Comparison of A. callidryas hatching enzyme (AcHE) expression and hatching gland cell (HGC) development.Images in left panels show in situ hybridization staining for AcHE expression; images in right panels show whole-mount scanning electron micrographs at the same stage. A, B, at 2 days, HE expression was strong on the snout (A), but HGCs were not yet visible on the surface of the embryo (B). C, D, in 3-day-old embryos, AcHE expression was strong on the snout and top of head (C), and early HGCs were abundant on the snout (D). E, F, in 4-day-old embryos, AcHE expression matched the distribution of HGCs and was absent from neuromasts (E); early HGCs were regressing and late HGCs were appearing on the snout (F). G, H, in a 5-day-old embryo, AcHE expression was diminished but still matched the distribution of HGCs (G), which were very abundant on the snout (H). Abbreviation: d, days.