376 
Mammalian Trypanosomes of A frica 
used above trypanosomes were never very numerous, but often showed up 
to two or three per field with the Jth objective. 
With the human strain all attempts at direct transmission failed com¬ 
pletely. Trypanosomes were always rare in the blood, though on some of the 
days on which the feedings were carried out they were present up to one in 
two fields. Here, doubtless, the susceptibility factor comes in, as I have had 
the same experience as Bruce and others of the difficulty with which early 
passages of this strain are effected. Reference to Table I will show that there 
is no appreciable alteration of virulence as the result of the passages so far 
carried out. 
Table VI shows that full cyclical development took place in laboratory- 
bred flies fed on Monkeys 88 (second passage, Exp. 48), 117 (sixth passage, 
Exp. 51), and 161 (eleventh passage, Exp. 85). 
Conclusions. 
(a) As far as the direct-transmission series has as yet progressed, i.e. to 
the eleventh passage, there is no sign of any loss on the part of the trypano¬ 
some of the capability to undergo cyclical development in the fly. 
(b) Up to the present no evidence has come to light that the virulence 
of a strain is enhanced by continued passage by this method in the same 
species of mammal. It is, however, too soon to draw any conclusion on this 
point. 
( c) Under natural conditions this method of transmission is likely to 
operate only when trypanosomes are present in fair numbers in the peri¬ 
pheral blood. Typical T. gambiense infections in man, as commonly recog¬ 
nised, do not, in the later stages at any rate, fulfil this requirement. 
PART V. EXPERIMENTS WITH LABORATORY-BRED FLIES AND 
THE LAKE-SHORE AND HUMAN STRAINS. 
The experiments presented in this section were devised to ascertain the 
extent to which the three strains of trypanosomes—the wild-fly, the antelope 
and the human strains—are transmissible by laboratory-bred G. palpalis ; 
and further, to investigate the effect of different kinds of blood on these 
strains during their development in the fly. 
The pupae from which the flies were hatched were obtained from the 
lake-shore in the neighbourhood of Entebbe. In some of the experiments 
notes were kept on the number of pupae produced under the different dietetic 
conditions, out of consideration for Roubaud's statement that mammalian 
blood is necessary for the reproductive processes of the tsetse (Roubaud, 
1919). My experiments show, however, that full-formed larvae and pupae 
can be produced by flies fed solely upon Varanus blood, a result similar to 
that of Kleine with flies fed with crocodile blood (Kleine, 1909 and 1911a). 
A whitish papular eruption, limited to the area bitten by the tsetse, signalises 
the first few applications of flies to a new monkey; this rash disappears in a 
