We show that magnetic field annihilation depends strongly on the behav
iour of the vorticity and is quite different in 2D and 3D. In 3D the v
orticity can be increased locally by the stretching of vortex lines (a
n effect that is absent in 2D). This leads to the onset of cellular fl
ow at quite low vorticities and the fragmentation of the simple curren
t sheet.