Tail extension and axial termination signaling schematic. During tail extension (depicted on left), somitogenesis is actively proceeding, with new somites forming from PSM at the determination front. Activities from Cdx proteins, Wnts, and Fgfs establish a posterior Wnt3a/Fgf8 gradient, which opposes an anterior RA gradient. These opposing gradients allow the creation of the determination front, and activation of the Notch pathway. Cycling expression patterns of Wnt, Fgf, and Notch pathway genes follow a clock wave-front model, promoting somite induction, segmentation and differentiation in successive waves, to add somites sequentially, rostral to caudal, down the vertebrate axis. During tail termination (right), the RA gradient is unopposed, due to progressively decreasing concentrations of Wnts and Fgfs. Contributions from RA (increased in chick via RALDH2), Hox genes, decreased concentrations of Cyp26a1 (mouse), Wnts and Fgfs, inhibition of the Notch pathway, apoptosis, and loss of cell division and cell recruitment in the CNH act to terminate the tail. Abbreviations: CNH, chordoneural hinge; RA, retinoic acid.