After several cleavages, the group of cells sitting atop the yolk is called the blastoderm. There are about 60,000 cells at this time. This is the most active site of ingression. Hensen's node is equivalent to the organizer in amphibians. In chick, as in other higher vertebrates, this organization begins in the anterior midline and gradually spreads posteriorly and laterally.
Cells move down, displacing the hypoblast.
Most of cells passing into the interior become mesoderm. Mesoderm that will form notochord, somites, head and heart passes in through Hensen's node. Primitive streak is now fully formed. This is equivalent to the migration around the ventral and lateral lips of the blastopore in amphibian gastrulation.epiboly superficially similar to amphibian. Formation of the gut Gut forms after gastrulation.