First Army. Under Canadian Corps--Third Canadian Division: Acheville-Mericourt-Avion sector. Under Canadian Corps--Fourth Canadian Division: Gavrelle-Oppy sector. On April 7, 1918, the First Canadian Division relieved the Fourth British Division astride the Scarpe and came under orders of Canadian Corps; the army boundaries being altered so as to include the sector taken over by the First Canadian Division in the First Army front. In the meantime, on the night of March 28th-29th, 1918, owing to operations astride the river Scarpe, the front-line system had been abandoned under orders of the Thirteenth Corps and the troops withdrawn to the Blue line in front of the Bailleul-Willer-val-Chaudière-Hirondelle line, as far north as the Mericourt sector. This Blue line was originally sited and constructed as an intermediate position, and consisted in most parts of a single trench none too plentifully supplied with dugouts. This meant that until a support line was dug and made continuous the troops had to be kept in strength in the front line, subject to heavy casualties from hostile shelling and to probable annihilation in case of an organized attack. Any advance beyond the Blue line on the Fourth Canadian Division front would have brought the Germans within assaulting distance of the weakest part of the Vimy Ridge, and the severity of the shelling seemed to indicate that a renewal of their attacks was probable. Every effort was made to give more depth to the new front-line system by pushing forward a line of outposts and by digging a continuous support line, as well as by constructing reserve lines at certain points of greater tactical importance. Switch lines facing south were also sited and dug or improved. To increase the depth of the defenses, machine-gun detachments were extemporized by borrowing men from the machine-gun battalions, who had then completed their organization on an eight-battery basis. Some fifty extra machine guns were secured from ordnance and other sources, and also a number of extra Lewis guns. Personnel from the Canadian Light Horse and the Canadian Corps Cyclist Battalion were organized in Lewis and Hotchkiss gun detachments and sent forward to man the defenses in Vimy and Willerval localities, under orders of the Third and Fourth Canadian Divisions. The machine-gun companies of the Fifth Canadian Division had arrived in France on March 25, 1918, and in view of the extreme urgency of the situation the personnel and armament had been moved by lorries, sent specially by Canadian Corps, from Le Havre to Verdrel, where they were in corps reserve. Their horse transport having now arrived, these machine-gun companies (Seventeenth, Eighteenth, and Nineteenth) were moved to the Vimy Ridge and allotted definite positions of defense on March 30, 1918.