In the spinal cord and brainstem, the dorsal and ventral parts grow larger than the intermediate part. In this way, a groove is formed longitudinally on each side in the inner wall: the sulcus limitans. This sulcus extends over the entire length of the spinal cord and brainstem and can be observed up to the rostral end of the mesencephalon. The part of the neural tube located ventrally to the sulcus limitans forms the basal plate (lamina ventrolateralis), and the dorsal part forms the alar plate (lamina dorsolateralis). Motor neurons differentiate in the basal plate, and sensory neurons in the alar plate. Both basal plates are connected ventrally by the floor plate (lamina ventralis). The alar plates are connected by the roof plate (lamina dorsalis).

Spinal cord

Two longitudinal nuclear columns develop in the area of the spinal cord and in each of the two basal and alar plates. These columns are aligned in the following order along the ventro-dorsal axis: somatic motor neurons (general somatic efferents, GSE), visceral motor neurons (general visceral efferents, GVE), visceral sensory neurons (general visceral afferents, GVA), and somatic sensory neurons (general somatic afferents, GSA). Due to the increase in the number of neurons, the spinal cord expands outwards and also towards the interior. The internal space of the neural tube narrows and becomes the central canal (canalis centralis); it often obliterates in the adult.