• During implantation, a cavity appears in the inner cell mass; amnion forms around this amniotic cavity; derived from amnioblasts from the epiblast
• Cells of the embryonic disc flatten and differentiate into two layers:
  -epiblast - a tall, columnar layer of cells forming the "floor" of the amniotic cavity (primitive ectoderm)
  -hypoblast - a short, cuboidal layer of cells forming the "roof" of the exocoelomic cavity (primitive endoderm)
• The exocoelomic cavity and membrane form the primary yolk sac
• Thus, bilaminar embryonic disk is sandwiched between two "balloon-like" cavities, the amniotic cavity and the primary yolk sac

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