Neurogenesis is the process by which new brain stem cells are generated by the hypocampus in the brain. This process starts with embryos while the brain is being constructed and continues past birth throughout the life of the organism. This contradicts a previously held supposition that once the brain was established and the offspring born, no new neural cells could be added. This belief was held as late as the 1980s. Neurogenesis is measured with the use of chemical markers and powerful microscope analysis. The migration from the hypocampus to the ventricles of the brain occurs throughout the life of the organism, slowing down as the hypocampus ages and decreases in both size and neural cell output. Many of the neural cells created by this process die as new cells, but many establish themselves as new neural pathways, helping to replace dying neural cells.