Establishment of anterior-posterior polarity in avian embryos.

In pre-streak chick embryos, the extraembryonic posterior marginal zone is able to induce an embryonic axis at an ectopic site without contributing cells to the induced primitive streak. This region expresses mesoderm-inducing factors that are capable of inducing an ectopic streak. Downstream of these events, chordin and bone morphogenetic protein acting within the central disc may play mutually opposing roles influencing streak formation. Although extraembryonic regions are important in establishing the embryonic axis, there does not appear to be an anterior region with head-inducing activity similar to that of the anterior visceral endoderm of the mammalian embryo.