primitive streak

primitive streak, a dense area on the central posterior region of the embryonic disk. It is formed by the morphogenetic movement of a rapidly proliferating mass of cells that spreads between the ectoderm and endoderm, giving rise to the mesoderm layer. This seamlike elongation indicates the cephalocaudal axis along which the embryo develops, and it corresponds to the blastopore of lower animal groups. Also called primitive line.