Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Between days 12 and 14 postinoculation, young, cellular fibrous polyps of collagenous tissue can be observed that develop into discrete fibroblastic polyps in the alveolar ducts (20, 21).
Category: