One of the reasons that we know so little about the control of vertebrate gastrulation is that there are very few systems available in which the process can be studied in vitro. In this paper, we suggest that one suitable system might be provided by the use of mesoderm-inducing factors. In amphibian embryos such as Xenopus laevis, gastrulation is driven by cells of the mesoderm, and the mesoderm itself arises through an inductive interaction in which cells of the vegetal hemisphere of the embryo emit a signal which acts on overlying equatorial cells. Several factors have recently been discovered that modify the pattern of mesodermal differentiation or induce mesoderm from presumptive ectoderm. Some of these mesoderm-inducing factors will also elicit gastrulation movements, which provides a powerful model system for the study of gastrulation, because a population of cells that would not normally undertake the process can be induced to do so. In this paper, we use mesoderm-inducing factors to attempt to answer four questions. How do cells know when to gastrulate? How do cells know what kind of gastrulation movement to undertake? What is the cellular basis of gastrulation? What is the molecular basis of gastrulation?

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