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Amnion structure and cell movements during human gastrulation. (A) Human embryo and uterine connections at day 15 of gestation. In the upper view, the embryo is cut sagittally through the midline; the lower view looks down upon the dorsal surface of the embryo. (B) The movements of the epiblast cells through the primitive streak and Hensen's node and underneath the epiblast are superimposed on the dorsal surface view. At days 14 and 15, the ingressing epiblast cells are thought to replace the hypoblast cells (which contribute to the yolk sac lining), while at day 16, the ingressing cells fan out to form the mesodermal layer. (After Larsen 1993.)
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