1906.] , Encystation in Trypanosoma grayi Novy, etc. 39 



For all these reasons it seems to me in the highest degree improbable, 

 indeed I may say impossible, that a tsetse-fly would ever infect itself by 

 sucking up cysts dropped by another fly, or that a parasite which had to 

 depend on this method of dissemination could maintain its existence in the 

 tsetse-fly. The only possible destiny I can imagine for these cysts is to be 

 swallowed accidentally by some vertebrate, the (as yet unknown) host of 

 Trypanosoma grayi, in order to germinate in its digestive tract, to pass 

 thence into the blood, and to be taken up again with the blood by the 

 tsetse-fly. A cycle of this type is as yet unknown, but there are abundant 

 analogies for all parts of it. In the first place, it is a common thing for 

 animals to have protozoan parasites in the gut, which they take up in the 

 encysted condition after they have been dropped by another individual. 

 Without multiplying instances unnecessarily, I may point out that Schaudinn 

 proved the infection of Amoeba coli to originate in this way, and that it is 

 a common human parasite in regions where sanitation has not advanced 

 beyond the primitive condition of epandage par terre. In the second place, 

 there are many instances among Sporozoa of cysts germinating in the 

 intestine and liberating motile forms which then pass through the wall of 

 the gut into other organs of the body. 



In a former communication by my colleagues,* Lieutenants Gray and 

 Tulloch, and myself, we were able to confirm Bruce's results as to the 

 existence of direct mechanical infection by means of the tsetse-fly, which, if 

 it stabs its proboscis first into an infected animal and then soon after into 

 a healthy one, can infect the latter. We were not able to demonstrate, 

 however, what I may term cyclical infection, which at present has not been 

 shown to exist. I suggest that there are two possible modes of cyclical 

 infection, in the dissemination of protozoan blood-parasites by biting insects 

 generally. In one method, which I may term inoculative, the parasite, after 

 going through developmental changes in the insect, passes back again into 

 a second vertebrate host through the proboscis, as in the case of malaria 

 transmitted by a mosquito. In the other method, which I propose to term 

 contaminative, the parasite taken up by the biting insect, after going through 

 developmental changes within its gut, would pass out of it through the anus, 

 and infect the vertebrate host by contaminating its food or drink. We have 

 all of us (I speak for myself) been imbued hitherto with the idea that the 

 cycle of the trypanosome in the tsetse-fly must be of the inoculative type, 

 and have failed to find it. I wish to suggest strongly to those working on 

 the subject of trypanosome-infection the desirability of making experiments 

 and observations to prove or disprove the existence, in the insect which 

 * ' Roy. Soc. Proc.,' B, vol. 78 (1906), p. 242. 



