BACTERIA AND TUMORS 155 



tumor sizes and weights in treated and untreated groups of mice bear- 

 ing sarcoma, after a period of 48 hours of growth. The groups were 

 matched as to initial size of the tumors. The selective damage of sar- 

 coma cells, as compared with normal cells, said to be caused by penicillin 

 (150a) was later shown (525a) to be due not to the pure penicillin it- 

 self but to some impurity present in crude penicillin preparations. 



