Fertilization of immature frog eggs: cleavage and development following subsequent activation.
Frog eggs are normally fertilized after reaching metaphase II. When eggs are inseminated prior to that, several sperm enter, but entry does not activate the egg. When such inseminated, immature eggs were maintained until they became mature and then were artificially activated, the eggs began to cleave. The cleavage furrows were irregular and often multiple, but the eggs developed to blastulae or partial blastulae. About 2 leads to 5% of the eggs developed to tadpoles. Typical asters were not associated with the entering sperm; rather, asters appeared only after activation. The sperm nucleus often formed chromosomes which were attached to small spindles. It is clear that sperm which remain for a time in unactivated egg cytoplasm, retain their ability to promote cleavage and development. Aster formation required not only sperm centrioles but also activated egg cytoplasm. Sperm which entered either near the equator or in the animal half of mature eggs usually produced normal cleavage furrows. Sperm which entered the animal half of immature eggs produced multiple animal half furrows when the egg was subsequently activated. In contrast, sperm which entered near the equator of immature eggs often failed to induce furrowing on subsequent activation or produced unusual equatorial furrows. The difference in the type of furrow between eggs inseminated in the animal half or at the equator is interpreted as a consequence of dissociating sperm entry from the cortical contraction which occurs in activation.