The word Oogenesis is formed from the Greek words Oon meaning egg and genesis meaning origin. It means egg production. The process by which an egg is produced in the ovary of a sexually fertile woman is called oogenesis.
Oogenesis is a continuous process. It is described under 4 headings-
1. Multiplication phase: Ovary contains primary germinal cells. Primary germinal cells divide repeatedly to form new cells in the process of mitosis. As a result, the number of cells increases. This is called the numerical growth phase. The resulting cells are called oogonia or egg mother cells.
2. Growth phase: Each oogonium grows in size by taking nutrients. Its nucleus is also enlarged in size. In this condition the cells are called primary oocytes.
3. Maturation phase: Primary oocyte divides in the process of meiosis to form haploid cells. Each primary oocyte produces two unequally shaped haploid cells in the process of meiosis. The larger one is called the secondary oocyte and the smaller one is called the primary polar body. Then by meiosis division one oocyte and one polar body are produced from the secondary oocyte and two polar bodies from the first polar body. That is, from a primary oocyte a ootide and 3 polar bodies are formed.
4. Metamorphosis phase: The ovoid transforms into an egg. Polar bodies are destroyed.