Embryos have an enlarged brain, irregular ventral nerve cord formation and disorganized peripheral ganglia; these defects first appear in 10-12 hour embryos and include a delay in germband contraction and a failure of head involution. The CNS of mutant embryos fails to complete the normal condensation that leads to compact paired cerebral hemispheres and a ventral ganglion. In late mutant embryos, the brain, which has protruded dorsally through a hole in the cuticle, remains extended and flattened against the vitelline membrane and the ventral neuromeres are extended posteriorly; discontinuities or thinnings of the ventral nerve cord can frequently be observed. Normal development of the mouth parts and cephalopharyngeal apparatus is disrupted.