Wnt3 and Wnt3a are required for MDO formation. Knock-down of Wnt3 and Wnt3a leads to a down-regulation of the expression of indicated markers in the central part of the MDO (A-L). Time-lapse analysis of Shh::RFP shows an independent induction of the dorsal remaining organizer spot (M, N). Treatment of wnt3/wnt3a morphant embryos with the Wnt signaling agonist BIO from 10 to 28 h leads to a rescue of shh expression at the MDO (O, P). Brackets mark the extent of the MDO, whereas the arrows indicate the gap between the basal plate and the remaining spot of dorsal MDO identity. Eth, epithlamus, HyTh, hypothalamus, MHB, midbrain-hindbrain boundary, Tel, telencephalon.