protected override void Tesellate()
        {
            vpnt = new VertexPositionNormalTexture[(slices + 1) * (stacks + 1)];

            float phi, theta;
            float dphi = MathHelper.Pi / stacks;
            float dtheta = MathHelper.TwoPi / slices;
            float x, y, z, sc;
            int index = 0;

            for (int stack = 0; stack <= stacks; stack++)
            {
                phi = MathHelper.PiOver2 - stack * dphi;
                y = radius * (float)Math.Sin(phi);
                sc = -radius * (float)Math.Cos(phi);

                for (int slice = 0; slice <= slices; slice++)
                {
                    theta = slice * dtheta;
                    x = sc * (float)Math.Sin(theta);
                    z = sc * (float)Math.Cos(theta);
                    vpnt[index++] = new VertexPositionNormalTexture(new Vector3(x, y, z), 
                                                                    new Vector3(x, y, z), 
                                                                    new Vector2((float)slice / (float)slices, (float)stack / (float)stacks));
                }
            }

            indices = new int[slices * stacks * 6];
            index = 0;
            int k = slices + 1;

            for (int stack = 0; stack < stacks; stack++)
            {
                for (int slice = 0; slice < slices; slice++)
                {
                        indices[index++] = (stack + 0) * k + slice;
                        indices[index++] = (stack + 1) * k + slice;
                        indices[index++] = (stack + 0) * k + slice + 1;

                        indices[index++] = (stack + 0) * k + slice + 1;
                        indices[index++] = (stack + 1) * k + slice;
                        indices[index++] = (stack + 1) * k + slice + 1;
                }
            }
        }