THE HAEMOFLAGELLATES 201 



per cent, and in from two to four days was able to observe various 

 developmental phases of the parasites. This further development 

 continued on the flies being fed upon healthy animals, but only in 

 about 10 per cent of the individuals; in the rest it gradually dis- 

 appeared. This percentage, it is instructive to observe, was about 

 the same as that of the Tsetses (G. fusca) found to be infected with 

 T. brucii (in all probability) in nature. 



It will be seen that it is impossible to draw any certain con- 

 clusions from the present position of the problem. Nevertheless, 

 there is good reason to suppose that, for a given lethal Trypano- 

 some, there is a particular insect which is a true alternate host. 1 

 It seems very probable that here, as among leeches, there are 

 right and wrong hosts for the parasites ; that while the com- 

 plete normal development, culminating in the transfer back to the 

 Vertebrate, can only take place in a certain species of fly, attempts 

 at development which are, to a varying degree, partially successful 

 may go on in other biting- flies ; these latter, however, being 

 able to act in relation to the Vertebrate host only as mechanical 

 carriers. 



Before leaving this question of the mode of transmission of 

 Trypanosomes, it is to be noted that Minchin has put forward (57) 

 an entirely new view with regard to the method of infection. His 

 idea is based especially upon the highly interesting discovery made 

 by him of the occurrence of cysts, doubtless for external dissemina- 

 tion by way of the anus, in one of the " fly - Trypanosomes," 

 Trypanosoma grayi. Minchin suggests that there may be two 

 varieties of cyclical infection among the Haemoflagellates ; in the 

 one, the parasite undergoes cyst-formation in the insect, resulting 

 in a conlaminative infection of the Vertebrate, by means of its food 

 or drink ; in the other, distinguished as the inoculative type, the 

 infection takes place through the proboscis of the fly (as, for 

 .example, in the malarial parasites). Up to the present, however, 

 T. grayi remains the only known form in the case of which infection 

 is most probably of the first type. 2 From what has been learnt so 

 far of the development of other Trypanosomes whether in leeches 

 or in insects the distribution of the parasites in the body (see 

 under " Habitat ") points at any rate to inoculative infection of the 

 Vertebrate. The possibility of the occurrence of both modes in any 

 one Trypanosome is not, so far as is known, excluded ; but there 

 is, as yet, no definite evidence in favour of this. 



1 For further remarks bearing on this point, see pp. 230-231, 261. 



2 Although the Vertebrate host of T. grayi has not been actually demonstrated, 

 both Minchin and others have made an important observation in connection with the 

 biology of the Tsetse-fly, which taken in conjunction with the non-occurrence of 

 hereditary infection seems to show that it is impossible for the parasites to be 

 merely fly-Trypanosomes. This is to the effect that the Tsetses, unlike mosquitoes, 

 feed only on blood, never on foul or decaying matter of any kind. 



