Fig. 10. Biomechanical contribution of convergent extension to neural tube closure. During normal neurulation, mechanisms that advance the elevated neural folds toward the midline (i.e. apical wedging, pushing by the epidermis, etc.) produce a finite amount of medial movement, arbitrarily defined as `X' in this figure (red bars). Convergent extension narrows the midline by a finite amount, defined as `Y' in this figure (blue bar). So, the distance between the forming neural folds can be defined as X+Y; at the end of normal neurulation this distance is reduced to zero, and the folds can meet and fuse. In embryos that lack PCP function, the distance X is covered by the normally functioning mechanisms (see Fig. 4). However, in the absence of convergent extension, the midline fails to narrow and at the end of neurulation, the folds cannot fuse as they remain separated by a distance roughly equal to `Y.'