The mesoderm is a plate of cells lying between the endodermic and ectodermic plates.
endoderm en·do·derm (ěn'də-dûrm') or en·to·derm (ěn'tə-)
The innermost of the three primary germ layers of an embryo, developing into the gastrointestinal tract, the lungs, and associated structures. Also called hypoblast.
The innermost of the primary germ layers of an animal embryo. In vertebrates, the endoderm gives rise to the respiratory tract, gastrointestinal tract (except mouth and anus), glands associated with the gastrointestinal tract, bladder, and urethra. Compare ectoderm, mesoderm.