part of develops into developing embryonic structure primordium somatic muscle primordium SoMusP2 fly_anatomy.ontology FBbt:00005518 somatic muscle specific anlage P1 SoMus Primordium of the larval segmentally-repeated body wall musculature. It arises from the outer layer of the trunk mesoderm. During germ band shortening (stage 13), it segregates into loose segmental clusters. Within these clusters, muscle formation proceeds by fusion of fusion competent myoblasts to muscle founder cells. Formation of the embryonic/larval somatic musculature is complete by stage 17. presumptive embryonic/larval muscle system somatic trunk mesoderm derivative larval somatic muscle