A new area of research is trying to get embryonic stem cells to replicate steps that happen in an embryo. In this case they took these embryonic stem cells and cultivated them in a special culture system. They then allowed it to develop to something equivalent to the 14th day of development of an embryo. What they had at the end of it didn't precisely mirror what you have in a natural embryo but what they did have were the precursors of the cells that would go on to become the placenta and the yolk sac. It's one of the first things to start to develop in an actual embryo.