A single midventral prominence deriving from the ventral bases of pharyngeal arches 2, 3 and 4 - provides the covering of the posterior of the tongue //The furcula is at first separated from the tuberculum impar by a depression, but later by a ridge, the copula, formed by the forward growth and fusion of the ventral ends of the second, third, and part of the fourth branchial arches[WP]. [ http://en.wikipedia.org/wiki/Copula_linguae ]