Sculpture relies upon the contour, architecture upon the line.  The distinction is vital, and were it not for the number and importance of the exceptions, from Michael Angelo down to Alfred Stevens, one would think that the sculptor-architect would be an anomaly.  In describing the pursuits of Donatello and Brunellesco during their first visit to Rome, Manetti says that the former was engrossed by his plastic researches, “senza mai aprire gli occhi alla architettura.”  It is difficult to believe that Donatello had no eyes for architecture.  There are several reasons to show that later on he gave some attention to its study.  Like the Roman Tabernacle, the Niche on Or San Michele[80] is without any Gothic details.  Albertini mentions Donatello as its sole author, but it is probable that Michelozzo, who helped on the statue of St. Louis, was also associated with its niche.  It is a notable work, designed without much regard to harmony between various orders of architecture, but making a very rich and pleasing whole.  It is decorated with some admirable reliefs.  On the base are winged putti carrying a wreath; in the spandrils above the arch are two more.  The upper frieze has also winged cherubs’ heads, six of them with swags of fruit and foliage, all of exceptional charm and vivacity.  The motive of wings recurs in the large triangular space at the top; flanking the magnificent Trinity, three grave and majestic heads, which though united are kept distinct, and though similar in type are full of individual character.  This little relief, placed rather high, and discountenanced by the bronze group below, is a memorable achievement of the early fifteenth century and heralds the advent of the power and solemnity, the Terribilita of Michael Angelo.  Donatello’s aptitude for architectural setting is also illustrated by the choristers’ galleries in the Cathedral and San Lorenzo.  The former must be dealt with in detail when considering Donatello’s treatment of childhood.  As an architectural work it shows how the sculptor employed decorative adjuncts such as mosaic and majolica[81] to set off the white marble; he also added deep maroon slabs of porphyry and bronze heads, thus combining various arts and materials.  Having no sculpture, the Cantoria of San Lorenzo is perhaps more important in this connection, as it is purely constructive, while its condition is intact:  the Cathedral gallery having been rebuilt on rather conjectural lines.  In San Lorenzo we find the same ideas and peculiarities, such as the odd egg and dart moulding which reappears on the Annunciation.  The colour effects are obtained by porphyry and inlaid marbles.  But we see how much Donatello trusted to sculpture, and how indifferently he fared without it.  This gallery does not retain one’s attention.  There is a stiffness about it, almost a monotony, and it looks more like the fragment of a balcony than a Cantoria, for there is no marked terminal motive to complete and enclose it