INHERITANCE IN TRYPANOSOMES 691 



but also in form. Their method of procedure was as follows : 

 The cold-blooded animals were inoculated in the peritoneum 

 with a small quantity of undiluted blood containing the 

 trypanosomes under investigation. After an interval of a few 

 days some of the blood of the inoculated cold-blooded animal 

 was injected into the peritoneal cavity of a normal rat. The 

 rat frequently became infected with a modified form of the 

 original race of trypanosomes and by infecting other animals 

 from this rat the virulence of the parasites towards their host 

 could be enormously increased. A large variety of cold-blooded 

 animals were found to harbour infection, e.g. grass-snakes 

 (Tropidonotus natrix), lizards, tortoises and even beetles and 

 molluscs. 



The most interesting results were obtained by the passage 

 of T. lewisi through grass-snakes. This common species of 

 trypanosome is not known to exercise any harmful effect on the 

 rats in which it lives (about 30 per cent, of all rats are naturally 

 infected with it) and consequently it has always been regarded 

 as one of the non-pathogenic forms. Moreover, ordinary races 

 of T. lewisi will not live in the blood of any other mammal 

 except the rat, the normal host. When the blood of a rat 

 containing many trypanosomes (7". lewisi) is inoculated into a 

 grass-snake the latter often becomes infected, even though the 

 parasites may never actually be seen in the infected snake. 

 Their presence can only be detected by inoculating some of the 

 snake's blood into a healthy rat, which subsequently may become 

 infected with a modified race of T. lewisi. The most remarkable 

 of the modifications is an alteration in virulence which, however, 

 is not usually apparent in the rat infected directly from the 

 snake but only after inoculating other rats from the first one. 

 After passing through a few rats in this way, the new race 

 becomes pathogenic towards its hosts and when injected into 

 other rats invariably kills them after a short period of incuba- 

 tion. One of these new virulent races of T. leivisi killed rats 

 only four days after they had been inoculated with it, the 

 incubation period {i.e. the time elapsing between the inoculation 

 and the first appearance of parasites in the general circulation) 

 being shortened from three or four days down to only twelve 

 hours. These pathogenic races of T. lewisi maintained their 

 peculiar characters through many generations of rats without 

 showing any diminution in virulence. It was also found 



