C. Strickland and N. H. Swellengrebel 
447 
culture tube is to a certain extent justified. The differences which 
exist are differences of degree, not of kind : the development of T. lewisi 
in the flea proceeds a further stage than it does on agar. 
We quote here an observation which may have some significance 
to the biology of trypanosomes. We found in an experiment that 
several fleas taken from a heavily infected rat and then fed for a 
forfnight on a rat which was immune to T. letuisi were nevertheless 
infective; which shews that serum which would have prevented the 
growth of T. letuisi in the rat has no power to do so in the flea. 
During the initial period of infectivity of the flea, during the first 
day when mechanical transmission is effected, the trypanosomes are 
found in the midgut of the flea, and may possibly be the forms which 
cause infection, perhaps by regurgitation of the gut contents, but we 
think that probably this mechanical transmission is effected by a ‘ wet ’ 
proboscis, or in some cases by the rat chewing up a flea which is full of 
T. lewisi and becoming infected by way of the mucous membrane of the 
alimentary tract. 
(ii) Experiments with lice. 
(a) By previous authors. Many workers have apparently suc¬ 
ceeded in transmitting T. lewisi by the agency of lice ; namely, MacNeal 
(1904), Swingle (1907), Nuttall (1908), Manteufel (1908), Baldrey 
(1909), Bensen (mentioned by Baldrey 1909), and Breinl and Hindle 
(1909). 
On the other hand failures to transmit the infection by lice have 
been common. Thus, MacNeal records but one out of a number of 
experiments which he performed which was successful. Swingle also 
succeeded in infecting rats by means of lice, but he does not state how 
many times he failed to do so. Manteufel (1908) although he seems 
several times to have been able to transmit T. lewisi by lice, yet records 
40 failures. Prof. Nuttall (1908) succeeded once (with 60 lice) and 
failed once (with 40 lice). Baldrey (1909) has succeeded and failed, 
but gives no figures. Breinl and Hindle (1909) quote three positive 
experiments but they also had negative results. 
Rabinowitsch and Kempner (1899) and Prowazek (1905) could not 
succeed at all in their efforts at transmission of the parasite. 
Further observations are required, but those which we review 
certainly indicate that transmission experiments by means of lice 
frequently yield negative results, even when large numbers of lice are 
transferred from infected to uninfected animals. 
