Fig. 1. (A) Dorsal and lateral views of the sinus rhomboidalis of an E1.5 quail embryo after the excision of the axial-paraxial hinge (APH) (V, ventral; D, dorsal). APH is the region encompassing caudal Hensen’s node (HN) and the rostral primitive streak (PS). (B) One day after the operation (at E2.5) the neural tube caudal to somite 20 (S20) is deprived of midline structures (floor plate and notochord) and is smaller in diameter than normal as seen in cross section (C). (D) In a second experiment, the quail neural tube (NT) deprived of midline cells is enzymatically isolated and grafted in a stage-matched chick embryo in the place of a segment of its own neural tube-notochord complex: (a) above a notochord; (b) above a floor-plate fragment; and (c) above a layer of SHH-producing cells.