“The special rapidity of the growth of the tissue depends so much on the composition of the medium that it may become possible to use as a reagent of the dynamic value of the humors of the organism a strain of cells adjusted to life _in utero_.” If human connective tissue could be preserved in the condition of permanent life as the connective tissue cells of a chicken are preserved, the value of the plasma of an individual might be approximated by the cultivation in it of a group of these cells and by the observation of the rate of their multiplication. Such observations do suggest some indication of certain values of the blood of an organism and may give us some clue to old age. Thus in the course of development the activity of the tissue is apt to vary in the body as a whole and in its parts. It therefore became a question whether each particular condition was permanent or whether the dynamics of the cell changes through the action of the medium upon it. To determine this, several bits of tissue, each having its own dynamic power, were cultivated in media exactly alike and differences in the character of the growth were noted. Then the influence of the medium began to tell. Measurements of the changes undergone on the part, in turn of a fast- and slow-growing tissue, showed that the former had lowered its activity one-half in forty-eight hours, while the latter had multiplied its activity by six. This process continued until the level of uniformity was reached, when the conditions of growth remained equal in all cases. Thus it appears that though, in the beginning, certain substances that the tissues had accumulated had the effect of accelerating or retarding its activity in the medium, yet in time the latter overcame these conditions and growth was brought under the laws of its own special mechanism. Thus the sum of the investigations on the influence of the medium on cells is that it may not only change the dynamic possibilities of the tissue but the character of the change may be regulated by a carefully considered modification of the medium (Grandcourt). All this work involves the theory that the cells make such demands upon the nutrition supplied by the medium that they deplete it and then become indirect means of introducing into the life process a chemically destructive activity (catabolism). The result is a gradual slowing down of cell growth, which is progressive aging and death. A very analogous course was that followed in the earlier artificial cultivations. The tissues lived a short span of days and then died. But the process of degeneration could be obviated by salt solutions and other processes so that tissues now grow _in vitro_ for a year and a half and may continue to multiply faster than those of the embryo. Thus for such tissues senility does not exist and the question naturally arises whether we can ever hope to accomplish anything of this sort inside the body.