15 
the parasites will be seen to show agglutination during the period of their 
decline in numbers and disappearance from the blood. When auto- 
agglutination is well advanced we see very few parasites occurring 
singly. They are collected into masses; but these masses in turn 
show a tendency to collect close together, which must necessarily leave 
certain drops of blood almost free from all parasites while other drops 
will show typical fields of auto-agglutination. On this account a 
single hanging drop or a single stained preparation taken from a rat 
is not sufficient to give a correct idea of the condition of the parasites 
in the blood. As many as eight to ten slides may be made in which 
will be found only a few scattered parasites, and the next slide will 
show pictures such as are seen in Plate II, figure 8, and Plate IV, 
figures 13 and 14. Figure 13 represents a single focus of agglutina- 
tion, and in figure 14 there are three such foci near to each other. 
A close examination of these agglutinations will show that they 
have nothing to do with the rosettes of multiplication. They are 
found in the blood after the period of multiplication has passed, and 
they are made up of adult parasites instead of young forms. We con- 
sider these agglutinations as an evidence of the presence of agglutinin 
in the rat’s blood and as an omen of an impending rapid disappear- 
ance of the parasites from the blood. We have in the laboratory in 
several instances seen the agglutination of the parasites in a rat’s 
blood for a few successive days before a sudden disappearance of all 
parasites over night. We conclude from this observation that agglu- 
tination is a step toward dissolution, and that it foretells the disap- 
pearance of the trypanosomes from the blood. 
From a daily study of the blood of numerous cases in which agglu- 
tinations of parasites were present we were able to prophesy that 
active immunity was near at hand. 
It is not every case of infection which shows auto-agglutination. 
We would explain its absence on the ground of an insufficient gener- 
ation of agglutinin. As will be referred to later, there is a difference 
in the agglutinating power of the immune sera of different rats. As 
soon as a rat loses his parasites he is considered an immune and his 
serum is admitted to have agglutinating power on fresh parasites to 
which it may be added. The agglutinating power of the immune 
serum is, moreover, attributed to the agglutinin which it contains. 
Now, we think that the production of this agglutinin is a gradual 
process which is begun before a rat loses his parasites, and we think 
that just previous to the disappearance of all parasites from a rat’s 
blood there is a short period during which he may generate sufficient 
agglutinin to agglutinate the parasites circulating in his own blood. 
We would offer as a possible explanation of our own cases that per- 
haps our trypanosomes were more virulent than those used by other 
observers. This view seems to be supported by the shorter time 
which elapsed between inoculation and heavy blood infection, the 
