The first cleavage furrow demarcates the dorsal-ventral axis in Xenopus embryos.

To determine the relationship between the first cleavage furrow and the dorsal-ventral axis of the Xenopus embryo, a heritable intracellular marker was injected into one blastomere at the two-cell stage. Embryos were selected in which the cleavage furrow bisected the crescent-shaped region of pale pigmentation or in which it formed 45-90 degrees from this region. This region, which is located in the animal hemisphere of the Xenopus embryo, meets the criteria of the grey crescent as defined in other amphibian species. At tailbud stages the interface between the labeled and unlabeled halves was always coincident with the midsagittal plane. This correlation shows that the first cleavage furrow demarcates the dorsal-ventral axis. The labeling pattern was the same whether the first cleavage furrow bisected the region of pale pigmentation or whether it formed 90 degrees from it. However, when this region was bisected (70% of embryos) it always was located on the dorsal side of the embryo. Thus the region of pale pigmentation indicates the dorsal side of the embryo only when it is bisected by the first cleavage furrow.