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 understood, enter fresh blood corpuscles and appear there as pale specks in the hæmoglobin. These pale specks, if watched in perfectly fresh blood, are seen to be possessed of very active amœboid movement. They throw out pseudopodia in various directions and wander about through the hæmoglobin of the corpuscle. After a time they increase in size by assimilating the hæmoglobin. By and by there appear somewhere in the parasite those specks of black pigment which we saw in the mature animal. Later they increase still further in size until they come to occupy half, and finally nearly the whole, of the blood corpuscle. Again there is concentration of pigment and the formation of little sporules. This is the cycle, as described by Golgi, of the tertian and quartan parasite. The cycle of the tropical or æstivo-autumnal parasite corresponds in plan almost exactly with that of the quartan and ordinary tertian parasite.