XVIl] BIOLOGICAL CHARACTERS 297 



Some trypanosomiases are transmitted entirely by coitus, 

 others by tsetse-flies, and yet others by Stomoxys and Tabanids. 

 In those few cases in which the hfe-cycle of the trypanosome 

 in the tsetse-fly (Glossina) has been followed, the evolution of 

 the flagellate within the body of the fly is found to differ in 

 different species, and in future this may furnish an important 

 means of distinguishing them. At present it is possible to 

 distinguish those trypanosomes which infect the whole length 

 of the alimentary canal of the fly {infection totale), from those 

 that are restricted to the proboscis. 



Cross immunity reactions. The most certain method of 

 distinguishing any two races of trypanosomes is by means of 

 their immunity reactions. Thus, given two races of trypano- 

 somes A and B, an animal having recovered from A is inocu- 

 lated with B, in order to determine whether or not it possesses 

 immunity against the latter, and vice versa. If an animal 

 which has recovered from A is still susceptible to B, one as- 

 sumes that A and B constitute distinct species. On the other 

 hand if an animal which has recovered from A, and therefore 

 is immune against this strain, is also immune against B, the 

 two are considered to be identical. 



This method is only capable of application in the laboratory 

 and cannot be used during expeditions in the tropics. 



Sero-diagnostic methods. Laveran and Mesnil have also 

 called attention to the possibility of employing sero-diagnostic 

 methods in the identification of trypanosomes. The serum of 

 an animal which has acquired immunity against any particular 

 trypanosome, is often active when mixed with blood containing 

 this trypanosome, and inactive when mixed under similar 

 conditions with other species of trypanosomes. Unfortun- 

 ately the activity of the immune sera is too variable to give any 

 very certain results, and this method, therefore, only gives 

 useful indications. 



The production of agglutination by means of the addition 

 of immune serum of the same species of trypanosome is also 

 of use in some cases, but is very inconstant. 



Classification. The identification of the various species of 

 trypanosomes has become a matter of great difficulty, especially 



