N. H. SWELLENGREBEL AND C. STRICKLAND 
105 
We would state here that in our expei’imeuts 83 fleas were allowed 
to feed on a rat infected with T. letvisi and these fleas were successively 
dissected on the following 18 days. During that time they were fed on 
uninfected rats. By this method we were able to follow all the changes 
in the morphology of T. lewisi, each stage of the development being 
connected to the foregoing by an uninterrupted series of forms. The 
life-cycle which we described was therefore not an “arrangement” of 
different forms, perhaps not related to one another (as Swingle seems to 
think possible), but represents the natural changes in the morphology 
of the trypanosome during its stay in the flea’s gut. 
A systematic research such as this was not carried out by Swingle, 
and in consequence he reaches the conclusions stated above regarding 
the life-cycle. His cycle is indeed a pure “ arrangement ” of the forms 
found by him in fleas caught haphazard on infected rats. 
It was quite natural that he should find “small trypanosomes” as these 
represent the last stage of the development of T. letoisi in the flea. In 
such fleas small crithidiae may also be found and it was the inevitable 
outcome of his unsystematic method of investigation that he should 
think that the “ small trypanosomes ” changed into round forms, as 
we ourselves thought before having observed our complete series of 
fleas. 
Swingle failed to observe that trypanosomes, when ingested by 
the flea, are first transformed into large crithidiae, and then become 
rounded, that these round forms produce “ small crithidiae ” (seen also by 
Swingle); afterwards these small flagellates become larger, and some¬ 
times herpetomonad forms appear. Finally all these forms are converted 
into the “small trypanosomes.” Swingle only found the first and last 
stages of this life-cycle and arranged them as well as possible. 
With regard to the small round cells found in the heads of fleas, we 
need only say that no proof whatever is given that they belong to the 
life-cycle of T. lewisi or of any other parasite of the flea’s gut. 
We shall now offer some criticisms as to the identity of H. pattoni. 
It seems that after Swingle had constructed the artificial life-cycle of 
T. lewisi, he found some of the intermediate stages of our life-cycle, and 
as these did not fit in with his views, it was only natural that he should 
create a new species for them. 
Swingle did not work with fleas free from T. lewisi, and so he 
was unable to prove that H. ■pattoni is a species independent of 
T. leiuisi. We think that he does not prove in the least that H. pattoni is 
