Blockage of Wnt signaling lead to lack of MDO tissue. Lateral views and dorsal views (marked by ') of embryonic heads at 28 hpf. Wnt3/wnt3a morphant embryos lack the organizer tissue and the lhx5 positive prethalamus (PTh, blue) abuts the irx1b thalamus (Th, red) (A-B', n = 54/78). A similar phenotype is observed in embryos treated with 30 μM IWR1 from 10 to 28 h to inhibit canonical Wnt signaling (n = 15/17). Hence, the otx2 positive MDO (yellow) is lacking if Wnt3/Wnt3a function is knocked-down (n = 44/80, D-E'), likewise after inhibition of canonical Wnt signaling (F, F', n = 14/24).