Named after Victor Hensen (1835-1924), and also called the primitive knot, it is the thickened area at the anterior end of the primitive streak or groove through which gastrulating cells invaginate to form the notochord. Found in birds, mammals and reptiles, it is homologous to the dorsal lip of the blastopore in amphibian embryos, and a primary organizer of the embryo. Its importance derives from the fact that it serves as a landmark rather than it gives rise to particular structures in the developing embryo (see figure below).
Dorsal view, magnified 14 times, of chick embryo at about 16 weeks of incubation (primitive streak stage).
See Blastopore, Embryogenesis, Induction (embryology), Neural tube, Notochord, Organizer (embryology)