140 THE MAIN CURRENTS OF ZOOLOGY 



of a blood corpuscle. This organism is sharp pointed 

 at each end and is provided with a vibratile mem- 

 brane and a flagellum which produce motion. This 

 parasite called a trypanosome was, by Bruce and 

 other observers, found in the blood and spinal fluid 

 as a constant concommitant of the disease. 



Its mode of transmission was obscure and a 

 problem of great difficulty. Studies on the wild and 

 domesticated animals of the district were a means of 

 throwing light on the matter. Similar protozoan 

 parasites were known to infect domesticated animals 

 and to live in the blood of native wild animals such 

 as the antelope, buffalo, and others. These wild 

 animals having been long infested with these par- 

 asites had acquired a tolerance for them and re- 

 mained unharmed, but recently infected domesti- 

 cated animals were not immune. These parasites 

 were shown to be conveyed by the bite of a tsetse-fly 

 from the wild game inhabiting the district in whose 

 blood they produce no deleterious result to the 

 animals that were susceptible to poisons produced 

 by the parasites. By analogy, suspicion was fastened 

 on the tsetse-fly as the probable source of transmis- 

 sion, but it was a very difficult matter to prove it. In 

 searching for the agent of the transmission of human 



