mesoderm mes·o·derm (měz'ə-dûrm', měs'-)
The middle embryonic germ layer, lying between the ectoderm and the endoderm, from which connective tissue, muscle, bone, and the urogenital and circulatory systems develop.
The middle of the three primary germ layers of the embryos of vertebrates and other complex animals. In vertebrates, the mesoderm gives rise to the muscles, bones, cartilage, connective tissue, blood, blood and lymph vessels, dermis, kidneys, and gonads. The mesoderm develops during gastrulation from either the ectoderm or the endoderm. The embryos of simpler animals lack a mesoderm. Compare ectoderm, endoderm.