Only one-fifth of the expected number of homozygotes reach the adult stage, the rest die as pupae. This semi-lethality may be due to unrelated lethal mutations induced with fs(2)abc^fs27 during mutagenesis. The egg laying rate is reduced to approximately one-tenth of normal in hemizygous females. Embryos derived from hemizygous females are usually arrested at the 8 nuclei stage or earlier. The embryos contain large nuclei that are erratically distributed and show mitotic asynchrony. Early nuclei are aligned along the antero-posterior axis, in contrast to wild-type. The endochorion has few pillars, the roof is thin and there is often debris in the space between the pillars. The dorsal appendages are sometimes abnormal; they may be shorter and fatter than normal and they may be fused at the base in the ventral midline. The mutant phenotype is not paternally rescued. Homozygous larval neuroblast chromosome structure and mitotic indices are normal.