TRYPANOSOMES IN TSETSE-FLIES AND OTHER DIPTERA. 183 
forms give rise to all those found later, it is implied that 
the slender twenty-four-hour forms die off; it is, of course, 
possible that they may persist and give rise to the later 
slender forms, and I must confess that my observations do not 
enable me to decide this point with certainty. 
In. my simears of flies dissected on the third day, I find 
great differences in the condition of the trypanosomes. In 
those from one batch (Oct. 18th, p. 235) the trypanosomes were 
almost without exception excessively granular and frequently 
also very vacuolated. They gave me the impression of being 
degenerate forms, with impaired vitality (figs. 82a-88). In 
another batch (Oct. 22nd, p. 237), however, I found in my 
smears abundant healthy-looking trypanosomes (figs. 89-92), 
not vacuolated nor excessively granular, and in the living 
condition they were extremely active,so much so that I thought 
I had to do with T. grayi. 
In both cases alike, however, no trypanosomes were to be 
found in the flies dissected the day following. 
On the fourth day (ninety-six hours or so) I have very 
rarely found trypanosomes present in the fly, and only once in 
smears of this period (Sept. 12th, p. 230). My preparations of 
them are, unfortunately, very poor, but it is at least possible 
to trace the form and features of the trypanosomes, which 
are of a large type, differing in no essential particular from 
those of the day before (figs. 97-101). 
In no case did I ever find T. gambiense in the fly after 
the fourth day. 
The disappearance of T. gambiense from the gut of the 
tsetse-fly, on or after the fourth day after infection, may 
-mean either that the trypanosomes die out completely, or 
that they pass into some form which has not been recognised. 
If they really die out in the fly without completing any life- 
eycle, it would indicate, in my opinion, that Glossina 
palpalis is not the true host for this trypanosome, and that 
some other invertebrate host must be sought for it. I discuss 
this question more fully below. If, however, the disappear- 
ance of the trypanosomes is only apparent, and they really 
