Solution:
The process in the ovary that leads to production of haploid female gametes called ova from diploid oogonia is called as oogenesis. It is a discontinuous process that is initiated during foetal development and terminated during pregnancy.
Oogenesis occurs in the following three phases:
- Multiplicative phase –
· During this phase, the Follicle cells differentiate from the ovarian germinal epithelium due to repeated mitotic divisions.
Few of these follicle cells enlarge and are now called egg mother cells. The egg mother cells undergo mitosis to multiply and produce oogonia.
· Growth phase – One of the oogonia differentiates while the others change into surrounding nutritive follicular epithelium. The differentiated and isolated oogonium grows in size due to the nourishment provided by girdling follicle cells. As a result, it transforms into a
diploid primary oocyte.
· Maturation phase – The diploid primary oocyte undergoes two maturation
divisions.
- Meiosis I – It is the first meiotic division that splits the diploid primary oocyte into two haploid cells where the larger cell is the secondary oocyte and the smaller cel is the polar body(polocyte).
- Meiosis II- It is the second meiotic division in which the secondary oocyte splits forming one large ootid and a smaller second polar body. Further, the first polar body splits through mitosis to form two polar bodies and the ootid goes on to mature into a functional haploid ovum.
Therefore, one primary oocyte produces one large ovum and three polar bodies that go on to degenerate since they do not participate in reproduction. This leaves behind one functional ovum.