How far should science go in making human-animal chimeras?

Human cells or DNA are planted into laboratory animals as a matter of routine, and nonhuman parts end up in people, too. Both forms of interspecies mixing—from us to them, and them to us—hold tremendous opportunities for therapy and could lead one day to biological enhancement. But they also bring to light some important moral quandaries.

The word hybrid, in the scientific sense, describes an organism that carries DNA from different species in each of its cells. Human-animal combos of this kind have indeed been created by several different labs—though only at the very early stages of development, and never in the hopes of making full-grown, crossbred monsters. For an old-fashioned test for male fertility, doctors would try to fertilize a set of hamster eggs with a patient's sperm. (If they succeeded, the hybrids would be destroyed.) More recently, British scientists implanted cow eggs with nucleic acids drawn from human skin as a means of growing human stem cells in a dish. (The constructs would never progress beyond this point.)

Full article:
http://www.slate.com/articles/health_and_science/superman/2013/05/human_animal_hybrids_chimeras_with_mice_pigs_and_goats.html