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A Critical Review , etc. 
referred fully to this work above and have clearly shown that there is 
no evidence to support Prowazek’s view of the flagellates he found in 
the rat louse. Keysselitz and Mayer, being unable to get sufficient 
material to breed out flies, G.fusca, studied the development of T. brucei 
in freshly caught insects. Judging from the contents of their intes¬ 
tines it would appear that all the flies had fed solely on mammalian 
blood, as no nucleated red blood corpuscles were seen. It seems to us to 
be very dangerous to presume that these caught flies had fed on animals 
infected with T. brucei alone, but this is apparently what Keysselitz 
and Mayer have done. 4'6°/o of these flies were, according to them, 
infected with T. brucei, whereas 11 *2 °/ 0 which had been fed on healthy 
animals after being caught, were similarly infected ; this difference is 
explained by saying that the parasites enter a swarming period and 
multiply after a meal when they are found between the “ Epithel und 
Darmwand.” The authors found the parasites in the proventriculus, 
proboscis, fore- and midgut, and in all cases in which they were in the 
proboscis they were also found in the other situations. All the flagellates 
we are told had the same general character, so that Keysselitz and Mayer 
regard them as representing part of the cycle of T. brucei as proved by 
Stuhlmann. We consider this proof is wanting for the reasons we 
have mentioned above. 
In one hungry fly Keysselitz and Mayer saw many amoeboid non- 
flagellate forms as well as motile pai’asites between the “ epithelium and 
intestinal wall.” In the juice of the proboscis they found agglutinated 
stages of small trypanosomes which were attached in the proboscis on 
the oral side of the openings of the salivary glands; they conclude that 
they might be washed into the wound when the salivary glands empty 
themselves. The authors, however, do not say whether they tried to 
inoculate animals with these forms. 
Keysselitz and Mayer fed their freshly caught flies on cattle which 
had become spontaneously infected with T. brucei ; the blood of the 
cattle, they say, contained male and female forms of T. brucei, as described 
by Prowazek (1905). We are not convinced of the certainty of Prowazek’s 
male, female and indifferent forms of T. brucei, and we have not seen 
any process of conjugation in this parasite. Even if it is admitted that 
there may be sexual dimorphism, Keysselitz and Mayer do not tell us 
how the zygote develops into the parasites they speak of in G.fusca. 
It is not at all clear to us why Keysselitz and Mayer fed their flies on 
infected animals, for they state that the flies had fed at large on animals 
and were infected with T. brucei. In these flies further development was 
