Furthermore, the very idea of a church itself is descended from the early Christian meeting-places in the catacombs or at the tombs of the martyrs, which are universally allowed to have been the primitive Christian altars. {035}We know now that the cruciform dome-covered plan of Christian churches is derived from these early meeting-places at the junction of lanes or alleys in the catacombs; that the nave, chancel, and transepts indicate the crossing of the alleys, while the dome represents the hollowed-out portion or rudely circular vault where the two lines of archway intersect. The earliest dome-covered churches were attempts, as it were, to construct a catacomb above ground for the reception of the altar-tomb of a saint or martyr. Similarly with the chapels that open out at the side from the aisles or transepts. Etymologically, the word chapel is the modernised form of _capella_, the arched sepulchre excavated in the walls of the catacombs, before the tomb at which it was usual to offer up prayer and praise. The chapels built out from the aisles in Roman churches, each with its own altar and its own saintly relics, are attempts to reproduce above ground in the same way the original sacred places in the early Christian excavated cemeteries. We will recur to this subject at much greater length in subsequent chapters. Thus Christianity itself is linked on to the very antique custom of worship at tombs, and the habit of ancestor-worship by altars, relics, and invocation of saints, even revolutionary Protestantism still retaining some last faint marks of its origin in the dedication of churches to particular evangelists or martyrs, and in the more or less disguised survival of altar, priesthood, sacrifice, and vestments.