Fig. 1. (A) Dorsal and lateral views of the excision of the axial-paraxial hinge (APH) in chick or quail embryos, at E1.5 (V, ventral; D, dorsal). The APH is the region encompassing the caudal Hensen’s node (HN) and the rostral primitive streak (PS). (B) One day after the operation (i.e. 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. (C) In experiment A, operated embryos are kept in ovo, (1) without a graft, as control; (2) grafted with a fragment of notochord or floor plate; (3) grafted with SHH-producing cells. (D) In experiment B, the neural tube (NT) deprived of midline cells is enzymatically isolated and grafted into a stage-matched chick embryo in place of a segment of its own neural tube-notochord complex, (1) alone; (2) above a notochord or a floor plate fragment; (3) above a layer of SHH-producing cells. (E) In experiment C, the neural plate (NP) and paraxial mesoderm (PM) are separated from the notochord (No) and floor plate (FP) by a slit penetrating the 3 germ layers, from the level of the last formed somites (S) to Hensen’s node (HN) level, in chick embryos at the 10- to 12-somite stage. (F) In experiment D, the neural tube (NT) is separated from the paraxial mesoderm or somites (So) by a slit extending from the level of the 2 or 3 last formed somites (S) to the non-segmented region in chick embryos at the 12- to 15-somite stage. Ao, aorta.