Fig. 5. The neural regulatory network. Animal cells are directed towards a neural fate beginning at the late blastula stage and continuing through gastrulation. Neural induction requires both suppression of the BMP-Smad1-Msx pathway by the BMP antagonists chordin, noggin and follistatin, and the activation of FGF signaling. Cells begin to express proneural genes (Sox2, SoxD) at the gastrula stage. By the end of gastrulation, the deeper layer of the pro-neural ectoderm activates specific neuronal precursor markers, including Xngnr1, which in turn regulates primary neuronal differentiation via XneuroD and N-tubulin. Primary neurogenesis requires the function of the maternal chromatin remodeling protein Brg1.