A group of cells that acts as an organizer of development in the early embryo of birds and mammals. It is located immediately anterior to the primitive streak in a developing gastrula and is analogous to the dorsal lip of the blastopore in the blastula of an amphibian embryo. Named after its discoverer, German zoologist Victor Hensen (1835–1924), it controls gastrulation by secreting diffusible protein signals, initiating a signalling cascade in surrounding cells that regulates the differentiation of such structures as the neural tube and notochord.