Thus experiment seems to establish the fact that even connective tissue, composed of not the most highly developed but of vigorous though low-level cells, is immortal. Senility and death result because in normal conditions the blood does not succeed in removing waste products. Could science only wash them away in a living organism, life might be indefinitely prolonged. It is these connective tissues that give support to the textures that compose the body and that chiefly make up bone, cartilage, ligaments, and the lymph network, the cells of which are endowed with special properties of growth and play a great rôle in rejuvenating injured tissue. All this work, in a sense, started from Claude Bernard’s principle that the life of an organism is dependent on the interaction of its cells and the medium in which they grow. Thus, to understand the process by which the body develops and why it must yield to decay and death, we must inquire into the cause of the loss of character of these interactions; and this was impossible until tissue could be grown outside the body so that the processes might thus be brought within the range of the microscope and all its conditions under control. Carrel’s first effort, thus, was directed toward the way in which the medium affected the life of the cell and in constituting this medium of plasma from the blood of dogs and chickens he found that the older the animal from which the blood was taken, the less rapidly and extensively the tissues grew in it. In the blood of a relatively old animal the increase became so slight as to be practically _nil_. These comparative experiments were made, Grandcourt tells us, with the blood of animals from five months to five years of age, and there was enormously greater activity on the part of the blood of growing animals. Thus it would seem that when an animal attains its size and stops growing, its blood undergoes progressive changes till it lacks, more and more, the dynamic power of youth. So the problem was whether the plasma could be given the force of youth so far as its action on growing cells was concerned and this was accomplished by mixing it with juices extracted from the embryo. Experiments, too, were made with a strain of connective tissue cells that had been kept in artificial life for more than sixteen months. It was divided into two parts, one of which was grown on adult plasma and the other in a mixture of two parts, one of plasma and the other of embryonic juice. In two days the ring of tissue around the second part was three times as great as that around the first. Some of these tissues, passed through a salt solution 130 times, doubled their area in forty-eight hours. Another, washed 57 times, increased in volume fifteen times in ten days, etc. These rapid growths, however, could not be duplicated in normal plasma which was then further modified. Thus the different media have a pretty constant effect upon the rate of growth. Carrel says: