Cоmplete the fоllоwing pаrаgrаph about limb bud anatomy by selecting the correct term for each blank: The developing limb bud can be divided into three functionally distinct regions. The [1] is a structurally distinct ridge of epithelium at the most distal tip of the limb bud that bisects the dorsal and ventral halves; it secretes FGF-8 and is required for limb outgrowth. Directly beneath it is the [2] , a mass of proliferating progenitor cells. Restricted to the posterior-most portion of the bud, the [3] is a source of Sonic Hedgehog signaling, and when transplanted to the anterior side of a forelimb bud, causes mirror-image digit duplication.