296 TRYPANOSOMES [CH. 



chromatin merely becomes aggregated around each pole and 

 division is more or less direct. In either case the connecting 

 line eventually disappears and the two daughter nuclei gradu- 

 ally assume the usual form. Meanwhile the new flagellum 

 has become nearly as long as the old one. The animal then 

 splits longitudinally, the fission commencing at the anterior 

 end and extending down until the two halves are connected 

 merely by their posterior extremities, in which position they 

 may remain for some little time before finally separating. 



In some species, e.g. T. lewisi, the trypanosomes multiply 

 by a process of schizogony, and multinucleate forms are not 

 infrequently found in many other species. The presence or 

 absence of such forms occasionally gives some help in the 

 determination of the species. 



Biological characters. Certain trypanosomes will only live 

 in one particular vertebrate host and when injected into other 

 species are unable to survive. None of the pathogenic 

 trypanosomes, however, are restricted to one species, the 

 nearest approach to it occurring in the case of T. simicz, which 

 only affects monkeys and goats. 



The great majority of trypanosomes have little or no effect 

 upon the health of their hosts and accordingly are termed non- 

 pathogenic. On the contrary, the most important parasites 

 from the present point of view the pathogenic trypano- 

 somes have a marked injurious effect on the health of the 

 animals they inhabit. 



The effect of these parasites on their hosts is of help in 

 distinguishing the various forms, as the incubation period, 

 duration of the disease, symptoms, etc, etc., all furnish useful 

 indications. 



Yet another means of distinguishing these pathogenic 

 trypanosomes is furnished by the results of inoculating the 

 parasites into various experimental animals. The smaller 

 animals are refractory to certain trypanosomes that are patho- 

 genic to ruminants, and on the other hand some parasites have 

 a much more pathogenic effect on small animals than on large 

 ones. 



The method of infection is another important character. 



