Fig. 3. Delta1 expression is restricted to the epidermal side of the ectoderm. Neighboring sections of stage 7 (one-somite stage) and stage 9 (five-somite stage) were hybridized with Notch1, Delta1, Slug and Sox2 probes. Notch1 is expressed in broadly in the ectoderm (A,B). Delta1 mRNA expression (C,D) is highly localized in the epidermal ectoderm, and shows a sharp boundary juxtaposed to the neural crest domain, marked by Slug expression (E,F). Sox2 is expressed in the neural plate, and downregulated in the neural crest domain (G,H).