FIGURE 2.5 Neural tube In the 4th week, the neural tube begins to develop into the brain and spinal cord. In this photo, the neural groove, which fuses together first at the center and then outward in both directions as if two zippers were being closed, has been “zipped shut” except for one part still open at the top. Spina bifida, a congenital disorder in which the skin over the spinal cord is not fully closed, can originate at this point. After closing, the top of the neural tube will develop into the brain.

LENART NILSON/SCANPIX