Fig. 4. (A-D) Graft of a 9 ss quail notochord (No') lateral to the caudal neural tube of a 10 ss chick embryo. Serial sections performed 2 days after the graft (E4) show that the neural epithelium has increased in size on the side of the graft when compared with the contralateral side. Shh (A) and HNF3ß (B) are expressed ectopically in the region facing the graft. Pax6 (C) is not found in this region. (D) QCPN labelling of the graft. (E-L) Serial sections 4 (E-H) and 5 (I-L) days after the same experiment (E6 and E7) show that the molecular characteristics of a complete floor plate with its medial and lateral components are progressively acquired in the region close to the graft: wide expression of Shh (E, I) and Netrin1 (K); presence of HNF3ß transcripts in a restricted medial region (F,J) where Shh (E,I) and Netrin1 (K) are upregulated and CSox1 is downregulated (L); expression of Nkx2.2 (G) and Sim1 (H) lateral to the HNF3ß+ region. Arrowheads, MFP limits; arrows, LFP lateral limit.