Middle layer of cells in the embryo, lying between the ectoderm and the endoderm. It includes the following tissues:

1. Lateral mesoderm: peripheral portion of intraembryonic mesoderm.
2. Intermediate mesoderm: origin of the nephrogenic cord.
3. Parietal (somatic) mesoderm: cell source for the formation of the lateral and ventral body wall.
4. Visceral (branchial, pharyngeal, splanchnic) mesoderm: inner layer of lateral mesoderm that, with the endoderm, provides the cells from which the gut and lungs and their coverings arise.