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



ondary growth ; they also determined that a certain number of 

 tumors apparently originate in tissue that has been misplaced during 

 embryonic development. In other cases, long irritation, and occa- 

 sionally a traumatism, may be held responsible for the origin of 

 cancer. Apparently, however, no further progress could be made 

 by these means of observation. The investigations seemed to have 

 arrived at a dead point. 



After a few isolated previous attempts, mainly since the year 

 1899, the attention of the investigators was directed to the occur- 

 rence of tumors in animals ; to the fact that cancer in animals fre- 

 quently occurs endemically. This means that a number of animals 

 are affected with cancer simultaneously in a certain locality. Fur- 

 thermore, they observed that certain kinds of tumors are charac- 

 teristic for certain species of animals ; and that the tumors occurring 

 endemically in a species of animals are all of the same type. 



The most important fact, however, which was fully developed only 

 within the last eight years, is that it is possible to transplant a certain 

 number of cancers into other animals of the same species. Many at- 

 tempts have been made to transplant cancers into animals of other spe- 

 cies and make them grow' in these animals, but without any success. A 

 certain kind of cancer found in the dog can be made to grow in some 

 related species, as, for instance, in the fox. Other tumors found in 

 white rats may be transplanted into hybrids between white and gray 

 rats, and the cancer of white mice can occasionally be made to grow 

 in gray mice. The cancer of a Japanese mouse could not be success- 

 fully transplanted into white mice, however, but only into the Japa- 

 nese mice. No such tumors can be transplanted into more distantly 

 related animals, nor can the cancer of man be transplanted into lower 

 animals. A very malignant tumor from a mouse can occasionally 

 be made to groiMifor a few days in a rat, but the growth soon stops. 

 In a similar wa^^inormal tissues of the body, for instance the epithe- 

 lium, may beldrfensplanted into other animals of the same species, 

 and kept therQiafliive after an initial growth ; but if transplanted into 

 an animal of aootlier species, it grows for a short period and then 

 it dies. 



Some tumors, and probably the majority of them, can be trans- 

 planted only into the same animal in which they have originated. 



