Embroyonic Disc Amniotic Cavity Yolk Sac

The embryonic disc with amniotic cavity and yolk sac represents the early post-implantation stage, where the bilaminar disc of epiblast and hypoblast forms, then gastrulation creates a trilaminar disc with ectoderm, mesoderm, and endoderm; the amniotic cavity expands above the epiblast as a protective fluid-filled space, while the yolk sac below from hypoblast provides early blood cells and germ cells before placental dominance. Extraembryonic mesoderm supports these structures, setting up chorion for future placenta; this flat disc stage is transient but foundational for all subsequent body layers and protective membranes.