During the present century oogenesis and maturation have been extensively studied in the Digenea. The earlier observations, however, were based on limited methods of histological and cytological techniques and were concerned principally with the general features of the germ-cells and with their chromosomes. Consequently, until recently, little information has emerged regarding the extra-nuclear components of oogonia and oocytes and the changes that these cells undergo during oogenesis. The earlier work on the Digenea is reviewed by Cable (1931), Anderson (1935), Rees (1939), and more recent research is cited in the present paper.