Each oogonium grows in size by taking nutrients. Its nucleus is also enlarged in size. In this condition the cells are called primary oocytes.

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.

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.

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.

The next narrow and long part of the middle section is the tail. There are two centrioles at the front of the tail. Axial filaments arise from a centriole and extend to the tip of the tail. Mitochondria are present along the axial nucleus. The tail is covered by a sheath.

The next widest part of the gribar is called the middle piece. It contains ring centrioles.

The narrow and constricted part behind the sperm head is called the neck. It has two centrioles. Proximal centriole and distal centriole.

The oval or oval part in front of the sperm is called the head. It is covered by a plasma screen. It has such a large nucleus. Nucleus contains 23 chromosomes. At the front of the head is the cap-like acrosome. Acrosome is composed of Golgibodies and filled with hydrolytic enzymes.

Human sperm consists of 4 parts. namely-
1. Head: The oval or oval part in front of the sperm is called the head. It is covered by a plasma screen. It has such a large nucleus. Nucleus contains 23 chromosomes. At the front of the head is the cap-like acrosome. Acrosome is composed of Golgibodies and filled with hydrolytic enzymes.
2. Neck: The narrow and constricted part behind the sperm head is called the neck. It has two centrioles. Proximal centriole and distal centriole.
3. Middle piece: The next widest part of the gribar is called the middle piece. It contains ring centrioles.
4. Tail : The next narrow and long part of the middle section is the tail. There are two centrioles at the front of the tail. Axial filaments arise from a centriole and extend to the tip of the tail. Mitochondria are present along the axial nucleus. The tail is covered by a sheath.

The shape of spermatozoa in animals is species specific. Human sperm are very small, narrow, long and frog-like. It is about 50 microns in length and 2.5 microns in diameter.