part of blastoderm embryo yolk sac membrane FBbt:00005376 Cellular envelope that surrounds the yolk mass in the center of the syncytial blastoderm embryo at the end of blastoderm stage. It contains the yolk granules, yolk cells (vitellophages) and some remnants of egg cytoplasm. At the end of stage 5, the cytoplasm of the yolk sac and the one from peripheral blastoderm cells are connected via wide cytoplasmic bridges. fly_anatomy.ontology extraembryonic structure cell component