144 
Transmission of S. duttoni 
three negative ones in the case of ticks which had been kept at 21° C. 
It may reasonably be assumed, therefore, that at moderate tempera¬ 
tures these organs often harbour the infection, although spirochaetes 
were never found by microscopical examination. On the other hand, 
in the case of those ticks which had been kept at 36° 0., the Malpighian 
tubules, with one exception, were all infective and spirochaetes were 
found in three out of the four examined microscopically. The result 
of incubation in this case, therefore, is to increase the possibility of the 
Malpighian tubules harbouring infection and also to cause the appear¬ 
ance of spirochaetes in their tissue. Moreover, as in the case of the 
‘ gut + contents,’ the incubation period is slightly curtailed. 
(c) The salivary glands are' probably never infected in ticks which 
have been kept at moderate temperatures. The two positive results 
which have been obtained out of eight experiments may be explained 
on the supposition that the salivary glands had been contaminated by 
infective material from the gut. On the other hand, the effect of 
raising the temperature of the tick is markedly shown in this case, 
for, with one doubtful exception (the mouse died before it had time 
to become infected), the salivary glands were always infective. 
The results of the microscopical examination for spirochaetes are 
not definite. In the case of ticks which had been kept at 21° C. the 
parasites were never found, but even in the case of the incubated ticks, 
whose salivary glands were infective, in only one case were spirochaetes 
found. It is possible that they are present only in very small numbers 
and might easily be overlooked, but the possibility of their being present 
in another form is also to be considered. 
{d) The sexual organs of ticks which had been kept at 21° C. ate 
generally infective, for out of eight experiments only two were negative. 
When the ticks are incubated at 36° C. the sexual organs were always 
found to be infective and therefore there is a slight increase in their 
infectivity as a result of raising the temperature. The incubation 
period is also slightly curtailed. 
The microscopical examination of the sexual organs for spirochaetes 
was uniformly negative in the case of ticks kept at 21° C., but when the 
latter had been incubated for some time at about 36° C., spirochaetes 
were invariably found in considerable numbers. 
The effect of raising the temperature in this case, therefore, is to 
increase the infectivity of the sexual organ, and also to cause the 
development of spirochaetes in its tissues. 
