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.