During the formation of the Embryo at the Métazoaire S, the cells are always organized in layers of cells, which are distinguished from/to each other when this one carries out its Gastrulation: the external layer is called then Ectoderme and the interior layer, Endoderme. At the métazoaires Diploblastique S or diblastic S, it has there only these two layers, separated by a hydrated extracellular matrix called Mésoglée. At the métazoaires Triploblastique S or triblastic S, the intermediate layer forms true a fabric called Mésoderme. Each one of these layers has as a role to constitute the bodies of the future individual:

• the ectoderm produces the skin and the Nervous system

• the endoderm produces the digestive Tract and its additional glands (pancreas)
• the mésoderme produces the Muscle S, the Squelette, the blood-vessels

Let us announce that part of the muscles and skeleton of the head come from the ectoderm via the cells from the Crête neurale: one speaks then about mésectoderme.