A prominent difference in axis formation in the chick on the one hand and in fishes and the amphibians on the other is the intermediate structure of the primitive streak in the chick . The primitive streak has the same orientation and position as the later-formed definitive AP axis and is thus frequently regarded as an AP-structure. On the other hand, it is regardet as the blastopore. i.e., a structure perpendicular to the AP axis. The following provides a unifying scheme:
The proliferating embryonic cells on the yolk form the area pellucida. The outer border of this region corresponds to the blastopore (light green). Thus, the pattern of bag-like ancestral organism became flattened to a circle, posterior is the outer margin, anterior the centre. The dorsal organizer (posterior marginal zone, Koller's sickle; dark green) forms on this ring , analogous to the Spemann-Organizer on the marginal zone in amphibians or the shield on the germ ring in the zebrafish:
However, the yolk of a chicken egg is much to large that the whole sphere can be covered by a sheet of cells as it occurs during fish epiboly. In a process that can be best described as partial epiboly (b), cells on both sides of the organizer move over the yolk (black arrows). This is in sharp contrast to the complete and radial-symmetric epiboly in the fish. The organizing region remains in place, giving rise to Hensen's node. Again this is in contrast to the fish where the shield moves with the germ ring. The deformed blastopore forms a hairpin-like loop, the primitive streak (c). Although the orientation of the streak is the same as the future AP axis, it is equivalent to the marginal zone in amphibians or to the germ ring in the fish, i.e., it is a pattern conventionally seen as a dorsoventral pattern. (d) With the regression of Hensen's node, the proper posterior part of the AP axis, the trunk (red), is generated. The brain is laid down in the ancestral, original central part of the area pellucida. The anterior midline is generated by the prechordal plate (PP, yellow) that moves as in the frog and fish away from the (here deformed) blastopore underneath the brain ectoderm (blue).
For a recent attempt to pin down molecules involved in chick patterning see [2,3].
Meinhardt, H. (2004). Different strategies for midline formation in bilaterians. Nat Rev Neurosci 5,502-510 [PDF]