The early embryo before the formation of somites. It is a flat disk of tissue bounded dorsally by the amniotic cavity and ventrally by the yolk sac. The formation of the primitive streak and archenteron in the embryonic disk determines the orientation of the embryo, which then becomes progressively elongated.
An oval disk of cells in the blastocyst of a mammal from which the embryo proper develops. Its lower layer, the endoderm, forms the roof of the yolk sac. Its upper layer, the ectoderm, forms the floor of the amniotic cavity. The primitive streak develops on the upper surface of the disk.