We have investigated whether superficial cells of the blastula contribute to mesodermal structures in the anuran Xenopus laevis and the urodele Ambystoma mexicanum. The superficial cells alone of late blastulae of both embryos were labelled with Bolton-Hunter reagent and the embryos were allowed to develop. The progeny of the labelled cells were identified at later stages and the results demonstrate that superficial cells of Xenopus blastulae make no significant contribution to the mesoderm, whereas those of the axolotl Ambystoma always contribute.