220 



THE HAEMOFLAGELLATES 



importance altogether undeserved, owing chiefly to the fact that 

 many of these parasites have been studied, so far, only in strange 

 and unaccustomed hosts hosts to which they are unadapted, and 

 for which they, on their part, prove markedly pathogenic. 



Trypanosomes appear to be, in most cases, able to support, for a 

 longer or shorter period, unfavourable conditions of environment, 

 whether due to the reaction of the host itself or to the transference 

 of the parasites to a strange medium. Sooner or later, however, 

 the organisms feel the effects of such changed circumstances and 



Fia. 15. 



Involution and degeneration forms of different Trypanosomes. A-E, T. gatribiensc (A, C, and 

 B after Bruce and Nabarro ; B and D after Castellani). F, K-P, T. brucii (F after Br. and 

 PI. ; K-P after L. and M.). G-J, Q and B, T. equinum (after Lignieres.) S, T. ftrucn, plas- 

 modial mass, from spleen pulp (after Br. and PI.). 



become markedly altered. The strange forms and appearances 

 frequently described are probably for the most part l abnormal ; i.e. 

 they do not represent phases in the typical life-cycle, but are vary- 

 ing stages in a process of degeneration. Nevertheless, it by no 

 means follows that the parasites rapidly die off. On the contrary, 

 many of these involution-forms, on entering the blood of a fresh 

 host, are able to infect it, though they may even have been kept for 

 some time in artificial surroundings. 



The course which involution takes varies in different cases, but 

 the process generally follows one or another of three lines, which 



1 See footnote to p. 222. 



