MASTIGOPHORA 



743 



and is continued forward as a short flagellum. The posterior end is 

 usually drawn out to a stubby point. The fully developed organism 

 is large, being 40 to 80 microns long by 5 to 40 wide. One striking 

 thing about this parasite is its tendency to pleomorphism. 



Multiplication in the blood stream of the frog is by binary fission ; 

 in addition, a form of multiple division occurs in the viscera, preceded, 

 according to Machado, by conjugation of sexually differentiated forms. 

 The merozoites liberated from the mother cell are small trypanosomes, 

 which in turn grow to large size, thus explaining the pleomorphism of 

 the parasite. 



Cultures have beeji obtained by Lewis and Williams on the blood 

 agar of Novy and MacNeal in which a great variety of forms may be 

 seen; the method of transmission is unknown, but the infection is 

 probably conveyed by leeches. Many other trypanosomes have been 

 found in fishes, frogs, and reptiles all over the world. 



Trypanosoma lewisi (Kent). This, one of the longest known and 

 commonest forms, has been studied more completely than any other 

 organism of its class. It occurs in 

 a large proportion of rats through- 

 out the world, twenty-five to one 

 hundred per cent being infected, 

 and since it is non-pathogenic, it is 

 a convenient organism for research. 

 It may be passed from wild to 

 white rats without difficulty, by in- 

 oculating the latter with a small 

 quantity of citrated blood contain- 

 ing the organisms. At first the 

 parasites are few, but after the 

 lapse of three or four days, large 

 numbers may be found ; the condi- 

 tion of rapid multiplication lasts 

 from eight to fourteen days, and 

 is succeeded by a period of a month 

 or more, during .which time the 

 parasites gradually diminish in 

 number, finally disappearing com- 

 pletely, rendering the animal immune from further infection, the 

 immunity being complete. The serum of an immune rat has a certain 

 protective power, and when inoculated simultaneously with blood con- 



FIG. 169. TRYPANOSOMA LEWISI. 

 (After Doflein and Minchin. Mac- 

 Neal, "Pathogenic Microorganisms," 

 published by P. Blakiston's Sons & 

 Go.) 



