Amphibian blastula cells capable of inducing the organizer form the:
Fate mapping at blastula stage identified vegetal cells capable of inducing organizer without contributing to it. At 32-cell stage dorsal vegetal blastomeres termed Nieuwkoop center accumulate nuclear beta-catenin after cortical rotation. Transplanting them to ventral vegetal position induces ectopic organizer and secondary axis, while organizer itself forms from dorsal marginal zone equatorial cells above. Nieuwkoop showed these vegetal cells emit Nodal-related signals cooperating with beta-catenin targets. Therefore inductive cells form Nieuwkoop center, not Hensen's node, hypoblast or primitive streak characteristic of amniotes. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.
Ref: Gilbert, Developmental Biology 12th ed., Chapter 10: Nieuwkoop center induction of Spemann organizer.