Mechanism of Maturation
The basic mechanism of maturation of ovum is more or less similar in all organisms. The primordial germ cells that enter into the ovary divide mitotically and produce oogonial cells. The oogonial cells grow in size and become primary oocyte. The primary oocyte undergoes the first meiotic division and forms two haploids cells-a secondary oocyte and a polar body.
The secondary oocyte undergoes the second meiotic division to produce an ovum and a polar body. In most vertebrates the second meiotic division is completed only outside the ovary after fertilisation. The polar body formed as a result of first meiotic division may divide again to produce two more polar bodies. All the polar bodies eventually degenerate. Thus the meiotic process in oogenesis results in one ovum from a single oogonium unlike spermatogenesis where each spermatogonial cell gives rise to four sperm.