The mesoderm is one of the three germ layers in the early developing embryo, the other two layers being the ectoderm and the endoderm. The mesoderm gives rise to tissues including connective tissue, muscles and the circulatory system. The mesoderm is also believed to be responsible for the formation of the central nervous system, for example the notochord is responsible for releasing certain factors which induce the ectoderm to become neural tissue.

In a developing vertebrate embryo, the mesoderm differentiates into these areas:

• The chordamesoderm lies along the central axis, under the neural tube and gives rise to the notochord.

• The paraxial mesoderm, at the sides of the neural tube, gives rise to the somites and head mesoderm. The later will develop into facial muscle and cartilage.

• The intermediate mesoderm, between the paraxial mesoderm and the lateral plate, develops into the urogenital system (kidneys and gonads).

• The lateral plate mesoderm is found at the periphery of the embryo. It will, in turn, split into two layers, the somatic layer/mesoderm and the splanchnic layer/mesoderm. Enclosed between the two is a space called coelom. The somatic layer forms the future body wall, and the splanchnic layer forms the circulatory system and future gut wall.

Because the mesoderm can form different ways in relation to the developing embryo's ectoderm, this is used as a method of categorizing animals. They are catagorized as: Acoelomates, Pseudocoelomates, and Coelomates respectively.