The following classification of ovarian tumors has an embryologic foundation, with the cellular background of the tumors determined in embryonic life and their differentiation into the various types determined by the genetic potentialities of their cells of origin. This results in attempts by the tumor to recapitulate in morphology the various stages in the development of the urogenital ridge, namely, the mullerian apparatus, germ cells, primitive gonads, and adrenals.
The genital system first appears in the fifth to sixth week of embryonic life as an undifferentiated structure, with the occurrence of a localized thickening, the urogenital ridge, on each side of the dorsal mesentery bulging ventrally into the coelom. This common fold subdivides longitudinally into a more lateral mesonephric ridge and a medial genital ridge. Both sexes develop a pair of female ducts, the mullerian ducts, as the result of a groove developing in the thickening epithelium of