Fig. 3. HpTb is activated zygotically and is expressed transiently during the blastula and gastrula stages. (A) Developmental northern blot. Two micrograms of total RNA extracted from sea urchin embryos at various developmental stages were run in parallel. The lower panel shows the rRNA bands in the ethidium bromide-stained gel. U, unfertilized egg; 16, 16-cell stage; M, morula; UB, unhatched blastula; HB, hatched blastula; MB, mesenchyme blastula; G, gastrula; Pr, prism; Pl, pluteus. (B-D) Whole-mount in situ hybridization showing spatial expression of HpTb transcript during embryogenesis. (B,C) Hatched blastulae, (B) lateral view and (C) vegetal view. (D) Early mesenchyme blastula, lateral view. PMCs migrating into the blastocoel show a positive signal. Scale bar: 50 µm.