The Meaning of Shavuot and its Unique Offering

While Pesach and Shavuot are linked by the counting of the Omer, they differ radically in their relationship to chametz and matza: on Pesach chametz is forbidden and matza is compulsory, while on Shavuot we offer two loaves of chametz.  How are we to understand this?  A proper explanation of the significance of chametz and matza, analyzing the various sources that appear in the Torah, shows that these are all details of a complete, unified system.