Mesoderm is one of the three germ layers, groups of cells that interact early during the embryonic life of animals and from which organs and tissues form.
Pander, Christian Heinrich, 1794-1865
A germ layer is a group of cells in an embryo that interact with each other as the embryo develops and contribute to the formation of all organs and tissues. All animals, except perhaps sponges, form two or three germ layers.
Ectoderm is one of three germ layers—groups of cells that coalesce early during the embryonic life of all animals except maybe sponges, and from which organs and tissues form. As an embryo develops, a single fertilized cell progresses through multiple rounds of cell division.