Laveran observed that certain forms of the parasite presented “flagella,” _i.e._ filaments exhibiting very active movements. Manson having observed that flagella were not found in blood first drawn, but only appeared after a little time had elapsed, conceived the idea that the function of these must be that of spores. Having previously observed that a microscopic worm, filaria, is drawn with the blood into the stomach of a kind of mosquito (page 278), and finds in the latter a secondary host, he concluded that a similar cycle of events might occur in malaria. Ross tested this theory, and by causing mosquitoes bred in bottles from the larva to bite persons affected with the crescent form of malaria, after repeated unsuccessful attempts, was eventually able to find in comparatively rare mosquitoes which had thus bitten a malarious patient, small rounded bodies embedded in the wall of the stomach. These were watched and found to present appearances identical with those of the parasite of malaria. Similar pigmented bodies were subsequently found in other mosquitoes. The malarial parasite belongs to the Protozoa, of which it is one of the smallest members. Man is its _intermediate_ host, and the anopheles its _definitive_ or final host. In the red blood corpuscle of man it is a unicellar organism, from 1 µ to 8 µ in diameter. It has two methods of reproduction, endogenous by spore formation and exogenous or sexual. The former occurs in man; the latter in the mosquito. Without the latter, the parasite being unable to pass from man to man, would die with its host. In endogenous multiplication spores are formed which separate from the original parasite and gain access to other red blood corpuscles. The large pigmented spheres and the crescent bodies require to enter the stomach of the anopheles to attain full development. In the anopheles the crescents become spherical, flagella are shot out, having a length of 4 to 5 times the diameter of a red blood corpuscle. These represent the male element, while other spheres without flagella are the females. By the fusion of these two a fertilised cell is produced (_the travelling vermicule_), which now assumes the shape of a spear-head and is actively mobile. The travelling vermicule pierces the stomach wall of the mosquito and develops into a _zygote_. If an infected mosquito is examined on a succession of days under the microscope, the following stages can be traced. The zygote consists of pigmented spheres 7 to 8 µ in diameter, lying in the muscular fibres of the mosquito. These grow, and become surrounded by a capsule. Smaller spheres form and subdivide, bud-like processes develop on their surfaces; these gradually become sickle-shaped and protrude into the body cavity. They increase in size. until they attain dimensions of from 40, to 60 µ. Eventually they rupture, and the sickle-shaped bodies (_sporozooites_) escape and are carried in the body fluid of the mosquito to its salivary glands.