206 PARASITISM 



The early history of animal cancer has a certain historical interest 

 in medical circles, but the present-day activity dates back only to 1902, 

 when Jensen, of Copenhap;en, discovered that mouse cancer (adeno- 

 carcinoma) can be transplanted from one mouse to another. With 

 more than usual breadth of view and scientific generosity, Jensen 

 distributed his cancer material to all who wished it, and the result is 

 that the "Jensen strain" of mouse cancer is being studied and trans- 

 planted in all parts of the civilized world, while special laboratories 

 for the exclusive study of cancer have been established in Buffalo, in 

 London, Heidelberg, and other places. Investigation has brought 

 out the fact that this mouse tumor differs but little from human car- 

 cinoma, while similar primary tumors are now known to occur in one 

 mouse in every 2500 (Bashford). Hundreds of such primary cancers 

 have been transplantable, so that today many in addition to the Jensen 

 strain are being studied. Malignant growths in other animals (rats 

 and dogs especially) have been discovered, and are all contributing 

 data for the ultimate control of human cancer. This dreaded disease, 

 therefore, which is still impossible to control and the cause of which is 

 still unknown, is at present in the full swing of experimental study. 



It was early shown by Jensen and his followers that a tumor induced 

 in a normal animal by inoculation is derived not by the abnormal 

 division of cells of the normal animal, but by proliferation of the 

 transplanted cancer cells of the diseased mouse. The induced tumor, 

 therefore, is not equivalent to a primary tumor, but may be regarded 

 as equivalent to a metastasis from such a primary growth. Further- 

 more, it was early shown that human cancer when similarly trans- 

 planted in mice, or any other lower animal, will not grow; nor will 

 the mouse or rat tumor grow in any other animal than the definitive 

 species. Cancer in lower animals, therefore, need not cause appre- 

 hension, although it is always possible that the unknown cause or 

 causes may be the same or similar in all cases. 



The Jensen tumor, to take only one example, has now been trans- 

 planted through nearly 100 generations, or possibly more, counting 

 as a generation the successive tumors produced by inoculation. The 

 average length of time required by the Jensen strain to develop into 

 a cancer fatal to the inocidated mouse varies from three to four weelvs, 

 but it may be reduced to ten daj's or two weeks, or increased to three 

 or four months or longer. 



This long-continued transplantation and the fact that each new 

 transplantation results in the formation of a mass of cancer cells 

 derived from the transplanted cells, yielding a growth which, up to 

 the present time, amounts to a small moinitain of mouse tissue, indi- 

 cates that the cancer cells are somehow endowed with the possibility 

 of an indefinitely continued division energy. The cancer cell, there- 

 fore, is different from any animal organism that we know, for in all 



