283. What is the meaning of transubstantiation?Transubstantiation means the change of the whole substance of bread into the substance of the Body of Christ and of the whole substance of wine into the substance of his Blood. This change is brought about in the eucharistic prayer through the efficacy of the word of Christ and by the action of the Holy Spirit. However, the outward characteristics of bread and wine, that is the "eucharistic species", remain unaltered.
Further reading: CCC 1376-1377, 1413284. Does the breaking of the bread divide Christ?
The breaking of the bread does not divide Christ. He is present whole and entire in each of the eucharistic species and in each of their parts.
Further reading: CCC 1377