The excavation of the cavity results in the separation of the extraembryonic mesoderm surrounding the amniotic cavity (together called the amnion) and yolk sac (together called the exocoelomic or Heuserπs membrane) from that extraembryonic mesoderm remaining with the trophoblast (together known as the chorion).t this time cells of the embryo, thought to delaminate from the cytotrophoblast and/or epiblast, contribute the extraembryonic mesoderm, a disorganized layer between the cytotrophoblast and the yolk sac as well as between the cytotrophoblast and the cells surrounding the amniotic cavity (Fig.