124 
HaevnojlageTlates, etc. 
of fleas ( C . fasciatus and C. agyrtes) by placing on them a number of 
these insects, which have recently been removed from an infected wild 
rat. Now we know this trypanosome does not undergo any develop¬ 
mental changes in the intestinal tracts of the two fleas, and that the 
parasites are most probably transmitted mechanically after finding then- 
way to the proboscides of the insects. 
In from six to seven days after the fleas have been placed 
on a rat, if the experiment is going to prove successful, the 
blood of the rat will be found to contain large numbers of young 
flagellates. Some of these are quite round and have short or long 
flagella, and are exactly like certain Crithidici, especially those under¬ 
going division (see Laveran and Mesnil’s (1904) excellent figures 
illustrating these changes in T. lewisi). The multiplication rosette of 
T. lewisi is also very suggestive of a Crithidia (see figures 6, 7 and 8 
Laveran and Mesnil (1904)); the flagella of the developing parasites are 
directed externally and each flagellum at an early stage is attached to 
the body of the parasite (compare the above figures with those of 
C. gerridis). T. rotatorium, also exhibits similar appearances in its 
developmental stages in the blood of frogs. We do not wish here 
to include the cultural forms of a number of trypanosomes, as we do not 
consider they represent the true developmental cycles of the parasites, 
but are rather pseudo-developmental forms; certain trypanosomes 
readily exhibit these changes, when they are cultivated on artificial 
media, such as blood agar; why they do so we cannot explain. We 
think then these facts suggest that the two trypanosomes ( T. lewisi and 
T. rotatorium ) have descended from crithidia-like ancestors. So far the 
majority of Crithidia have only been found in the intestinal tracts and 
appendages of blood-sucking and non-blood-sucking arthropods and 
leeches, but quite recently Wenyon (1909) has recorded a flagellate from 
the blood of a snake ( Erythrolamprus aesculapii ), which is very like 
some Crithidia ; and again the flagellate which Brimont has recently 
found in the red blood corpuscles of Cholaepus didactylus may quite 
well prove to be but a stage of a Crithidia. We may then, in the case 
of these parasites, be dealing with two haemal forms, which perhaps 
exhibit the peculiarities of ancestral haemoflagellates. The fact that 
one of these parasites penetrates the red blood corpuscles of a vertebrate, 
suggests that it may probably be the more primitive of the two. We 
would then suggest that the ancestors of the present trypanosomes may 
probably have been intraglobular crithidia-like forms. We will leave 
this question at this point, as it is unwise with our present limited 
