TUMOR GROWTH 441 



normal into cancerous tissue, consist, and what the chemical factors are which 

 different tumors have in common. 



By means of transplantation of tumors a considerable plasticity in certain 

 functions of the cells composing tumors has been revealed ; this plasticity of 

 function is superimposed upon and interacts with the constancy of the in- 

 dividuality differentials of these tumors. In the case of normal tissues, adap- 

 tive processes are either lacking or they are very much weaker than in tumors, 

 and by comparison with the latter, normal adult tissues appear relatively rigid. 

 Because of the complexity and the relatively great plasticity in the reactions 

 of tumors, the behavior of tumors presents problems of great biological in- 

 terest, especially in view of the fact that tumors are transformed normal tis- 

 sues, and that various attributes applying to tumors apply, therefore poten- 

 tially, also to normal tissues. But these attributes become manifest only when 

 normal cells, undergoing transformation into tumors, have reached the 

 equilibrium of cancer cells. 



