Embryos hemizygous for armadillo produce a "segment polarity" phenotype in which the naked posterior two-thirds of each segment is replaced by denticles with reversed polarity. Small patches of homozygous arm cells induced by mitotic recombination also form such denticles, indicating that the changes in cellular fate observed in homozygous arm embryos are autonomous at the level of single cells. Clonally derived arm patches do not, however, show the characteristic arm polarity reversals, arguing that this feature of the phenotype depends on cell interactions in fully mutant embryos. Few, if any, clones were found in the posterior-most regions of the naked cuticle, and none were found in the posterior compartments of the thorax.