In June 1515 Michelangelo was still working at the tomb of Julius. But a letter to Buonarroto shows that he was already afraid of being absorbed for other purposes by Leo: "I am forced to put great strain upon myself this summer in order to complete my undertaking; for I think that I shall soon be obliged to enter the Pope's service. For this reason, I have bought some twenty migliaia [measure of weight] of brass to cast certain figures." The monument then was so far advanced that, beside having a good number of the marble statues nearly finished, he was on the point of executing the bronze reliefs which filled their interspaces. We have also reason to believe that the architectural basis forming the foundation of the sepulchre had been brought well forward, since it is mentioned, in the next ensuing contracts. Just at this point, however, when two or three years of steady labour would have sufficed to terminate this mount of sculptured marble, Leo diverted Michelangelo's energies from the work, and wasted them in schemes that came to nothing. When Buonarroti penned that sonnet in which he called the Pope his Medusa, he might well have been thinking of Leo, though the poem ought probably to be referred to the earlier pontificate of Julius. Certainly the Medici did more than the Delia Rovere to paralyse his power and turn the life within him into stone. Writing to Sebastiano del Piombo in 1521, Michelangelo shows how fully he was aware of this. He speaks of "the three years I have lost."