224 
with animals infected with T. brucei, T. gambiense and T. dimoTfhn 
was instituted specially to make observations on this point. In the 
case of the infections with T. brucei it was found that the virulence 
increased after attack with atoxyl, but was diminished after trj’pan- 
red. A slight increase in virulence was seen in the few experiments 
done with T. dimorphon, and a slight decrease in the case of 
T. gambiense, but the number of experiments and alteration in the 
virulence are both too small to warrant a definite conclusion in the 
case of these two species. The number of experiments with 7 . brmi 
is larger, and we are of opinion that this parasite after escape from 
atoxyl is, in the majority at least of the cases, more virulent thau 
before treatment. 
Previous experiments, without treatment with any drug, gave as 
the average duration of life after appearance of infection in the case 
of each strain as follows : — 
T. brucei 6-3 days Average of 146 rats 
T. dimorphon 17-6 „ „ 56 „ 
T. gambiense 287 „ 36 „ 
No sudden alteration in virulence was found in the case of any of 
the strains. This liad been specially watched for in the case of the 
strain of T. brucei, which had been under observation he® 
4th November. 1906, till i6th July, 1907; as stated, no increase of 
virulence was seen in the strain passed in the usual way from animal 
to animal without coming under the influence of any drug. 
In order to infect the animals, an injection subcutaneously of 
o 4 c.c. of infected blood, containing 6 to 8 parasites to the 
(Zeiss, ocular 4, objective DD), was given in each case. 
The results are shown in the following tables, which show the da) 
of reappearance of trypanosomes in the peripheral circulation after 
their disappearance under the influence of the drug mentioned in each 
case, the number of animals then infected by the recurrent strain fro® 
the drug, the duration of life under this strain, and the increase m 
virulence marked +, or decrease marked — . 
