Periodic changes in the rigidity of activated anuran eggs depend on germinal vesicle materials

The eggs of Xenopus laevis and Bufo bufo japonicus matured in vitro by progesterone were denuded and activated by electrical shock or pricking, and their diameters were determined periodically until the postactivation time equivalent to the 8- to 16-cell stage. Normally matured eggs showed a quick rounding-up and flattening immediately after activation (activation response), followed by a periodic flattening and rounding-up (postactivation response) corresponding to the intervals of early cleavage cycles. When denuded eggs that were induced to mature after removal of the germinal vesicle (GV) were activated, they showed a normal activation response, but retained the rounded-up state without exhibiting the periodic postactivation changes. Injection of GV materials back into enucleated Bufo oocytes and determination of the postactivation flattening 120 min after activation revealed that the extent of flattening correlated well with the amount of GV materials injected, provided injection was made before the first polar body was emitted. Thus, postactivation flattening, but not activation flattening, is an active property of eggs established under the influence of the GV and is linked to the periodical changes of rigidity during early cleavage cycles.