Changes Before Gregory the Great
Even before the time of Pope Gregory I (pontificate 590-604), we can see a number of trends which will influence the evolution of Western liturgy for the next few centuries. Although the books of Scripture were used for proclamation in liturgy from the earliest times, other texts and prayers were delivered from oral tradition or improvised according to generally accepted forms. In this period of history the texts become fixed and written down in various liturgical books. Another important trend is the spread of Roman liturgy to territories using Gallican liturgy and the mixing of Roman and Gallican elements. Although Roman liturgy during this period remains faithful to the traditional shape seen in the early sources such as the Apologia of Justin, the third trend we see is the modification and reordering of a number of secondary elements in the celebration of Eucharist.
The first liturgical book to develop is the sacramentary, a collection of prayers used by the presiding bishop or presbyter for Eucharist or other sacraments celebrated in the context of the Eucharist. Although such books are mentioned in the late fifth century, the oldest example that comes down to us is the Verona Sacramentary, sometimes called the Leonine Sacramentary because it was formerly believed to have been compiled by Pope Leo I. It is apparently a collection of libelli, booklets containing formularies for single celebration. It has been argued that the Verona was not a true sacramentary in the sense of a book used in the course of celebrating liturgy, but a private collection of libelli used as a reference. 
The formulary for each Mass (as the Roman celebration of Eucharist came to be called, form the Latin word, missa, dismissal) generally contains an opening prayer said before the readings, an offertory prayer said over the gifts before the Eucharistic Prayer begins, a prayer after communion, and sometimes a blessing prayer said over the people at the conclusion of the celebration. Note that with the exception of the last, these collect prayers are used at the conclusion of a liturgical action: the entrance procession, the bringing forward of the gifts of bread and wine, and the procession of people to the altar to receive communion. In addition to these short collect prayers, some of the formularies include Prefaces for the Eucharistic Prayer. The Verona formularies are for use in Roman presbytural liturgies, although the prayers are based on Papal models, most of them composed during the the fifth and sixth centuries. It is incomplete, lacking formularies for masses from January through April, including Easter.
The second and more influential of the early Roman sacramentaries is known as the Gelasianum Vetus, or Old Gelasian Sacramentary. It was compiled near Paris around 750 and contains a mixture of Gallican and Roman elements dating from around 650. It is similar to the Verona, but contains formularies for the entire year as well as the text of the Canon, the Roman form of the Eucharistic Prayer. Some formularies in this sacramentary contain two collects with the same apparent liturgical function, perhaps to provide an option for the celebrant. Although the only surviving copy of the Old Gelasian was written about fifty years after the leadership of Gregory I, most of its contents reflect a practice before his reforms. The Gelasian is divided into three parts according to the liturgical year.
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Reforms of Gregory I and His Successors
Sacramentaries directly influenced by Gregorian reforms are referred to as Sacrementaria Gregoriana. They are distinguished from the earlier sacramentaries by different arrangement of formularies without the three-part division. Eighth Century Gelasian sacramentaries are based on the Old Gelasian, but also contain ample Gregorian elements as well as more Gallican influence.
With the appearance of these earliest sacramentaries, the Western liturgy begins to show a characteristic that distinguishes it from Eastern liturgical traditions. Aside from scripture readings and chants, Eastern liturgies are generally composed of invariable texts. In contrast, Roman and other Western liturgies have a series of prayers that change depending on the feast and liturgical season; in addition to the variable Prefaces, the Roman Canon has variable phrases that change to reflect the specific images of the feast being celebrated. These characteristics reflect a Western taste for more variability and the desire to make the liturgy reflect the particular feast which it celebrates, compared with the Eastern tendency to prefer textual stability; these differences may be thought of as distinctions of liturgical style rather than a fundamental divergence in structure.
The Roman Canon reaches its mature form in these early sacramentaries, although they seem to represent a much older oral tradition of this prayer. The version quoted by St. Ambrose in 390 is substantially the same prayer recorded in the Gelasian Sacramentary. It appears that the Roman Canon was originally composed in Latin rather than being a translation of an older Greek Eucharistic prayer. It lacks the explicit Epiclesis (a prayer addressed to God the Father asking Him to send down the Holy Spirit to empower a sacramental action) in the Eastern sense. Without mentioning the Holy Spirit directly, the Roman prayer merely calls upon God to "bless" the gifts of bread and wine so that they may become the Body and Blood of Christ, while the priest extends his hands over them in a gesture of epiclesis.
Toward the end of the prayer a similar blessing is invoked upon the people who will receive the gifts as communion. This two-fold invocation over the gifts and assembly is known as the Roman "split epiclesis." In the Roman Canon, the Eucharist is portrayed as God's gift to the Church through Christ, and the gift of the assembly to the Father in Christ. In contrast to Eastern Eucharistic Prayers' emphasis on praise, the Roman prayer stresses the celebration of the local assembly and the gifts offered. 
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Changes in Secondary Liturgical Elements
In addition to Sacramentaries, other important liturgical books developed during this time. These include lectionaries containing citations of scripture readings for use in particular celebrations, graduals containing chants for the celebration of Eucharist, and ordines containing rubrics — the instructions for ritual action.
