These phenomena we can better take up later in our course, when we have dealt with the general processes of development and growth. From the study of regeneration we shall be able to confirm the explanation of old age, which I want to lay before you. This confirmation is so important that it will be better taken up in a separate lecture, than slipped in now when the hour is nearly by.
Old age, after what I have said, I think you will all recognize as merely the advanced and final stage of cytomorphosis. Old age differs but little in its cytomorphosis from maturity; maturity differs much from infancy; infancy differs very much indeed from the embryo; but the embryo differs enormously from the germ in its cytomorphic constitution. We know that in the early time comes the great change, and this fact we shall apply for purposes of interpretation later on. Cytomorphosis is then a fundamental notion. It gives us in a general law, a comprehensive statement of all the changes which occur in the body. None, in fact, are produced at any period in any of us except in accordance with this general cytomorphic law. There is, first, the undifferentiated stage, then the progressive differentiation; next there follows the degenerative change ending in death, and last of all the removal of the dead cells. Such we may conveniently designate as the four essential stages of cytomorphosis. This cytomorphosis is at first very rapid; afterwards it becomes slower. That is a significant thing. The young change fast; the old change slowly. We shall be able, when we get a little farther along in our study, to see that in differentiation lies the explanation of a great deal of biological knowledge, lies the explanation of our conception of cell structures; and in it also lies not only the explanation of the death of cells, but also, as it seems to me—and this is one of the points that I shall want particularly to bring forward before the close of the course—of general death, that which we mean by death in common parlance, when the continuation of the life of the individual ceases, and is thereafter bodily impossible. The explanation of death is one of the points at which we shall be aiming in the subsequent lectures of the course. Now we know that in connection with age there is always growth. I propose, therefore, in the next lecture, to take up the subject of growth. We shall arrive at some paradoxical conclusions, for it can be shown by merely statistical reckonings that our notion that man passes through a period of development and a period of decline is misleading, in that in reality we begin with a period of extremely rapid decline, and then end life with a decline which is very slow and very slight. The period of most rapid decline is youth; the period of slowest decline is old age, and that this statement is correct I shall hope to prove to you with the aid of tables and lantern illustrations at the next lecture.
