The most complex life forms ever developed entirely in Petri dishes can pump blood through tiny beating hearts, gradually growing nerves and muscles in a laboratory. These little collections of mammalian cells form rudimentary mouse embryos, built from scratch out of stem cells - cells that have the potential to develop into any other cell type in the body. "Watching an embryo develop is a marvelous thing to behold," said developmental biologist Christine Thisse from the University of Virginia, one of the authors of the study.