Embryonic tissue is continuously rotating downward but is displaced upward by the dense yolk. These cells can induce an ectopic primitive streak if transplanted. They also act to inhibit cells in the rest of the marginal zone from inducing primitive streak formation. This is the most active site of ingression. In chick, as in other higher vertebrates, this organization begins at the future anterior end of the emryo and gradually spreads posteriorly and laterally.
Cells move down, displacing the hypoblast.
Mesoderm that will form notochord, somites, head and heart passes in through Hensen's node. The primitive streak is now fully formed. While the primitive streak regresses, gastrulation continues at the posterior while at the anterior end of the chick, gastrulation is already complete. This is equivalent to the migration around the ventral and lateral lips of the blastopore in amphibian gastrulation.epiboly superficially similar to amphibian. Become bottle shaped, as in amphibians, but then delaminate and migrate as single cells rather than a cell layer.