Soon after Spemann and Mangold’s (1 ) famous demonstration in 1924 that the dorsal lip of the blastopore of the gastrulating amphibian embryo has the unique ability to induce a second axis when grafted into an ectopic site in a host embryo, Waddington (2 ,3 ) showed that Hensen’s node is its equivalent in amniotes. After transplanting this region into an ectopic site in interspecific combinations of rabbit, duck, and chick embryos, he found that a second axis developed, where the nervous system was derived from the host ectoderm (4 ). Hensen’s node is situated at the anterior (cranial) tip of the primitive streak during gastrulation, and in chick embryos appears as a bulbous thickening, some 100 μm in diameter, centered around a depression, the primitive pit. At this point, the three germ layers of the embryo are in very close apposition.
