The avian organizer

The development of avian embryos is characterized by the large amount of yo lk present from the one-cell stage until late phases of organogenesis, In t he chick, an axis of bilateral symmetry is established already before egg l aying, when the egg rotates in the uterus, There is evidence for an active Wnt-catenin pathway in the vegetal cells in the periphery of the multi-cell ular embryo. It overlaps with the posteriorly restricted expression of gene s characterizing the vegetal hemisphere in amphibia. The zone of overlap be ars several functional characteristics of a Nieuwkoop center, which is firs t apparent in the posterior marginal zone, but continues into the early pri mitive streak. Only the anterior part of the late streak is capable of dire ct neural induction, and only its tip, Hensen's node, can induce an anterio r neural identity. This latter activity leaves the node together with the c ells representing the anterior mesendoderm. Thus, although the constraints and dynamics of avian development make comparisons with the amphibian situa tion a complex undertaking, Hensen's node comes as close as possible to an organizer in Spemann and H. Mangold's definition.