1906.] Glossina palpalis and Trypanosoma gambiense, etc. 245 



certainty within a very few days, whereas had we used T. gambiense the 

 results of our experiments would have remained uncertain for a very long 

 while. 



It has also been proved by the experiments of Bruce and by ourselves 

 that freshly caught specimens of G. palpalis, at Entebbe, are capable 

 of infecting animals with the trypanosome of Sleeping Sickness, but in this 

 case all experiments seem to show that the number of fly-bites required to 

 produce infection is a very variable one indeed, since over and over again 

 more than 1000 flies have fed on a susceptible animal without infecting it. 

 The smallest batch with which we ourselves have been successful in 

 producing infection consisted of 134 flies. 



Observations on the fate of trypanosomes introduced into the digestive 

 tract of the tsetse-fly by feeding it in the laboratory upon animals infected 

 with T. gambiense gave the following results : — The trypanosomes, never very 

 numerous in the ingested blood, show at the end of 24 hours a slight 

 increase in number, and many of the parasites are observed in stages of 

 division. At the same time they have become differentiated into two very 

 distinct forms. The first is a very slender type with cytoplasm free from 

 granules, with the nucleus sometimes rounded but more usually compressed, 

 and with a considerable length of free flagellum (Plate 12, figs. 1 to 6). 

 Many of these slender forms are observed at this stage to be in the act of 

 extruding granules of chromatin from the nucleus (Plate 12, figs. 4 to 6). 

 The second form of parasite is relatively very large and bulky with granular 

 and deeply-staining cytoplasm, with very large spherical nucleus, with 

 short free flagellum, and with the blepharoplast often some distance from 

 the posterior end (Plate 12, figs. 7 to 14). These two forms may be regarded, 

 on the analogy of developmental facts recorded of other trypanosomes, as 

 male and female respectively. In both forms stages of division were 

 observed, but in no case have we succeeded as yet in observing with 

 certainty any process of conjugation. The two forms are easily distinguished 

 in the living condition, the slender males being also characterised by much 

 greater activity than the bulky females. 



Male and female forms could also be recognised in the blood of the 

 experimental animals (monkeys), especially in films fixed with osmic vapour. 

 In films dried in the ordinary way the characteristic differences were much 

 less distinct. In either case the differentiation of sexual characters is far 

 less marked than it becomes in the intestine of the fly. Trypanosomes 

 of male character (Plate 12, fig. 16) are common in blood-films, but those 

 of female character (Plate 12, fig. 17) are very scarce, and only two were 

 found, both of which were remarkable for having the nucleus composed of 



