In Grip84^R20 homozygous late third instar larvae, the mitotic index of larval brain neuroblasts is elevated 4-5 fold over wild-type levels: cells accumulate in pro-metaphase and metaphase; fewer cells are found in anaphase or telophase. More than half these neuroblasts have overcondensed chromosomes and most have disorganised metaphase plates. In addition around 25% of these neuroblasts are polyploid. Dividing neuroblasts frequently have abnormal spindles: monopolar, devoid of polarity or asymmetric. Marker analysis suggests that pericentriolar matrix is frequently deformed and sometimes fragmented.