Fig. 2. Timing and tissue interactions underlying stem zone specification. 3c epiblast explants (A) do not express Sax1 (B) or Hoxb8 (C), but do express bra (D) and Sox2 (E). 4c explants (F) express Sax1 (G), Hoxb8 (H), bra (I) and Sox2 (J). 4a and 4b explants each include a region medial (a) or lateral (b) to 4c (K). 4a explants express Sax1 (L), Hoxb8 (M), bra (N) and Sox2 (O). 4b explants do not express Sax1 (P), Hoxb8 (Q) or bra (R), but are Sox2 positive (S). All explants assessed after 24 hours. (T) Explants 3c, 4b, 7CPM (caudal paraxial mesoderm) and combination experiments (a,b,c and d, respectively). Stage 7 CPM induces expression of Sax1 (U) and Hoxb8 (V) in 3c explants. In 4b explants, the CPM induces Sax1 (W) but not bra (X) after 24 hours. Expression of Sax1 (Y) and bra (Z) is also undetected after 8-12 hours. Scale bar: 100 µm.