Fig. 7. (A-L) Back grafts of quail neural tubes deprived of midline cells by APH excision into chick embryos from which a fragment of their own neural tube and notochord had been previously excised (see Fig. 1). Fixation was 5 days after the operation (E7). (A-D) Grafted over a notochord (No') the quail neural tube, labelled with the quail specific mAb QCPN (A), develops a typical floor plate with its medial and lateral components distinguished by the characteristic distribution of the Shh (B), HNF3ß (C) and (D) NKx2.2 transcripts. (E-H) Grafted over a floor plate (FP'), the QCPN+ quail neural tube (E) weakly expresses Shh (F) and very little HNF3ß (G) but strongly expresses Nkx2.2 (H) in the region close to FP'. This pattern recalls that of the lateral floor plate (LFP) as defined in Fig. 2. (I-L) In contact with SHH-producing cells (arrow in J), the quail neural tube deprived of midline cells develops a large LFP-like structure that expresses CSox1 (I), Shh (J), little HNF3ß (K) and Nkx2.2 (L). Arrowheads, MFP limits; arrows, LFP lateral limit.