Figure 1.

Whole mount RNA in situ hybridization of Hh (A–F), Ptc (G–I) and Smo (J–L). In (A) and (B, ventral view), Hh expression is first detected at mid to late mesenchyme blastula stages as a single ring of cells in the endoderm. By early gastrula (C) the ring expands to a ring two – three cells wide, and during gastrulation (D,E) Hh expression is detected throughout the gut but not in the NSM at the tip of the archenteron. In pluteus larvae Hh expression spans the entire gut to the boundary of the coelomic pouches seen budding off the side of the foregut in (F). Ptc is first observed localized to the NSM at the beginning of gastrulation (G). At late gastrulation Ptc is still seen in the NSM and a new site in the PMCs surrounding the base of the archenteron (H). At the pluteus stage Ptc is expressed in the two coelomic pouches (I). Smo is first localized to the NSM at the beginning of gastrulation (J). At the end of gastrulation Smo is observed in the NSM at the tip of the archenteron (K), throughout the PMCs surrounding the NSM, and perhaps at a low level in the hindgut, though most of the staining observed in is in out-of-focus PMCs. At the pluteus larva stage Smo is observed in coelomic pouches (L).