Even before Gregory I, secondary elements in the Roman celebration of Eucharist begin to show some change. Even as late as 426 the Roman liturgy began abruptly with the Scripture readings; St. Augustine describes an Easter Sunday Mass where the first scripture reading is preceded only by a simple greeting. The addition of a prayer before the readings found in the earliest sacramentaries indicates the initial development of introductory rites. By the sixth century the introductory rites were enriched with the Kyrie, a litany where each invocation is answered by the assembly by the Greek phases for "Lord have mercy" or "Christ have mercy." This addition is either a direct importation from the East or a reworking and shift of position of the intercessory prayers concluding the word service mentioned by Justin Martyr. The Kyrie found in the Gelasian Sacramentary is certainly intercessory in nature. Note that in the Kyrie we find the Greek language used in a Liturgy that is otherwise almost completely Latin.
The Gloria, a hymn of praise originating in the Eastern churches, is also added at this time. It is an example of psalmi idiotici, non-biblical texts composed in the style of psalmody. Earlier the Gloria was used in the Office; at first it was only used at Masses as a special sign of solemnity when the Pope presided. At its introduction its use in non-papal liturgy was limited to Easter Sunday. In later practice it was used at all Masses celebrated by a presbyter on Sundays and solemnities except during penitential seasons. The Gloria is always perceived as a festive addition; it is not replaced by another element when omitted.
The reforms of Gregory the Great (590-604) and his successors, such as Honorius I (625-638) and Gregory II (715-731) affected the secondary elements of the Roman Mass that give it a distinctive form. The Kyrie litany was stripped of its invocations, so that only the responses "Kyrie eleison" and "Christe eleison" remained, perhaps in order to shorten the time required for the introductory rites. The number of readings was fixed at two; an additional reading from the Old Testament now occurred only on special days. The singing of the Alleluia before the Gospel became a standard part of the Mass except in during Lent and on other penitential days; formerly it was used only during the fifty days of Easter. The joyful, paschal nature of the Alleluia must have seemed out of character with the more somber character of these penitential times. Although the Kiss of Peace had already been transferred to a position after the Eucharistic Prayer in the early fifth century, the preliminary rites of communion were reordered so that the Lord's Prayer was recited directly after the Canon, followed by the Kiss of Peace and fraction rite.
Later Sergius I (687-701) introduced the singing of the Agnus Dei to accompany the fraction rite — a litany, possibly of Byzantine origin, giving solemnity to the ritual of the breaking of the consecrated Bread before communion. Gregory II added Mass formularies (sets of collects and other variable texts proper to the feast ) for the Thursdays in Lent, which had been aliturgical up to that time.
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Reasons for Liturgical Reforms
The changes discussed so far came about for various reasons. The rise of various heretical groups caused bishops to become scrupulous about the orthodoxy of their prayer texts, so having a codified collection of written formulae assured them their collects were doctrinally sound. The rearrangement and addition of secondary liturgical elements reflected a Western comfortability with limited liturgical change and adaptation, in contrast to the Eastern tendency to hold fast to traditional forms. Perhaps the political instability brought about by barbarian invasions provided a sense of need for liturgical codification and careful ordering of worship.
Other changes in society and the Church influenced its liturgical practice. As more and more of the general population became Christian, the order of catechumens began to decline, so that infant baptism became the norm. The rise of monasticism is also evident during this period with its liturgical influence. Cloisters were built by the popes near Roman basilicas and these communities took charge of celebrations of the Divine Office bringing their monastic liturgical practices with them. In the city of Rome, two types of liturgy became more apparent — the city or parish liturgy used in general, and the more elaborate Papal liturgy used whenever the Pope presided.
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Evolution of Gallican Liturgies
Since many of the Roman sacramentaries were copied in Gallican areas, we must assume the the Roman liturgy was being used as a model in Gallican areas. Some Eastern influence is evident in both Gallican and Old Spanish liturgy, such as the introduction of the Trisagion before the readings in both of these rites. The trinitarian interpretation of this text emphasized orthodox belief in areas beset with heretical groups challenging the doctrine of the Trinity. In general, the Gallican liturgy welcomed liturgical innovation from outside sources in contrast to Rome which was much more conservative in adopting foreign elements. The Milanese liturgy developed introductory rites similar to, but in a slightly different order than the Roman rite. Milan maintained the older tradition of the Kiss of Peace before the preparation of the gifts and the use of the Old Testament reading during the celebration of Eucharist.
Adam, Adolf. Foundations of Liturgy: An Introduction to Its History and Practice. Translated by Matthew J. O'Connell. Collegeville, MN: The Liturgical Press, 1992.
Jungmann, Josef. The Mass of the Roman Rite: Its Origins and Development. II vols. translated by Francis A. Brunner. New York, 1951.
Metzger, Marcel. History of the Liturgy: The Early Stages. Translated by Madeleine M. Beaumont. Collegeville, MN, 1997.
Vogel,Cyrille. Medieval Liturgy: An Introduction to the Sources, trans. and revised by William Storey and Neils Rasmussen. Washington, DC, 1986
Wegman, Herman. Christian Worship in East and West. Translated by Gordon W. Lathrop. New York, 1985.
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 Cyrille Vogel, Medieval Liturgy: An Introduction to the Sources, pp. 38-46.
 For an analysis of the distinctive characteristics of the Roman Canon see: Herman Wegman, Christian Worship in East and West, pp. 130-132.