§ 2. However, while the _atrium_ of the Christian basilica is merely an outer court, the secular basilica, when planned, like the Basilica Ulpia at Rome, with direct relation to a _forum_, was a principal building in connexion with the _forum_, but not a building of which the _forum_ was a mere annexe. Further, when we begin to seek for a complete identification of the Christian with the secular basilica, we are met by the obstacle that the secular basilica had no fixed plan. If we try to trace any principle of development in its plan, we find that this development is directly inverse to that of the Christian basilica. The secular basilica, in earlier examples a colonnaded building with its central space open to the sky, became at a later time a roofed hall, either, as in the case of the basilica at Trier, without aisles, or, like the basilica of Maxentius or Constantine in the Roman forum, with a series of deep recesses at the side, the vaulted roofs of which served to counteract the outward pressure of the main vault. The Christian basilica, if it were a mere imitation of this type of building, would follow the same line of development; but, as a matter of fact, the highest type of Christian church is always a colonnaded or aisled building. And, even if the Christian apse derived its arrangement from the apse or apses which projected from the ends or sides of the secular basilicas, there is again a difference. The apse with its altar was the main feature of the interior of the Christian church: it was the place in which the chief rite of Christian worship was performed before the eyes of all. In the secular basilica the apse was devoted to special purposes which set it apart from the main business of the body of the building: it was an appendage to the central hall, not necessarily within view of every part of it. In fact, the relation of the apse to the main building was totally different in the two cases.