692 SCIENCE PROGRESS 



possible to infect other species of mammals, such as mice and 

 guinea-pigs, with this new race — a further proof of its altered 

 characters. 



In addition to this alteration in pathogenic effects, the viru- 

 lent race of leivisi presents certain morphological peculiarities 

 distinguishing it from the normal forms. The most conspicuous 

 feature is a great increase in size combined with marked granu- 

 lation of the protoplasm. The virulent trypanosomes stain 

 much more readily and appear darker than the normal form ; 

 the nucleus also is more irregular. The increase in size is 

 very remarkable : under normal circumstances, the extreme 

 length of the rat trypanosome is about 30/z, but the augmented 

 race contained individuals measuring as much as 40/i and even 

 more. Also parasites may be found in which the posterior end 

 is enormously drawn out into a long beak almost equal in 

 length to the remainder of the body. This form has only once 

 been described as occurring in normal races of lewisi but is 

 comparatively common in the virulent strains. 



The above-described results show clearly that both the 

 pathogenic character and form of trypanosomes may undergo 

 considerable modification when external conditions are varied ; 

 it may therefore be questioned whether in nature it' be possible 

 for harmless parasites to acquire harmful characters suddenly. 

 In the case that we have described practically a new rat 

 disease has been created, ordinary races of T. lewisi exercis- 

 ing no injurious effect upon their hosts, whereas the virulent 

 form is highly pathogenic, soon killing the rats which become 

 infected with it. These experiments seem to show that at the 

 present time under suitable conditions new diseases may arise 

 and it is not impossible that the recently described human 

 trypanosome (T. rhodesiense), which is the cause of sleeping 

 sickness in Rhodesia, may have arisen in:modern times as the 

 result of some modification of a previously harmless parasite. 



(c) Drug-resistant Trypanosomes. — One of the more interest- 

 ing and also the most thoroughly investigated of the responses 

 of trypanosomes to changes in their environment is their power 

 of becoming resistant to the action of drugs. 



The existence of this property was discovered by Franke 

 and Roehl * whilst working in Ehrlich's laboratory on the treat- 



1 Vide Ehrlich (1907), " Chemotherapeutische Trypanosomen-Studien," Bert, 

 klin. Wochenschr. pp. 233-6 ; 280-3 ; 310-4 ; 341-4. 



