Gastrulation partitions the early embryo into three primary germ layers — endoderm, mesoderm, and ectoderm — and the developmental fate of each layer is deterministic, so that adult organ systems can be assigned to a layer of origin. Endoderm gives rise to the gastrointestinal tube and its outgrowths (lungs, liver, pancreas), mesoderm to muscle including cardiac muscle, bone, deeper skin layers, and the genitourinary and gonadal structures, and ectoderm to the outer skin with its appendages and to the entire nervous system. This germ-layer derivation scheme belongs to embryology and developmental biology, and it supplies the organizing principle that links early embryonic patterning to adult anatomy and to the grouping of organs by shared developmental lineage rather than by anatomical proximity.
Gastrulation partitions the early embryo into three primary germ layers — endoderm, mesoderm, and ectoderm — and the developmental fate of each layer is deterministic, so that adult organ systems can…