Cell movements, germ layer formation, invagination and ingression
Gastrulation transforms a single-layered blastula into a trilaminar embryo with three primary germ layers: ectoderm (outer, nervous system + skin), mesoderm (middle, muscles + skeleton + heart), and endoderm (inner, gut + lungs + liver). In the sea urchin, primary mesenchyme cells ingress at the vegetal pole via epithelial-to-mesenchymal transition, followed by invagination of the archenteron (primary gut) driven by apical constriction and convergent extension. In vertebrates, epiblast cells migrate through the primitive streak, undergoing EMT to generate mesoderm and endoderm beneath. The spatial pattern of ingression and the duration of exposure to Nodal and BMP gradients encodes the eventual fate of each cell.