May 14, 1915] 



SCIENCE 



705 



not the cells of the host grow into a new 

 cancer. He suggested in 1901 that this 

 claim might be extended to somatic cells in 

 general. 



The idea suggests itself that not only the germ 

 cells can he immortal, but that perhaps also the 

 somatic cells, like conneetive tissue cells, might, 

 under certain conditions, live for a long period, 

 much longer than the individual life of the organ- 

 ism of which they were a part, that they might 

 perhaps also be immortal ia the same sense as the 

 ovum is. 3 



Eeturning to the same problem in 1907 

 he added the following remarks : 



There exists another very striking phenomenon 

 in the growth of malignant tumors, to which I 

 called attention in my first communications on the 

 transplantation of tumors, namely the fact that 

 tumor cells have apparently an unlimited existence 

 and that they seem to resemble in this respect the 

 germ cells. It is certain that their life and growth 

 exceeds that of the other somatic cells of the in- 

 dividual, from which they are taken. But at 

 present we are not yet justified iu saying that the 

 tumor cells differ in this specifically from certain 

 other somatic cells. It has been tacitly assumed 

 thus far that the somatic cells of the metazoa have 

 only a limited existence, but no attempt has been 

 made to determine exactly the possible duration 

 of life of somatic cells. We must therefore con- 

 sider the possibility that certain somatic cells pos- 

 sess the same apparently unlimited duration of life 

 as somatic tumor cells. . . . This seems to be a 

 biological problem of great bearing to which the 

 experimental investigation of tumors has led, and 

 it might be possible to decide experimentally 

 whether or not other cells resemble tumor cells in 

 this respeet.4 



The experimental decision seems to have 

 been furnished, since Carrel has succeeded 

 in keeping conneetive tissue cells from a 

 chick embryo alive for over three years, 

 and these cells are still growing and divi- 

 ding. It should be added, however, that 

 similar attempts with other cells have not 

 yet met with the same success. 



3 Leo Loeb, ' ' On Transplantation of Tumors, ' ' 

 Jour. Med. Bes., VI., 28, 1901. 



■* Leo Loeb, ' ' Beitrage zur Analyse des Gewebe- 

 wachstums," Arch. f. EntwioTclungsmech., XXIV., 

 655, 1907. 



While thus theory and experience seem 

 to agree to some extent, a closer examina- 

 tion of actual conditions reveals a some- 

 what different and more complicated situa- 

 tion. The egg cell, for which Weismann 

 claimed immortality, can not grow and 

 develop and will die quickly if it is not 

 fertilized at a certain stage of its existence. 

 The cells in the body will not grow con- 

 stantly as our definition seems to demand, 

 but their growth is followed by a period of 

 rest from which they may be aroused by 

 special substances or by a wound. More- 

 over, all differentiation of form in animals 

 and plants depends on the fact that the 

 different parts grow with different veloc- 

 ities, since otherwise all organisms would 

 be perfect spheres. 



In reality then the resting condition of a 

 cell seems to be as much a part of real life 

 as growth and cell division. Yet the defi- 

 nition from which we started is apparently 

 correct, and it may be that we have to de- 

 fine the additional conditions which make- 

 a resting cell possible and which will wake 

 a resting cell from its slumber. 



In the usual treatment of the problem of 

 growth the increase of mass of the whole 

 organism is taken into consideration. 

 While this method is adequate for the study 

 of the relation of nutrition to growth, it is 

 not adequate for the study of the stimula- 

 tion of growth. In the latter ease we must 

 remember that it is the individual cell 

 which grows, and that we must therefore 

 study the mechanism of this stimulation in 

 the individual cell and not in the organism 

 as a whole. The ideal object for this study 

 is the egg cell, since we can observe it in 

 the condition of rest as well as of ceU divi- 

 sion and growth. 



Since usually cell division follows growth 

 and is possibly a consequence of the in- 

 crease of mass of the cell, this rule does not 



