Fig. 1. Comparison of early neural, mesodermal and stem zone-specific gene expression adjacent to the primitive streak during body axis extension. Expression of Sox3, Sox2, bra and Fgf8 at HH3-3+ (A-D'), HH4 (E-H') and, additionally, Sax1 and Hoxb8 at HH6-7 (M-N'), HH8+-9 (S-T') and HH10+ (U-Z'). Levels of transverse sections are indicated with a black bar and epiblast adjacent to the primitive streak lies between arrowheads. Arrowhead in Z indicates somite 5. Diagrams show the positions of the stem zone from HH stages 4-10.