Encyclopedia.Com: Our Sunday Visitor's Catholic Encyclopedia, pg. 437 The rite of committing consists of placing a small particle of the consecrated Bread into the chalice. This custom derives from the fermentum practice (literally, "leaven"). The fermentum was originally part of the Eucharistic Bread consecrated by the Pope in Rome (as attested in the letter of Pope St. Innocent 1 in A.D. 416 to Bishop Decentius of Gubbio) and then sent to the various tituli (parish churches) in the city to denote the unity of the one Catholic community despite their having to celebrate separate Eucharists.
Encyclopedia.Com: Our Sunday Visitor's Catholic Encyclopedia, pg. 255 The origins of this rite [liturgical commixture] are to be found in fermentum, during which a piece of the consecrated Bread was broken off and sent to be part of another Eucharistic celebration to show the essential unity of the Church in the Eucharistic Sacrifice. When this was no longer done, the piece was dropped into the chalice and medieval allegorical explanations were developed to explain the practice.
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Eusebius of Caesarea's Church History, Book 5, Chapter 24, Paragraph 17 But though matters were in this shape, they communed together, and Anicetus conceded the administration of the eucharist in the church to Polycarp, manifestly as a mark of respect. And they parted from each other in peace, both those who observed, and those who did not, maintaining the peace of the whole church.
