Whole Mount of Chick Blastoderm

AO = area opaca, AP = area pellucida

P = region of thick blastoderm that marks the future posterior region of the embryo; area where gastrulation begins

Arrows = movement of rapidly dividing cells of epiblast layer

Chick Embryo - Primitive Streak Stage

AO = area opaca, AP = area pellucida, P= posterior

The primitive streak is the region where epiblast cells converge (solid arrows) and move through a groove into the blastocoel (dashed arrows). The primitive steak consists of a lightly staining groove bordered on each side by darker staining ridges. Hensen's node marks the anterior of the streak and P marks the posterior.

Cross sections through Primitive streak

Key to Upper Cross section: AO = area opaca, . AP = area pellucida, ^^ = remnants of yolk granules, arrows = direction of ingression of embryonic cells

Lower Cross Section shows the three germ layers formed by the process of gastrulation.