462 
The slight fluctuations in the periodicity, namely, the variations 
in the time-intervals between the crests of the waves, can be 
explained by the fact that the resistance of the animal varies from 
time to time. Again, we might note that the rat, the guinea-pig, 
and the rabbit are not the natural hosts of the parasites of 
sleeping sickness. 
We should also note that the strains with which we have been 
dealing are very virulent, as judged by the records of Thomas and 
Linton (1904),* Moore and Breinl (1907), and others, whose 
inoculated animals lived much longer than ours. In less virulent 
strains the time-intervals between the crests of the waves in the 
graphs might tend to be longer. 
The periodic variation or oscillation that we have described is of 
practical importance in the administration of drugs to eliminate 
trypanosomes in a host. If the drug be administered on the 
downward slope of the curve, the fall in numbers of trypanosomes 
is not due to the drug alone. But if the drug be administered on 
the upward slope of the curve, while the parasites are increasing in 
numbers, and yet a reduction in numbers of the parasites is 
brought about, then the drug may legitimately be inferred to be 
efficacious. 
SUMMARY 
>• In these investigations two strains of trypanosomes were 
used, namely, ( a ) T. rhodesiense (Stephens and Fantham) derived 
from a patient, W.A., suffering from sleeping sickness contracted 
m Rhodesia, and (/?) T. gambiense, an old laboratory strain. 
Of these two strains the Rhodesian was the more virulent, 
as judged by sub-inoculations in rats, guinea-pigs and rabbits, for 
the duration of life of animals inoculated with the Rhodesian strain 
was shorter than in those inoculated with T. gambiense. 
Stephens and Fantham (1910) have already pointed out a 
distinct morphological peculiarity in T. rhodesiense in the presence 
°t a posterior nucleus in some of the stout forms. 
3 - Enumeration of trypanosomes in the peripheral blood of 
•ats, guinea-pigs and rabbits inoculated with each of these strains 
* I.ancct, May ,4, IQ04j pp . ,337.^ 
