Of all the various liturgical accretions, the hymn is the oldest.  Hymns, texts using popular melodies and folk tunes, were designed to teach religious doctrine and were polemic in nature.  As a weapon against established dogma, their influence was great.

The true father of the Catholic hymn is St. Ambrose (340? - 397).  His goal was the combating of heresy.  Sung congregationally, his hymns immediatley became popular and soon were absorbed into the liturgy.

These elements are basic to the hymn:  1) written to be a musical weapon against heresy, 2) popular in both words and music, 3) musically simple, with a strophic technique, that is, each strophe sung to the same music, and 4) congregational performance as a primary goal.

In general, to the 8th century, the Ambrosian hymn was the model.  During this period, the steady absorption of the hymn into the Hours was constant, except in Rome, where there was resistance until the the 9th century.

Just as the hymn acted as a creative outlet within the Hours, so the sequence and trope acted withing the Mass.