The developmental roles of factors associated with the nuclear complex of Halocynthia roretzi and Boltenia villosa oocytes were investigated by cutting mature oocytes into animal and vegetal merogons before and during GVBD. Animal and vegetal merogons were cultured in sea water until the GV cytoplasm had dispersed within the cytoplasm of control oocytes and then they were cross-fertilized and scored for their ablility to undergo normal development. Halocynthia oocyte fragments produced from the animal region of oocytes containing intact GVs exhibited a low frequency of polyspermy, a high frequency of fertilization and cleavage, and a high frequency of expressing an epidermal antigen, Epi-2. In contrast, merogons produced from the vegetal region of Halocynthia oocytes in which GVs were intact exhibited a high frequency of polyspermy, did not undergo cell division, and expressed a high frequency of Epi-2 expression. When vegetal fragments were produced after the dispersal of approximately 50-70% of the GV nucleoplasm, these merogons exhibited a low frequency of polyspermy, high frequencies of cell division (including the formation of epidermal layer), and in most cases expressed Epi-2. Vegetal Boltenia fragments produced during GVBD in some cases developed into larvae. These results suggest that the ascidian GV nucleoplasm may contain factors required for fertilization and cell division and that epidermal determinants reside in the oocyte cytoplasm.