Fig. 9. Summary of results. The cartoon in the middle represents a lateral view of a gastrula stage embryo. Prospective epidermis is light green; prospective CNS is purple, with dark colour indicating prospective anterior fate and progressively lighter colour indicating progressively more posterior fate (as indicated in the legend to Fig. 2; the organisation of prospective dorsal versus ventral prospective CNS structures is not indicated). Prospective mesendoderm is yellow. Bmp activity is graded during gastrulation: high ventrally and low dorsally (i,ii). This gradient is established, at least in part, by an opposing gradient of dorsally derived Bmp antagonists such as Chordin (e.g. Hammerschmidt and Mullins, 2002). Fgf may also have graded activity during blastula and gastrula stages: high vegetally and low at the animal pole (iii,iv) (Figs 2, 3) (Furthauer et al., 2002; Roehl and Nusslein-Volhard, 2001; Tsang et al., 2002). In the animal gastrula ectoderm, high levels of Bmp activity promotes epidermal fate, while Bmp antagonists promote neural fate (i). Conversely, in vegetal ectoderm, Bmp activity promotes caudal neural fate, whereas Bmp antagonists promote the adoption of more rostral neural fate (ii). The different consequences of activation or suppression of Bmp activity in animal and vegetal ectoderm are influenced by Fgf-dependent promotion of prospective neural fate in vegetal ectoderm (iii). In addition to promoting neural specification, Fgf signalling posteriorises neural tissue in the vegetal ectoderm, most obviously on the dorsal side (iv).