The myoplasm of ascidian eggs is a localized cytoplasmic region containing a unique cytoskeletal domain. During ooplasmic segregation, the myoplasm moves first to the vegetal pole and then to the future posterior region of the fertilized egg, where it subsequently enters the muscle cell lineage during cleavage. In the vegetal pole region, the myoplasm defines a developmental center which later controls gastrulation and embryonic axis formation. In the posterior region, the myoplasm defines another developmental center, which specifies muscle cell development. Evidence is described suggesting that the integrity of the myoplasmic cytoskeletal domain is required for normal embryonic functions of the myoplasm.