This sounds like one placenta could produce 100x more stem cells than an aborted embryo. Amazing how quickly we've overcome Bush's shortsightedness and anti-research bias.
/sarc off
The placental encapsulation is the first organ the newly conceived human builds for his or her survival in the water world of Mom's body and the construction starts even before implantation because it is the early stage of these cells which allow for implantation without being rejected by the Mother's body immune system. The organ does the job of breathing and digestive/waste, etc. systems during the nine months in the water world and is discarded upon birthing into the air world. The amniotic sac is a portion of this placental encapsulation which includes the umbilicus, and cell systems in this system also turn inward to build the organ systems of the body for the air world. [That's an over simplification, if you will.]