<P> In vertebrates, landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory . </P> <P> All bilaterian animals at an early stage of development form a gastrula, which is polarized, with one end called the animal pole and the other the vegetal pole . The gastrula has the shape of a disk with three layers of cells, an inner layer called the endoderm, which gives rise to the lining of most internal organs, a middle layer called the mesoderm, which gives rise to the bones and muscles, and an outer layer called the ectoderm, which gives rise to the skin and nervous system . </P> <Table> <Tr> <Td> Human embryo, showing neural groove </Td> <Td> Four stages in the development of the neural tube in the human embryo </Td> </Tr> </Table> <Tr> <Td> Human embryo, showing neural groove </Td> <Td> Four stages in the development of the neural tube in the human embryo </Td> </Tr>

Types of neurons in the human nervous system