Theoritical explanation of oogenesis
The diploid germ cells that have the potential to develop into ova are called oogonia. In humans, all of a female's oogonia that she will make in her lifetime are created when she's still a fetus and hasn't even been born yet. In fact, about one or two months before a baby girl is born, most of her approximately seven million oogonia die, and the remaining surviving oogonia enter meiosis I and become primary oocytes. These primary oocytes press the pause button on their development in prophase I, after they've replicated their genomes, but before they've made the first meiotic division. They stay arrested at this stage of development for over a decade until the girl begins her first menstrual cycle. Then, for about the next 30 to 45 years, on a monthly basis, primary oocytes resume meiosis where they left off and complete the first meiotic division.
When the primary oocyte does finally complete its first meiotic division, it divides the chromosomes evenly, just as you would expect. However, it does not divide its cytoplasm equally. Almost all of the cytoplasm remains in one of the two daughter cells, which becomes a secondary oocyte. The other daughter cell, which gets half of the chromosomes but very little cytoplasm, is called a polar body. The polar body is not a functional oocyte, instead it degenerates and dies. The formation of a polar body allows the primary oocyte to reduce its genome by half and conserve most of its cytoplasm in the secondary oocyte.
The secondary oocyte still has two copies of each chromosome, so if it's going to become a fully-functional ovum, it must undergo the second meiotic division. This division is also uneven, like the first one, with half of the chromosomes going to another very small degenerate polar body and half of the chromosomes being retained by the ovum along with almost all of the cytoplasm. In this way, the ovum achieves its haploid state while conserving as much cytoplasm as possible
The diploid germ cells that have the potential to develop into ova are called oogonia. In humans, all of a female's oogonia that she will make in her lifetime are created when she's still a fetus and hasn't even been born yet. In fact, about one or two months before a baby girl is born, most of her approximately seven million oogonia die, and the remaining surviving oogonia enter meiosis I and become primary oocytes. These primary oocytes press the pause button on their development in prophase I, after they've replicated their genomes, but before they've made the first meiotic division. They stay arrested at this stage of development for over a decade until the girl begins her first menstrual cycle. Then, for about the next 30 to 45 years, on a monthly basis, primary oocytes resume meiosis where they left off and complete the first meiotic division.
When the primary oocyte does finally complete its first meiotic division, it divides the chromosomes evenly, just as you would expect. However, it does not divide its cytoplasm equally. Almost all of the cytoplasm remains in one of the two daughter cells, which becomes a secondary oocyte. The other daughter cell, which gets half of the chromosomes but very little cytoplasm, is called a polar body. The polar body is not a functional oocyte, instead it degenerates and dies. The formation of a polar body allows the primary oocyte to reduce its genome by half and conserve most of its cytoplasm in the secondary oocyte.
The secondary oocyte still has two copies of each chromosome, so if it's going to become a fully-functional ovum, it must undergo the second meiotic division. This division is also uneven, like the first one, with half of the chromosomes going to another very small degenerate polar body and half of the chromosomes being retained by the ovum along with almost all of the cytoplasm. In this way, the ovum achieves its haploid state while conserving as much cytoplasm as possible
