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HIS lecture is devoted to a description of the parasite and of its life cycles. The existence of a parasite in malarial disease has been suspected for a long time, but only very recently have we had absolute assurance that such a parasite exists. Some time in the thirties Meckel described in the human blood certain black particles which he found in leucocytes and in certain pale, leucocyte-like bodies the nature of which he did not know. When he saw these bodies he certainly saw the malarial parasite. His observations were repeated and extended in the forties and the fifties by Frerichs and Virchow, and they, too, undoubtedly saw the malarial parasite. But it is one thing to see and quite another to recognize; discovery is recognition.

The discoveries of Laveran, Golgi, Marchiafava, Bignami and others resulted in considerable knowledge of the life history of the malarial parasite and of the correspondence between its life cycle and the clinical cycle of the disease. Laveran discovered the parasite; Golgi described the cycle of the tertian and quartan forms; the others added new data, especially concerning the more malignant parasites. The malarial parasite in its mature form has the appearance—I shall take the tertian parasite as a type—of a mass of pale protoplasm occupying practically the whole of the red blood corpuscles. Scattered through this mass of protoplasm are a number of black specks or little rods of intensely black pigment. Later in the life of the parasite a peculiar thing happens: all these little specks of black pigment concentrate usually towards the center of the organism whilst the pale protoplasm arranges itself into little spherules, the whole constituting what is known as the 'rosette body.' Later in the life of the parasite the surrounding blood corpuscle breaks away and this rosette body floats free in the liquor sanguinis and then breaks up into its constituent spores, setting free at the same time the black pigment clump. Phagocytes attack many of these free spores and probably absorb most of them, as well as the little pieces of pigment. The result is the pigmented leucocyte, so characteristic of malarial blood. A few of the spores escape and in virtue of some peculiar faculty, which is not at present