polys = vtk.vtkAppendPolyData()

Nslices = 10
p01 = (0,0,0)
p02 = (0,1,0)
for ks in range(Nslices):
    p0 = (0,p01[1]+(p02[1]-p01[1])/float(Nslices)*ks,0)
    plane = vtk.vtkPlane()
    plane.SetNormal(0,1,0)
    plane.SetOrigin(p0)

    cut = vtk.vtkCutter()
    cut.SetInput(ugrid)
    cut.SetCutFunction(plane)
    cut.Update()
    output = cut.GetOutput()
    polys.AddInput(output)