In Minnesota, pigs are being born with human blood in their veins. In Nevada, there are sheep whose livers and hearts are largely human. In California, mice peer from their cages with human brain cells firing inside their skulls. Biologists call these hybrid animals chimeras, after the mythical Greek creature with a lion's head, a goat's body and a serpent's tail. Chimeras are allowing scientists to watch, for the first time, how nascent human cells and organs mature and interact ... inside the bodies of living creatures. But with no federal guidelines in place ... how human must a chimera be before more stringent research rules should kick in? Chimerism becomes a more sensitive topic when it involves growing entire human organs inside animals. And it becomes especially sensitive when it deals in brain cells. Imagine, said Robert Streiffer, a professor of philosophy and bioethics at the University of Wisconsin, a human-chimpanzee chimera endowed with speech ... what some have called a humanzee." Perhaps the most ambitious efforts to make use of chimeras come from Irving Weissman, director of Stanford University's Institute of Cancer/Stem Cell Biology and Medicine. Weissman helped make the first mouse with a nearly complete human immune system. More recently his team injected human neural stem cells into mouse fetuses, creating mice whose brains are about 1 percent human. Now Weissman says he is thinking about making chimeric mice whose brains are 100 percent human.