(a) Explain the process of oogenesis.
(b) What is the significance of fertilisation?
(c) What is spermiation?

(a) Oogenesis is the process of formation of ovum in the ovaries. The process begins in a 25 week old foetus (even before the birth). 

• Oogonium is the germ cell which undergoes several mitotic divisions to form primary oocytes. The primary oocyte starts to undergo meiosis I, but is arrested at prophase I. A female is born with millions of primary follicles with primary oocytes arrested at prophase I in her ovaries. Many follicles degenerate and rest starts developing at puberty.
• The follicular cells proliferate and form a stratified cuboidal epithelium. Such cells are known as granulosa cells. These cells secrete glycoproteins to form zona pellucida around the primary oocyte. The fluid-filled area, present between granulosa cells, combines to form a central fluid-filled space called the antrum. These are known as secondary follicles. In every month cycle, these secondary follicles develop into tertiary follicle under the influence of follicle-stimulating hormone and luteinising hormone. 
• Primary oocyte complete meiosis I within the tertiary follicle and  results in the formation of two haploid cells of unequal sizes within the tertiary follicle. One of the daughter cells that receive less cytoplasm forms the first polar body. The second daughter cell is secondary oocyte which undergoes meiosis-II. Secondary oocyte is arrested at metaphase stage of meiosis-II. Tertiary follicle matures into Graafian follicle.  LH surge causes rupture of Graafian follicle to release ovum (secondary oocyte arrested at metaphase II). This process is ovulation. 
• Meiosis-II is completed at fertilisation resulting the formation  of  a large ootid and small second polar body. If the fertilisation does not occur, the oocyte degenerates.

(b) Fertilisation is a process that leads to variability in the population and restores 
 diploidy.
(c) The process of release of sperms from seminiferous tubules is called spermiation.