During the 3rd and 4th weeks of development, the ectoderm on the midline of the embryo undergoes neurulation. During this process, the neuroepithelial cells proliferate, and the lateral edges of the neuroectoderm rise up. The end result is the production of a fluid filled neural tube surrounded by mesenchyme, just underneath the surface ectoderm, which has resealed on the dorsal midline.

In the screen on the left, a three-dimensional depiction of the neural tube is shown in blue. The embryo is shown as a red outline for orientation. Removal of the embryo outline allows us to focus on continued growth of the neural tube. At this early stage, the neural tube can be subdivided into four distinct regions.

From caudal to cranial, these regions are the spinal cord, rhombencephalon, mesencephalon, and prosencephalon. These regions are color coded in the 3-dimensional image on the left. A color coded longitudinal section is indicated in the panel to the right.