During earlier rounds of the debate over research on human embryos — back in the days of the George W. Bush administration, when the issue was a major political controversy — the advocates of allowing and federally funding that research explained that carefully drawn ethical limits would be observed. Research would not be permitted after the embryos had developed to the 14-day mark.
The opponents, mostly pro-lifers, said that these advocates were at best fooling themselves. There was no principled difference between killing a human embryo at 14 days and killing one at 15 days,
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