Heat shock at the end of gastrulation produces strong and ubiquitous fgf8 expression in transgenic animals. (A-C, G-K) Following a 30 minute heat shock, strong and ubiquitous expression of fgf8 can be observed in the transgenic embryos (G-K) up to 1.5 hours after heat shock, masking the endogenous fgf8 expression domains that are observed in the wild-type embryos (A-C). (D, I) Ectopic fgf8 mRNA is gradually lost and the endogenous fgf8 expression domains emerge at 2 hours after heat shock. (D, I) At 2.5 hours after heat shock only scattered cells show ectopic fgf8 expression in transgenic embryos. Dorsal views of 2–5-somite stage embryos with anterior towards the top. h, hours after heat shock; mhb, midbrain-hindbrain border; tb, tail bud. Scale bar: 100 μm.