as a system of mutually dependent parts and not a specific inherent and inevitable cellular process. Cells in culture _in vitro_ do not grow old. We see none of the characteristic senescent changes in them.” Thus it may be inferred that when cells show characteristic senescent changes it is because they are “reflecting in their morphology and physiology a consequence of their mutually dependent association in the body as a whole and not any necessary progressive process inherent in themselves. Thus senescence is an attribute of the multicellular body as a whole consequent upon its scheme of morphologic and dynamic organization.” The lethal process, thus, does not originate in the cells themselves. “In short, senescence is not a primary attribute of the physiological economy of cells as such.” It has long been known, as we have seen, that unicellular organisms could go on dividing indefinitely and that germ plasm had a potential mundane immortality; but no one had suspected that highly organized and differentiated somatic cells, which had lost the power of producing the whole individual and could only produce cells of their own special tissue, had this power. Recent experiments, however, indicate that under certain highly elaborated conditions they, too, can be made to live and even grow indefinitely and that this growth can not only be observed but measured under the microscope. Many attempts had been made by many individuals to grow tissues artificially to see their development, their functions, and decay, in both health and disease. This can now be done by taking pieces of living tissue from the body, for science has never produced a single living cell, and placing it in artificial media made out of blood plasma especially prepared, for nutrition for such a bit of tissue deprived of access to the normal circulation of the blood is the prime condition for such growth.[179] Indeed, until Carrel, who had long been interested in the regenerative processes of scars, succeeded in actually causing cells of the connective tissue to grow after being deprived of the circulation of the blood, this was supposed to be impossible. Leo Loeb had already produced artificial growth within and without the body as early as 1907, and in such processes that utilized the body fluid it was found that the same course was followed as in nature, so that the processes in such culture media approximated those that followed grafting. In 1907 Harrison gave details of such a process that seemed convincing, although he worked only on cold-blooded animals, cultivating nerve fibers from the central system of the frog. Carrel extended this method to warm-blooded creatures and mammals, studying especially the laws of regeneration of tissues after surgical wounds.