Mesoderm is one of three principal germinal layers of cells that are formed in early in embryonic development. Mesoderm comprises the middle germinal layer that lies between the ectodermal and endodermal layers. Mesodermal cells ultimately form tissues, organs, and systems including blood vessels, blood, connective tissues, muscles, cartilage, bone, lymph glands, pericardium, peritoneum, pleura, gonads, and kidneys.

In the embryonic disk ectoderm and endoderm sandwich mesoderm, the third primitive germinal layer. When the embryonic disk ultimately folds into a tube the basic "tube within a tube" plan of development becomes evident. A core endodermal tube establishes a primitive digestive pathway bounded by an oral orifice and an anal orifice. Around that innermost tube is an outer tube comprised of ectoderm. The ectoderm serves as a protective layer and the layer from which the nervous system and sense organs develop. Mesodermal cells fill the space between the inner (endodermal...