TRYPANOSOMES IN TSETSE-FLIES AND OTHER DIPTERA. 221 
disseminated as such parasites are now; it formed resistent 
cysts in the gut which passed out with the feeces, were scattered 
abroad, and contaminated the food of fresh vertebrate hosts. 
(2) When such a form succeeded in penetrating the intes- 
tinal wall and passing into the circulatory system, it found 
itself in a situation from which there was no escape or outlet 
by natural channels. Hence, if it did not at once come into 
relation with blood-sucking invertebrates, it could only have 
infected new hosts by coming back to the intestine of the 
vertebrate, becoming encysted there, and passing out with 
the fesces, as in (1). There is absolutely no evidence that any 
trypanosomes develop in this way; but a cycle of this kind 
has been described for Lankesterella ranarum by Hinze 
whose statements are usually, though perhaps not very 
logically, considered to be refuted by the observations of Siegel 
on the Hemogregarina stepanowi of the tortoise. 
(3) Our trypanosome in the vertebrate blood may be sup- 
posed to have been taken up sooner or later by a blood-suck- 
ing invertebrate, the digestive juices of which it succeeded in 
resisting. It has acquired now a channel of escape from the 
vertebrate blood and is no longer obliged to become encysted 
in the vertebrate gut. Becoming adapted to the inverte- 
brate gut, where it finds the nutriment, namely blood, to 
which it was accustomed, it now forms in the invertebrate gut 
the cysts which it formerly produced in the vertebrate. ‘The 
cysts pass out with the faeces, are spread abroad, and reinfect 
the vertebrate host by contamination. This is the condition 
which I believe to be represented by T. grayi, described 
above; though it is possible that in this case contaminative 
infection is combined with inoculative, definite proof of either 
being as yet lacking. 
(4) The trypanosome having become thoroughly adapted 
to the invertebrate gut acquires the power of passing for- 
wards till it reaches its proboscis, and becomes inoculated 
into the vertebrate host, thus establishing the commonly- 
occurring inoculative type of infection. | Now intestinal 
cysts become unnecessary and cease to be produced. 
