86 EXPERIMENT A L RE SEA RCHES 



even in so simple an organism as this proto- 

 zoon. 



Although in the mouse atoxyl-resistance usu- 

 ally develops only after a fairly long period, I 

 have occasionally seen it after a fortnight. This 

 observation is very important, because it makes 

 it possible that the negative results observed 

 by Ayres Kopke, Broden, Rodhain, Todd and 

 van Campenhout, in cases of sleeping-sickness 

 which had received a long-continued treatment, 

 may also be due to such a resistance. 



In the same way as against atoxyl, trypano- 

 somes can also be rendered resistant to para- 

 fuchsin, trypan-red and trypan-blue, and further, 

 it was shown in my Institute by Franke in his 

 work with apes, that it is even possible for try- 

 panosomes to become resistant to those pro- 

 tective substances which are formed in the ani- 

 mal after its recovery from a trypanosome infec- 

 tion. A similar discovery has recently been 

 made by Levaditi for the spirochaetes of relaps- 

 ing fever. This is therefore a general law, and 

 it is especially important, because this change 

 in the trypanosomes, having once become ac- 

 quired, remains an hereditary property. Thus I 

 have cultivated for 125 generations my atoxyl- 

 fast strain, without finding any decrease in its 



