(A,A') Wild type embryo in frontal and lateral views. (B,B'-D,D') Three classes of Twsg1 mutants. Class A - normal appearance of the BA1; Class B - partial midline fusion of BA1 with some preservation of the forebrain; Class C - severe midline fusion with ventral displacement of BA1 and severe reduction of anterior head structures. (E) Micro-dissected wild type BA1 at E10.5. (F) Mutant BA1 class C with severe midline fusion of the mandibular components and only partial separation of the maxillary components at E10.5. Dashed lines outline the mandibular prominences of BA1; mx, maxillary component of BA1; md, mandibular component of BA1. Scale bar: 200 μm.