Once the neural plate edges have fused to become a neural tube, the tissues that were at the edges of the plate now form the __________ region the neural tube. Fill in the blank.

The neural plate is a region of the dorsal ectoderm that develops into the neural ectoderm. It is a flat structure, with columnar appearing cells, and the formation of the neural ectoderm is the first step in the neurulation process. This region is generated from the thickened mid-dorsal surface opposite the primitive line of the ectoderm. From this epithelial zone the various types of neuronal and glia cells belonging to the nervous system are derived. For the neural plate to be built, it is necessary that the epiblast cells converge towards the midline of the embryonic disc forming a groove of the primitive line, delimiting a longitudinal axis of bilateral symmetry around which the embryonic structures and their organs will be aligned. From this moment on, the embryo will have a rostral (cephalic) and caudal (tail) region as well as a left and right side and a dorsal (back) and ventral (front) surface. From the folding of the neural plate or neurulation, the neural tube and the development of the entire nervous system will be formed.

The neural tube is tubular structure that undergoes modifications and will give rise to a central nervous system structure (encephalon and spinal cord). The neurulation process begins by means of signals sent by the dorsal mesoderm and pharyngeal endoderm to the ectodermal cells located above these germ layers. These signals cause this area of the ectoderm to elongate into a columnar plate of cells. The elongation allows the differentiation of the cells of the future neural plate, with respect to the pre-epidermal cells surrounding the area.

So, the neural tube is formed by invagination from the dorsal region of the ectoderm (neural plate) induced by the notochord while it is forming. This occurs at the beginning of the third week of conception by a process called neurulation.