Practice question

Amphibian gastrulation initiates specifically where dorsal determinants have accumulated before cleavage. Following cortical rotation, gray crescent region opposite sperm entry acquires high nuclear beta-catenin, expresses organizer genes siamois and goosecoid, and forms bottle cells by apical constriction. This site becomes dorsal lip of blastopore, first point of marginal zone involution and archenteron initiation. Involution spreads laterally and ventrally around blastopore forming lateral and ventral lips. Initiation at gray crescent ensures chordamesoderm internalizes dorsally to form notochord beneath ectoderm where it secretes BMP antagonists inducing neural plate formation.