was decided, despite the protestations of Stern (see below), to ship the 25 training vehicles in Britain to France, where they joined the other 20 Mark IIs and 15 Mark Is at the Battle of Arras in April 1917. The Germans were able to pierce the armour of both the Mark I and Mark II tanks at Arras with their armour-piercing machine gun ammunition. The Mark II was built from December 1916 to January 1917 by Foster & Co and Metropolitan (25 Male and 25 Female respectively). Five Mark IIs were taken for experiments on improved powerplants and transmission. They were provided to firms to show what improvements they could make over the Mark I system in an open competition. In the demonstrations held in March 1917, only three of them were able to compete alongside Mother, which had been fitted with a Daimler petrol-electric system. Wilson's epicyclic gear system, which replaced the secondary gear and the gearsmen, was clearly superior and adopted in later designs. Mark III The Mark III was a training tank and used Lewis machine guns and a smaller sponson for the females. Fifty were built. It was originally intended that the Mark III was to have all the proposed new design features of the Mark IV. This is why there were two distinct training types, the Mark II being little more than a slightly improved Mark I. However, development of the new features was so slow that the change from the Mark II was very gradual. The last two Mark IIIs were melted down in the Second World War. They did not see action overseas. Mark IV The Mark IV was a more heavily armoured version of the Mark I, and went into production in May 1917. Fundamental mechanical improvements had originally been intended, but had to be postponed. The main change was the introduction of the QF 6-pounder 6 cwt Hotchkiss, a shorter-barrelled version of the earlier gun. Improved 12 mm armour made it impervious to German armour-piercing rifle bullets. Lewis guns replaced the Hotchkiss and Vickers machine guns in both the male and female variants, although the air-cooling system of the Lewis proved problematic and the Hotchkiss was later readopted. It had all its fuel stored in a single external tank, located between the rear track horns, in an attempt to improve crew safety. The smaller sponsons could be swung inwards on hinges