Once the artificially «fertilized» eggs began dividing into the hollow balls of cells known as blastocysts, Kono's team inserted them into female mice. (sciencemag.org)
They shrank and clumped into balls of cells, reversing their developmental course and eventually becoming surprisingly similar to embryonic stem cells. (protomag.com)
Even though the reproductive age for humans is around 15 — 45 years old, the precursor cells that go on to produce human eggs or sperm are formed much earlier, when the fertilized egg grows into a tiny ball of cells in the mother's womb. (scientificamerican.com)