3. In the third mode of insect conveyance the insect does not play a merely passive role, but becomes a part of the disease, itself undergoing infection, and a period in the life cycle of the organism takes place within it. In all these cases quite a period of time must elapse before the insect is capable of transmitting the disease; in malaria, which is the best type of such a disease, this period is ten days. Malaria is due to a small protozoan, the _Plasmodium malariae_, which was discovered by Lavaran, a French investigator, in 1882. The organism lives within or on the surface of the red blood corpuscles. It first appears as a very minute colorless body with active amoeboid movements, and increases in size, attacks 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.]