Morphological and marker analyses indicate that stage 11 Alk^mili homozygous embryos lack muscle founder cells in their circular visceral mesoderm. The remaining cells in the visceral mesoderm differentiate as fusion competent myoblasts and express genes that are characteristic for this differentiated cell type. Muscle founder cells form normally in the somatic mesoderm of these embryos, and the resulting somatic musculature has only mild fusion defects. Similar phenotypes are seen in Alk^mili/Df(2R)ΔAlk21 and Alk^mili/Alk¹ embryos.