Mesoderm: One of the three primary germ cell layers, the others being the ectoderm and endoderm. The mesoderm is the middle layer. In the embryo, it differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the middle layer of the skin. Some cells in mesodermal tissues retain the capacity to differentiate in diverse directions. For example, some cells in the bone marrow (mesoderm) can become liver (endoderm).