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We have studied the embryonic development of Drosophila hemocytes and their conversion into macrophages. Hemocytes derive exclusively from the mesoderm of the head and disperse along several invariant migratory paths throughout the embryo. The origin of hemocytes from the head mesoderm is further supported by the finding that in Bicaudal D, a mutation that lacks all head structures, and in twist snail double mutants, where no mesoderm develops, hemocytes do not form. All embryonic hemocytes behave like a homogenous population with respect to their potential for phagocytosis. Thus, in the wild type, about 80-90% of hemocytes become macrophages during late development.

In mutations with an increased amount of cell death (knirps; stardust; fork head), this figure approaches 100%. In contrast, in these mutations, the absolute number of hemocytes does not differ from that in wild type …