The cavity, lined with mesoderm, that surrounds the embryo from the earliest stages of development. It communicates temporarily with the coelomic cavity within the embryo (peritoneal cavity). Late in pregnancy it becomes almost entirely obliterated by the growth of the ‘amnion, which fuses with the chorion.
In humans, the cavity in the developing blastocyst that lies between the mesoderm of the chorion and the mesoderm covering the amniotic cavity and yolk sac.