478 
the spleen and bone marrow on the downward slope of the curve 
representing the numbers of the parasites in the peripheral blood of 
the host. Change of the latent forms into trypanosomes takes place 
on the rise or upward slope of the curve. 
There is, of course, a mutual action and reaction of the host and 
the parasite, the resistance of the host probably being greatest when 
the flagellate trypanosomes within it arc beginning to decrease,thus 
helping to bring about the assumption of the rounded form by 
many of the flagellates, so that latent or resistant non-flagellate 
stages of the parasite are then numerous. 
The occurrence of latent bodies also helps to explain the 
successful inoculation of animals with trypanosomiasis when no 
flagellates can be found in the blood inoculated from a previously 
infected animal. Although it might be urged that flagellate 
trypanosomes in numbers too few to recognise may actually be 
present in the infected blood inoculated, yet it is possible to 
inoculate only latent, non-flagellate bodies, and give the inoculated 
animal trypanosomiasis. In other words, persistent infectivity in 
the case of trypanosomes is explained by rounded bodies. 
I have performed this experiment (inoculation of latent bodies 
on two occasions. In the first experiment a rat was inoculated with 
one drop of spleen-pulp (from Rat 7) mixed with a little sterile 
physiological salt solution, the mixture containing no flagellates. 
I he inoculated rat developed trypanosomiasis on the 6th day, dying 
on the 1 2th day. The daily counts of this rat were as follows: — 
Day ... ... . 
1 
2 
3 
4 
5 
6 
7 
Parasites per c.mm. ... . 
— 
— 
— 
— 
- 
4 
86+ 
Day ... . 
8 
9 
10 
it 
12 
Parasites per c.mm. ... . 
2 U)fi 
4308 
30.600 
10,000 
l 
_ 
Secondly, a further experiment was tried, since it was considered 
that the parasites in one drop of spleen-pulp solution might be too 
numerous to count accurately, and that a few flagellate 
