TRYPANOSOMES IN TSETSE-FLIES AND OTHER DIPTERA. 211 
proboscis is “cleaned” for a second one. Hence, if the 
infection of I’. brucii were only by the direct method, the 
flies caught off a healthy animal, in Bruce’s experiment, 
should have been non-infective. The experiment seems to 
me, therefore, to indicate that in the case of T. brucii 
there is infection of a type other than the direct—that is to 
say, that cyclical infection occurs, doubtless in addition to 
direct infection.! 
Turning from experiment to observation, we have the 
results obtained by Koch (19) and Stuhlmann (41), who have 
studied the development of 'T. brucii in Glossina fusea, 
and have found a state of things which are in sharp contrast 
with my results for T. gambiense and G. palpalis.? 
Koch (19) found a multiplication of the trypanosomes, 
with differentiation into stout and slender forms; next, 
multinuclear forms, believed to be zygotes, from which arose 
round forms, which in their turn gave rise to small forms 
with 2 in front of N. Long narrow forms, with » far in front 
of N, were also found. In the proboscis fluid trypanosomes 
were found resembling blood-trypanosomes in form and size, 
in addition to other forms. Attempts to infect rats with the 
trypanosomes in the digestive tract of tsetses were always 
without result. Koch gives reasons for believing that the 
infection produced in the fly differs according to the state of 
the trypanosomes in the blood of the sick animal ; in flies fed 
on animals freshly infected, with many trypanosomes in their 
1 The great infectiveness of tsetse-flies in the case of nagana is notorious. 
According to the unanimous testimony of travellers, confirmed by Bruce’s 
observations, it is only necessary for susceptible animals, such as horses or 
cattle, to pass through the ‘‘fly-belts,” in order to acquire the disease, practi- 
cally as a certainty. This is not the case in sleeping sickness. In Uganda 
I was struck not so much by the number of sick persons in affected areas as 
by the number of healthy persons living under precisely the same conditions. 
The infection of sleeping sickness by the bites of tsetses may be compared to 
a lottery, in which the prizes are few and the blanks are many. 
_? Bruce (4) found, in tsetses infected with nagana, trypanosomes in the 
stomach up to the end of the fifth day, so long as any blood remained 
undigested, 
