This problem of synthesis leads to the assumption of immortality of the living cell, since there is no _a priori_ reason why this synthesis should ever come to a standstill of its own accord as long as enough food is available and the proper outside physical conditions are guaranteed. It is well known that Weismann has claimed immortality for all unicellular organisms and for the sex cells of metazoa, while he claimed the necessity of death for the body cells of the latter. Leo Loeb was led by his investigations on the transplantation of cancer to assume immortality not only for the cancer cell but also for the body cell of the organism. He had found in transplanting a malignant tumor from one individual to another that the tumor grew; that it was not the cells of the host but the transplanted tumor cells of the graft which grew and multiplied, and that this process could be repeated apparently indefinitely so that it was obvious that the transplanted tumor cells outlived the original animal. Such experiments have since been carried on so long that we may now say that an individual cancer cell taken from an animal and transplanted from time to time on a new host lives apparently indefinitely. Leo Loeb had found that these tumor cells are simply modified somatic cells. He therefore suggested that the somatic cells might be considered immortal with the same right as we speak of the immortality of the germ cells of such animals.[23] [23] Loeb, Leo, _Jour. Med. Res._, 1901, vi., 28; _Arch. f. Entwicklngsmech._, 1907, xxiv., 655. This view receives its support first from the fact that certain trees like the _Sequoia_ live several thousand years and may therefore be considered immortal and second, from the method of tissue culture. The method of cultivating tissue cells in a test tube, in the same way as is done for bacteria, was first proposed and carried out by Leo Loeb, in 1897,[24] but his test-tube method did not permit the observation of the transplanted cell under the microscope. This was made possible by a modification of the method by Harrison, who established the fact that the axis cylinder grows out from the ganglionic cell. Harrison and Burrows then perfected the method for the cultivation of the cells of warm-blooded, animals, and with the aid of these methods Carrel succeeded in keeping connective-tissue cells of the heart of an early chick embryo alive more than four years, and these cells are still growing and dividing.[25] Only very tiny masses of cells can be kept alive in this way since all the cells in the centre of a piece die on account of lack of oxygen; and every two days a few cells from the margin of the piece have to be transferred to a new culture medium. [24] Loeb, Leo, _Über die Entstehung von Bindegewebe, Leucocyten und rothen Blutkörperchen aus Epithel und über eine Methode isolierte Gewebsteile zu züchten_. Chicago, 1897.