Mutant embryos show grossly normal morphology, normal segmentation, normal epidermal denticle belts and normal patterning of the neuromusculature. They show greatly reduced movement compared to wild-type. The muscle pattern is largely unaltered in mutant embryos, although a few missing muscles are occasionally seen. Mature embryos show striking defects in individual muscle fibre morphology. Unfused myoblasts are seen. The number of muscle nuclei is reduced compared to wild type. All muscles show a similar quantitative reduction in nuclei numbers. The ordered sarcomere structure seen in wild-type muscles is totally disrupted. The parallel myofilament array seen in wild-type muscles is disrupted and myofilaments are arranged irregularly, both longitudinally and transversely. The unit of a thick myosin filament surrounded by an array of thin actin filaments can form correctly, whereas the overall arrangement of myofilaments is largely disrupted.