part of organism substance material anatomical entity multicellular organism organism subdivision acellular anatomical structure extraembryonic structure multi-cell-part structure gross anatomical part Anatomical structure that is part of a multicellular organism and is at the gross anatomical level, e.g. above the level of a cell. Included are portions of organism substances such as blood, multi-cell-part structures such as axon tracts, acellular anatomical structures such as hair, and organism subdivisions such as head. Excluded is the whole organism and more granular parts of the organism, such as atoms, molecules, macromolecular complexes and cells. multicellular anatomical structure