C. DOBKLL 217 



supposed to be directly produced by a modification of the individuals 

 of the old race'. 



Erhlich's views in this respect are not shared by some other workers. 

 Levaditi, with Mutermilch (1909) and Twort (1911), interpreted his own 

 results as due to selection by the poison employed. The toxin of 

 B. sttbtilis was found to kill or affect many Trypanosomes, when observed 

 in vitro. And it was concluded that " certain races of Trypanosomes, 

 considered as homogeneous, are only, in reality, a mixture of a large 

 number of individuals endowed with unequal susceptibility towards a 

 given trypanocidal poison." (Levaditi and Twort [1911].) In other 

 words, resistant pure lines may be formed from a mixed population 

 by the selective action of a poison — onl}' those naturally most tolerant 

 Trypanosomes being able to survive, and to perpetuate the race. 



It therefore seems uncertain how resistant races of Trypanosomes 

 arise. It is possible, however, that both a direct action of the drug and 

 an indirect selection by it play a part in their formation. 



In conclusiou the more important results noticed in the foregoing 

 pages may be very briefly summarized. I will limit myself to only 

 those conclusions which appear to me to be justified at the present 

 moment. 



(A) It has been stated that the passage of certain Trypanosomes, 

 which normally occur in mammals, through cold-blooded vertebrates 

 and certain invertebrates, causes them to undergo certain structural 

 changes which persist during subsequent divisions (VVendelstadt and 

 Fellmer). This work has not yet been confirmed. 



It has further been stated (Werbitzki) and confirmed (Laveran and 

 Roudsky, Kudicke) that certain dyes can destroy a definite organ 

 (kinetonucleus) in a Trypanosome, without killing or injuring it or 

 impairing its power of propagation. Thus new races of Trypanosomes 

 may be produced which completely lack this organ. It has, moreover, 

 been rendered highly probable that the dyes which have this power 

 possess a certain chemical structure (orthoquinoid substances of Ehrlich): 



* According to Ehrlich (1911), resistant races of Trypanosomes are of two quite different 

 sorts: (1) Serom-resistant, i.e. resistant to specific antibodies; (2) Chemo-resistant, i.e. 

 resistant to various chemicals. Such races are supposed to arise in different ways. In the 

 terms of Ehrlich's theory, a serum-resistant race is formed by the serum causing a certain 

 receptor (nutriceptor) to disappear, when it is replaced by an altogether new kind of 

 receptor. A chemo-resistant race, on the other hand, is produced, not by the replace- 

 ment of one receptor by another, but by the diminution (" Uerabminderung '') of a certain 

 chemical function. 



