The first great distinction, then, between the second-century façades of Petra and the third-century façade of Ctesiphon is that the mock architecture at Petra is organically coherent, whereas at Ctesiphon it is incoherent, i.e. it is a pattern covering the wall-face rather than a simulation of plastic construction. The second great distinction is the systematic use of the archivolt at Ctesiphon for all the secondary intercolumniations in the wings. It is perhaps not without importance to observe that the same change from architrave to archivolt took place, though at a rather later date, in the stone-building regions of western Asia. In Syria, for example, the arched window almost entirely replaced the rectangular window in the course of the fifth century.[347] In the lower and central zones of Ctesiphon the arches are framed by groups in a rectangle composed of engaged piers and architraves; in the upper zone this system is abandoned. The principle of the arched niche within a rectangular frame appears, as has been seen, as early as Assyrian stelae, but for the use of the motive in a continuous series upon the façade there is, so far as I am aware, no example earlier than the Tabularium.[348] In the Augustan age it is found upon the Porta Praetoria at Aosta,[349] and thenceforward it governs the decorative scheme of Roman city gateways. Whether it was derived from Hellenistic Alexandria, together with the whole city gateway type, as Schultze surmises;[350] or whether it was evolved out of such wooden superstructures as gave birth to the decoration upon the Etruscan gates at Perugia;[351] or whether it was a specifically Roman (Stadtrömisch) conception, it is impossible to say. Nor does it signify. We know it as Roman, not only in the gateways, but also in the theatres and amphitheatres of the Roman empire, and I cannot doubt that the perfected Roman scheme is at least as directly responsible for Mesopotamian wall-surface decoration as is the western Asiatic development of Hellenistic façades. The gateway at Aosta, the Storied tomb at Petra, may well be taken as representing the immediate progenitors of Ctesiphon. Five hundred years later, in round figures, comes Ukhaiḍir--five hundred years of architectural growth and of fairly continuous intercourse with the West. The architrave has vanished from the principal orders; it is retained only to form the old rectangular framework for the small niches at the top of the wall. Symmetry and organic cohesion rule over the two lower zones. But in the details of its composition there is nothing at Ukhaiḍir which might not have been foretold from the façade of Ctesiphon.