In mutant embryos, the muscle founder cells are restricted to the ventral part of the visceral band and become less elongated. Within these cells some nuclei move into a more dorsal position, but no signs of myoblast fusion are visible. At the dorsal side, there are many unfused globular cells, which in wild-type embryos have already fused with the ventral myoblasts at that stage of development. All fusion competent myoblasts retain their globular shape. During further differentiation the spindle-like founder cells stretch out ventrally and dorsally. Although no fusion processes are seen, these cells can differentiate to a certain degree. At stage 16, besides the absence of the first midgut constriction severe aberrations are seen in the visceral musculature of the anterior part of them midgut. In comparison to wild-type the longitudinal muscles are almost absent and circular muscles are completely disorganised.

Dorsal vessel and visceral mesoderm is unaffected but major distortions occur in somatic mesoderm. Medium allele: Late stages of embryonic development show many unfused myoblasts. First distortions are visible during germ-band retraction. Unfused myoblasts are often arranged in specific locations where in the wild type ventral, pleural and longitudinal muscles form.