Weissman hailed the guidelines overall as "a significant step forward for the field of stem cell research." But he also underscored the importance of chimerical animals as "crucial" to modeling human genetic diseases and testing therapies to treat them.

Previous to the release of the guidelines, Weissman had proposed a new project to create chimerical mice that had 100 percent human nerve cells in their brains -- a plan that he had already asked a committee of bioethicists and medical researchers to review.  Weissman says he will not pursue further experiments without the agreement of that panel and his colleagues, and that, if he does move forward, he will be careful to "ensure that these mice don't take on any human characteristics, such as altered brain structures."

"Experiments like these," Weissman says, "need societal approval -- as well as our scientific enthusiasm for them."

Indeed, while these guidelines do provide an overarching ethics framework for researchers, they do not regulate their work or impose penalties for violating the suggested guidelines. Some opponents of human-chimera research believe that the United States should set federal limits, with the force of law behind them, or outlaw the creation of chimeras altogether (see The Notebook). Canada has already taken a stand by effectively banning the creation of chimeras last year, with the introduction of its Assisted Human Reproduction Act, which prohibits placing animal cells in a human embryo or human cells in an animal embryo.

The NAS guidelines, though, stop short of recommending those types of sanctions, and leave the door open to lots of potential experiments -- and what crosses the line is still up to interpretation. 

But even some within the scientific community are questioning whether these cross-species creations are blurring what should be a distinct line between human and animal. 

Stuart Newman, a developmental biologist with New York Medical College in Valhalla, New York, filed a patent application to create a human-chimpanzee hybrid more than seven years ago, which was turned down by the U.S. Patent Office earlier this year. The idea behind filing the patent, Newman says, was not to actually create a so-called "humanzee," but to "alert people of some of the more troubling things that [could] come out of this scientific work."

In recent years, Newman says this field of biomedical research has been "distorted" by the potential for financial gain, in light of the spectacular attention cloning and other areas of bioengineering have received. He's concerned that these early chimerical developments -- many of which he believes are justified -- could lead to the creation of animals that border too closely on human, or to creating "custom-made" humans. 

Others in the medical field have expressed a more near-term -- and decidedly more ecumenical -- disdain for the ambitious development of animal-human hybrids.