696 SCIENCE PROGRESS 



These experiments clearly show that during the first ten 

 days after being taken into the gut of the louse the trypanosomes 

 maintain their acquired character of resisting the drug ; from 

 the tenth to the twelfth day some change takes place in the 

 parasite which involves the disappearance of the acquired 

 character and after this date the trypanosomes are quite normal. 

 Another interesting feature brought out by these results is 

 the proof that the louse acts as a true intermediate host in the 

 transmission of Trypanosoma lewisi from one rat to another. 



Prowazek 1 has described a life-cycle of T. lewisi occurring in 

 the rat louse and it is a noteworthy circumstance that he describes 

 a sexual process taking place in the gut of the louse about ten 

 days after the trypanosomes have been ingested. The date 

 of the disappearance of the acquired character of drug-resist- 

 ance and the appearance of the so-called sexual process of the 

 trypanosome are thus identical ; there can be no doubt that 

 these results afford strong support to Prowazek's account of 

 the life-cycle of T. lewisi in the intermediate host. 



In addition to the above-described experiments, Gonder 

 also attempted to decide whether the power of resistance was 

 lost when the resistant race of trypanosomes was merely grown 

 on culture media. It was found that trypanosomes which had 

 been grown on blood-agar during a period of more than three 

 months still remained drug-resistant. As a result, rats injected 

 with these " cultured " trypanosomes became infected with a 

 drug-resistant race of the parasites. The acquired character 

 was not lost. 



There is obviously a difference, therefore, between the 

 changes that take place in the trypanosomes in the gut of the 

 intermediate host and those occurring in culture tubes. The 

 view of certain authors that the gut of the louse acts in the 

 same way as a culture-tube is thus shown to be erroneous. 



In conclusion, attention may be drawn to the especial 

 interest these experiments have for those who believe that the 

 sexual process involves an elimination of acquired characters. 

 The trypanosomes that become drug-resistant in direct response 

 to an external influence maintain this character through an in- 

 definitely large number of generations, so long as the parasites 

 are transmitted from one rat to the next by direct inoculation. 



1 Prowazek (1905), " Studien iiber Saugetiertrypanosomen," Arb. a. d. kais. 

 Gesundheitsamt. vol. xxii. pp. 351-95. 



