Scientists may have taken a big—and controversial—leap by mixing human cells into monkey embryos. who operates a lab at the Salk Institute in California, has been working working with monkey researchers in China to perform the disturbing research. Their objective is to create “human-animal chimeras,” in this case monkey embryos to which human cells are added. Why, why? The idea behind the research is to fashion animals that possess organs, like a kidney or liver, made up entirely of human cells. Such animals could be used as sources of organs for transplantation. Making chimeras: The technique for making chimeras involves...