Spatial gene expression patterns in Xenopus embryos. (A) The ventx gene is expressed ventrally. (B) The gsc gene is expressed dorsally. (C) The bix1 gene is expressed in both the ventral and dorsal regions. (D) The gata4 gene is expressed in the vegetal region. The rectangular bar across the embryo indicates the portion of the image viewed for classification purposes, with the bar under the image showing the 1-dimensional representation of gene expression.