Eggs are contained within the ovaries, where they are surrounded by organized groups of other cells. These "functional units" of eggs and the cells that surround them are referred to as ovarian "follicles."
Within the first year of life (by 1 year of age), a female is thought to have contained all of her eggs within ovarian follicles. These immature early follicles are called "primordial follicles" which contain an immature egg surrounded by spindle shaped cells (which may eventually develop into the granulosa cells).
After puberty, collections of primordial follicles begin to undergo a series of developmental changes (which take place over a several month time frame) to become invested in multiple layers of granulosa cells. The number of primordial follicles that are recruited into development for any given month is unclear, but it appears that there are essentially "waves" of follicular development taking place continuously throughout the reproductive years.
The tonic (continuous) growth phase of egg development is characterized by a tremendous (greater than 600 fold) increase in the number of granulosa cells, a (greater than 15 fold) expansion in follicular diameter, and formation of an antrum (a cavity filled with follicular fluid). This tonic growth phase (also) takes place over a several month period and essentially presents a monthly cohort (collection) of small antral follicles for "FSH regulated growth."
A few days prior to the onset of menses, circulating FSH increases to begin to recruit the cohort of follicles within the ovaries that have (been prepared over the past several months to) become capable of further rapid development. These follicles essentially "race" toward ovulation, and generally only one of the follicles that undergo FSH regulated growth during a particular menstrual cycle will make it all the way to ovulation.
FSH containing fertility medications may overwhelm the normal physiologic barriers limiting multiple mature egg development during a particular cycle to allow ovulation of several mature eggs within a single cycle.
Eggs viewed with a microscope undergo a series of identifiable structural changes as they develop from an "immature egg" to a "mature egg" (one that is capable of fertilization).