Practice question

Organizer at dorsal blastopore lip is signaling center and mechanical engine of amphibian gastrulation. Initial phase involves apical constriction forming bottle cells at blastopore, then involution of head and axial mesoderm around lip, convergent extension driven by Wnt-PCP, and epiboly of ectoderm toward vegetal pole. Organizer secretes BMP antagonists and expresses Brachyury and goosecoid coordinating these movements. Removal impairs blastopore formation and mesoderm internalization arrests. Thus primary function during gastrulation is initiating and orchestrating gastrulation movements rather than producing yolk or purely endodermal fates. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.