Primitive streak

The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.

Formation
The formation of the primitive streak is one of the first signs of gastrulation. It is characterized as a furrow in the midline of the embryonic disk at the future caudal end of the embryo. This furrow is formed by the ingression of epiblast cells which will go on to form the definitive endoderm and mesoderm by replacing hypoblast cells. The epiblast cells then develop into the definitive ectoderm.

The primitive streak establishes a visible longitudinal axis of bilateral symmetry around which all embryonic structures will organize and align.

Components
The primitive streak consists of the following:
 * Primitive groove - shallow valley extending along the length of the streak.
 * Primitive knot - a raised circle of cells at the rostral tip of the primitive streak. (Hensen's node)
 * Primitive pit - a depression in the center of the primitive node.

Ethical implications
The primitive streak is an important concept in bioethics, where some experts have argued that experimentation with human embryos is permissible only before the primitive streak develops, generally around the fourteenth day of existence. The development of the primitive streak is taken, by such bioethicists, to signify the creation of a unique, potential human being.