Serial sections of individual embryos of a staged series were hybridized with radioactive probes for Sonic hedgehog, Hoxd-IO, Hoxd-? I, and Hoxd-72 Several embryos were analyzed at each stage with an effective maximum of 35 pm separating any two sections analyzed with a given probe. Thus, within the limits of probe sensitivity, it is unlikely that any expression domains have gone undetected.(A) Transverse sections at the level of the posterior wing bud showing the relative onset of expression of these four genes. The first detectable signal of each gene is indicated by an open arrow. Hoxd-70 is expressed prior to Sonic hedgehog, and Sonic hedgehog and Hoxd-77 expression initiate coincidently and are followed by h’oxd-72. Only the right wing bud in each section should be used for comparison as sections were cut obliquely. Embryo stages (St.) and probes used are indicated in the left and top margins, and panels to the right are bright field images. Abbreviations WB, wing bud; No, notochord; NT, neural tube; and G, gut endoderm.(B) Serial sections of a stage 23 wing bud hybridized with Sonic hedgehog, Bmp-2, Hoxd-77, and Hoxd-73 probes. Note that the Sonic hedgehog and Brnp-2 expression domains are both centered at the posterior wing margin. Hoxd-77 expression extends throughout the PD extent of the bud and anteriorly to mid-bud level, while the Hoxd-73 expression domain has shifted distally. Anterior is to the top; distal is to the right.