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The tiny mouse embryos were off to a promising start, many of them progressing from a one-cell fertilized egg to two-, four-, and even eight-cell blobs resembling raspberries-to-be. As the eager scientists carefully tracked their development (in lab dishes), many of the embryos formed characteristic structures called pronuclei that housed the DNA of egg and sperm, following the normal developmental script. And they began turning specific genes on or off, again as happens during normal gestation, seemingly on their way to becoming mouse fetuses and, eventually, newborns.

But then the embryos hit a wall. Not a single one divided past the eight-cell stage. And that is cause for either relief or disappointment, depending where you stand on one of the wilder ideas in reproductive medicine: These embryos came from eggs produced from stem cells, which are easily created from skin and other cells. Their apparent inability to develop into baby mice suggests that using, say, skin cells to create baby-to-be eggs in humans is further off than some dystopian scenarios have envisioned.

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