Fig. 8. The movement of cells derived from the gastrula organizer relative to the morphogenetic movement of the mesoderm during gastrulation from the ES (early-streak) to the EB (early-bud) stage. Left column: lateral view of the embryo with anterior towards the left; right column: flattened dorsal view, anterior towards the top. The early gastrula organizer (EGO, gray) cells are first found in the posterior epiblast at a distance of about 50 µm anterior to the newly formed primitive streak (black) of the ES embryo. These cells initially remain static in a tight cluster before the elongating primitive streak reaches them. Cells in the posterior region (Region IV, Fig. 5A) of the EGO are incorporated first into the advancing primitive streak and they ingress (grey arrows) to join the mesoderm (pink) lateral to the anterior end of the primitive streak. The cells in the anterior region of the EGO (the bulk of the progenitor of the prechordal mesoderm) stay in the midline and are carried anteriorly to meet the progenitors of the prechordal mesoderm and the head process in the epiblast of Regions I and II. These cells collectively constitute the mid-gastrula organizer (MGO). Cells leaving the MGO move in a tight column anteriorly along the midline of the embryo (black arrow, pointing anteriorly) and form the prechordal mesoderm and the head process. These progenitors of the axial mesoderm are moving in concert with the paraxial stream of EGO-derived cells in the mesoderm. The movement of these two streams of EGO-derived cells is part of the global movement of the mesoderm and the definitive (gut) endoderm towards the anterior region of the gastrulating embryo (Tam et al., 2001). At the MS stage, the bulk of the precursors of the trunk notochord is not found in the MGO and is localized in the primitive streak immediately posterior to the MGO. These cells are recruited into the node after the departure of the anterior axial mesoderm and they are laid down as the notochord as the node is displaced posteriorly (black arrow, pointing posteriorly) with the regression of the primitive streak.