Final answer:
The ovary of a newborn girl contains about 1,000,000 primary oocytes at birth, but this number decreases over time due to natural processes.
Step-by-step explanation:
The ovary of a newborn girl houses about 1,000,000 primary oocytes. These oocytes are contained within ovarian follicles, which grow and develop through a process called folliculogenesis
It is during this process that approximately once a month after puberty, one follicle reaches maturation and results in ovulation. However, not all the oocytes will survive until maturity; they experience atresia or natural death, resulting in a decline to about 40,000 by the time a girl reaches puberty. This life-long decrease in oocyte count continues until menopause.