth” (p. 161). He even holds that the first stages of the segmentation of the ovum must be called rejuvenation. On page 167 he says: