The process of oogenesis occurs in female gonad called ovary. Oogenesis is also completed in three phases:
1) Multiplication phase
2) Phase of Growth
3) Maturation phase
1) Multiplication phase :
Germinal cells undergo mitosis to form large number of oogonia. Oogonia in human beings are formed in ovary of female baby even before her birth.
2) Phase of growth :
Just before puberty, under the influence of follicle stimulating harmone, one of the oogonium grows in size. Growth in size of oogonium is larger than that seen in spermatogenesis. This grown up cell is called Primary Oocyte.
3) Maturation phase :
Primary oocyte undergoes maturation or meiotic division. Meiotic I division of primary oocyte shows equal nuclear division but unequal cytoplasmic division. So at the end of meiosis I division, large sized haploid secondary oocyte and haploid small sized polar body are formed. Unequal division is meant for sufficient supply of food for developing embryo.
Secondary oocyte and polar bodies undergo II meiotic division. II meiotic division is arrested at metaphase stage and secondary oocyte is released from ovary. Remaining part of division is completed at the time of fertilization. This division is also unequal by which and functional female gemete ovum is formed. This ovum is ready for fertilization.