( 6 )
The sexual
cycle in the
mosquito-
contd.
following development there. As a result of the mixture of the blood with
the fluids in the mosquito's stomach, aided by the digestive action of the
stomach juices, the thin envelope of red blood corpuscle which protected the
parasite while it was in the blood of man, becomes disintegrated, and the
parasite escapes and is free in the fluid of the mosquito's stomach. Imme-
diately following its escape from the corpuscle great changes are seen to take
place in the character of the parasite. The female sexual form becomes
a granular spherical body (Pl. I, 13). The pigment of the male sexual
form is seen to be agitated violently, and then, from the periphery
of the parasite, three or four long filaments [microgametes] are suddenly pro-
truded (Pl. I, 12). After lashing about for a few moments the filaments
break off from the main body of the parasite and swim about in the fluid in
the mosquito's stomach. They are the true male element in the sexual pro-
cess. When one of them meets a female sexual form it enters it and fertilizes
the true female element [macrogamete] contained in it (Pl. I, 14). After this
act of fertilization has occurred, the female parasite acquires new characters.
Changing from the shape of a sphere to that of an ovoid with a pointed end
[ okinet] (Pl. I, 15), it begins to move about, and making for the inner wall
of the stomach of the mosquito it passes through the internal coats and comes
to rest between the epithelial and muscular layers. Here a thin capsule is
formed round it, and it begins to absorb nourishment from the host and to grow,
until in a few days it has attained a comparatively large size (Pl. I, 16). At
this stage it is usually called a zygote [ocyst]. By a process of division
(called sporogony, which distinguishes it from the process already described
under the term schizogony) an enormous number of embryo parasites called
sporozoiles, elongated in shape and provided with a nucleus, are formed in the
full grown zygote (Pl. I, 17, 18). After a time the capsule of the zygote
bursts, and the sporozoites are liberated into the lymph sinuses which surround
the outer surface of the stomach. Thence they are conveyed by the circula-
tion to the salivary glands, and penetrating the outer wall of the gland cells
they come to rest in them and in the salivary duct, which has its external
opening at the point of one of the piercing stylets of the proboscis which enter
the skin in the act of biting. This completes the history of the life of the
parasite in its second host, and if this host dies before the parasite can be