The question now remains, whether the semi-spherical shell was abandoned during Michelangelo's lifetime and with his approval.
There is good reason to believe that this may have been the case: first, because the tambour, which he executed, differs from the model in the arching of its windows; secondly, because Fontana and other early writers on the cupola insist strongly on the fact that Michelangelo's own plans were strictly followed, although they never allude to the third or innermost vault.

It is almost incredible that if Della Porta departed in so vital a point from Michelangelo's design, no notice should have been taken of the fact.

On the other hand, the tradition that Della Porta improved the curve of the cupola by making the spring upward from the attic more abrupt, is due probably to the discrepancy between the internal aspects of the model and the dome itself.

The actual truth is that the cupola in its curve and its dimensions corresponds accurately to the proportions of the double outer vaulting of the model.
Taking, then, Vasari's statement in conjunction with the silence of Fontana, Poleni, and other early writers, and duly observing the care with which the proportions of the dome have been preserved, I think we may safely conclude that Michelangelo himself abandoned the third or semi-spherical vault, and that the cupola, as it exists, ought to be ascribed entirely to his conception.

It is, in fact, the only portion of the basilica which remains as he designed it..