The only researchers to publicly show that they have cloned a human embryo said on Tuesday that they had successfully repeated the experiment, growing an embryo to the 16-cell stage. Researchers at Advanced Cell Technology of Worcester, Massachusetts, have also repeatedly created embryos using a process called parthenogenesis -- using only a human egg cell and no sperm, and without cloning. The company says the experiments, reported in the January issue of Wired magazine, are not breakthroughs but a natural progression of its efforts to create human embryonic stem cells to use for medical treatments.
