The dorsal lip of the blastopore forms due to:
After cortical rotation, vegetal endoderm undergoes vegetal rotation where large yolky dorsal marginal cells move inward and upward, forming wedge thrust pushing dorsal marginal mesoderm outward creating lip. This movement of bottle cells and involution starts at gray crescent region, generating dorsal blastopore lip visible as pigment-free arc. Rotation driven by Brachyury and Wnt11 dependent cell intercalation and apical constriction via Shroom3. Animal pole elongation not involved. Process establishes site of gastrulation and marks organizer region preparing for mesoderm involution and archenteron formation during amphibian gastrulation.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Vegetal rotation and formation of dorsal blastopore lip.