Explanation of the Prayers and Ceremonies of Holy Mass, Dom Prosper Guéranger - Introit

The ceremony of the Incensing completed, the Priest says the Introit.  Formerly, this was not done.  The Ordo of St. Gregory tells us, that the Priest vested in the Secretarium, and then went to the Altar, preceded by the Cross and Torches; during which time, the choir sang the Introit, which was longer than we now have it, for the entire Psalm was sung, and not merely one or two of its verses, with the Gloria Patri, as at present.  In like manner, it was the choir alone that took the remaining portions, which were to be sung during the Mass.  The custom of the Priest's reciting these several portions, originated with that of Low Mass, which custom was, at last, introduced into High Masses.

These remarks will explain how it is, that the ancient Missals differ considerably from those which are now in use.  They simply contain the Prayers: Collects, Secrets, Postcommunions, Prefaces, and the Canon.  They were called Sacramentaries.  Whatever was sung by the choir was inserted in the Antiphonarium, which now goes under the name of Gradual.  (Most of the chanted portions of the Mass are, really, nothing more than Antiphons; only, they have more notes than what ordinary Antiphons have.)  In more modern times, ever since Low Masses were introduced, our Missals contain everything that used, formerly, to be sung by the Choir; as also the Epistles and Gospels.

Both the Priest and the Choir make the sign of the Cross at the beginning of the Introit, because it is considered as the opening of the Readings.  In Masses for the Dead, the Priest makes the Cross over the Missal only.