motionless, non-parasitic vacuole (Fig. 17). The other common forms—the larger or smaller intracorpuscular pigmented parasites—occupy anywhere from a sixth to nearly the entire area of the affected corpuscles. They are recognized by their pale protoplasm; by the black hæmozoin particles scattered about or, if towards the period of rigor, concentrated in their interior; and by their more or less active amœboid movements. In quartans and tertians, but especially in the former, segmenting rosette forms are seen occasionally.
Examination of blood for flagellated bodies.—When the student has become familiar with these appearances, and has thoroughly seized the fact that the segmenting forms are to be found only or usually during, just before, or soon after the rigor stage of fever, he should endeavour to follow up the initial steps of the exogenous, sexual or mosquito phase of the parasite. So far as ordinary preparations permit, this phase is best studied in the "zone of heaped-up corpuscles" and in the "zone of rouleaux"; because in these zones the parasite, not being subjected to pressure, has more freedom to undergo its evolutionary change into the flagellated body.
In ordinary quartans and tertians flagellated bodies are but seldom encountered. The best time to find them in such cases is said to be during the hot stage of the fever. In cases of crescent infection they are much more frequently met with, as, in this form of malaria, flagellated bodies are usually more numerous, appear at any time of the clinical cycle, and persist in the circulation perhaps for several weeks after fever has disappeared.
In most cases of crescent infection the gradual evolution of the flagellated body from crescent through oval and sphere can, with patience, be easily followed.
Diagnostic value of the "zone of free hœmoglobin."—The zone of free hæmoglobin is of value as enabling the practised observer to pronounce very rapidly on the presence or absence of pigmented parasites in the blood. The relatively large quantity of blood in each field of this zone, and therefore the
