Practice question

When micromeres surgically removed from 16-cell stage sea urchin embryo, remaining animal hemisphere and macromeres lack sufficient vegetal Wnt/β-catenin and Delta-Notch signals needed for endomesoderm induction. Without these inductive cues, archenteron never forms, no primary or secondary mesenchyme ingresses, and gut differentiation fails. Instead embryo becomes permanently ciliated, hollow epithelial ball composed predominantly of expanded apical ectoderm and ciliary band, termed dauerblastula or animalized embryo. It swims but never gastrulates, analogous to β-catenin inhibition phenotype. This demonstrates conditional specification of animal cells requiring micromere signals; replacement of micromeres rescues gastrulation and normal pluteus formation.