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210 peripheral circulation. In smear preparations they are often free or in clusters of various numbers, sometimes arranged with great regularity like the merozoites in the segmenting quartan or tertian malaria parasites. Sometimes as many as 50, or 200, or even more parasites are found together embedded in a structureless matrix or stroma, probably the remains of the original host cell.

In cultures (Fig. 57) the parasites enlarge very rapidly. They retain at first their shape, the cytoplasm becoming granular, opaque, and vacuolated. Having attained a diameter of 7 to 9 $$\mu$$, they assume an

Fig. 57.—Kala-azar parasites in cultures. (After Leishman.)

elongated pyriform shape and become flagellated. The flagellum arises from the blepharoplast at the rounded end of the parasite, and projects at once clear of the body as in Euglena. There is no undulating membrane as in trypanosomes. These flagellated forms measure from 12 to 20 $$\mu$$ in length. They multiply by longitudinal fission; sometimes throwing off exceedingly fine linear forms (Leishman), comparable in their delicacy to spirochætes. They move actively with the flagellum in front, and towards the twelfth day tend to agglomerate in rosette groups, the flagella being directed centrally.

The culture medium used by Rogers was blood to which a small quantity of sterile, slightly acid citrate of soda solution was added to prevent