It appears certain that up to this date no models of his on anything like a large intelligible scale had been provided for S.Peter's; and the only extant model attributable to Michelangelo's own period is that of the cupola.

This may help to account for the fact that, while the cupola was finished much as he intended, the rest of his scheme suffered a thorough and injurious remodelling.
He wrote to his nephew Lionardo on the 13th of February 1557 about the impossibility of meeting the Grand Duke's wishes and leaving Rome.

"I told his Lordship that I was obliged to attend to S.Peter's until I could leave the work there at such a point that my plans would not be subsequently altered.

This point has not been reached; and in addition, I am now obliged to construct a large wooden model for the cupola and lantern, in order that I may secure its being finished as it was meant to be.

The whole of Rome, and especially the Cardinal of Carpi, puts great pressure on me to do this.