Fgf signalling is required for lung and elements of thyroid differentiation in Xenopus. When embryos were treated with SU5402 at either stage 12, 20 or 26, expression of nkx2.1 was lost in the lung (green arrows) but not the thyroid (red arrows) or anterior neural tissue. Although treatment with SU5402 at stage 20 caused clear deformations in the lens ectoderm expression (yellow arrows) of foxe4, there was no clear effect on the thyroid expression of foxe4. Expression of pax2 in the thyroid was lost when SU5402 was applied at stage 12 but expression of pax2 was observed in most embryos at stage 20 and all embryos when treated at stage 26. Note the severe tail defects in embryos treated with SU5402 at stage 12 demonstrating the effectiveness of the block to Fgf signalling. (side - side view, ventral - ventral view).