igoS.] LOEB— TUMOR GROWTH AND TISSUE GROWTH. 9 



conditions. This explains a fact very familiar to surgeons ; namely, 

 that after an operation a recurrent tumor is frequently more malig- 

 nant than the original tumor. 



But it is also possible to decrease the power of propagation of 

 tumor cells without killing them by exposing the cells to chemical 

 and physical injurious influences, in a way similar to that pursued 

 in the case of bacteria. Here, also, we may, not without some hope, 

 look forward to the preparation of some vaccine that may, some day 

 in the future, help us to combat the dreaded disease. Even in this 

 case, however, tumor tissue probably differs only in degree, and not 

 in principle, from normal tissue. At least, this conclusion is indi- 

 cated by the fact that such an organ as the normal thyroid gland 

 may, without being entirely destroyed, be markedly weakened in its 

 power of growth through a short exposure to the air before trans- 

 plantation. 



There exist, however, some interesting differences of 'another 

 kind between tumor tissue and normal tissues or organs. Normal 

 organs have a specific metabolism and, in connection with or as a 

 part of this metabolism, they exert distinct specific functions. We 

 understand by functions those physical and chemical processes which 

 attract our attention by their real or apparent significance for the 

 organism as a whole. The normal female mammary gland, for 

 instance, secretes milk under the influence of certain chemical 

 stimuli which are present in the circulation at the end of pregnancy ; 

 and it also grows during pregnancy, under the influence of similar 

 stimuli. If we now transplant the mammary gland of a nonpreg- 

 nant animal into a pregnant animal, the foreign transplanted gland 

 may secrete milk at the end of pregnancy in a similar way to that 

 of the animal's own gland. The circulating chemical substance 

 exerts the same stimulus upon the transplanted as upon the autoch- 

 thonous gland, and the transplanted gland responds to the stimulus 

 in the normal way. 



There exist certain conditions in which a tumor-like hypertrophy 

 of the mammary gland is found in the white rat. The structure of 

 the gland is slightly modified, but the tumor is not infiltrating. We 

 do not call it a cancer, but a benign tumor — an adenoma. If we 

 transplant such a tumor to another place in the original animal, it 



