?(   5   )
the hœmoglobin into dark granules of pigment (melanin), which can be
plainly seen in the parasite (Pl. I, 3). When it has grown to its full size.
so that it nearly or quite fills the corpuscle, the grains of pigment collect
together into a mass, and the parasite begins to divide up into a number of
small parts (Pl. I, 4, 5), each of which is capable of becoming a new para-
site identical in all respects with the original one. These parts or segments
(" spores ") [merozoites] remain in contact with each other for a short time
and then burst through the red blood corpuscle-which forms at this time
little more than a thin shell round the parasite-and become free in the blood
(Pl. I, 6). Each of them now seeks out and enters a red blood corpuscle,
in which it begins to grow in the same way as the original one did,
ultimately reaching its full size and dividing up into a number of embryo
parasites which again enter other corpuscles and go through the same cycle.
This is the asexual method of multiplication [schizogony] by simple division of
a full grown parasite into a number of small ones, and it is clear that even if
originally only one corpuscle contained a parasite, a very large number of
corpuscles will, by this method, soon become infected.
The sexual
cycle in the
mosquito.
After this method of multiplication has gone on for a number of
days, some of the parasites in the red blood cells instead of going on
to their full size and dividing up into a number of small parasites, proceed
to the formation of sexual forms [gametocytes]. These sexual forms are,
as a rule, readily distinguished from the full grown asexual forms. Two
kinds are present, viz., male forms [microgametocytes] and female forms [mac-
rogametocytes]. Having attained to their full size, they appear as coarsely
pigmented round or crescent shaped bodies enclosed within the thin shell or
envelope of the red blood corpuscle (Pl. I, 8, 9). In the blood of man they
undergo, as a rule, no further development, and if they remain in this host they
gradually die off.* It is at this stage of the life history therefore that a
second kind of host in which these forms can continue to live and develop
becomes necessary. Such a host is found to be a mosquito of a particular
kind, namely, an Anopheline. When a mosquito of this kind bites a person
with these sexual forms in his blood, some of them are carried into the
mosquito's stomach with the blood which the insect extracts, and undergo the
* See, however, the remarks regarding parthenogenesis on page 3.