Fig. 3. In retinoic acid-deficient chick embryos, r4 forms a defined domain from the r3/r4 border to the level of the sixth somite. Dorsal view of whole mounts of control embryos (a,c,e) and BMS493-treated (5x10^-6 M) chick embryos (b,d,f). In treated embryos, follistatin expression is ectopically expressed in the neural tube (b, stage11), Wnt8c expression is not restricted to the normal position of r4 (compare d with c, stage8⁺) and Cepu-1 expression is throughout the posterior hindbrain (f, stage10). r3, r4, rhombomeres 3 and 4. s1, s6, first somite and sixth somite.