as otherwise the synthesis of specific proteins would have to come to a standstill.

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.'

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, 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 warmblooded 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. 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;

3

'Loeb, Leo, Jour. Med. Res., 1901, vi., 28; Arch. f. Entwicklngsmech., 1907, xxiv., 655.

Loeb, Leo, Über die Entstehung von Bindegewebe, Leucocyten und rothen Blutkörperchen aus Epithel und über eine Methode isolierte Gewebsteile zu züchten. Chicago, 1897.

3 While this has been demonstrated thus far only for connectivetissue cells it may be true also for other cells.

and every two days a few cells from the margin of the piece have to be transferred to a new culture medium.

This effect of lack of oxygen explains also why the immortality of the somatic cells is not obvious. Death in a human being consists in the stopping of heart beat and respiration, which also terminates the action of the brain or at least of consciousness. Immediately after the cessation of heart beat and respiration the cells of muscle and of the skin and probably many or most other organs are still alive and might continue to live if transferred to another body with circulation and respiration. As a consequence of the lack of oxygen supply in the dead body they will, however, die comparatively rapidly. It may be stated that hearts taken out of the body after a number of hours can still beat again when put into the proper solutions and upon receiving an adequate oxygen supply.

The idea that the body cells are naturally immortal and die only if exposed to extreme injuries such as prolonged lack of oxygen or too high a temperature helps to make one problem more intelligible. The medical student, who for the first time realizes that life depends upon that one organ, the heart, doing its duty incessantly for the seventy years or so allotted to man, is amazed at the precariousness of our existence. It seems indeed uncanny that so delicate a mechanism should function so regularly for so many years. The

mysticism connected with this and other phenomena of adaptation would disappear if we could be certain that all cells are really immortal and that the fact which demands an explanation is not the continued activity but the cessation of activity in death. Thus we see that the idea of the immortality of the body cell if it can be generalized may be destined to become one of the main supports for a complete physico-chemical analysis of life phenomena since it makes the durability of organisms intelligible.

5. This generalized idea of the immortality of some or possibly most or all somatic cells has a bearing upon the problem of the origin of life on our planet. The experiments of Spallanzani, Schwann, Schroeder, Pasteur, Tyndall, and all those who have worked with pure cultures of micro-organisms, have proved that no spontaneous generation of living from non-living matter can be demonstrated; and the statements to the contrary were due to experimental errors inasmuch as the new organisms formed were the offspring of others which had entered into the culture medium by mistake.

In the last chapter of that most fascinating book Worlds in the Making,' Arrhenius discusses the possibility of life being eternal and of living germs of very small dimensions-e. g., the spores of micro-organismsbeing carried through space from one planet to another

'Arrhenius, S., Worlds in the Making, London and New York, 1908, P. 212.

or even from one solar system to another. If it be true that there is no spontaneous generation; if it be true that all cells are potentially immortal, we may indeed seriously raise the question: May not life after all be eternal? Such ideas were advocated by Richter in a rather phantastic way and more definitely by Helmholtz as well as Kelvin. The latter authors assumed that in the collision of planets or worlds on which there is life, fragments containing living organisms will be torn off and these fragments will move as seed-bearing stones through space. "If at the present instant no life existed upon this earth, one such stone falling upon it might . lead to its becoming covered with vegetation." Arrhenius points out the difficulties which oppose such a view, as, e. g., the fact "that the meteorite in its fall towards the earth becomes incandescent all over its surface and any seeds on it would therefore be deprived of their germinating power."

Arrhenius suggests another and much more ingenious idea based on the fact that for particles below a certain size the mechanical pressure produced by light wavesthe radiation pressure can overcome the attractive force of gravitation.

Bodies which according to Schwarzschild would undergo the strongest influence of solar radiation must have a diameter of 0.00016 mm. supposing them to be spherical. The first question is therefore: Are there any living seeds of such extraordinary minuteness? The reply of the botanist