Small pieces of the consecrated bread from the episcopal Mass, called fermentum (leaven), were sent to parish churches and placed in the consecrated wine at the eucharist to signify the unity of the Christian assembly with their bishop. This practice dates from the fifth century in Rome. It continues today in some dioceses of the Episcopal Church on Maundy Thursday. A fragment of bread from a eucharist celebrated by the bishop during Holy Week is placed in the cup after the eucharistic prayer at the evening parish liturgy. A related custom is called commixture, in which the presider places a fragment of consecrated bread in the cup at every eucharist. It is infrequently practiced in the Episcopal Church today. See Commixture.
Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from "An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians," Don S. Armentrout and Robert Boak Slocum, editors.