Adhesion is the final process in the sequence of primary neurulation events. Such physical zippering state of the neural tube has been suggested, in previous studies, as evidence that neural tube closure is a continuous process . However, a debate exists as to whether the physical process of neurulation actually equates to continuous zippering or, more accurately, to a button-like process in which neural tube adhesion initially occurs at various slightly separated points along the axis. According to the latter idea, neural tube adhesion is actually a discontinuous process of closure .