In situ hybridization of selected genes. Representative images of whole-mount in situ hybridizations ordered according to developmental time to allow visual correlation with the corresponding microarray profile (from left to right: stages 2-3, 5, 6-7, 8-9, and 10-12). Embryos are oriented with the anterior region to the left and the dorsal region facing upward; all are lateral views. Gene symbols are indicated above the series of images. To compare the expression profile obtained by our microarray study and in situ hybridizations we transformed the gene expression values as follows: normalized intensity values (IVi = stages 2-3, 5, 6-7, 8-9, 10-12) of each gene at each developmental interval were used to build a gene expression vector (gev) with coordinates calculated as gevi = log2 (IVi/IVS2-3). Then, gevi arithmetic means (Mgev) for each gene were used to build a color scale according to the following rules: black, gevi = 0; dark red, 0 < gevi ≤ 0.5 Mgev; red, gevi ≥ 0.5 Mgev. The scale is represented by color bars at the bottom of each series of in situ images. Known genes, c1 to c5; uncharacterized genes, a to p.