Animation of cell movements during the formation of the neural tube of an embryo, the earliest stage in the formation of the central nervous system (CNS). The CNS is derived from the ectoderm, the outermost tissue layer of the embryo. In the third week of development, the neuroectoderm appears and forms the neural plate (red, centre) along the dorsal side of the embryo. A groove forms in the neural plate and cells become columnar, bringing the folds closer together until they meet at the neural crest cells (green), so forming the neural tube. The neural tube sinks into the tissue of the embryo, while the ectodermis forms the outer layer of the skin. The neural crest cells themselves differentiate into various other cell types (yellow), including the nerve cell ganglia of the head and body, sensory neurons, connective tissue and the pigments cells (melanocytes) of the skin and eye.