functionally. Large portions of the tissues, which arise in the embryo, are destroyed at the time of birth, and take no share in the subsequent development of the child. If we follow out with the microscope the various changes which go on in the developing body we see revealed to us a very large number of cases of death of tissues, followed by their removal. Thus the cartilage which exists in the early stages dies and is dissolved away, and its place is taken by bone. Those things which we know as bony elements of the skeleton in the adult, in the embryo exist merely as cartilage, but the cartilage is not converted into bone but it is destroyed and its place taken by bone. There is overlying the heart of a child at birth a well-developed gland known as the thymus. After childhood this undergoes a retrograde development; it becomes gradually absorbed and persists only in a rudimentary condition. "With the loss of the teeth occurring during infancy, you are familiar, and know that the first set of teeth are but for a short period, and are to be replaced by the permanent set. In very old persons we see a great deal of the bony material absorbed, and this absorption of the bone is a phenomenon which occurs at almost every period of the development. Portions of the epidermis or outer skin are constantly shed, as is well known, and the loss of hair and the loss of portions of our nails are so familiar to us that we hardly heed them. Of the constant destruction of the cells, which are found in the lining of the intestine, I have already spoken. At all times in the body there is a vast amount of destruction of blood corpuscles going on, a destruction which is physiologically indispensable, for the material which the blood corpuscles furnish is used in many ways. For instance, the pigment which occurs in the hair is supposed to be derived from the chemical substances the use of which the body obtains by destroying blood corpuscles. One of the most familiar instances of destruction is that of the tail of the tadpole. The young frog and the young toad during their larval stages live in the water and each of them is furnished with a nice tail for swimming purposes. As the time approaches for the metamorphosis of the tadpole into the adult, the tail is gradually dissolved away. It is not cast off, but it is literally dissolved, resorbed, and vanishes ultimately altogether.
It is evident that such a vast amount of destruction of living cells could not be maintained in the body without the body going entirely to destruction itself, were there not some device for making good the losses which are thus brought about. We find in fact that there is always a reserve of cells kept to make good the loss which it is essential should be made good. Some losses apparently do not have to be repaired, but the majority of them must be compensated for, and this is done by having in the body a reserve supply of cells which can produce new cells of the sort required. This leads us to consideration of the phenomenon of regeneration and of the repair of parts.
