The dislocation structure of slip lines in silicon iron has been examined by an etch-pit technique. It has been observed that the arrays when viewed normal to the edge component are straight but not coplanar over long distances while the arrays where the screw component emerges from the surface show much curvature. In bend specimens the increase in strain from the neutral plane toward the surface is accomplished mainly by the lateral growth of the first bands formed rather than by the formation of new slip bands. Observations made in the slip plane itself demonstrate that the motion of the screw components is considerably more limited than that of the edges. Evidence is presented for the existence of interaction between the edges and screw components of adjacent loops in agreement with the theoretical predictions. The observations can be most adequately explained by the double cross-slip model of the formation of a slip band. Evidence is presented for the transfer of portions of a loop from one plane to another as this model requires.