We observed a posterior to anterior gradient of convergence in the epiblast associated to a parallel gradient of ingression in the PS. A posterior to anterior gradient of cell motility of ingressed mesodermal cells has been documented along the regressing PS (Zamir et al., 2006). These graded movements in the mesoderm are largely controled by a posterior to anterior gradient of Fgf8 established in the PS and acting as a repellent on newly ingressed mesodermal cells at this stage (Yang et al., 2002). This cellular dynamics suggests that the movement away from the posterior PS could act as a sink for the epiblastic territories generating these progenitors. These cell movements in the epiblast and the ingressing mesoderm could explain the progressive exhaustion of the PS from its posterior end first described for the extraembryonic territory (Spratt Jr., 1947). Importantly, our **data** show that the precursor territories along the PS do not disappear at the same rate. We observe a sequential posterior to anterior exhaustion of the territories of the extraembryonic mesoderm, the lateral plate and the SOX2 negative paraxial mesoderm progenitors (Figure 3k). Combined to an increased proliferation in the NMP region, this would explain why the SOX2/T territory eventually remains as the major remnant of the PS in the tail bud after PS regression. Thus, the tail bud is composed of a mosaic of monopotent territories such as the precursors of the notochord, or the hindgut, and the multipotent NMPs, which give rise to descendants in the ectoderm and mesoderm. This likely explains why the tail bud appears as a site where gastrulation movements are still ongoing but also shows blastema characteristics (Catala et al., 1995;Davis and Kirschner, 2000;Gont et al., 1993;Holmdahl, 1925).