Development of the posterior body (lumbosacral region and tail) in ver
tebrates is delayed relative to gastrulation. In amniotes, it proceeds
with the replacement of the regressed node and primitive streak by a
caudal blastema-like mass of mesenchyme known as the tail bud. Despite
apparent morphological dissimilarities, recent results suggest that t
ail development in amniotes is in essence a continuation of gastrulati
on, as is the case in Xenopus. However, this has been inferred primari
ly from the outcome of fate mapping studies demonstrating discrete, re
gionalized cell populations in the tail bud, like those present at gas
trulation, Our analysis of the tail bud distribution of several molecu
lar markers that are expressed in specific spatial domains during chic
k gastrulation confirms these results. Furthermore, we present evidenc
e that gastrulation-like ingression movements from the surface continu
e in the early chick tail bud and that the established tail bud retain
s organizer activity. This 'tail organizer' has the expected propertie
s of being able to recruit uncommitted host cells into a new embryonic
axis and induce host neural tissue with posteriorly regionalized gene
expression when grafted to competent host cells that are otherwise de
stined to form only extra-embryonic tissue. Together, these results in
dicate that chick tail development is mechanistically continuous with
gastrulation and that the developing tail in chick may serve as a usef
ul experimental adjunct to investigate the molecular basis of inductiv
e interactions operating-during gastrulation, considering that residua
l tail organizing activity is still present at a surprisingly late sta
ge.