Syn.: _Hæmamœba vivax_, Grassi and Feletti, 1890; _Plasmodium malariæ_ var. _tertianæ_, Celli and Sanfelice, 1891; _Hæmamœba laverani_ var. _tertiana_, Labbé, 1894; *Hæmosporidium tertianum*, Lewkowitz, 1897; _Plasmodium malariæ tertianum_, Labbé, 1899: _Hæmamœba malariæ_ var. _magna_, Laveran, 1900, p.p.; _Hæmamœba malariæ_ var. _tertianæ_, Laveran, 1901. This species, _P. vivax_,[201] is the causal agent of the simple or spring tertian fever and is, therefore, named directly the tertian or benign tertian parasite (figs. 80, _3_-_8_; 85). During the afebrile period in the patient, the young trophozoites or amœbulæ appear on or in the red blood corpuscles as pale bodies of 1·5 µ to 2 µ diameter which at first show only slow amœboid movements. Their nucleus is difficult to recognize in the early stage. Soon the food vacuole is formed and this grows concomitantly with the trophozoite and the parasite has a ring-like appearance. Afterwards the vacuole diminishes, and at this period the first brownish melanin granule appears. From this time the activity and number of the pigment granules increase with continuous growth. When the parasite has grown to about one-third the diameter of the erythrocyte the latter shows characteristic red Schüffner’s dots or “fine stippling,” after staining with Romanowsky’s solution. Later, after about twenty-four hours, the blood corpuscles begin to grow pale, then to increase in size, and after thirty-six hours, that is, about twelve hours before the next attack of fever, schizogony of the parasite is initiated by the division of the nucleus. The parasite at this time occupies half to two-thirds of the enlarged blood corpuscle. The daughter nuclei continue dividing until sixteen, and occasionally twenty-four, daughter nuclei are produced. The pigment which, up till now lies nearer the periphery, moves to the middle, while the nuclei lie nearer the surface. [201] See Schaudinn, F. (1902), _Arb. a. d. kaiserl. Gesundheits._, xix, pp. 169–250, 3 plates. [Illustration: FIG. 85.--Development of the tertian parasite in the red blood corpuscles of man; on the right a “Polymitus.” (After Mannaberg.) See also fig. 80, _3_--_7_.] Around each nucleus a portion of cytoplasm collects and thus young merozoites are produced. These separate from each other and from the little residual masses[202] which contain the melanin and pass from the blood corpuscles, which now can hardly be recognized, to the blood plasma, where they soon attack new erythrocytes. [202] The pigment masses (melanin or hæmozoin) are taken up by the leucocytes, particularly the mononuclear ones, and are carried especially to the spleen, and also to the liver and the bone-marrow. From this circumstance arises the well-known pigmentation of the spleen in persons who have suffered from malaria.