It is not the province of this essay to enter into a discussion of the origin of the ambulatory and its introduction into the church plan.[402] It is sufficient to note that a passage around a semicircular apse appears even in Roman times in the imperial tribune of the so-called stadium of Domitian on the Palatine at Rome which dates from the second century A.D.,[403] and that a similar passage was added around the apse of San Giovanni in Laterano by Pope Sergius II (844-845).[404] Such ambulatories were mere service galleries, not directly connected with the apse and in fact shut off from it by a solid wall, but when once adopted as a feature of the church plan, the ambulatory rapidly became an aisle around the apse corresponding in all respects to that which flanked the rectangular nave or choir.[405] It was natural, therefore, that this added aisle should have been vaulted and such is the case in the two earliest ambulatories of any size which still exist, namely, those in Santo Stefano at Verona (end of tenth century) and the cathedral of Ivrea (973-1001 or 1002),[406] while the early ambulatories in France, like those of Saint Martin at Tours (end of eleventh century) and the cathedral of Clermont-Ferrand, which have unfortunately been destroyed, were doubtless also vaulted. ORIGIN OF AMBULATORY VAULTING That the form which such vaulting assumes owes its origin to that of the concentric aisles in earlier buildings of circular plan would seem a most natural supposition since the problems in the two cases were precisely alike. As a matter of fact, a comparison shows that all or nearly all the methods of vaulting developed in the Roman or Byzantine period for the aisles of circular buildings were tried by the Romanesque builders when they added an ambulatory to the semicircular apses of their churches. ANNULAR TUNNEL VAULTS