Figure 1. The known left–right cascade of gene activity in chick embryo. (a)–(d) show four successive stages (see text) as seen in plan view from above, with anterior at top. (a) Primitive streak stage; (b) a short ‘head process’ stage, with mesoderm emerging laterally and ahead of the regressing Hensen’s node; (c) a late headfold or neurula stage (about four somites segmenting in mesoderm); (d) stage with the newly looped heart tube, and onset of embryo torsion (twisting); (e) transverse section through posterior Hensen’s node at stage of (b). Note consistent anatomically asymmetrical appearance, as well as gene expressions, in node whose deep structure is in continuity with (upper) epiblast layer. Separate bilateral lower layer is mesoderm; (f) transverse section, ahead of regressed node but posterior to segmented somites, at stage of (c). Colour code for gene expressions: green, Act-RIIa; blue, Shh; yellow, nodal; red, cSnR. Note that in (f), the lefty genes known from other vertebrates (see text) would occupy a left lateral position similar to nodal but also a narrow domain at the left lateral side of the Shh expression domain at the neural midline (floor plate) above the notochord. Pitx2 is not shown. Hn, Hensen’s node; nc, notochord; s, somite; h, formed heart tube.

Figure 2. Autonomy and interference in left–right patterning between twinned axial plans. (a) Shows a plan view from above of a case in which anteriorly opposed streaks have formed by earlier events at near-opposite sites at a blastular (10³–10⁴ cell) stage. The gene expression colours around each node, and corresponding asymmetrical appearance, represent how in such a case each streak autonomously sets up the same, correct left–right asymmetry cascade in relation to its anteroposterior (and dorsoventral) aspects, ruling out prior use of any blastoderm-wide left–right information (symbolized by double arrowheads from right). (b) shows a similar view of a case where events initiating streaks were not fully opposed, so that convergence movements of streak development have led them to a more parallel, though separated, conformation. Frequent disturbance or reversal of laterality in just the right-hand axis of such cases suggests that a preferential gene expression on the right normally first ‘converts’ a molecular chirality aligned in the vicinity of each streak into differential right- and left-specific gene expressions. Right-hand twin members are thus at risk of abnormal symmetrization. Colour code for gene expressions: green, Act-RIIa; blue, Shh; yellow, nodal; red, cSnR.