Fig. 1. The `default model' in Xenopus. (A) A rough fate map of a blastula-stage embryo. Prospective territories are organizer in red, ventral mesoderm in pink, neural tissue in blue, epidermis in yellow and yolky endoderm in green. The red lines represent BMP antagonist activity emanating from the organizer. (B) A `genetic' diagram of the inductive interactions proposed by the model: ectoderm cells have an autonomous tendency to differentiate into neural tissue, but are prevented from doing this and are directed instead to epidermis by BMP4, which is expressed ubiquitously. Near the organizer, BMP antagonists block BMP4 signalling, allowing neighbouring ectoderm cells to develop according to their `default' neural fate. D, dorsal; V, ventral. Modified, with permission, from Stern (Stern, 2004).