6 
certain differences. Still, an accurate differential diagnosis on the 
basis of structnre alone is not altogether practicable. 
As a harmless inhabitant of the blood still other trypanosomes are 
harbored by the hamster, certain fish, frogs, and birds. 
OCCURRENCE AND MORPHOLOGY OF TRYPANOSOMA LEWISI. 
Our studies were made entirely with the rat trypanosome {Trypan- 
osoma Lewisi). The natural host is the wild gray rat and the sewer 
rat. These rats harbor the trypanosomes in their blood and they 
infect each other spontaneously. 
Crookshank (2) investigated a number of rats in London and found 
that 25 per cent of the apparently healthy rats harbored trypanosomes. 
Rabinowitsch and Kempner (3) examined the blood of fifty wild 
rats caught about the city of Berlin and found that eighteen of them 
were infected with trypanosomes. 
Laveran and Mesnil (4) found two cases of infection among forty- 
three sewer rats examined in Paris. 
W e examined sixty of our home rats caught in different parts of 
'Washington and found no cases of infection. 
Xatural infection of the white or spotted rats has never yet been 
found, although they are very susceptible to the infection by inocula- 
tion. We inoculated the trypanosome into white rats and examined 
the blood in fresh and stained preparations. 
The parasite may be considered in three parts: The body, the 
undulating membrane, and the fiagellum. The body is the elongated 
portion which has attached to its side the undulating membrane and 
to one of its ends the flagellum. The body of the adult parasite 
measures 13 to 25 micra in length, which is about two to three times 
the diameter of a red blood corpuscle. Its breadth is 2 to 4 micra. 
The body runs to a beak-like process at one end, while at the other 
end there is a free flagellum 7 to 15 micra in length. The flagellate 
end is considered the anterior or front end because locomotion takes 
place in the direction of the flagellum, while the beak end is desig- 
nated the hind end, since it follows the body in locomotion. In the 
hinder fourth of the body, about 4 micra from the end, there is a 
short rod-shaped structure, the long axis of which is at a right angle 
to the long axis of the body; this spot is called the centrosome. It 
can with difficulty be seen in a fresh preparation. In the anterior 
fourth of the body of the parasite there is a larger strongly retractile 
spot, which is regarded as the nucleus. 
The undulating membrane is a clear, fine, transparent membrane 
which presents an attached border, a free border, and a web. ^ It takes 
its rise from the centrosome, and is fixed by its attached border along 
the length of the body of the parasite and terminates at the front end, 
while the outer free margin of the membrane is thickened into a rod- 
like border which is continued past the front end into the long free- 
