(A,AI) Org-1 protein (Org-1) is found during stage 10 in all cells of the trunk visceral mesoderm primordia, visualized by bap3-lacZ expression (β-Gal). (B,BI) At stage 11 the visceral mesodermal expression domains of Org-1 are narrowed down to the founder cells of the circular visceral musculature (cFC), whereas the visceral fusion competent myoblasts (FCM) have become negative for Org-1 protein. (C,CI) Visceral muscle fusion takes place in stage 13, leading to the activation of Org-1 expression in the newly incorporated nuclei in the binucleated muscle syncytia. (D,DI) From stage 14 until the end of embryogenesis (E,EI) all nuclei of the circular visceral musculature continue to express Org-1.