a succession of corpuscles, and finally attains a size as large as or larger than a corpuscle.  The corpuscles attacked become pale by the destruction of haemoglobin, swell up and disintegrate, the haemoglobin becoming converted into granules of black pigment inside the parasite.  Having attained a definite size the organism forms a rosette and divides into a number of forms similar to the smallest seen inside the corpuscles; these small forms enter other corpuscles and the cycle again begins.  This cycle of development takes place in forty-eight hours, and segmentation is always accompanied by a paroxysm of the disease shown in a chill followed by fever and sweating which is due to the effect of substances liberated by the organism at the time of segmentation.  A patient may have two crops of the parasite developing independently in the blood, and the two periods of segmentation give a paroxysm for each, so that the paroxysms may appear at intervals of twenty-four hours instead of forty-eight (Fig. 20).  This cycle of development may continue for an indefinite time, and there may be such a rapid increase in the parasites as to bring about the death of the individual; but with him the parasite would also perish, for there would be no way of extending the infection and providing a new crop.  The disease has been transmitted by injecting the infected blood into a normal individual.

[Illustration:  FIG. 20.—­PART OF THE CYCLE OF DEVELOPMENT OF THE ORGANISM OF MALARIA, a-g, Cycle of forty-eight hour development, the period of chill coinciding with the appearance of f and g in the blood.  The organisms g, which result from segmentation, attack other corpuscles and a new cycle begins. h, The male form or microgametocyte, with the protruding and actively moving spermatozoa, one of which is shown free. i and j are the macrogametes or female forms. k shows one of these in the act of being fertilized by the entering spermatozooen.  The differentiation into male and female forms takes place in the blood, the further development of the sexual cycle within the mosquito.]

If a mosquito of the species anopheles bites the affected person, it obtains a large amount of blood which contains many parasites.  Within the mosquito the parasite undergoes a further development into male and female sexual forms, which may also form in the blood, termed respectively microgametocyte and macrogamete.  From the microgametocyte small flagellate bodies, the male sexual elements microgametes or spermatozoa, develop and fertilize the macrogametes; after fertilization this develops into a large body, the ooecyst which is attached to the wall of the stomach of the mosquito.  Within the ooecyst, innumerable small bodies, the sporozoites, develop, make their way into the salivary glands and are injected into the individual who becomes the prey of the mosquito, and again the cycle of development begins.  The presence of the parasite within the mosquito does not constitute a disease.  So far as can be determined, life goes on in the usual way, and its duration in the insect is not shortened.