**The Second Maturation Division of the Mammalian Ovum**

The second maturation division of the mammalian ovum, also known as meiosis II, occurs after fertilization. Once the sperm successfully penetrates the egg, it triggers a series of events that lead to the second maturation division. This division is crucial for the formation of a diploid zygote, which will eventually develop into an embryo.

**Events Leading to the Second Maturation Division:**

1. **Activation of the Egg:** Upon fertilization, the release of calcium ions within the egg cytoplasm activates various enzymes and metabolic processes. This activation prepares the egg for further development and initiates the second maturation division.

2. **Resumption of Meiosis II:** Fertilization triggers the resumption of meiosis II, which had been arrested in metaphase II. The second meiotic spindle reforms, and the chromosomes align along the equatorial plane.

**Stages of the Second Maturation Division:**

1. **Anaphase II:** In this stage, the centromeres split, separating the sister chromatids. The microtubules of the spindle fibers shorten, pulling the chromatids towards opposite poles of the cell.

2. **Telophase II:** The separated chromatids reach the opposite poles and become individual chromosomes. Nuclear envelopes begin to form around each set of chromosomes, creating two distinct nuclei.

3. **Cytokinesis:** Following telophase II, the cytoplasm of the cell divides through cytokinesis. This process results in the formation of two daughter cells, each containing a haploid set of chromosomes.

**Significance of the Second Maturation Division:**

The second maturation division is essential for the formation of a diploid zygote. During fertilization, the fusion of the sperm and egg nuclei restores the diploid number of chromosomes in the zygote. The second maturation division ensures that each gamete contributes only a single set of chromosomes to the zygote, preventing an excessive number of chromosomes in the offspring.

In conclusion, the second maturation division of the mammalian ovum occurs after fertilization. It involves the resumption of meiosis II, followed by the separation of sister chromatids and the formation of two daughter cells. This division is crucial for the formation of a diploid zygote, ensuring the appropriate number of chromosomes in the offspring.

In females, oogenesis starts at embryonic age and forms millions of oogonia. These cells start dividing and temporarily arrested at prophase-1 called primary oocyte. these oocytes acquire granulosa cells and form primary follicles which degenerate from birth to puberty. Further, they form secondary and tertiary follicle. When tertiary follicles grow in size it completes its first meiotic division and releases first polar body and secondary oocyte. Secondary oocyte completes its second meiotic division after the entry of sperms and releases second polar body.