Growth factors are known to have pleiotropic effects on many cell types ranging from the control of cell proliferation to inducing cell differentiation. FGF and TGF beta are members of two growth factor families which are thought to be involved in embryogenesis of the frog, Xenopus laevis. These two growth factors are equivalent to the embryonic "morphogen(s)" which induce one of the first differentiation events during embryogenesis, the formation of the mesoderm. Embryonic induction events are crucial for the development of most organisms and, therefore, these growth factors may be involved in induction events during mammalian embryogenesis. Thus, the structure and function of TGF beta and FGF molecules appear to be conserved throughout vertebrate evolution and during ontogeny, growth factors and their signalling pathways may be used for different functions depending upon the nature of the target cell.