Practice question

Spemann-Mangold organizer responsible for secondary axis induction derives from dorsal marginal zone. After cortical rotation, dorsal vegetal cells with nuclear beta-catenin and Nodal signaling induce overlying dorsal marginal zone to become organizer expressing goosecoid, chordin, noggin. At gastrulation, this region forms dorsal lip of blastopore containing bottle cells and involuting chordamesoderm forming notochord. Fate maps show dorsal lip contributes to prechordal plate and notochord, while ventral lip forms ventral mesoderm. Transplantation of dorsal lip to ventral side induces secondary axis with neural tissue, whereas ventral lip lacks organizing ability, confirming dorsal origin and inductive power.