8 LOEB— TUMOR GROWTH AND TISSUE GROWTH. [January 17, 



an organism that dies, and because a certain number of cells, espe- 

 cially of nervous character, cannot survive. 



The inevitable fate of all metazoan organisms is death ; and 

 this conception deeply influenced all our valuations and directions of 

 thought, as Metchnikoff only recently pointed out in his book on the 

 " Nature of Man." Weismann added one consoling idea : not all 

 of our cells must necessarily die, but only the so-called somatic cells ; 

 the germ-cells, ova and the sperm-cells, of each individual may 

 propagate forever, may be immortal. The results of the tumor 

 investigations just mentioned may, perhaps, enlarge the number of 

 cells that may remain alive for so long a period that we cannot see 

 the end at present ; ordinary somatic cells may propagate through 

 many generations, long after their brother cells that remained in 

 the original organism have been transformed into simple chemical 

 substances, and who can at present deny the possibility that they 

 may have the potentiality of immortality, as well as the germ cells? 

 Thus the work on tumors leads us into different realms of general 

 biology, and opens up new fields that are not without interest. 



The experimental work on tumors has given some other results 

 of an unexpected nature. One of the great achievements of the last 

 century was the development of bacteriological technique by which 

 it is possible for us not only to cultivate bacteria on artificial culture- 

 media, but also to influence markedly their behavior, functions, vital- 

 ity, and virulence. It has been found to be possible to raise the viru- 

 lence of certain bacteria by inoculating them into animals through 

 several generations ; on the other hand, it is possible to decrease their 

 virulence by subjecting them to certain injurious chemical or physical 

 agencies. Such a bacterial culture with artificially decreased viru- 

 lence has been used as a vaccine ; that means, as a substance that, 

 when inoculated into human beings or animals, without causing the 

 disease, confers immunity against the virulent bacilli. 



In experimenting with tumor cells, the surprising result was 

 obtained that, through successive transplantations, by cutting out 

 pieces of tumor, an artificial stimulus is given to the tumor cells, 

 so that they begin to grow more rapidly and more extensively. In 

 other words, their virulence has been increased. This is due to a 

 direct stimulating action upon the tumor cells, and not to secondary 



