part of somatic muscle primordium somatic muscle embryonic somatic muscle 2022-07-05T13:30:44Z fly_anatomy.ontology FBbt:00059033 Somatic muscle of the embryo, found prior to the fully-formed larval muscle pattern visible at stage 17. At stage 12, myoblasts of the somatic musculature are found in the superficial layer of the mesoderm, these differentiate and fuse to form muscle fibers by the end of stage 13. Between stages 13 and 17, the muscle fibers insert at apodemes formed by the epidermis and stretch during dorsal closure and head involution.