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How does a single cell become a multicellular embryo? Every embryo begins as a single cell. The first cellular divisions produce a multicellular ball with a fused outer cell layer and an internal fluid filled space. As development proceeds, the cells of the inner cell mass divide into two major groups by the process of delamination: the hypoblast and the epiblast. The epiblast contains the cells that will eventually make up the embryo and the amnion, while the hypoblast forms extra-embryonic structures such as the yolk sac. (Carlson, 38)
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