Hedgehog crosses the snail's midline

According to the dorsoventral axis-inversion theory1, protostomes (such as insects, snails and worms) are organized upside-down by comparison with deuterostomes (vertebrates)2,3,4,5, in which case their respective ventrally (belly-side) and dorsally (back-side) located nervous systems, as well as their midline regions, should all be derived from a common ancestor5. Here we provide experimental evidence for such homology by showing that an...