Photomicrographs showing gradients of body plan determinants. (a) mRNA for the gene bcd is shown localized to the anterior (left-hand) tip of the embryo. (b) mRNA of the nos gene is localized to the posterior (right-hand tip of the embryo). The distribution of the proteins encoded by these genes and other genes determines the body axis.

[(a) Courtesy of Ruth Lehmann; (b) Courtesy of James Langeland.]