The process by which the growing embryo gets implanted in the uterine wall is called implantation. Within 6-9 days of fertilization, the embryo is implanted in the uterine wall. The embryo is implanted in the uterus by the action of trophoblast cells.

1. Fertilization results in the formation of a zygote.
2. Characteristics of parents are transferred to offspring through fertilization.
3. The diploid number of organisms is established.
4. Ovarian protein synthesis and metabolism rate increases.
5. New arrangements of genes occur.
6. The sex of the fetus is determined.
7. Ensures continuity of lineage protection.
8. Variations in organisms are created.
9. Evolution occurs in the living world.

Fertilization is a complex process. Fertilization occurs when the secondary oocyte and spermatozoon meet in the fallopian tube of the female body. The stages of fertilization are:
1. Hyaluronidase enzyme is secreted from sperm acrosome. This enzyme creates a pathway for sperm to travel to the egg.
2. The spermatozoon travels to the zona pellucida of the egg. Later, the sperm collector and egg collector form a bond.
3. When the bond is formed, the sperm is stimulated and an enzyme is released from the sperm head. This enzyme breaks down the zona pellucida of the ovum to create an entrance. Through the entrance, the sperm head enters the egg.
4. Lysozyme enzyme is released from the egg as soon as the sperm head enters the egg. This enzyme hardens and thickens the zona pellucida. As a result, no more sperm can penetrate through the zona pellucida.
5. After sperm penetration, the secondary oocyte divides to form the ovum and polar body. Polar bodies are destroyed. The sperm and egg nuclei are swollen. It is called pronucleus.
6. Pronuclei of sperm and egg fuse to form zygote.

The process by which sperm and egg unite to form a zygote is called fertilization. Human fertilization occurs in the woman’s fallopian tubes. 1.5-4.0 ml of semen is secreted each time and contains about 40-100 million (4-10 crore) spermatozoa. Only one of these sperm unites with the egg. If fertilization does not occur within 12-24 hours of intercourse, the sperm die.

1. Gamete formation: Sperm and ovum are produced through gametogenesis. It causes sexual procreation.
2. Maintenance of chromosome parity: Diploid zygote is formed by the union of haploid sperm and egg. It maintains the equality of chromosomes in organisms.
3. Crossing over: Crossing over occurs between non-sister chromatids during cell division.
4. Protection of heredity: The heredity of organisms is preserved through gametogenesis.
5. Variation: Gametogenesis causes variation in organisms.
6. Evolution: Gametogenesis has a role in evolution.

Cytoplasm of egg is called ooplasm. Euplasm contains different types of cells. Most eggs contain yolk or deuteroplasm as a stored food. Mammalian eggs are microscopic because they do not contain deuteroplasm. Hence human ovum is of alecithal or microlecithal type.

Human ovum is covered by 3 membranes. namely-
(i) Corona radiata: The outermost layer of ovum is called corona radiata. In amphibians it is jelly-like. In birds it is complex in nature and has four layers.
(ii) Zona pellucida: The inner layer of corona radiator is called zona pellucida. It has a protective coating.
(iii) Vitelline: The innermost layer of the envelope is called vitelline. It covers the cytoplasm.

1. Membrane: Human ovum is covered by 3 membranes. namely-
(i) Corona radiata: The outermost layer of ovum is called corona radiata. In amphibians it is jelly-like. In birds it is complex in nature and has four layers.
(ii) Zona pellucida: The inner layer of corona radiator is called zona pellucida. It has a protective coating.
(iii) Vitelline: The innermost layer of the envelope is called vitelline. It covers the cytoplasm.
2. Ooplasm: Cytoplasm of egg is called ooplasm. Euplasm contains different types of cells. Most eggs contain yolk or deuteroplasm as a stored food. Mammalian eggs are microscopic because they do not contain deuteroplasm. Hence human ovum is of alecithal or microlecithal type.
3. Nucleus: Ovum may have one or more nuclei. The nucleus contains 23 haploid chromosomes.

The gamete or gamete produced in the ovary of a sexually fertile female is called an ovum.

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.