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eight hours, we should find in the blood at any 
particular time parasites in two phases of development 
corresponding to each cycle. The accompanying 
chart shews how, in the case of what proved to be the 
malignant tertian parasite, we were able to establish 
the cycle of development. We proceeded to make blood 
examinations at frequent intervals (four hours). We 
found that at any particular time parasites of various 
sizes might be found, but by counting several hundred 
parasites in each film and estimating their size with a 
micrometer we found that at any particular time 
there was a preponderance of parasites of one size. 
Thus, at ten p.m. on the 2nd, there are numerous small 
forms, z.<?., about one-seventh to one-eighth of a red 
cell in diameter, and it is not till, ten p.m. (about) on 
the 4th that the same condition of blood is found again, 
accordingly the parasite had a developmental cycle 
of forty-eight hours (approximately). And, further, 
we determined the periods taken to develop from small 
forms to largest forms in the peripheral blood (about 
eighteen hours) and the disappearance of these and 
the reappearance of numerous youngest parasites 
(about thirty hours). So that by determining these 
three periods we were able to conclude that the 
parasite was the malignant tertian. 
In order then to determine the cycle of a parasite 
it is necessary :— 
1. To estimate the size and percentage of 
parasites of each size at any particular time, e.g., 
starting with the onset of the attack. 
2. To follow each group to its period of maxi¬ 
mum development in the circulation. 
3. To estimate the time between this period and 
the next appearance of young forms. 
4. To estimate the time between the appearance 
