By injecting animal embryos with human stem cells, scientists can create ‘chimera’ hybrids. By creating and nourishing these beings, letting the hybrid embryos develop into fully-grown animals with human cells and organs, scientists can then use the human/animal for experiments, ostensibly to learn about and treat diseases. Researchers say they might be able to breed sheep, pigs, and cows with human hearts, kidneys, etc. for human transplantation.