They are carefully outlined, with the precision of a working architect; but the sculptural details bear the aspect of what may be termed a generic Florentine style of draughtsmanship.
Two important letters from Michelangelo to Fattucci, written in October 1525 and April 1526, show that he had then abandoned the original scheme, and adopted one which was all but carried into effect.

"I am working as hard as I can, and in fifteen days I shall begin the other captain.

Afterwards the only important things left will be the four rivers.

The four statues on the sarcophagi, the four figures on the ground which are the rivers, the two captains, and Our Lady, who is to be placed upon the tomb at the head of the chapel; these are what I mean to do with my own hand.

Of these I have begun six; and I have good hope of finishing them in due time, and carrying the others forward in part, which do not signify so much." The six he had begun are clearly the Dukes and their attendant figures of Day, Night, Dawn, Evening.