C. Strickland and N. H. Swellengrebel 
449 
similar to that in culture, which agrees with most other workers’ 
observations. We noted that either owing to lice being very short¬ 
lived, or to the parasite dying out in lice relatively rapidly, or to some 
other cause, the growth was found to persist for only a very short time 
in the louse. 
With regard to the coeloraic crithidial forms in the louse which 
have been described by Baldrey, it is possible that they were inde¬ 
pendent flagellates, and we think that the developmental cycle which 
the same author describes is partly described from young blood forms 
of T. letvisi ingested by the louse. Baldrey says indeed that the 
developmental forms are only to be found in lice which have fed on 
rats just infected with T. lewisi. 
We would like to emphasize here that the development of the 
trypanosome in the louse is a very rare occurrence. One of us has 
previously reported (0. S. 1910) not finding any stage of development 
whatever in 275 lice, while the other has examined a great number 
in Holland with negative results. Other observers also have never 
observed any cycle of development. Thus, Breinl and Hindle say that 
they examined so many lice from rats infected with T. lewisi with 
negative result.'^ that they were about to give up the search when they 
suddenly came upon developmental forms. Notwithstanding this these 
authors go on to make the remark that it seemed surprising ‘ that 
Nuttall and his co-workers had failed ’ in the search. 
Whatever the cycle of development of T. lewisi in the louse 
may be, it is not very clear as to its connection with infection of rats 
by lice, as will be seen in the following list of observations made by 
different authors. Prowazek (1905) and Rodenwaldt (1909) both 
observed a developmental cycle in lice, but could not succeed in trans¬ 
mitting the parasite by their agency. We have also, as stated above 
(1910), found a developmental cycle in the louse, but were unable to 
transmit the parasite with such lice. 
Baldrey (1909) however found the developmental forms in lice, and 
succeeded in transmitting T. lewisi by means of them. He correlated 
what he thought was ‘early’ and ‘late’ transmission with different 
stages in the developmental cycle of the parasite. Similarl}' Breinl 
and Hindle (1909) succeeded in transmitting T. lewisi from rat to rat 
by lice and they saw developmental stages of T. letvisi in such lice. 
On the other hand some of those observers who could find no sign 
of any development of T. letvisi in the louse, were able to transmit 
infection from infected to non-infected rats by the louse, while others 
