E. Hindle 
148 
In this series of experiments also it will be noticed that two of the 
ticks were uninfected even though they had been fed some time 
previously on a heavily infected animal. 
The results of the remaining six positive experiments have been 
summarized in the following table:— 
Tick material injected 
Result of inoculation 
Gut + contents 
6 + 
0 - 
Malpighian tubules 
5 + 
1- 
Salivary glands 
5 + 
1 ? 
Sexual organs 
6 + 
0- 
In the above described experiments five out of the nineteen ticks 
used were uninfected, and therefore must be considered to have been 
immune to infection with S. duttoni. These results confirm Schuberg 
and Manteufel’s (1910) observations on the immunity of certain 
individuals of 0. moubata against spirochaetal infection; and also show 
that a rather large proportion (about 30 “/o) of ticks possess this 
immunity. When performing transmission experiments, or attempting 
to follow the life-history of the spirochaetes in the intermediate 
host, it is important to remember this fact. 
Discussion of the positive expe7'iments. 
(а) The ‘ gut-f contents ’ were found to be infective in all cases, 
and in many instances the spirochaetes could be seen by means of 
microscopical examination. There is no difference between the ticks 
which had been kept at 21° C. and those which had been kept for 
some time at 37° C., as far as the infectivity of the ‘gut-t- contents’ is 
concerned, except that the incubation period in the latter is, on the 
whole, one or two days shorter. There is a marked difference in the 
result of the microscopical examinations, however, for whereas, in the 
case of ticks kept at 21° 0., spirochaetes were only seen in one case, 
when the tick had fed on an infected mouse eight days previously, 
parasites were invariably found in the ‘ gut + contents ’ of those which 
had been kept for some time at about 36° C. 
These experiments clearly prove that the gut together with its 
contents always harbours a spirochaetal infection, except in the case of 
those ticks which are immune. 
(б) The Malpighian tubules are less constantly infected than the 
‘ gut-f contents,’ but nevertheless there are five positive results against 
