380 
Trypanosoma lewisi 
the same negative result; in both cases the rats were in the chronic 
stage of infection, i.e. the trypanosomes in their blood did not divide 
any more. In May 1910 we at last found lice showing developmental 
stages on the insects occurring on a rat in the last period of the infection. 
Fifty-two lice were dissected ; 33°/o were not infected at all, 43°/o showed 
only normal and degenerating forms, 23°/o showed developmental forms. 
We are unable to state what the influences are which favoured the 
development. Generally these forms were only to be found in lice 
which had not sucked blood quite recently (gut with black contents). 
We often kept the lice for 24—48 hours in a moist chamber at 
13—16° C. (after 48 hours all the lice had died) and were then 
generally able to find the developmental forms in the black contents 
of the hindgut. Manteufel (1909) was also able to find these forms 
under similar conditions. During the course of this experiment our 
rat lost its trypanosomes suddenly. Directly afterwards the normal 
forms disappeared, but the developmental forms were to be found till 
three days afterwards. This corresponds with the fact mentioned by 
Manteufel that the lice may be infective even five days after feeding. 
It remains to be determined if these developmental forms found in the 
louse are really connected with T. lewisi or if they are perhaps natural 
flagellates. The control observations are very difficult in this case; as 
has been pointed out already, developmental stages may not be found 
even in a large number of infected lice, so it is not enough to dissect a 
large number of lice and make sure that no flagellated forms are to be 
found, because if there is a natural infection it is very scanty. In our 
experiment the condition of the production of crithidiae being evidently 
very favourable, there was no reason (supposing that these forms were 
natural ones) why they should disappear after the rat ceased to be 
infective. We therefore continued dissecting lice for ten days after 
that time. Developmental forms were to be found during the first 
three days, but not later—for 61 lice were dissected with negative 
results. Although the number of our controls was not so large as that 
of Breinl and Hindle, we think we may safely conclude that the crithidiae 
are indeed connected with the trypanosomes, because their appearance 
and disappearance were so manifestly connected with presence or absence 
of trypanosomes in the rat on which the lice fed. (Table IV shows the 
rate of infection of each day.) 
The course of infection being so very irregular, and it being im¬ 
possible to keep the lice off the rat for more than 48 hours, we could 
not follow the changes occurring in the trypanosomes as regularly as in 
