13 
can be gotten from Griibler, which is to be dissolved in meth}d 
alcohol. 
W e have gone at some length into these stains, for without some 
one of them the investigator wdll attain little in the way of getting 
instructive preparations of trypanosomes and because their more gen- 
eral use may bring out some points still unmentioned in the structure 
of animal parasites generally. Since a thorough understanding of 
the stains is necessary to fine work, the reader is referred to the orig- 
inal articles mentioned in the bibliography. 
ACTIVE IMMUNITY. 
With very few exceptions a single infection with trypanosomes ren- 
ders the rats free from parasites thereafter. Rabinowitsch and 
Kempner had no second infections following the injection of heavily 
infected trypanosome blood into the peritoneal cavity of rats, which, 
after artificial inoculation, had become spontaneously free of para- 
sites. Laveran and Mesnil, in their series of thirty, found two suscep- 
tible to a second infection. One of our rats infected by feeding and 
another infected by intrastomachal injection proved susceptible to a 
second infection by intraperitoneal inoculation in tw^o and five months, 
respectively, after they had become free of j)arasites. Their second 
infections lasted only three days. 
PASSIVE IMMUNITY. 
From our knowledge of the antitoxins of the bacterial diseases, we 
are led naturally to the investigation of the protective property of the 
serum of immunized rats, and it is found that there is produced a 
specific immune serum. The serum of rats wdiich have been immu- 
nized by one or more inoculations of trypanosome blood does give pro- 
tection to other rats within certain limits. Laveran and Mesnil found 
that their most active serum came from a rat which had been given 13 
inoculations. 
If we add in vitro 1 c. c. of immune serum to 1 c. c. of trypanosome 
blood and inject the mixture into a fresh rat, no infection will follow. 
We also separated the two injections in time to see whether immune 
serum would ijrevent infection if injected before the trypanosome 
blood; likewise, whether the immune serum would prevent infection 
if it were injected after the injection of infected blood. Our results 
were somewhat variable, but in general they corresponded to the limits 
of time which have been set by Rabinowitsch and Kempner and con- 
firmed by Laveran and Mesnil, namely, that 1 c. c. of immune serum 
injected into a fresh rat twenty-four hours before or twenty-four hours 
after the injection of trypanosomes will prevent infection. They 
found that emulsions of the spleen, bone marrow, liver, or brain con- 
ferred no passive immunity. 
The limits of the preventive and curative power of the immune 
