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What makes something (or someone) an embryo—as opposed to what is actually, and not just in biotech parlance, a collection of cells? This question has come to the fore in recent years with proposals for producing embryonic (or pseudoembryonic) stem cells for research. While some of those opposed to use of standard embryonic stem cells emphasise that adult (including umbilical) cells have a clinical track record, others argue that there may be further benefits obtainable from cells very like those of embryos, provided such cells can be derived in new ways. Rather than deriving them in ways that kill or otherwise endanger a living human embryo, they could be obtained from an entity that merely resembles a human embryo sufficiently closely for its cells to be of use. Such an entity might be created after introducing genetic changes to an ovum before it is activated by, for . . . [Full text of this article]