The mesoderm of Xenopus laevis and other amphibia is formed through an inductive interaction during which cells of the vegetal hemisphere act on cells of the animal hemisphere. Two groups of factors mimic the effects of the vegetal hemisphere. One group consists of members of the fibroblast growth factor (FGF) family, while the other is related to transforming growth factor typeβ(TGF-β). In this paper we discuss the evidence that the FGF family represents ‘ventral’ mesoderm-inducing signals, and the TGF-β family ‘dorsal’ signals. The evidence includes a discussion of the cell types formed in response to each type of factor, the fact that only XTCMIF (a member of the TGF-β family) and not bFGF can induce animal pole ectoderm to become Spemann's organizer, and an analysis of the timing of the gastrulation movements induced by the factors.

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