The process of formation of a mature female gametes is called oogenesis.

Oogenesis is initiated during the embryonic development stage when a couple of million gametes mother cells are formed within each fetal ovary.

No more oogenia are formed and added after birth.

Primary oocytes : These cells start

division and enter into prophase-$1$ of the meiotic division and get temporarily arrested at that stage called primary oocytes.

Primary follicle : Each primary

oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle.

A large number of these follicles degenerate during the phase from birth to puberty.

Secondary follicle : The primary follicle get surrounded by more layers of granulosa and a new theca and are called secondary follicles.

Tertiary follicles : The secondary follicles soon transforms into a tertiary follicle which is characterised by a fluid called filled cavity called antrum. The theca layer is organised into an inner theca interna and an outer theca externa.

Now, the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of large haploid secondary oocyte and a tiny first polar body.

The secondary oocyte retains bulk of the nutrient rich cytoplasm of the primary oocyte.

The tertiary follicle further changes into the mature follicle or Graafian follicle

The secondary oocyte forms a new membrane called zona pellucida surrounding it.