N. H. SWELLENGREBEL AND C. STRICKLAND 
377 
Miss MacKinnon found in Ctenophthalmus agyrtes and Hystricho- 
psylla talpae two flagellates which in many stages of their development 
perfectly resemble our small oval and round forms. 
We were able to find in Ctenocephalus serraticeps flagellated forms 
(Diagram XIII) which are almost identical to the small oval and round 
forms described here as developmental stages of T. lewisi ; the same is 
the case with Crithidia ctenophthalmi of Patton and Strickland (1908). 
Diagram XIII. 
Flagellates found in the hindgut of Ctenophthalmus canis. G. 
Some of these fleas ( Ctenophthalmus agyrtes, C. serraticeps ) may- 
attack rats and so it is possible that some of these Crithidiae or her- 
petomonads are really developmental stages of trypanosomes, but even 
if we consider all these flagellates to be natural forms we do not think 
that this invalidates in the least our view that the forms we found in 
the fleas fed on an infected rat, are really developmental forms of 
T. lewisi. The resemblance with the natural forms is very striking, 
but not greater than the resemblance with the cultural forms of 
T. lewisi and nobody will doubt that the latter are connected with 
T. leioisi. One point remains to be considered; two out of sixty of our 
control fleas were infected, the flagellates found in their gut belonging 
all to the type of the small oval forms, so one might consider these 
forms not to belong to the cycle of T. lewisi. This however is a 
mistake: the small oval forms were found after the seventh day in 
15 fleas out of 83 = 18°/ 0 - In the control fleas they were found only 
in 3'3°/„, so the only possibility is that a part of the small oval forms 
found in fleas of Series F did not belong to the cycle of T. lewisi. 
Summary. Diagram XIV will serve better than words to give a 
summary of the life-cycle of T. lewisi, as observed by us in Geratophyllus 
fasciatus. The following table (p. 379) gives the measurements of the 
different types described (out of the flea and of cultures). 
As already stated, the small trypanosomes seem to be in the 
final stage of development, but we are unable to say if these are the 
