<Li> Cervical enlargement--corresponds roughly to the brachial plexus nerves, which innervate the upper limb . It includes spinal cord segments from about C4 to T1 . The vertebral levels of the enlargement are roughly the same (C4 to T1). </Li> <Li> Lumbar enlargement--corresponds to the lumbosacral plexus nerves, which innervate the lower limb . It comprises the spinal cord segments from L2 to S3 and is found about the vertebral levels of T9 to T12 . </Li> <P> The spinal cord is made from part of the neural tube during development . There are four stages of the spinal cord that arises from the neural tube: The neural plate, neural fold, neural tube, and the spinal cord . Neural differentiation occurs within the spinal cord portion of the tube . As the neural tube begins to develop, the notochord begins to secrete a factor known as Sonic hedgehog or SHH . As a result, the floor plate then also begins to secrete SHH, and this will induce the basal plate to develop motor neurons . During the maturation of the neural tube, its lateral walls thicken and form a longtitudinal groove called the sulcus limitans . This extends the length of the spinal cord into dorsal and ventral portions as well . Meanwhile, the overlying ectoderm secretes bone morphogenetic protein (BMP). This induces the roof plate to begin to secrete BMP, which will induce the alar plate to develop sensory neurons . Opposing gradients of such morphogens as BMP and SHH form different domains of dividing cells along the dorsal ventral axis . Dorsal root ganglion neurons differentiate from neural crest progenitors . As the dorsal and ventral column cells proliferate, the lumen of the neural tube narrows to form the small central canal of the spinal cord . The alar plate and the basal plate are separated by the sulcus limitans . Additionally, the floor plate also secretes netrins . The netrins act as chemoattractants to decussation of pain and temperature sensory neurons in the alar plate across the anterior white commissure, where they then ascend towards the thalamus . Following the closure of the caudal neuropore and formation of the brain's ventricles that contain the choroid plexus tissue, the central canal of the caudal spinal cord is filled with cerebrospinal fluid . </P> <P> Earlier findings by Viktor Hamburger and Rita Levi - Montalcini in the chick embryo have been confirmed by more recent studies which have demonstrated that the elimination of neuronal cells by programmed cell death (PCD) is necessary for the correct assembly of the nervous system . </P>

Hole in the tube surrounding the spinal cord