A chimera, in the current scientific world, is an organism composed of cells or genes obtained from two or more different species. Human-animal chimeras can be created by transplanting human stem cells into animal fetuses or human genes into the genome of animal fetuses. Thus, a human-mouse chimera may contain a human liver in a mouse or may have a liver constituted of both human and mouse cells. These kinds of experiments are relatively recent—the first interspecies chimeras were engineered in 1989 by I. J. Davis, a researcher at the University of California. His work produced the geep, a new intraspecies chimera obtained by fusing a sheep embryo with a goat embryo. Most of the ethical and scientific discussion to this point has used the ...