BrdU incorporation in third larval instar brains is reduced in homozygotes compared to heterozygotes. There is a 3.3 fold increase in the percentage of mitotic cells in the mutant larval brain compared to wild type. Telophase and prophase figures are not seen, and anaphase figures are rare. 91% of figures appear arrested in a metaphase-like state. A large number of metaphase-like figures are seen in which a highly condensed chromosome mass is present. Abnormal mitotic spindles are seen that are either anastral or have poorly focused poles. 51% of figures (Class I) have chromosomes aggregated at the metaphase plate (in some cases the individual chromosomes can be resolved in the chromosome aggregate), while 41% (Class II) have chromosomes scattered along the metaphase-like spindle. The spindle poles are often clumpy and the spindle fibres are not present. The mitotic chromosomes are abnormally condensed, with three classes of defects based on severity, in third instar larvae; 38% (Class I) have mild defects where chromosome morphology is close to wild type, but their arrangement is abnormal - metaphase spreads are quite crowded and chromatids are often fused. In 56% (Class II), mutant centromeres cannot be distinguished and chromosome arms are decondensed. In addition, sister chromatid cohesion is often lost. 6% (Class III) have underwound centromeres and chromosome arms are more decondensed than in Class II, having a "dumb-bell" morphology.