Practice question

Gastrulation converts radially symmetrical blastula into triploblastic gastrula through morphogenetic movements epiboly, emboly, involution, invagination, and convergent extension that reorganize cells without net growth. Animal cap ectoderm spreads over embryo, marginal zone mesoderm rolls inside over blastopore lip, and vegetal endoderm internalizes to line archenteron cavity. Molecularly BMP, Nodal, Wnt, and FGF pathways pattern germ layer identities before movements, with Nodal highest vegetally. End result is establishment of three definitive germ layers ectoderm, mesoderm, endoderm positioned appropriately for organogenesis, with dorsal mesoderm forming organizer inducing neural tissue via inhibition of BMP signaling.