106 
Raemoflagellates, etc. 
of our criticism of the so-called developmental cycles of certain verte¬ 
brate trypanosomes in tsetse flies we have clearly shown that there is no 
evidence to support the views of those who maintain that these parasites 
of Africa undergo a true evolutionary cycle in the alimentary tracts of 
the flies. We firmly believe that Koch, Minchin, Stuhlmann, Keysselitz, 
Mayer and Roubaud have all confused these vertebrate trypanosomes 
of Africa with the natural Crithidia of the Glossinae and that therefore 
their conclusions are erroneous and misleading. 
Owing to the great importance of knowing exactly how these 
pathogenic trypanosomes are transmitted toman and the lower animals, 
we now propose reviewing in detail the many observations that have 
been recorded. In order to make the subject quite complete it is 
necessary to begin with Bruce’s (1896) experiments in Zululand. It 
will be remembered that Bruce on capturing some Glossinae (G. morsitans 
and probably G. pallidipes ) kept them for a few days (exact time not 
being stated) and then fed them continuously on the same dog; in all, 
three such experiments were carried out. In one case five flies were 
fed on the same dog (1) every second day for nine weeks; in the second 
experiment, thirty flies were fed every day for ten days on the same 
dog (2) and in the third experiment fifty flies were fed daily for fourteen 
days on another dog. As a result of these three experiments Bruce 
found that none of the dogs became infected with T. brucei. It should 
be noted Bruce does not mention the length of time the flies were kept 
before they were fed on the dogs. 
Bruce then carried out another series of experiments, this time 
feeding tsetse flies (presumably G. morsitans) on Zulu dogs infected with 
T. brucei and obtained the following results : 
1. Interrupted feeding. In one case infection occurred after thirty- 
two feeds fifteen days after the first feed and in another case after 
twenty-four feeds with an incubation period of eight days. 
2. Twelve liotirs interval. A dog became infected after nine flies 
had been fed 333 times. 
3. Twenty-four hours interval. Twelve flies were fed on alternate 
days about 228 times and produced an infection in thirty-eight days. 
4. Forty-eight hours interval. Twelve flies were fed on a dog for 
thirty-one days, seventy feeds were necessai’y before infection was 
produced. 
5. Three days intervals gave negative results. 
Bruce’s experiments then clearly demonstrate that quite a small 
number of flies (G. morsitans ) are able to infect clean animals immediately 
