For example, Stanier et al. They continue to flatten, and they relocate by epiboly towards the vegetal pole. Irrespective of their early positions, the EVL cells differentiate directly as periderm, an extremely flattened protective outer single-cell layer covering the entire embryo. Near the time that involution begins, other DEL cell movements and rearrangements occur both within the epiblast and hypoblast. During convergence, cells stream from all sectors of the blastoderm towards the dorsal side.
Intercalations repack the cells in both layers.
As gastrulation proceeds the cells undergo many such directional intercalations, such that single labeled clones descended from marked blastula founders become markedly spread apart. Formation of the embryonic shield visibly indicates that rapid convergence movements have begun. The shield is an accumulation and condensation of DEL cells along a local stretch of about 30 degrees of longitude of the germ ring, and marks the future dorsal side of the embryo. Hence, when the shield forms one can reliably distinguish the orientation of the embryo's eventual DV axis. These first cells, expressing goosecoid , stream anteriorwards, and appear to correspond to precursors of the tetrapod prechordal plate.