Dermatomes, sensory innervation areas of different spinal nerves
During the segmentation of the embryo, the primordial vertebrae (somites) develop out of the mesoderm lateral to the neural tube and notochord. Out of the dorsolateral part, the skin develops. This construct is called a dermatome. Due to this segmental origin of the skin areas, there is a segmental assignment to the correspondent spinal nerves.