198 Major R. Ross and Dr. D. Thomson. Sleeping [Oct. 15, 
The greater the resistance of the infected animal the longer is the cyclical 
period, as in guinea-pigs and man. 
By an “unconditional cycle” we mean one of which the period is not 
affected by the resistance of the host or by therapeutic agents—as, for 
instance, that of Plasmodium. By a “ conditional cycle” we mean one which 
is so affected—as in the present case. 
Gi) The Increase of Parasites is Due to their Sudden and Active Multipli- 
cation.—The presence of numerous dividing forms during the cyclical rise 
would seem to support this statement. 
(iii) The Multéplication of Trypanosomes is extremely Rapid.—As before 
stated they seem capable of dividing in man three to four times in 24 hours. 
In rats the rate of multiplication would seem much greater, as many as 
10 divisions or generations may occur in 24 hours. 
(iv) The Rate of Multiplication depends on the Suitability of the Infected 
Blood to the Parasite-—The blood of rats seems more suitable to the parasite 
than the blood of man and guinea-pigs. 
Also this suitability would appear to vary from time to time in the same 
animal. In our case there occurred high and low rises in the numbers of 
trypanosomes. Moreover the three highest rises recorded in the chart were 
followed by the three lowest rises recorded. This is a very significant fact. 
It would appear to indicate that a high rise sets up some reactionary 
condition. This reaction, if great, not only causes the decrease of 
trypanosomes but extends its influence so far as to reduce the next rise. 
(v) Lhe Fall in Number of Parasites 1s not Due to the Toxins they Develop.— 
In man the numbers may reach only 200 per cubic millimetre of blood and 
then they diminish, whereas in rats they may reach over 200,000 per cubic 
millimetre of blood and yet continue to rise, and the toxins in the latter 
case must be much more abundant than in the former. | 
It would seem, therefore, that the explanation may be reduced to some 
of the following causes, all of which may come into play. 
(1) Lhe Increase of Trypanosomes is due to their Active Multiplication, the 
rate of multiplication depending on the following conditions :— 
(a) The liberation of a reproductive stimulant from the dead trypanosomes 
of the previous fall. | 
(6) The small number of leucocytes, especially mononuclears. 
(c) The habituation of the trypanosomes to their anti-bodies. 
(@) The absence or the diminution of anti-bodies to the trypanosomes. 
(ii) The Decrease of Trypanosomes is Due to their Rapid Death and to a 
