In cutting-edge experiments, scientists have injected human brain cells into monkey fetuses to study the effects. Critics warn that these experiments may accidently produce monkeys with brains more human than animal, posing a new set of ethical issues.
Researchers are debating the ethics of creating chimeras -- creatures that are part animal and part human.
North American scientists are considering whether to implant stem cells from a human embryo into a mouse embryo ? a controversial experiment that could result in a mouse-human mix reminiscent of mythological monsters.