Chemical Basis of Genus and Species 45 



the so-called lymphocytes, which develop in the lym- 

 phatic tissue, gather at the edge of the graft in great 

 numbers, and he suggested that these lymphocytes (by 

 a secretion of some substance?) rid the host of the 

 graft. He applied two tests both of which confirmed 

 this idea. First he showed that when small fragments of 

 the spleen of an adult chicken are transplanted into the 

 embryo the latter loses its tolerance for foreign grafts. 

 The second proof is still more interesting. It was known 

 that by treatment with Roentgen rays the lymphocytes 

 in an animal could be destroyed. It was to be expected 

 that an animal so treated would have lost its specific 

 resistance to foreign tissues. Murphy found that this 

 was actually the case. On fully grown rats in which 

 the lymphocytes had been destroyed by X-ray^s (as 

 ascertained by blood counts) tissues of foreign species 

 grew perfectly well. These experiments have assumed 

 a great practical importance since they can also be 

 applied to the immunization of an animal against 

 transplanted cancer of its own species. Murphy found 

 that by increasing the number of lymphocytes in an 

 animal (which can be accomplished by a mild treatment 

 with X-rays) the immunity against foreign grafts as 

 well as against cancer from the same species can be 

 increased. It is quite possible that the apparent im- 

 munity to a transplantation of cancer produced by 

 Jensen, Leo Loeb, and Ehrlich and Apolant through 

 the previous transplantation of tissue in such an animal 



