How best to describe the pharyngeal arch arteries when the fifth arch does not exist?

In the accompanying article appearing in this issue of the Journal, Prabhu and his colleagues, from Bengalaru in India, describe their experience with patients having a right aortic arch. They discuss the fact that the anomalous arrangements they encountered can all be interpreted on the basis of the hypothetical double arch proposed by Edwards. They point to the fact that interpretation of the developmental changes underscoring the production of the double arch is currently confused by reference to the so-called Rathke diagram, in which six sets of arteries are shown extending through the mesenchyme of the pharyngeal arches. As the authors point out, Graham and his associates have now shown that the alleged fifth set of pharyngeal arches do not exist. Based on our own observations, we endorse this statement. It means that new explanations must now be provided for the lesions previously described on the basis of persistence of the alleged artery of the fifth pharyngeal arch. We have previously claimed to have observed such an artery in a human fetus. We now believe, on the basis of our latest findings, that our earlier observation is better explained on the basis of presence of a collateral channel. We suggest that the so-called "fifth arch arteries" are themselves then best explained either on the basis of existence of such collateral channels, or remodelling of the aortic sac, which is the manifold, during development, that gives rise to the pharyngeal arch arteries.