1. The third germ layer, lying between the ectoderm and the endoderm; which gives rise to the connective tissues, muscles, urogenital system, vascular system, and the epithelial lining of the coelom.

Any of three cellular layers, the ectoderm, endoderm, or mesoderm, into which most animal embryos differentiate and from which the organs and tissues of the body develop through further differentiation.

2. The middle of the three cell layers of the developing embryo.

It lies between the ectoderm and the endoderm where bone, connective tissue, muscle, blood, vascular and lymphatic tissue, and the pleurae (membrane that encloses each lung and lines the chest cavity) of the pericardium (membranous sac enclosing the heart) and peritoneum (membrane that lines the walls of the abdomen and the pelvis) are all derived from the mesoderm.