104 INSECTS AND DISEASE 
and the spores are liberated. Many of them are swallowed 
by the phagocytes of their host, but some escape and con- 
trive to enter fresh red cells, in which the cycle of develop- 
ment is renewed. It is a very significant fact that the 
parasites may be present abundantly in the blood during 
the period in which the patient is free from fever, but the 
febrile paroxysm follows immediately on the stage in which 
the spores are liberated from the broken-down corpuscles. 
The intervals between the paroxysms and the frequency 
of their recurrence depend on the life-history of the parasites. 
Of these, there are several varieties differing slightly in their 
morphology, and considerably in the symptoms of the 
disease, which they cause. Into these differences we 
need not enter; in the main points the various parasites 
are Closely similar. 
Though the discovery of these facts was a notable 
step in advance, we were nearly as far as ever from under- 
standing the causation of malaria. In what way, and 
under what conditions do the parasites enter the human 
body 2? For the answer to these questions we are mainly 
indebted to two British observers, Sir Patrick Manson 
and Sir Ronald Ross. Manson, starting from the pro- 
position, that every parasite has some way of getting 
out of the body of its host, drew special attention to a 
remarkable form of the parasite, which had been previously 
discovered by other observers, without its true significance 
having been recognised. This is the so-called flagellate 
body. It is never found in blood freshly drawn, but 
appears only when the blood has been exposed to examina- 
tion for some time after withdrawal. In such a 
preparation some of the parasites may be observed to 
slip out of their containing corpuscles, assume a rounded 
form, and suddenly project a half-dozen long whip-like 
arms, which perform rapid lashing movements, and 
finally break away and swim freely in the serum. 
This striking phase in the history of the malaria 
organism was rightly regarded by Manson as the first phase 
in its extra-corporeal life-cycle, and he suggested that this 
stage of its life was passed in the tissues of the mosquito, 
and that from the mosquito the parasite in some way 
again entered man. A young Indian army surgeon, Ronald 
