The schizogony can, however, only be repeated a certain number of times, supposing that the disease has not been checked prematurely by the administration of quinine, which is able to kill the parasites. It appears that after a number of attacks of fever the conditions of existence in man are unfavourable for the malarial parasites, and this brings about the production of other forms which have long been known, but also long misunderstood (spheres, crescents, polymitus). The merozoites in this case no longer grow into schizonts, or at least not all of them, but become sexual individuals called gametocytes (fig. 80, _9_-_12_), which only start their further development when they have reached the intestine of _Anopheles_. This does not take place in every case, nor with all the gametocytes which exist in the blood of patients with intermittent fever. Of those parasites which remain in the human blood the male ones (microgametocytes) soon perish, the females (macrogametocytes) persist for some long time, and perhaps at last acquire the capacity of increasing by schizogony. They might thus form merozoites which behave in the body as if they had proceeded from ordinary schizonts (fig. 80, _13c_-_17c_). If their number increases sufficiently, in course of time the patient, who was apparently recovering, has a new series of fever attacks, or relapses, without there having been a new infection. This is the view of Schaudinn, who from researches of his own concluded that relapses were brought about by a sort of parthenogenetic reproduction of macrogametocytes. R. Ross, on the contrary, believes that in the relatively healthy periods the number of parasites in the blood falls below that necessary to provoke febrile symptoms; relapses then result merely from increase in the numbers of the parasites present in the individual. Ross’s view is now generally accepted. [Illustration: FIG. 81.--Stages of development of pernicious or malignant tertian parasites in the intestine of _Anopheles macultpennis_. (After Grassi.) _a_, macrogametocyte (crescent) still attached to human blood corpuscles; _b_, macrogametocyte (sphere) half an hour after ingestion by the mosquito; _c_, microgametocyte (crescent) attached to the blood corpuscle; _d_, microgametocyte (sphere) half an hour after ingestion; the nucleus has divided several times; _e_, microgametes attached to the residual body (polymitus stage).] [Illustration: FIG. 82.--Oökinete of the malignant tertian parasite in the stomach of _Anopheles maculipennis_, thirty-two hours after ingestion of blood. (After Grassi.)]