The embryo proper will develop from part of the epiblast. The hypoblast will lead to part of the extra-embryonic membranes together with the trophoblast. It is the result of changes in cell-cell adhesion and cell junction. Several molecules cluster together forming large plaques. Cell-cell contact is increased.
Actin filaments attach to the periphery of the E-cadherin plaques.
There is no actin filaments within the plaque itself. Number of cells in each group varies and cell can be reallocated from one group to the other. Process of random approximation. These are located on the apical side of the trophectoderm cells.