Primitive node

Hensen's Node is the organizer for the early gastrula of birds. It is the equivalent of the dorsal lip of the blastopore found in the embryos of more primitive vertebrates, such as amphibians and fish.

Hensen's node starts as a regional knot of cells that forms on the blastodisc immediately anterior to where the outer layer of cells will begin to migrate inwards - the primitive streak. Posterior to the node is the primitive pit, where the cells initially begin to invaginate. This invagination expands posteriorly into the primitive groove as the cells layers continue to move into the space between the embryonic cells and the yolk and differentiate the embryo into the germ cell layers - endoderm, mesoderm, and ectoderm. Hensen's node migrates posteriorly as gastrulation proceeds, eventually being absorbed into the tail bud.

Hensen's node secretes many signals essential for gastrulation - including Fibroblast Growth Factors, and Retinoic Acid.