In research institutions studying the frontier between species, scientists have achieved what was once theoretical: human brain tissue, grown in a laboratory and transplanted into living mice, has taken root, formed connections, and functioned as part of a chimeric mind. The technique, called xenocortication, offers neuroscience a long-sought middle path — a way to observe human neural behavior in a living system without experimenting on human beings. It is a milestone that arrives not only with promise for understanding Alzheimer's, autism, and other conditions, but with questions about consc