For years, neurobiologists clung to a fundamenral truth: Once animals, or people, reach adulthood, they may lose brain cells but they can never grow new ones. There were a couple of exceptions - birds and rats but the thought was that these were peculiarities of nature and not evidence of a general principle. But now, in experiments that experts call amazing, that dogma has been overturned. Scientists have found that monkeys are constantly making new brain cells in the hoppocampus, an area of the brain used for forming long-term memories.