INHERITANCE IN TRYPANOSOMES 693 



ment of infections of T. brucei by parafuchsin. These observers 

 found that on feeding mice infected with brucei With parafuch- 

 sin the trypanosomes disappeared from the blood for a long time. 

 Eventually, however, they reappeared in the circulation of the 

 mouse but could be banished again by a second course of 

 feeding with parafuchsin. But this alternation did not continue 

 indefinitely, as the intervals between the reappearances (or 

 " relapses ") of the trypanosomes gradually became shorter 

 until finally the drug had no effect on the parasites. In other 

 words, a race of trypanosomes had been produced able to 

 resist treatment with parafuchsin. Moreover, when a second 

 mouse was infected with this race the trypanosomes still main- 

 tained their power of resisting the drug. 



It was next found possible to develop races of trypanosomes 

 resistant to other trypanocidal agents and thus the important 

 fact was established that these parasites are able to acquire 

 the power of resisting the effects of drugs. 



Shortly after atoxyl was introduced as a trypanocidal 

 agent, Browning produced in mice an atoxyl-resistant race of 

 trypanosomes. This resisting power persists through a large 

 number of generations, as long as the trypanosomes are kept 

 in the species in which the property was developed. Thus 

 one atoxyl-resistant race of T. brucei in mice still maintained 

 its power of resistance after having passed through 140 mice 

 during a period extending over fourteen months. In another 

 case, however, the resistance, though still marked after the 

 trypanosomes had passed through sixty-seven mice in six 

 months, was not apparent about seven weeks later after the 

 organisms had passed through eighty-nine mice. 



It was subsequently demonstrated by Mesnil and Brimont 1 

 that atoxyl-resistant races of trypanosomes were only resistant 

 in the species of animal in which this property had been 

 developed. .Their race of T. evansi maintained unimpaired its 

 acquired power of resisting atoxyl even after no passages 

 through mice without any fresh contact with the drug. Yet 

 when this race was injected into white rats, the trypanosomes 

 at once became susceptible to the action of atoxyl, all power 

 of resistance having disappeared. When this race had been 

 passed through 20 rats without becoming atoxyl-resistant, it 



1 Mesnil and Brimont (1908), " Sur les proprietes des races de Trypanosomes 

 r6sistant.es aux medicaments," Attn. I?ist.\Pasteur, vol. xxii. pp. 856-975. 



