Fig. 2. Arrested pectoral fin bud induction in hst mutant embryos. (A,B) Methylene Blue-stained cross sections through 48 hpf embryos. (A) Wild-type embryos have pectoral fin buds (arrows). (B) hst mutant embryos retain small populations of presumptive fin mesenchymal cells (arrowheads) that fail to proliferate. (C) Methylene Blue-stained transverse sections of 3 dpf mutant embryos. Presumptive fin mesenchyme is usually present as a flat patch (arrowhead), but occasionally cells turn perpendicular to the basement membrane (double arrowhead), but do not proliferate further.