That is the problem with the whole notion of regarding embryos as individual human beings. In fact, they are collections of raw materials which could still become one person, multiple persons, or a part of a person (at this stage, you can still squish two embryos together and get a single baby with mixed DNA -- happens naturally fairly often, though apparently less often than monozygotic twinning).
So far, I don't believe anyone has gotten a single cell plucked from an embryo to develop into a baby (even a baby mouse), though it may eventually be possible. I think a bare minimum of two cells is needed, because once the zygote has divided, the groundwork for inter-cell signalling about which cells will become up/down/right/left parts of the body is being laid, and the process doesn't seem to work with just one cell.