1909] rii, Life History of Trypanosoma Dimorphon. 133 



casl off and the eysts are carried by the blood into the spleen, and 

 other organs of the body, in which they may remain dormant for 

 some time. 



In Trypanosoma raiai cysts are formed in exactly the same 

 manner as in dimorphon, hut the process seems to take place onlj 

 in the crop of Pontoodella muricata, the intermediate hosl 

 (Robertson, 1909). 



[f rats infected with T. dimorphon are injected with 0.2 gm. 



^OII 

 of a solution of "Atoxyl" (C„ H 4 — AsO— ONa), all the trypano- 



^NH 2 



somes are killed off with the exception of certain of these encysted 

 forms which may si ill be found in the spleen. After an incuba 

 tion period of about four weeks they start to develop, and the 

 parasites reappear in the peripheral circulation. In infected rats 

 which have been treated with Atoxyl it is possible, therefore, 

 to obtain the developmental stages of the eysts without any other 

 types of try panosomes being present. Each cysl (PI. 16, fig. 19) 

 consists of an oval, or round mass of dense protoplasm sur- 

 rounded by a very distinct membrane, and contains a typical 

 oval-shaped trophonucleus beside which is the kinetonucleus. Al 

 the commencement of development an end-bead is thrown off from 

 the latter and from it a new flagellum grows forward. As it 

 develops the kinetonucleus travels backwards until it arrives al 

 a position close to the rounded posterior extremity. The growing 

 flagellum draws the protoplasm with it. and in this way the 

 ordinary trypanosome form is produced. At immature stages of 

 development, the parasites frequently multiply by longitudinal 

 fission before proceeding to the fully developed forms described 

 above. 



This type is characterized, therefore, by its external form; 

 absence of chromatoid granules; the rather dense protoplasm; 

 the shape and size of the two nuclei; and especially by the forma- 

 tion of eysts (or latent bodies). 



From this type of trypanosome are developed two distinct 

 forms and chiefly for this reason it is regarded as representing the 

 indifferenl type. The fact that it possesses a complete asexual 

 cycle (the cyst formation) which does not occur in the other two 



