Category: Biology Second Paper

Stages of embryogenesis are cleavage, gastrulation and organogenesis.
1. Cleavage: The process by which the zygote divides to form a multicellular embryo is called cleavage. As a result of division, the zygote becomes a sterile sphere with 16-32 cells. This is the marula stage and the cells are called blastomeres. The process of morula formation is called morulation. The cells of the marula dasa are arranged in a layer and a cavity filled with fluid is formed inside. This stage is called blastula. The cell layer of the blastula stage is called the blastoderm and the cavity is called the blastocoel. Cleavage process ends with the development of blastula stage.
2. Gastrulation: The process by which gastrula stage is formed from blastula is called gastrulation. In this process, the cells change rapidly and are arranged in two layers. The outer one is called ectoderm and the inner one is called endoderm. Later mesoderm is formed from ectoderm. A fluid-filled cavity forms inside the gastrula. It is called archenteron. The opening through which the archenteron is released is called the blastopore.
3. Organogenesis: The process by which various organs are formed from the embryonic stage is called organogenesis. A specific cell mass forms a specific organ. First, rudimentary buds are formed from the embryonic layer. Later, specific organs are formed from the organelles.

The process by which the zygote divides to form an embryo is called embryogenesis. Stages of embryogenesis are cleavage, gastrulation and organogenesis.

1. After fertilization the zygote divides by cleavage process to form blastomere. This condition is called Marula.
2. The morula enters the uterus and transforms to form a blastocyst. The outer layer of the blastocyst is called the trophoblast and the inner layer is called the blastocyst.
3. Within two days the blastocyst attaches to the endometrial epithelium of the uterus.
4. As the blastocyst attaches to the uterus, the trophoblast divides rapidly to form two layers. The inner layer is called cyto-trophoblast and the outer layer is called syncytio-trophoblast.
5. Syncytio-trophoblasts rapidly infiltrate the endometrial epithelium and endometrial stroma layers of the uterus.
6. Within seven days the endometrium of the uterus covers the blastocyst. As a result, the implantation is completed.
7. Trophoblasts divide rapidly to form numerous chorionic villi and amnion sheaths in the uterine endometrium.

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.