Ovum

Human ovum is barely visible to the naked eye.

Source: tatoeba (1020598)

Pregnancy only happens if the ovum is fertilized.

Source: tatoeba (1395479)

In short it was only after many failures, and after many barren epochs during which, for a variety of reasons, the enterprise was neglected, that the Fourth Men did at length fashion two individuals almost precisely of the type they had originally designed. These were produced from a single fertilized ovum, in laboratory conditions. Identical twins, but of opposite sexes, they became the Adam and Eve of a new and glorious human species, the Fifth Men.

Source: tatoeba (10606594)

The ovaries develop with the growth of the female, so that, finally, at the pubescent period, they ripen and liberate an ovum, or germ vesicle, which is carried into the uterine cavity through the Fallopian tubes.

Source: tatoeba (11641397)