The study of homeobox genes has substantially increased our understanding of vertebrate embryogenesis. Work in the past Eve years has shown that vertebrate homeodomain proteins, like their fruit fly counterparts, specify the antero-posterior (A-P) identity of body regions. We now propose to study how homeobox genes specify the gradual elaboration of the vertebrate A-P axis. We have recently found that microinjection of homeobox mRNA into Xenopus laevis embryonic cells is sufficient to cause induction of new axial structures after transplantation into a host embryo. This provides a powerful direct method for testing whether different homeobox genes confer regional specificity and, if so, which parts of the protein are responsible. Other in vivo methods, such as microinjection of purified mRNAs and antibodies, transplantation of germ layers, culture of explants, and the generation of transgenic animals, will also be utilized. Three crucial stages in amphibian development have been defined by embryological studies. We propose to study Xenopus homeobox genes: a) accumulated in the unfertilized egg b) activated in the dorsal lip of the blastopore c) expressed in the mesodermal mantle at the neurula stage. By studying these genes, which act at different stages of the same regulatory pathway, we hope to. understand how the vertebrate' body plan is gradually established. The proposed experiments exploit the accessibility of Xenopus embryos to manipulation at stages before the aids is determined, and during which equivalent experiments in mammalian embryos would be much more difficult. However, because all vertebrates develop similarly, it is expected that these studies will provide insights into the causes of congenital malformation and embryo wastage in all vertebrates, including man.