About a decade ago, biologists started to observe that stem cells, left alone in a walled plastic container, will spontaneously self-assemble and try to make an embryo. These structures, sometimes called "embryo models" or embryoids, have gradually become increasingly realistic. The University of Florida is trying to create a large animal starting only from stem cells--no egg, no sperm, and no conception. They've transferred "synthetic embryos," artificial structures created in a lab, to the uteruses of eight cows in the hope that some might take. At the Florida center, researchers are now attempting to go all the way.