58 NATURE 
after the fly had taken up the parasites, it would 
not be ripe, so to speak, to infect a healthy animal 
until after a certain period of time or a certain 
number of feeds, as is known to be the case with 
the mosquito in the transmission of malaria. To 
test this, and to discover the period necessary for the 
supposed cycle, batches of flies were fed first on an 
infected animal and then at regular intervals on a 
succession of healthy animals (monkeys), using a new 
healthy animal for each feed. In no case was an 
infection obtained later than forty-eight hours, 
although the experiments were extended over three 
weeks. 
On the other hand, conclusive evidence was obtained 
of the existence of what may be termed direct 
mechanical infection; that is to say, if a tsetse be 
allowed to have a partial feed on an infected animal 
and be then transferred at once to a healthy animal, 
on which it is allowed to finish its feed, the second 
animal may become infected. This confirms the 
results previously obtained by Bruce, both for nagana 
and sleeping sickness. The experiment was varied 
by making the fly feed first on an infected animal 
and then on two healthy animals in rapid succession; 
it was then found that the first healthy animal be- 
came infected, but not the second. If the tsetse dips 
; its proboscis for an in- 
stant into the skin of a 
healthy animal, it appears 
to clean the proboscis and 
render the fly non-infec- 
tious to other animals. 
This indicates that the 
direct mechanical trans- 
mission is effected by the 
proboscis alone. As is 
well known, if a tsetse-fly 
be fed on an_ infected 
animal, then decapitated, 
and its probescis exam- 
ined under the microscope, 
the cavity of the proboscis 
is seen to contain blood 
corpuscles and active try- 
panosomes, a fact which 
: : sufficiently explains the 
direct transmission. The experiments suggest that a 
tsetse which has fed on an infected subject is only in- 
fectious to the first healthy subject bitten by it after- 
wards. ; 
A second series of experiments had for its object 
to test the connection, if any, between the wild 
trypanosomes and sleeping sickness. An island called 
Kimmi, not far from Entebbe in the Victoria Nyanza, 
was found to teem with tsetse-flies to a degree almost 
incredible to anyone who has not been there. 
Although this island was uninhabited and hardly ever 
visited by human beings, it was found that the tsetse- 
flies there were more often infected with trypano- 
somes than on the mainland, since on the average 
between 7 per cent. and 8 per cent. of Kimmi flies 
were found to harbour trypanosomes, as against 
1.7 per cent. from the neighbourhood of Entebbe. 
The island of Nsadzi, adjacent to Kimmi, was found 
to be free from tsetse except in certain limited spots 
along the shore, and hence served as convenient 
ground for a camp and station for experimenting 
upon the flies of Kimmi. The method was to feed 
a batch of flies caught at Kimmi upon a given 
healthy experimental animal daily for a certain length 
of time, then, by dissection and microscopic examin- 
ation of every fly in the batch, to find out how many 
of them contained trypanosomes. In this way it was 
possible to make certain that animals susceptible to 
NO. 1933, VOL. 75] 
Fic. 3 —T7ypanosoma tullochii from 
the Intestine of the tsetse-fly. x 2000 
diameters. 
| NOVEMBER 15, 1906 
sleeping sickness had been fed upon by one or more 
tsetse-flies containing trypanosomes. Had these wild 
trypanosomes been identical with those of sleeping 
sickness, it might have been expected that some at 
least of the experimental animals would have become 
infected; but not in a single case did this occur. 
Attempts to infect experimental animals by direct 
inoculation of trypanosomes from the intestine of the 
fly proved equally futile. 
The microscopical observation of the trypanosomes 
within the tsetse-fly led to similar conclusions. If 
tsetse-flies were fed on animals infected with 
Trypanosoma gambiense, and subsequently dissected 
and examined after various intervals, it was found 
that the trypanosomes flourished and multiplied for 
the first twenty-four hours, becoming at the same 
time differentiated into two distinct types, the one 
slender, transparent, and active, the other bulky, 
granular, and sluggish in movement. Compared 
with what is known of developmental phases in other 
Protozoa, the slender forms may be called male 
(Fig. 1, a, b), the bulky forms female (Fig. 1, c, d). 
Up to forty-eight hours the multiplication continues, 
and a more “indifferent ’’ type of individual appears. 
At seventy-two hours, however, the trypanosomes 
have become greatly diminished, and by ninety-six 
hours, or slightly later, the trypanosomes have dis- 
appeared completely from the gut of the tsetse-fly, 
this disappearance coinciding with the complete 
absorption of the blood with which they were taken 
in. Trypanosoma gambiense appears, in short, to 
have very limited powers of maintaining its existence 
in the gut of the tsetse, and to be unable to pass 
forwards into the blood ingested at feeds subsequent 
to that at which it was taken up by the fly. 
In the case of the wild trypanosomes, onthe other 
hand, a very different state of things is found to 
exist. A study of the forms found in different flies 
shows that two distinct types occur, one or the other 
usually being present, though exceptionally both 
may be found together in the same fly. One of these 
types, which Novy has named Trypanosoma grayi, 
is distinguished in all its phases by the relatively 
large size of the smaller mass of chromatin (micro- 
nucleus or blepharoplast), which is elongated in a 
direction transverse to the axis of the body, and 
placed almost invariably in front of the nucleus 
(Fig. 2, a-d). The other type, which we have named 
T. tullochti, is more like T. gambiense in_ its 
characters, having a small rounded blepharoplast 
placed well behind the nucleus (Fig. 3, a, b). Both 
these types are remarkable for their very great 
activity, whereby they swarm forwards in the gut 
of the fly into the blood ingested by it at each feed, 
and by their own exertions penetrate from the hind- 
most portion of the gut into its most anterior regions. 
The conclusion drawn from these observations is 
that the ‘‘ wild’’ trypanosomes, those found occur- 
ring naturally in the tsetse-fly at Entebbe, are not 
stages of the trypanosome of sleeping sickness, but 
represent at least two entirely distinct species. It 
remains to be discovered whence the tsetse-fly obtains 
these trypanosomes. It may be that the tsetse obtains 
them from the blood of indigenous animals upon 
which they are parasitic; Trypanosoma grayi has 
some resemblance to certain trypanosomes of birds, 
while T. tullochii is more of the type of a mammalian 
trypanosome. It may be, on the other hand, that 
they are parasites of the fly itself, and have no other 
host of any kind. 
With regard to Trypanosoma gambiensc, experi- 
ment and observation alike show that in Uganda it 
does not pass through a developmental cycle in the’ 
tsetse-fly, but is only transferred mechanically by the 
