Endodermal cells which have replaced the hypoblast cover the umbilical vesicle on both sides of the notochordal plate. Once the notochord has detached from the endoderm, both sides of the endoderm join together to form a homogeneous layer above the umbilical vesicle.
At S6, the primitive streak expands over the caudal half of the embryo. At S8, it reaches its maximal length. During the following stages, it regresses to more caudal regions of the embryo. At S11, it exists only as a small remnant in the caudal eminence, and gives rise to only a few cells.