Vertebrate mesoderm forms a diversity of tissues, including notochord, somitic muscle, pronephros, mesenchyme, and blood. In the amphibian Xenopus laevis , mesoderm arises in the equatorial region (marginal zone) of the blastula embryo in response to inducing signals produced by vegetal pole cells (1 ). This model of Xenopus mesoderm induction is derived from tissue recombination experiments in which vegetal pole tissue (prospective endoderm) was cocultured with animal pole tissue (prospective ectoderm) and resulted in the conversion of animal pole tissue into mesoderm (2 ). Therefore, vegetal pole cells of the cleavage embryo secrete factors that can redirect animal pole cells from ectodermal fates to mesodermal fates. These studies identified the source of mesoderm-inducing signals (vegetal pole) and established the animal pole explant as a responsive tissue useful for identifying mesoderm-inducing factors (3 –5 ).