38 Mr. E. A. Minchin. On the Occurrence of [Nov. 3, 



the red-black blood the conditions were similar, but young forms are becoming 

 commoner. In the black blood young forms predominate, and large individuals 

 are comparatively infrequent. Thus in the gut of this fly as a whole we find 

 great rarity of differentiated sexual forms, but a swarm of indifferent forms 

 which in the hinder part of the intestine give rise to very numerous young 

 forms, and these in their turn would appear to pass into the small, 

 Herpetomonas-like forms found in the proctodeum, starved-looking creatures 

 which, in a medium where there is probably no nutriment, go through the 

 process of encystment already described. 



Having so far confined myself strictly to matters of fact, I will now offer 

 a few suggestions and speculations as to the probable significance of the 

 encystation in the economy of the life-cycle of the parasite. In the first 

 place, the resemblance of these cysts, and especially the pear-shaped forms, 

 to the " Schleim-Cysten " described by Prowazek* in Herpetomonas musece- 

 domesticce, Burnett, is very marked, and has struck everyone to whom I have 

 shown both my preparations and Prowazek's figure. I think there can be no 

 doubt that they are similar bodies, and have a similar function, that is to say, 

 that they are destined to pass out of the gut of the fly with its dejecta. Very 

 numerous analogies in support of this inference could be cited from other 

 parasitic Protozoa. The question which interests us most is : what becomes 

 of them after being cast out from the fly ? In the absence of any observa- 

 tions or experiments upon this point, one can only draw conclusions from the 

 analogy of what is known in other cases. In the case of H. muscm- domestical 

 the cysts are scattered about everywhere, and, as house-flies are far from 

 being particular in their feeding or sanitary in their habits, a fly runs every 

 chance of infecting itself with the Herpetomonas by swallowing with its food 

 cysts dropped by another fly. There can be no doubt whatever, it seems to 

 me, that this is the manner in which the house-fly acquires the infection of 

 the Herpetomonas. The case is, however, very different with the tsetse-fly, 

 which does not haunt the enclosed spaces and insanitary surroundings of the 

 house-fly, but lives a free, open-air life by lake and forest. Nor is the 

 tsetse-fly a foul feeder like the house-fly. My assistant, Mr. Degen, and 

 myself made many attempts to feed tsetse-flies on all kinds of food, but 

 always without the least success. On the other hand, the tsetse-fly is 

 a greedy blood-sucker, and will attack anything from a frog, lizard, or bird 

 to a hippopotamus, but, in my opinion, it does not feed in any other way. 

 Further, the tsetse-fly is not like the Stomoxys, which lays its eggs in dung ; 

 it is viviparous, as is well known, and nourishes its larva in the uterus until 

 full-grown. 



* « Arbeiten a. d. k. Gesundheitsamte,' Berlin, vol. 20 (1904), p. 446. 



