Fig. 2. Ectoderm maintains the epithelial structure of somites. (A) Schematic of the procedure: separation of the anterior PSM from overlying ectoderm by the introduction of an impermeable membrane (blue), followed by 3-24 hours of culture. The position of the membrane at the end of the experiment matches perfectly the region where no staining is observed (see C-H). (B-D) Expression of Fz7 after 3 (B), 12 (C) or 24 (D) hours of culture. The position of the membrane is indicated in B. (E,F) Expression of Pax3 after 12 (E) or 24 (F) hours of culture. (G,H) Expression of paraxis after 12 (G) or 24 (H) hours of culture. (I,J) Transverse sections of C (I) and D (J) at the positions indicated.