The nervous system starts to develop in the third week of development with the formation of the notochord in the mesoderm. The ectoderm above the notochord will condense into the neural plate—at this point, just a flat piece of tissue. The edges of the neural plates then rise to form the neural groove. The edges of the groove then start to come together forming the neural tube. The neural tube will drop down into the embryo to become surrounded by mesoderm. The ventral surface of the neural plate also gives rise to neural crest cells which migrate down towards the notochord. The neural crest cells give rise to numerous cell types, one of which is the dorsal root ganglia cells lying alongside the neural tube.