Regression of Primitive Streak
As the inward migration of cells by the primitive streak ends progressively along its anterior to posterior regions the streak slowly regresses in the same direction. As the streak shortens the Hensen's Node that forms its anterior end, also moves in a posterior direction at the same time, the presumptive notochordal cells still remaining in the epiblast migrate inward through the posteriorly moving node adding to the length of the notochord. So the regressing streak leaves behind in its path the dorsal axis of the embryo represented by the posteriorly lengthening notochord. By the time the streak disappears and the Hensen's Node arrives at the last position at the posterior border of area pellucida all the presumptive endodermal and mesodermal cells have left the epiblast that now contains only the ectodermal cells. Regression of streak starts after about 22 hours of incubation and is completed in the next about 20 hours. It should be noted that not like amphibian gastrulation differentiation of axial structures and some organs like neural tube, somites, foregut, heart, takes place cephalocaudally along with the progress of later stages of gastrulation in chick embryos.