Fig. 2. (A) Whole-mount in situ hybridisation of a 15 ss chick embryo with the CSox1 probe. (B,C) Cross-sections of this embryo at the neural tube level (B,C) as indicated in A. CSox1 transcripts are present only in the neuroepithelium and not in the floor plate (FP) or the chordo-neural hinge (CNH), which is present at the posterior neuropore level at this stage. (D) Cross-section of a 15 ss quail-chick chimera (quail Hensen’s node graft as shown in E) at the same level as C, stained with the quail specific mAb QCPN. The CNH is made up of quail cells. (E) Schematic representation of quail-chick grafts of Hensen’s node (in red) and posterior neural plate (in blue) at the 5-6 ss. (F-H) Serial cross-sections of a chimera grafted with a quail Hensen’s node (red in E), 2 days after the operation (E3.5). The expression pattern of HNF3ß (F) and Shh (G) genes is wider than the node-derived region revealed by the QCPN mAb (H). (I-M) Serial sections of another Hensen’s node chimera fixed 5.5 days after the graft (E7). CSox1 (I) is not expressed in the quail QCPN+ node-derived region (shown in L), which is where expression of HNF3ß is now restricted (J). This region also constitutes the medial floor plate (MFP). Shh (K) is expressed both in the node-derived region and in a neural plate-derived area where Nkx2.2 transcripts (M) are also present. The latter constitutes the lateral floor plate (LFP). (N-P) Serial sections of a quail-chick chimera grafted with a posterior neural plate (blue in E), 5.5 days after the operation (E7). As in Hensen’s node chimeras at the same stage, HNF3ß transcripts (N) are localised in the node-derived (host) region as seen in P, while Shh transcripts are distributed over a larger area covering both node-derived (MFP) and QCPN+ neural plate-derived tissues, including the LFP (O). Arrowheads, MFP limits; arrows, LFP lateral limit.