+PLANS.+ The remains of Byzantine architecture are almost exclusively of churches and baptisteries, but the plans of these are exceedingly varied. The first radical departure from the basilica-type seems to have been the adoption of circular or polygonal plans, such as had usually served only for tombs and baptisteries. The Baptistery of St. John at Ravenna (early fifth century) is classed by many authorities as a Byzantine monument. In the early years of the sixth century the adoption of this model had become quite general, and with it the development of domical design began to advance. The church of +St. Sergius+ at Constantinople (Fig. 74), originally joined to a short basilica dedicated to St. Bacchus (afterward destroyed by the Turks), as in the double church at Kelat Seman, was built about 520; that of +San Vitale+ at Ravenna was begun a few years later; both are domical churches on an octagonal plan, with an exterior aisle. Semicircular niches--four in St. Sergius and eight in San Vitale--projecting into the aisle, enlarge somewhat the area of the central space and give variety to the internal effect. The origin of this characteristic feature may be traced to the eight niches of the Pantheon, through such intermediate examples as the temple of Minerva Medica at Rome. The true pendentive does not appear in these two churches. Timidly employed up to that time in small structures, it received a remarkable development in the magnificent church of +Hagia Sophia+, built by Anthemius of Tralles and Isodorus of Miletus, under Justinian, 532-538 A.D. In the plan of this marvellous edifice (Fig. 75) the dome rests upon four mighty arches bounding a square, into two of which open the half-domes of semicircular apses. These apses are penetrated and extended each by two smaller niches and a central arch, and the whole vast nave, measuring over 200 × 100 feet, is flanked by enormously wide aisles connecting at the front with a majestic narthex. Huge transverse buttresses, as in the Basilica of Constantine (with whose structural design this building shows striking affinities), divide the aisles each into three sections. The plan suggests that of St. Sergius cut in two, with a lofty dome on pendentives over a square plan inserted between the halves. Thus was secured a noble and unobstructed hall of unrivalled proportions and great beauty, covered by a combination of half-domes increasing in span and height as they lead up successively to the stupendous central vault, which rises 180 feet into the air and fitly crowns the whole. The imposing effect of this low-curved but loftily-poised dome, resting as it does upon a crown of windows, and so disposed that its summit is visible from every point of the nave (as may be easily seen from an examination of the section, Fig. 76), is not surpassed in any interior ever erected. [Illustration: FIG. 77.--INTERIOR OF HAGIA SOPHIA, CONSTANTINOPLE.]