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A Critical Review, etc. 
pointed out that this stage is characterised by the flagellates collecting 
in the hindgut and rectum of their host, where they shorten, divide 
longitudinally, the flagella on degenerating are shed and the parasites 
finally round up into cysts. 
In the film made from the contents of the rectum of the infected 
flea, noted above, we were able to recognise every stage from the adult 
flagellate up to the formation of the cysts. Fig. 1 represents one of the 
adult flagellates as yet unchanged, and it will be seen that its anterior 
end is prolonged along the flagellum, a short portion of which is free. The 
blepharoplast is a large rod-shaped structure, and in the majority of 
these unchanged flagellates, is seen lying either alongside the nucleus or 
just anterior to it. The flagellum is a single stout filament and arises 
in close proximity to the blepharoplast. In our preparation we have 
not seen any basal granule. On looking at a series of these flagellates 
we were struck by the fact that the blepharoplasts appear to migrate 
towards the posterior end so that these parasites come to look very like 
trypanosomes; we have depicted some of these appearances in figs. 2, 
3 and 4. The explanation of this migration appears to us to be quite 
simple; in many of these forms it is seen that, though the flagellum is 
quite clearly stained at its free end, it is indistinct and hardly visible 
towards the blepharoplast and in some specimens this portion has quite 
disappeared (figs. 2, 3 and 4). We believe, therefore, that at this 
stage as the flagellum begins to degenerate the blepharoplast is freed 
and tends to pass towards the posterior end of the cell. We wish to 
draw attention to this peculiar appearance because it may, if studied 
alone, be quite well confused with a trypanosome. We have seen 
similar appearances in some Herpetomonads where the blepharoplast 
comes to lie behind the nucleus. At the same time as these changes 
are taking place, the anterior ends of the parasites become shortened 
