How was the continual "Memory" of Christ's Sacrifice to be re-lived and made present in history and the flow of His Life in the Spirit to be maintained for each generation?
The bread used by Jesus at the Passover was unleavened bread because the ritual prescribed it thus, "That same night they shall eat its (the lamb) roasted flesh with unleavened bread...." (Ex 12:8) This served as a reminder that the Jews had left Egypt in a hurry with unleavened dough which they baked into bread. (Ex 12:39) This is one reason why the Roman Church to this day uses only unleavened bread for the Eucharist.
Something not in the ritual was added by Christ probably after the second cup of wine:
" Jesus took bread, said the blessing, broke it, and giving it to his disciples said,
'Take and eat; this is my body.'"
Probably with the third cup of wine, "Jesus took a cup, gave thanks, and gave it to them, saying,
'Drink from it, all of you,
for this is my blood of the covenant,
which will be shed on behalf of many for the forgiveness of sins.'"
Now they understood the manner through which Christ gave them his his "body and blood" to eat as he had told them,
"Amen, amen, I say to you,
unless you eat the flesh of the Son of Man and drink his blood,
you do not have life within you."
The question is often raised as to how Christ could give of his sacrificial humanity before e his actual death and resurrection. This is a mystery to which I can only give the insight others have proposed. The personal presence of Christ at the Last Supper was rooted in the timeless dimension of his divinity which encompassed the totality of his earthly life. Thus in the bread and wine Christ was truly present with the totality of his life even if his death-resurrection had not yet being realized in history.
Mystery of the Eucharist