Oogenesis starts with the division of oogonia (gamete mother cells) giving rise to primary oocyte which enters into prophase - I of the meiotic division and gets temporarily arrested at this stage. These primary oocyte gets surrounded by primary, secondary, and tertiary follicles respectively. The tertiary follicle grows in size and completes its first meiotic division to give rise to the haploid secondary oocyte. This secondary oocyte forms a new membrane called zona pellucida surrounding it. During fertilization, the sperm enters into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This induces the completion of the meiotic division (2^nd division) of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid).