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Fig. 4 Time course of the expression of pax2a in embryos in which en-pou5f3 was induced later during somitogenesis. Embryos obtained by crosses of en-pou5f3+/− and wild-type fish were subjected to heat shock once at the 17-ss (17.5 hpf) (A) or twice sequentially at the 3-ss (11 hpf) and 17-ss (B), and examined for pax2a expression following heat shock by WISH. The expression of egr2b in r3 and r5 of the hindbrain was also examined. Lateral and dorsal views are shown with the anterior to the top. Solid and open triangles mark normal and reduced expression at the MHB, respectively. Head hypoplasia due to double en-pou5f3 induction is marked with asterisks. For all marker genes and genotypes, at least 7 embryos were examined, all of which showed the indicated patterns. os, optic stalk; r3/r5, rhombomeres 3 and 5. Scale bars, 100 μm.