The construction of the nervous system is one of the most complex processes in embryology. Development of the nervous system, or neural development (neurodevelopment), refers to the processes that generate, shape, and reshape the nervous system of animals, from the earliest stages of embryonic development to adulthood. In vertebrates, it begins with the formation of the neural tube from the ectoderm via neurulation. This tube then differentiates into the brain and spinal cord through regionalization and patterning by morphogen gradients. Subsequent stages include neurogenesis neuronal migration, axon guidance, synaptogenesis, and extensive activity-dependent refinement to produce functional neural circuits. This field of neural development draws on both neuroscience and developmental biology to describe and provide insight into the cellular and molecular mechanisms by which complex nervous systems develop, from nematodes and fruit flies to mammals.