Definitions for "Primitive streak"
Around the 14th day of development, the primitive streak begins to develop in the embryo which signals the start of the development of the nervous system.
This develops in an embryo at about day 14 when the cells separate into 2 groups: those that will eventually become a foetus; and those which will form the placenta and umbilical cord.
The initial band of cells from which the embryo begins to develop. The primitive streak establishes and reveals the embryo's head-tail and left-right orientations.