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An ovum, also called an oocyte, is the female cell of the human germ. It is produced in the ovaries and is one of the largest human cells, with a diameter of about 0.1 mm. An oocyte contains 23 chromosomes, which contains genetic information and is necessary for the development of an embryo. An ovum is ready for fertilization when it passes through ovulation and leaves the fallopian tube. When fertilized by a sperm, it forms a zygote, which develops into an embryo and later into a newborn.