A well known “Polonaise” pattern of epiblast cell movements accompanies formation of the amniote primitive streak (PS), which is the organizing center for gastrulation. Although the movements observed in classical (text book) and modern studies appear similar, the biophysical mechanisms driving these movements are unknown. In comparison to studies of dynamic cellular movements during PS formation, and more generally, gastrulation, there is a relative paucity of data regarding movement of the extracellular matrix (ECM) lying adjacent to the ventral surface of the epiblast. Electron microscopy and immunofluorescence studies demonstrated decades ago the presence of a nascent basement membrane-like structure, which we refer to as the sub-epiblastic ECM (SE ECM), containing, at least, fibronectin [1] and collagen [1]. Using ultrastructural markers, Sanders [2] found that the SE ECM is transported medially to the PS with the epiblast cells. Almost two decades later, Bortier et al. [3] grafted radiolabeled quail cells into the epiblasts of chicken blastoderms, and concluded that whole groups of epiblast cells slide across (move relative to) the SE ECM — thus, contradicting Sanders’ earlier findings.

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