A 3-D fate map of the chicken (Gallus gallus) embryo with the prospective point of ingression and yolk. The area where the primitive streak will form during gastrulation is shown. The anterior- posterior axis is shown by labeling the anterior and posterio ends (A) and (P). Different colors indicate prospective fates of different regions of the epiblast after gastrulation. The turquoise shaded region represents the prospective ectoderm, the lavender shaded region represents the prospective mesoderm, the dark blue shaded region represents the prospective endoderm, and the white shaded region represents the prospective extraembryonic area.

This image shows a chicken (Gallus gallus) embryo undergoing gastrulation in stage four (18-19 hrs after laying) according to the Hamburger-Hamilton staging series. At this point in time the chicken embryo is a blastoderm (shown in blue). The first magnification of the embryo shows that the blastoderm cell layers have thickened to form the primitive streak and Hensen's node. The primitive streak extends from the posterior (P) region to the anterior (A) region. The second rectangular magnification shows the blastoderm cross-sectioned through the primitive streak. The cross-section shows the blastoderm's two cell layers, the epiblast and the hypoblast. The fluid filled cavity between the two cell layers is the blastocoel. The space left between the hypoblast cell layer and the yolk is called the subgerminal cavity.