N. H. SWELLENGREBEL AND C. STRICKLAND 
361 
elongated crithidia-like forms, with the nuclear apparatus situated 
in the hind end ; then they shorten up and take the form of short 
oval critbidiae 1 (with an undulating membrane) or herpetomonads 
(without an undulating membrane), which divide actively, forming 
rosettes. Round forms may be found also, but the authors could not 
make clear their significance or the way in which they are formed. 
Some of the larger herpetomonads, with short flagella or aflagellar and 
with an indistinct blepharoplast, are considered to be ookinetes arising 
from a previous conjugation. Some authors (Prowazek, Baldrey, Roden- 
waldt) assert that they have seen a heterogamic sexual process, but 
their proofs for this statement are far from convincing. Apart from 
these forms, Prowazek described reduction of the blepharoplast and 
the nucleus, male and female forms, etc. 
Minchin and Thomson (1910) have pointed out that there is a 
distant transmission of T. lewisi from rat to rat, which is performed 
by the rat flea ( Ceratophyllus fasciatus), and so they think that this 
flea is the real host for the trypanosome, a statement which confirms 
Swingle’s assertion (1907), that there is development of this flagellate 
in the flea’s gut. 
As is well known, all these papers have been severely criticised by 
Patton and Strickland (1908), who assert that the Crithidiae found in 
the louse and the flea are natural flagellates and have nothing to do 
with T. lewisi. This argument must be specially considered in 
connection with the supposed development of T. lewisi in the rat flea, 
because Swingle (1907), Balfour (1906) and MacKinnon (1909) have 
described various herpetomonads and Crithidiae in Pulex cleopatrae 
(Balfour’s Herpetomonas ), Ctenophthalmus agyrtes (Herpetomonas cteno- 
phthalmi MacKinnon) and Hystrichopsylla talpae (Crithidia hystricho- 
psyllae MacKinnon). After the description of the forms we observed, 
we shall have occasion to discuss this question and the influence these 
observations have on the interpretation of our w’ork. 
The object of our experiments was : 
(1) To trace out the development of T. leiuisi (if there were any) in 
the rat flea {Ceratophyllus fasciatus). 
(2) To compare this development with that in the rat louse, in 
some other Arthropods and in artificial cultures, to make sure if the 
development in the invertebrate host is a specific life-cycle or only 
a sort of natural culture. 
1 The word is used in a descriptive sense throughout this paper when it is not italicised. 
