XVIII] MODE OF INFECTION 331 



seven days' march away ; these animals had been naturally 

 infected with nagana by the bites of G. morsitans. Fifty G. 

 palpalis were fed on three of these infected animals for three 

 successive days and from the fourth day onwards daily on fresh 

 healthy animals. From the fourth to the seventeenth day 

 inclusive the flies fed on a new animal each day, but none of 

 them became infected. From the eighteenth to the twenty- 

 fourth day the flies fed on the same sheep, and from the twenty- 

 fifth to thirty-ninth day on the same ox. On the twelfth day 

 after the flies were put on this ox, numerous trypanosomes were 

 found in the blood and the sheep was also found to be infected. 

 All the other animals remained healthy. Continuing this experi- 

 ment from the fortieth to the fiftieth day the flies, now reduced 

 in number to twenty-two, were fed on two goats, two calves 

 and two sheep. All these animals became infected after incu- 

 bation periods varying from five to eight days. 



This experiment clearly shews that G. palpalis remains non- 

 infective for many days after the ingestion of blood containing 

 T. brucei, but that after this negative incubation period the flies 

 become infective and may remain so for a considerable length 

 of time. 



Later these experiments were repeated employing Glossina 

 morsitans instead of palpalis. It was found that the animals 

 bitten during the three days following the feed on an infected 

 animal, all became infected, the transmission being direct, as 

 in the case of the Zululand experiments. The animals bitten 

 from the fourth to the tenth day remained healthy, whilst all 

 those that were bitten by the flies from the eleventh to the 

 forty-fourth day became infected. Moreover, Taute has shewn 

 that Glossina morsitans may remain infective for at least 

 83 days. 



The evolution of T. brucei within the intermediate host has 

 not been fully worked out, but the parasites seem to multiply 

 within the alimentary canal and subsequently invade the 

 proboscis in a manner similar to T. gambiense. 



