Answer:
Oogenesis
Step-by-step explanation:
In other words, oogenesis is the origin and development of the ovum. Ovum is the female reproductive cell or gamete of animals, which is capable of developing, usually only after fertilization, into a new individual.
Phases of Oogenesis:
1. Multiplication Phase:
The primary germinal cells of the ovary with diploid number of chromosomes (2n) divide several times mitotically so as to form a large number of daughter cells known as oogonia. (Attachment #1)
2. Growth Phase:
The oogonium does not divide but increases in size enormously to form a primary oocyte. The growth is associated with both nuclear and cytoplasmic growth. The nuclear growth is due to accumulation of large amount of nuclear sap and is termed as germinal vesicle. The cytoplasmic growth is associated with increase in number of mitochondria, endoplasmic reticulum and Golgi complex and accumulation of reserve food material called yolk or vitellin.
3. Maturation phase:
The primary oocyte undergoes two successive divisions by meiosis. The first division is meiosis-I and two unequal daughter cells are produced. The large cell is called secondary oocyte containing haploid (n) set of chromosomes (due to reductional or disjunctional division) and entire amount of cytoplasm. The smaller cell is called first polar body or polocyte containing ‘n’ number of chromosomes and practically no cytoplasm. The secondary oocyte and first polar body then undergo second maturation division by meiosis-II which is an equational division. As a result of this division one large ovum is formed containing entire amount of cytoplasm and ‘n’ number of chromosomes and a second polar body like the first polar body. Simultaneously, the first polar body may divide into two polar bodies may not divide at all. Thus only one functional ovum is formed and the two or three polar bodies soon degenerate. In vertebrates the first polar body is formed after the primary oocyte is released from ovary and has entered into the oviduct. The second polar body is formed only when the sperm enters into ovum during fertilization.