Penetration is when the sperm and egg meet and the sperm makes it way into the egg cytoplasm. The rest degenerate. In mammals, the first sperm penetration triggers a response that prevents further sperm entry. In mammals, activation also results in the second mitotic division. In other animals, meiosis is complete at this stage.
Fertilization is complete when the sperm and egg nuclei fuse.
In isolecithal eggs , resulting cells are all roughly the same volume. Cells of varying volume result from cleavage of telolecithal cells. Even holoblastic cleavage is seen in agnathans. Uneven holoblastic cleavage is seen in amphibians and some fish. This type of cleavage is seen in birds, reptiles and some fish.