Page:Popular Science Monthly Volume 71.djvu/124

known at the present time. These tables are taken from a lecture which I delivered in New York a few years ago, which was subsequently published. If any of you should care to make a closer acquaintance with them they are therefore readily accessible to you. How then, from the standpoint of cytomorphosis ought we to look upon old age? Cytomorphosis, the succession of cellular changes which goes on in the body, is always progressive. It begins with the earliest development, continues through youth, is still perpetually occurring at maturity and in old age. The rôle of the last stage of cytomorphosis, that is, of death in life, is very important, and its importance has only lately become clear to us. I doubt very much if the conception is at all familiar to the members of this audience. Nevertheless the constant death of cells is one of the essential factors of development, and much of the progress which our bodies have made during the years we have lived, has been conditional upon the death of cells. As we have seen, cytomorphosis, when it goes through to the end, involves not only the differentiation but the degeneration and death of the parts. There are many illustrations of this which I might cite to you as examples of the great importance of the destruction of parts. Thus there is in the embryo before any spinal column is formed an actual structure which is termed the notochord. In the young mammalian embryo this structure is clearly present and plays an important part, but in the adult it has entirely disappeared, and its disappearance begins very early during embryonic life. There are numerous blood vessels which we find to occur in the embryo, both those which carry the blood away from the heart and those which bring blood to the heart, which during the progress of development are entirely destroyed, and disappear forever. Knowledge of these is to the practical anatomist and surgeon often of great importance. Vast numbers of the smaller blood vessels which we know commonly by the name of capillaries, exist only for a time and are then destroyed. There is in the young frog, while he is in the tadpole stage, a kidney-like organ, which on account of its position is called the head-kidney, but it exists only during the young stage of the tadpole. There is later produced another kidney which, from its position, is called the middle kidney, and which is the only renal organ found in the adult, for the head kidney entirely disappears in these animals long before the adult condition is reached. In the mammal there is yet a third kindey. We have during the embryonic stage of the mammal always a well-developed excretory organ which corresponds to the middle or permanent kidney of the frog, yet during embryonic life the greater part of this temporary structure is entirely destroyed. It is dissolved away and vanishes, leaving only a few remnants of comparatively little importance in the adult. The new structure, the permanent kidney which we have, takes its place