In vertebrates, the notochord arises from the dorsal organiser. Originally identified by Spemann and Mangold in amphibians, the dorsal organiser is a region of a vertebrate gastrulae that, when transplanted into prospective lateral or ventral regions of a host embryo, induces the formation of a second embryonic axis, while only contributing to notochord and prechordal mesendoderm (Harland and Gerhart,1997; Spemann and Mangold,1924). In amphibians, this region is the dorsal lip of the blastopore. In other species, homologous structures have been found: the embryonic shield of teleost fish, Hensen’s node in the chick and the node of mouse embryos all possess essentially the same activities as Spemann and Mangold’s dorsal organiser (Beddington,1994; Oppenheimer,1936; Waddington,1930). The functions and activities of the dorsal organiser are complex and have been discussed in detail elsewhere(Harland and Gerhart, 1997).