part of compound eye morphogenesis mystery cell differentiation The process in which an undifferentiated cell acquires the features of a mystery cell. The mystery cells are a precluster of cells that emerge from the compound eye morphogenetic furrow, normally positioned between R3 and R4. They then disappear into the surrounding pool of undifferentiated cells and have no known fate in the mature ommatidium. An example of this process is found in Drosophila melanogaster. biological_process GO:0008583 cell differentiation