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A node, or primitive knot, is an enlarged group of cells located in the anterior portion of the primitive streak in a developing gastrula. The node is the site where gastrulation, the formation of the three germ layers, first begins. The node determines and patterns the anterior-posterior axis of the embryo by directing the development of the chordamesoderm. The chordamesoderm is a specific type of mesoderm that will differentiate into the notochord, somites, and neural tube. Those structures will later form the vertebral column. In the chick embryo, the node is referred to as Hensen's node because of its discoverer, Viktor Hensen, who first described the node in 1875. The discovery of Hensen's node has helped to answer questions of axis formation and has allowed experimental embryologists to further investigate vertebrate embryonic development.