C. DOBELL 213 



species of host animal alone in which it was acquired. Fur example, 

 they found that T. brucei in donkeys became resistant to atoxyl after 

 injections of the drug. Transplanted into rats, however, it rapidly lost 

 its resistance and became susceptible again. 



Races of Trypanosomes with a changed virulence, produced by 

 passages through a different host, have several times been recorded. 

 Fellmer (1907), for example, stated that the virulence of T. brucei 

 was diminished by passage through the hedgehog. The structure of 

 the parasites was also stated to be modified by a sojourn of the race 

 in this animal. Fellmer's experiments were repeated by Gonder and 

 Sieber (1909), who used both T. brucei and T. eqidpej-dum. They com- 

 pletely failed, however, to produce any change in either the virulence or 

 the structure of these Trypanosomes in this way. 



Wendelstadt (1909) and Wendelstadt and Fellmer (1909, 1910) also 

 announced that the passage of T. brucei and T. lewisi through cold- 

 blooded vertebrates — to which reference has already been made — greatly 

 modified their virulence. They found, for example, that T. lewisi when 

 passed through the grass snake becomes modified into a race which 

 is pathogenic for rats — in which the infection is normally harmless. 

 Inoculation of Trypanosomes from the snake back into the rat kills 

 the latter. Twenty-four passages with a similar result were thus made. 

 Laveran and Fettit (1909) repeated these experiments. They injected 

 both T. lewisi and T. evansi from rats into snakes, and then back into 

 clean rats. But they failed entirely to produce any change in the 

 virulence of the Trypanosomes. They state, moreover, that the blood 

 of the snake is very toxic to rats — which may account for the results of 

 Wendelstadt and Fellmer. 



It seems, therefore, that these experiments in which ciianges of 

 virulence are said to have been produced in Trypanosomes should 

 be regarded with considerable scepticism for the presents 



Levaditi and Mutermilch (1909) found that they could produce 

 races of T. brucei which were resistant to certain antibodies. Later, 

 Levaditi, in collaboration with Twort (1911), has shcnvn that a race 

 of T. brucei can be made which is resistant to the toxin produced by 

 Bacillus subtilis — a substance which is usually very toxic to T. brucei. 

 If a normal race of this Trypauosonie (from the blood of the mouse) 

 is subjected in vitro — even for only a few minutes — to the action of 



' It should be recalled, however, that the Werbitzki races of Trypanosomes have 

 undergone a diminution in their virulence — a fact which appears to be established (Laverau 

 and Eoudsky). 



