Spemann's organizer is derived from:
Gray crescent cytoplasm contains determinants accumulated via cortical rotation: dishevelled protein, GBP, kinesins and Wnt11 RNA enriched dorsally inhibiting GSK-3 beta. This permits beta-catenin nuclear accumulation in dorsal marginal cells overlaying crescent. These cells become Spemann organizer in dorsal blastopore lip expressing Goosecoid, Chordin and Noggin, inducing neural plate. Transplantation shows dorsal equatorial region derived from gray crescent involutes as chordamesoderm. Ventral blastopore cells lacking nuclear beta-catenin form blood and mesenchyme, not organizer activity, confirming developmental origin from gray crescent.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Spemann organizer derived from gray crescent equatorial region.