Homozygotes have approximately 80% fewer mitotic figures in third instar larval brains than wild-type larvae. Among those cells with detectable mitotic figures, a large number of cells are aneuploid, polyploid or have abnormal metaphase figures. Approximately 35% of the mitotic figures have more than the wild-type set of four chromosomal pairs. Lagging chromatids and chromosomal bridges are seen in 60% of anaphase figures. In addition, cells with two or more nuclei and nuclei with a high chromosomal content are sometimes seen. 80% of fbl¹/fbl³ primary spermatocytes have spindle abnormalities, with the majority of spindles being multipolar. Anaphase and telophase stages are difficult to define in these testes, due to the severe defects in metaphase spindle organisation. In some less severely affected cells, two asters are seen at a single pole. Despite this lack of aster separation, these cells contain a contractile ring positioned over a central spindle.