Final answer:
Primary oocytes halt their development in prophase I and stay in this suspended state until puberty. After puberty, the first meiotic division is completed resulting in a secondary oocyte, which is again arrested until fertilization.
Step-by-step explanation:
Primary oocytes press the pause button on their development in prophase I, before they make their 1st meiotic divisions. They stay arrested at this stage of development until puberty, which is when the girl begins her first menstrual cycle.
During this time, the primary oocytes are in a period known as the first meiotic arrest within the ovarian follicles. At puberty, anterior pituitary hormones stimulate a few follicles to develop each month, leading one primary oocyte to resume and finish the first meiotic division.
This division results in an unequal allocation of cytoplasm, forming one large secondary oocyte and a smaller cell called a polar body, which generally degenerates. The secondary oocyte then undergoes a second meiotic arrest at the metaphase II stage until it's ovulated.
If fertilization occurs, the secondary oocyte completes meiosis II, results in a second polar body, and the production of a haploid egg which then combines with a haploid sperm to form a diploid zygote.