The process of formation of ovum is called oogenesis. It occurs in the ovary . Each ovary is lined by germinal epithelium. At the time of gamete formation, some cells of the germinal epithelium will become active and detach from the epithelium and then enters into the cortex start dividing mitotically to produce large number of primordial germ cells (2n). The primordial cells pass through three phases before they transform into the ovum.
1. Multiplication phase
The primordial germ cells undergo repeated mitotic divisions to produce large number of cells; each cell is called oogonia or egg mother cell (2n).
2. Growth phase
In this phase egg mother cells grow in size due to synthesis and accumulation of food materials and these enlarged oogonial cells are called primary oocytes (2n).
3. Maturation phase
The primary oocyte undergoes first meiotic division to produce two unequal sized cells, which are haploid (n), a large cell with more amount of cytoplasm is called secondary oocyte and a small cell with negligible amount of cytoplasm is called primary polar body (polocyte).
The secondary oocyte and primary polar body undergo second meiotic division which is also unequal as a result; secondary oocyte produces one large cell and a smaller cell; larger cell is called ootid which develops into ovum. The smaller cell called secondary polar body and primary polar body produces two secondary polar bodies. Thus, in oogenesis, primary oocyte produces one ovum and three polar bodies. The polar bodies disintegrate later
