The Basilica of St. John had its beginnings in the tetrapylon martyrion erected over the traditional location of St. John's tomb as early as around 300. Probably around 450 a cruciform church with a wooden roof was built on the site, incorporating the tetrapylon at its crossing. The western arm, with one or two narthexes, contained a nave and two aisles, while the eastern arm had four aisles and terminated in an apse. The church was rebuilt under Justinian I, with work beginning before 548 and completed prior to 565. The cruciform plan was maintained but the building was now covered with a series of six domes resting on massive piers. The western arm, longer than the others, consisted of two such bays, while the crossing, north, south, and east arms each had a single bay in a design described by Prokopios as closely resembling that of the Church of the Holy Apostles in Constantinople, which was also imitated later for the Basilica of San Marco in Venice. The nave and the transept were housed with six domes, so that the shape of the cross was evident. The church was flanked by an octagonal baptistery built in the 5th century and a domed, octagonal skeuophylakion (or sacristy) erected in the late 6th or early 7th century.