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ormation of mesoderm begins at posterior end of embryonal disc by the formation of primitive streak as a faint groove.Thois establishment head and tail ends of the embryo
  The active proliferated of cells at caudal end of embronic disc causes a local thickening of cells in the form of the primitive streak.Proliferated cells later deatach from epiblast and move inward below the primitive dtreak to form mesoderm layer between newly formed endoderm and epiblast.The process of inward movement of presumptive mesodermal cells is called invagination.

Formation of ectoderm occurs after the separation and invagination of mesodermal cells,the remaining cells of epiblast forms ectoderm.
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