Several groups have tried to make human animal chimeric embryos. Scientists think that the cells from such different animals are simply too distinct to coexist and communicate as intimately as embryonic cells do. There is another way to generate transplantable organs, and maybe one day brain tissue that could be used to study and treat disease. In a method called blastasist complementation, scientists introduce mutations that prevent an embryo from generating a particular organ. They then take stem cells that can produce that organ from another animal and inject them into the blast assist.