224 THE SCIENTIFIC MONTHLY
tissne^ and no defensive reaction takes plaee^ no chemically inmiiinizing substance is formed against a spontaneous, autochthonous tumor. We find, therefore, that, after transplantation into another individual, cancers not rarely retrogress spontaneously. But such retrogression hardly ever occurs in a spontaneous tumor within the individual that produced it. And, moreover, not only does such a spontaneous retro- gression not usually occur in spontaneous cancers, but we usually are not even able to produce experimentally a defensive mechanism such as appears when a transplantation occurs into another animal of the same species. Vaccination with either normal tissue or with tumor tissue that does not grow is of little or no avail for restricting the growth of spontaneous tumors.
We have now discussed the origin of tumors and the protective reac- tion in the body against tumor growth. There remains the third field upon which cancer research has shed new light. I refer to the char- acter, conditions of growth and life of tumor cells in particular and of normal body cells in general. As has already been stated, the continu- ous transplantation of tumors has made very evident one fact of great biological significance, namely, the potential immortality of ordinary body cells. It had been assumed formerly that unicellular organisms like protozoa and bacteria as well as the germ cells — especially the ova of metazoa including the vertebrates — are potentially immortal. Pro- vided no accident takes place, one protozoon can give origin to an end- less chain of new individuals of the same species. In a similar way the germ cells of a multicellular organism, for instance, of a vertebrate, develop after fertilization into adult organisms; certain cells, how- ever, fail to undergo the diflPerentiation which other cells do and they form later on the germ cell — ^the ovum or sperm cells — ^in the new individual. Through fertilization they again are preserved and again give rise to a new generation of germ cells. In this way germ cells are potentially immortal, while all the other cells of the body, which, in contradistinction to germ cells, are called somatic cells, die with the individual. Now it was supposed that the death of the somatic cells was not an accident, but a necessary event distinguishing somatic cells from germ cells. This view cancer research has shown to be incorrect. We now know that most of the cells of the body are potentially tumor cells and we also know that tumor cells are potentially immortal, in the same sense as protozoa and germ cells. When we speak of tmmor- tality" we must of course take into consideration the fact that periods of time which man can observe are always finite quantities and that to use the term immortality is to deal with only approximations. If then somatic cells usually die, it is due to a more or less unfavorable environ- ment, conditions which we can not control at the present time.
There are some other interesting conclusions regarding cell life, to which cancer research has led. We have shown that it is possible
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