G-. H. F. Nuttall and S. Hadwen 
165 
centage of (00) parasites, but such an increase is not observable. The 
persistence of single rounded intracorpuscular parasites (0), following 
upon the administration of the drug, may be due to some extent to the 
after-effects of the drug upon free pyriform parasites whose sojourn in 
the plasma may have been so brief as not to affect their vitality to too 
great a degree. On the other hand, it is certain that, in the normal 
course of events, a number of round parasites degenerate and die before 
giving rise to pyriform parasites. Again, there can be no doubt that in 
some cases their development into pyriform parasites may be delayed 
by unfavourable conditions. The delay cannot, however, be indefinite 
since, to survive, the parasite must attain the pyriform stage before the 
degeneration of the containing corpuscle takes place. We know from 
the observations of Nuttall and Graham-Smith upon living parasites 
that the stages of the parasite other than the pyriform have always 
been seen to promptly degenerate and die upon their escape into the 
plasma. 
It is worthy of note in this connection that P. canis assumes a 
rounded form when it dies. This can be observed in fresh preparations 
under the microscope, in blood kept in vitro and in blood taken from 
dogs some hours after death. 
The fall in the total number of infected corpuscles which accompanies 
or ultimately follows the disappearance of the (PP) parasites also indi¬ 
cates that the action of the drugs is directed primarily against the 
pyriform parasites. It is the latter stage of the parasite which invades 
fresh corpuscles. 
In this connection the protocols relating to Dogs 12 and 10 are of 
interest. In the first instance the drug caused a considerable reduction 
of (PP) parasites, although it was given on the day when the dog died. 
Dog 12 had not been previously treated. On the other hand, when the 
relapse occurred in Dog 10, which had been previously treated, a second 
dose of Trypanblau seemed to exert no influence on the parasites. 
Although this observation requires confirmation by further experiment, 
it certainly suggests that the parasites which survive the first dose of the 
drug may have become resistant to its action. That trypanosomes may 
acquire a considerable resistance to the effects of drugs like those we have 
used has been clearly established. 
We shall, however, leave the further consideration of this subject for 
a future paper, in which we shall deal with observations upon the para¬ 
sites under the microscope, and with the effects exerted by the drugs 
upon their morphology. 
