ACCORDING to the present view, the forces responsible for the closure of the neural tube reside in the neural plate itself. On this view, the disproportion between the surface area and the volume of the neural plate is the real mechanical cause of neurulation movements. This disproportion can arise either by a diminution of the surface area, or by an increase in volume, or, occasionally, by a combination of both processes. In all these cases the neural plate cells must be firmly anchored in the developing internal limiting membrane system1,2. The first possibility has been generally conceded as the theory of active contraction3–6, which presupposes the existence of a contractile system settled either extra- or intra-cellularly at the level of the internal limiting membrane. Volume increase of the neural plate may occur either as a result of cell multiplication (proliferation hypothesis), or by an augmentation in size of particular cells (hydration hypothesis7). The active part of proliferation has generally been overlooked, in spite of the fact that only one research worker7 denies the existence of mitoses in the neural plate.