Mature vitelline follicles consist of cells in various stages of development, progressing from immature cells of gonial type near the periphery to mature ones toward the centre. Maturation, completed before the cell leaves the follicle, is characterized by: increase in cell volume; increase in nuclear surface area restoring the N/C ratio; nucleolar transformation; extensive development of large parallel cisternae of granular endoplasmic reticulum, the shell-protein producing units; development of Golgi complexes engaged in shell globule formation; formation and storage of glycogen in the cytoplasm; simultaneous, independent formation and storage of intranuclear glycogen; progressive increase in the number and size of shell globule clusters; and disintegration of endoplasmic reticulum, degenerative changes, and accumulation of glycogen and shell globule clusters within the cytoplasm associated with a massive accumulation of glycogen in the nucleus. The functional significance of the large amount of nuclear and cytoplasmic glycogen and numerous shell globule clusters is analyzed. Vitellogenesis in G. catostomi is compared with that in other cestodes and trematodes. Some conclusions, concerning the interrelationship between the vitellogenesis pattern and the type of embryogenesis following it, are drawn and discussed.