In those mediæval days, when the arts protected themselves by forming into constituted guilds, no one could call himself a Master unless he were trained and qualified in one of these guilds and had reached the higher grades. To trace Niccolò's place in the great chain of the Masonic Guild, we must go back a little, and gather together the threads of information we have been able to glean, as to the expansion of the guild itself, and here the valuable collections of archivial documents made by Sig. Milanesi from the books and archives of the Opera del Duomo at Siena, and by Sig. Cesare Guasti from those of the cathedral at Florence, will materially assist us. By studying these and putting facts and statements together the whole organization becomes clear, and our former glimpses into the threefold aspect of the lodges at Modena, Parma, and other northern cities become confirmed. Here in Tuscany we again find the three branches. First: There is the school where novices were trained in the three sister arts--painting, sculpture, and architecture. When pupils were received from outside the guild, they had to pass a very severe novitiate before being admitted as members; but the sons and nephews of _Magistri_ were, we learn, entitled to be members by heritage without the novitiate.[156] The hereditary aspect of the lists of Masters certainly displays this right of heritage very strongly. The qualified Masters were entitled to take pupils and apprentices in their own studios. The large number of pupils who studied under Niccolò Pisano suggests his eminent position in the guild. Second: There was the _laborerium_, or great workshop, where all the hewing of stone, carving of columns, cutting up of wood-work was done--in fact, the head-quarters of the brethren who had passed the schools, but were not yet Masters.[157] A graphic sketch from a Masonic _laborerium_ is given by Nanni di Banco, in the relief under the shrine of the _Quattro Coronati_ on Or San Michele at Florence, where the four brethren are all at work. In looking at it, one is reminded of the old story of the block of marble from which Michael Angelo's David was made, which had laid for many years in the stores of the Opera del Duomo at Florence, it having been once assigned to Agostino di Ducci, who was commissioned in 1464 to make a statue for the front of the Duomo, which was blocked out so badly that the marble was taken away from him, and he was expelled from the _laborerium_.[158]