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Trypanosoma lewisi 
that 96 “/o of the insects were found to contain large numbers of ‘ small 
trypanosomes ’ in the hind gut. 
The fleas in lot (b), 219 in number, were used for infection experiments 
on rats. They were placed on 14 clean animals about four days 
after taking them from the vicinity of the rat infected with T. lewisi. 
About 16 were placed on each rat. Eight of the 14 rats became 
infected; 45 “/o of the fleas distributed on six rats not being infective, 
i.e. although 96 "/o of tlie fleas in lot (a) contained ‘small trypanosomes’ 
only 55 “/o (at the most) of the fleas in lot (b) proved infective. We do 
not know upon what this discrepancy depends. 
Since we found no other forms than the small trypanosomes in the 
hindgut we assumed that these might represent the potentially infective 
forms, but this assumption gained no support from experiments in which 
two rats respectively were inoculated subcutaneously and intraperi- 
toneally with ‘small trypanosomes’ derived from the hindguts of four 
fleas and yet did not become infected. 
We therefore thought that possibly the parasites detected had not 
reached a stage when they were infective under the conditions in which 
the experiments were carried out. Although we found no parasites in 
the infective fleas other than those encountered in the hindgut, an 
emulsion of the salivary glands and alimentary tracts of 11 fleas from 
an infected flea-box were respectively inoculated into two rats, but the 
results were again negative. 
We have already said that we think the ‘small trypanosomes’ in 
the hindgut of the flea are the final forms which T. leiuisi takes in the 
flea, and that it is they which find their way back to the rat when the 
flea has passed the latent period of non-infectivity and become infective, 
(late infection). 
The forms of the parasite which are found in the flea prior to the 
‘ small trypanosomes ’ correspond in the time of their occurrence to the 
time when the fleas are non-infective. 
We therefore believe that while the ‘small trypanosomes’ are 
inherently able to grow in the rats’ blood and infect the rat, the forms 
which precede them ai’e not capable of doing so. This is we think 
analogous to the relation of the malarial parasite to its power of infection 
at different stages of its life cycle, and not to a bacterial culture, which 
would be infective continuously. 
We agree with Novy and MacNeal in the case of T. lewisi that the 
growth in the insect is similar to the growth on agar in ‘artificial 
culture,’ and therefore that their conception of the insect being a flying 
