Embryonic Development: Blastula and Gastrulation
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An animal embryo divides into a hollow blastula, then folds inward during gastrulation, forming layered germ layers that give rise to all adult structures.
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Gastrulation in a sea urchin embryo creates a pouch-like archenteron, the precursor to its future digestive cavity.
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Gastrulation is shared across the animal kingdom, not a trait found only in complex, later-diverging lineages.