Until as recently as the 18th century, the belief that an embryo exists as a pre-formed entity contained inside semen was common.
This concept, referred to as “preformation” held that a pre-formed, miniature human or “homonculus” was planted in the female during intercourse, which then grew into a larger being as it developed during pregnancy. The competing theory for the development of embryos, epigenesis, was proposed by Aristotle 2,000 years previously. Epigenesis refers to the unfolding development of an egg or seed through cell division and the formation of organs