The middle primary germ layer that originates during gastrulation [the other two being entoderm (inner layer) and the ectoderm (outer layer)], and which gives rise to a large number of structures: muscles, elements of the skeleton (bone and cartilage), the urogenital system, kidneys, gonads and the adrenal cortex. It also differentiates into bone marrow, connective tissues (mesenchyme), epithelia (lining of tissues), and the middle layer of the skin. Some mesodermal cells differentiate in different directions, one example being bone marrow that become liver (formed in the endoderm). As for the mesoderm itself, it seems that it is generated by inductive signals from the entoderm.
See Blastopore, Bone Morphogenetic Proteins, Chordin, Dermomyotome, Differentiation (embryology), Ectoderm, Embryo, Endothelium, Entoderm (or endoderm), Epimere, Gastrulation, Germinal (or germ) layers, Hypomere, Induction (embryology), Invagination, Limb bud, Meninges, Mesenchyme, Morphogenetic field, Noggin, Notochord, Organogenesis, Pia mater, Progress zone