AbstractHydrocortisone acetate (1–10 mg/egg), when injected into the extraembryonic celom of 3‐ or 4‐day chicken embryos, caused necrosis in limb‐bud mesoderm after further incubation for 24 h. The necrotic areas usually appeared in the central and subapical mesoderm. Similar treatment of 4‐day embryos with a higher dose (7.5 mg/egg) caused, in addition, hemorrhage in limb buds, micromelia, and slight growth retardation. The mechanism of the resulting necrosis and phagocytosis are