This is a top view of the chick blastodisc showing the general movement of the epiblast cells towards the Primitive Streak. The mesoderm cells involute over the edges of the Primitive Streak.

This is a diagram of a cross-section through a gastrulating avian embryo. Cells from the upper Epiblast Layer invaginate down through the Primitive Streak into the interior of the embryo. The invaginating cells form the Mesoderm Layer, displace the Hypoblast layer and form the Endoderm Layer.