The egg is a storehouse of material necessary to initiate growth. In all eggs, the meiotic divisions are highly asymmetric with regards to apportioning the cytoplasm, since the mature egg itself needs to retain a large fraction of the original cytoplasm. In vertebrates, the first arrest is broken by a maturation signal. Experiments with amphibian oocytes suggested that a factor in the cytoplasm can induce maturation. We will discuss it further in connection with cleavage.
Fertilization triggers completion of meiosis in most vertebrates.
Fertilization breaks the second meiotic arrest wherever it has occurred. The common mechanism for release from the various checkpoints is release of calcium. However, the mechanism is not totally understood. When the arrest is in M phase of meiosis II, the immediate effect is ejection of the second polar body. This can be a significant problem, especially for some marine dwelling animals where sperm are released into a very large volume of water.