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A bewildering array of causative factors for cancer have been found. They range from purely environmental, such as diet or environmental substances, over radiations of all types, carcinogens and mutagens (Berenblum, 1974) up to viruses (Bishop, 1978) and transforming factors (Todaro et al., 1981).

In Drosophila genetic factors cause malignant and benign neoplasms (Gateff, 1978a,b,c). In addition, compactly growing lethal tumors have been obtained from eye-antennal imaginal discs during serial subculture in the abdomens of female flies (Gateff, 1978a,b). The following tumor types have been found: (1) lethal-benign imaginal disc neoplasms, (2) malignant neuroblastomas, (3) malignant blood cell neoplasms, and (4) a benign gonial cell neoplasm. The fine structure of some of the above tumors has been studied and compared to the fine structure of corresponding wild-type cells. These will be discussed below.