Injection of endoplasm from large Xenopus oocytes which have undergone maturation into small (0.6-0.8 mm diameter) oocytes induces germinal vesicle (GV) breakdown. This process has been cytologically studied. Injection of cytoplasm from such small oocytes into large oocytes (1.0-1.2 mm diameter) is followed by the formation of a large spindle bearing many chromosomes at the site of injection.