The spinal nerves develop along a cranio-caudal gradient. Their segmentation is determined by the arrangement of the somites. The spinal ganglia form from the neural crest. Nerve cells of these ganglia send fibres towards the alar plate of the neural tube (afferent roots) and towards the periphery of the body. Axons of the motor neurons of the ventral horn start their growth from the basal plate of the neural tube and form the efferent ventral roots; these are linked with fibres of the spinal ganglia to form the spinal nerves. At S13, according to the cranio-caudal gradient of maturation, spinal ganglia appear in the cranial segments. By S14, the spinal nerves in the cervical area are fully grown and start to form the cervical plexus. By S16, the brachial plexus and the lumbosacral plexus also are beginning to appear. By S17, the peripheral nerves have already deeply penetrated the periphery of the upper limbs; by S20, those of the lower limbs have penetrated too.