308 T. GAMBIENSE [CH. 



relapses. It is possible that some latent forms are produced, 

 but up to the present the evidence for their existence is 

 inconclusive. 



One of the most noticeable features of the infection in the 

 blood is the periodic increase and decrease in the number of 

 trypanosomes in the peripheral circulation. It was formerly 

 believed that the parasites completely disappeared from the 

 circulation at certain times, but Ross and Thomson have shewn 

 that when once a person becomes infected trypanosomes are 

 constantly present in the blood. During the so-called negative 

 periods the parasites are in such few numbers that their pres- 

 ence can only be detected by the examination of large quantities 

 of blood, either by centrifuging or some similar means. These 

 alternating periods of increase and decrease in the circulating 

 blood are of irregular duration, and the circumstances which 

 bring about the disappearance of the majority of the parasites 

 are not thoroughly understood. After a period of depression 

 the following increase in the number of flagellates is always 

 accompanied by division in the peripheral blood. 



These variations in the number of trypanosomes are accom- 

 panied by differences in their infectivity when ingested by the 

 tsetse-fly. Thus it has been shewn that monkeys infected 

 with T. gambiense have negative periods during which they are 

 not infective to the fly, and moreover the percentage of flies 

 that become infected after any particular feed varies with the 

 stage of infection. 



From this point of view, the following table, given by Miss 

 Robertson, is of considerable interest. Monkey 113 was 

 infected by wild G. palpalis from the shore of Lake Victoria, 

 and first shewed trypanosomes in its blood on July 25th. 

 From August 23rd up to the time of its death, fresh lots 

 of flies were fed on this monkey and the percentage number 

 of these flies that became infected is shewn in the table. 



It is evident, therefore, that there is a marked difference in 

 the mlectivity of the parasite at different stages of its develop- 

 ment, but in addition, different strains vary considerably in 

 this respect. Thus, another monkey was infected by the 

 direct injection of blood from a bush-buck which had been 



