Google Answers: Totipotent cell count in human zygotes: "At approximately the 16 cell
stage there is a differentiation event, in which some cells are
directed to become the trophectoderm which produces the chorion and
the cells of the inner cell mass which will give rise to the embryo.
This is the first differentiation event. So, at this point, no cells
are left totipotent."
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