The human midline is particularly vulnerable to dysmorphogenesis. This property can be derived theoretically from models of developmental fields that rely on positional information for the control of the process of determination. The topological properties of the midline as an early plane of symmetry imply positional informational weaknesses that should decrease developmental stability in this area for 2 reasons: 1) the unique location on a cusp between 2 mirror image fields is a “null” position for differentiation, and 2) this location between 2 equal positional values eliminates a lateral gradient that aids in regaining information after disruptions. Under this model, midline malformations would be expected to be primarily determinative defects, with an especial, proclivity for patterning and tissue abnormalities. Evidence is presented in support of this hypothesis, which makes midline properties consequences of developmental field properties, even though, technically speaking, the midline would not itself be a field.