374 
Mammalian Trypanosomes of Africa 
syringe or by positive flies. A black mangabey species is extremely tolerant 
to these organisms. The chimpanzee is susceptible to infection by the wild-fly 
mainland strain; unfortunately, owing to an independent abdominal infection 
which proved fatal, it is not possible to report on the course of the disease 
in this animal. 
PART IV. DIRECT-TRANSMISSION EXPERIMENTS. 
These experiments were undertaken to test the hypothesis advanced by 
me in 1919 that this mode of transmission may play an important part in 
the production of strains of enhanced virulence, when the natural conditions 
result in the predominance of direct over indirect or cyclical transmission. 
It soon became evident, however, that a considerable time must elapse 
before any opinion can be advanced as to modification in the virulence of 
the directly-transmitted strains. 
In these experiments, boxes containing 30-35 flies were applied for a 
quarter of a minute or so to the infected monkey and then transferred to the 
clean monkey for a slightly longer period; the process was then repeated 
immediately, so that each day the flies had two “interrupted feeds." The 
flies were finally fed on the clean monkey as required, to keep them alive. 
At first wire-sided boxes were used, later on mosquito-net sides were 
substituted for the wire to ensure more rapid feeding by the tsetses. 
Table IV sets forth the direct-transmission experiments. 
The reason for the repeated applications of the boxes was that the main 
object of these experiments was infection of as long a series of monkeys as 
possible, to see whether there ensued any alteration in virulence in the later 
passages. 
Allowing for a prolonged incubation period, which it is clear from Exp. 98 
may occur in these direct-transmission experiments, it will be seen that in 
ten experiments the transmission was probably effected during the first appli¬ 
cation of the box. In some of the others, in which negative applications are 
recorded, it is possible that the incubation period may have been under¬ 
estimated, and that the first application of the box was effective in these cases 
also. 
It is, however, plain that in a considerable number of instances 35 hungry 
flies failed to transmit trypanosomes from sick to healthy monkey bv the 
direct method. Exps. 60, 109, 135 and 161 failed altogether. In the three-fly 
experiment trypanosomes were very numerous in the blood of the infected 
monkey, and here the infection probably occurred on the first day. 
As a general rule, though the point is not brought out in the Tables, the 
ease with which infection occurred depended on the number of trypanosomes 
in the peripheral blood. No doubt, as Roubaud has suggested, the success of 
direct-transmission experiments depends to a certain extent on the suscepti¬ 
bility of the clean animal to the trypanosome, and this factor must have contri¬ 
buted to Oeliler’s success with single parasites (Oehler, 1913). In the monkeys 
