Correlation between pluripotency factors and differentiation signals during Xenopus germ layer formation. Germ layers are formed in strict spatiotemporal patterns. Either POU-V factors or germ layer inducing signals have specific localizations and exhibit different levels of activities during the process of germ layer formation of Xenopus embryos. POU-V factors antagonize the differentiation signals, so as to ensure that germ layer formation can occur in the correct spatiotemporal patterns (See text for details). An: animal pole; Vg: vegetal pole; D: dorsal side; V: ventral side. Reproduced from  with modifications.