(Taken from Notes Made at the Conferences of Dom Prosper Guéranger).
The four Prayers of the Offertory are not very ancient; it was formerly left to the option of the different churches, to choose their own formula of prayer for this moment; the Canon alone has undergone no local changes; it has always been the very same everywhere. Since Pope Pius V issued his Missal, which is the one now in use, nothing may be altered in any of the formulae accepted therein by him; but the variety of epoch from which the several prayers date, explains the vast difference observable in the Latin of their composition and in that of the Canon, which is far more beautiful.
The Priest having finished the Oblation Prayer, makes the sign of the cross with the Paten and places the host on the Corporal. This form of the cross expresses the identity existing between the Sacrifice of the Mass and that of Calvary. Next, the Deacon puts Wine into the Chalice, and the Sub-Deacon approaches to fulfil his office, which consists in putting the water into this same Chalice; this act is the highest of all his functions. The prayer which accompanies this ceremony is very ancient; it dates back as far as the first ages of the Church, and indeed it is easy to see that the Latin was a spoken language at the time it was composed. In it is strongly brought before us what is the importance, what the dignity of the Water here used in the Holy Sacrifice. Why is Water put in the Chalice? Because, according to Tradition, Our Lord Himself when instituting the Holy Eucharist, mixed Water with the Wine, as the abstemious are wont to do, and the Church continues to observe this custom. She avails herself of this opportunity to speak to us in wonderful language, unfolding to us sublimest mysteries.