Human ovum. Colored scanning electron micrograph of a single human ovum (egg). Mature human ova develop in the ovaries from primordial follicles that although formed between the 4th and 7th month of fetal development, remain dormant until sexual maturity. In each ovarian cycle up to 20 primordial follicles are activated to form primary follicles; normally only one will reach full maturity and be released at ovulation.