The four animal blastomeres of the eight-cell stage of Xenopus laevis are intrinsically capable of differentiating into dorsal mesodermal derivatives.

Mesoderm formation in the amphibian embryo is thought to be induced in the ectoderm of the animal region by signals emanating from the endoderm of the vegetal region after cleavage up to the mid-blastula. During this process the dorsal vegetal zone is thought to stimulate the dorsal animal zone to establish the Spemann organizer, which will in turn trigger the overlaying neuroectoderm during gastrulation resulting in the development of the central nervous system. In this concept it is assumed that the animal hemisphere is an uncommitted area, which receives its instructions from the vegetal region of the embryo. However, the experiments of this paper show that the 4 animal blastomeres of the eight-cell stage will form dorsal mesodermal structures in over 50% of the cases. The results support the view that developmental determinants are distributed in distinct gradients already in the early cleavage stages and that in the embryo the mesoderm is determined by factors prelocalized in the marginal zone. The spatial and temporal activation of certain genes in a distinct pattern is not simply emanating from certain areas or centers, but is a result of complex interactions between the vegetal and animal hemisphere and vice versa.