3. Blood-plaques.--These are not colored by Ehrlich's stain, nor by eosin and methylene-blue. With Wright's stain they appear as spheric or ovoid, reddish to violet, granular bodies, 2 to 4 µ in diameter. When well stained, a delicate hyaline peripheral zone can be distinguished. In ordinary blood-smears they are usually clumped in masses. A single platelet lying upon a red corpuscle may easily be mistaken for a malarial parasite (Plate VI). Blood-platelets are being much studied at present, but, aside from the facts mentioned under their enumeration (p. 165), little of clinical value has been learned. They have been variously regarded as very young red corpuscles (the "hematoblasts" of Hayem), as disintegration products of leukocytes, as remnants of extruded nuclei of erythrocytes, and as independent nucleated bodies. The most probable explanation of their origin seems to be that of {186} J. H. Wright, who, from his recent studies, regards them as detached portions of the cytoplasm of certain giant-cells of the bone-marrow and spleen. VII. BLOOD PARASITES A study of blood bacteriology is useful, but is hardly practicable for the practitioner. Most bacteria can be detected only by culture methods. The _spirillum of relapsing fever_ can be identified by the method for the malarial parasite in fresh blood. The blood must be taken during a paroxysm. The organism is an actively motile spiral thread, about four times the diameter of a red corpuscle in length. The movements which its active motion causes among the corpuscles render it conspicuous. It can also be seen in stained preparations (Fig. 78). The disease has rarely been seen in the United States. [Illustration: FIG. 78.--Spirillum of relapsing fever (Karg and Schmorl).] Of the numerous animal parasites which have been {187} found in the blood, three are especially interesting clinically: _Plasmodium malariæ_, _Filaria sanguinis hominis_, and _Trypanosoma hominis_. 1. Plasmodium Malariæ.--This organism is one of a large group, the _hemosporidia_ (p. 247), many of which live within and destroy the red corpuscles of various animals. Three varieties are associated with malarial fever in man--the tertian, quartan, and estivo-autumnal malarial parasites. (1) Life Histories.--There are two cycles of development: one, the _asexual_, in the blood of man; and the other, the _sexual_, in the intestinal tract of a particular variety of mosquito.