Mesoderm is the layer of cells in a eukaryote that lies between the ectoderm and the endoderm. It is also the layer of cells that lies between the ectoderm and the endoderm in the developing embryo once it has reached the gastrula stage. This cell layer ultimately gives rise to the muscles, cartilage, bones, blood, and lymph, forming tissues and vessels, spleen, kidney, gonads, genital tract, linings of the body cavities, and adrenal cortex.