A pair of HMS biologists have hit upon a surprising answer to the last question. Mike Levin and Mark Mercola cut away the right lateral flank of a very early chick embryo and grew the remaining portion in isolation. The left side, deprived of the right flank, was unable to initiate normal asymmetrical gene expression. The findings will appear in Development in mid-November. Gap junctions had been known to play a role in Xenopus asymmetry, but not in chicks.
Gap junctions appear to allow greater passage of molecules dorsally than ventrally.
This differential could produce a current of molecules from one side of the embryo to the other. The accumulation of molecules on one side may initiate a signaling cascade that eventually causes the two genes to turn on. What kind of molecule is being communicated through the tiny portals is not known. Nor do the researchers know what tells the gap junctions to shepherd molecules in one direction over another. If that were true, identical twins conjoined at the belly, who would have originated from the same right-left prepatterned embryo, should have their right sides facing.