During cleavage stages embryos from chb^mast5 mutant mothers are characterized by the presence of monopolar spindles organized from one or two centrosomal foci with chromosomes dispersed within the aster. Bipolar spindles and isolated centrosomes are also regularly observed, as well as short bipolar spindles with the poles close to the chromosomes. Cellularized mutant embryos are characterized by the presence of highly polyploid cells that, in mitosis, organize monopolar or bipolar spindles with centrosomes at a single pole. Time-lapse analysis of cycle 12 embryos from chb^mast5 mutant mothers shows that, during early stages of mitosis, the centrosomes separate as in controls and a bipolar spindle assembles after nuclear envelope breakdown. However, during prometaphase, chromosomes never reach a stable position at the equatorial region and appear instead to drift back and forth along spindle microtubules. On average, only 32% of centromeres cluster within an area of the metaphase plate occupied by >95% of centromeres in wild-type. Centrosomes separate normally during prophase, but spindle extension during prometaphase fails. During metaphase the spindles start to shorten as in wild-type, but then end up collapsing toward the equatorial region, leading to the formation of monopolar spindles.