Page:Popular Science Monthly Volume 71.djvu/515

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Ladies and Gentlemen: I have referred in these lectures repeatedly to the cell and its two component parts, the nucleus and the protoplasm. To-night I shall have only a few references to make directly to these, and shall pass on for the latter part of the hour to another class of considerations bearing upon the problem of age. Before we turn to these new considerations, however, I wish to say a few words by way of recapitulation concerning the changes in the cells as corresponding to age. Cells, as you know from what I have told you, undergo in the body for the greater part a progressive change which we call their differentiation. We may say that there are four kinds of cells for purposes of an elementary classification to be used in a simple exposition like the present. The first kind are those cells of the young type, in which the protoplasm is simple, and shows as yet no trace of differentiation. These cells are capable of rapid multiplication, and some of them are found still persisting in various parts of the adult body, and serve to maintain the growth of the body in its mature stage. Another class of cells presents to us the curious spectacle of a partial differentiation; such are the muscle fibers by which we accomplish our voluntary movements. These fibers consisted originally only of protoplasm with the appropriate nuclei, but, as they are differentiated, part of the protoplasm changes into contractile substance. Another part remains pure protoplasm unaltered. If now the muscular or contractile portion of the fiber be destroyed, the undifferentiated part of the protoplasm then shows that it has still the power of growth. It has only been held back by the condition of organization, and we see in the regeneration of these fibers evidence of the fact that so long as the protoplasm is undifferentiated it has the power of growth, which, however, does not reveal itself unless an opportunity is afforded. Third, we come to the cells which are moderately differentiated; such, for instance, are the cells of the liver, and, if for any reason a portion of the liver be injured by accident or disease, we find that these partially differentiated cells reveal at once that they have a limited power of growth still left. If we pass on to the fourth class, that in which differentiation is carried to the highest extreme, we find that the cells do not have the power of multiplication. Such are the