The shift from verse to prose dates from the early 13th century. The Prose Lancelot or Vulgate Cycle includes passages from that period. This collection indirectly led to Thomas Malory 's Le Morte d'Arthur of the early 1470s. Prose became increasingly attractive because it enabled writers to associate popular stories with serious histories traditionally composed in prose, and could also be more easily translated. 
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--Midwest Book Review, Diane Donovan (über Verschwunden)