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.
With advancements in microscopy during the 19th century, increasing numbers of biologists could examine the embryo and see it changing as it moved through the different stages of development. Eventually the theory of preformation was abandoned in favor of the epigenesis concept.
Aside form Aristotle, much of the early work into embryology was performed by other great Italian anatomists including Leonardo da Vinci, Aldrovandi, Aranzio, Marcello Malpighi, Gabriele Falloppia, Emilio Parisanoand more. More recent pioneers of embryology include Charles Darwin, Gavin de Beer, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham.
A new lease of life was breathed into the field of embryology after the 1950s when the DNA helical structure was discovered. This increased knowledge in the area of molecular biology which led to the emergence of developmental biology. Developmental biology examines the correlations between genes and morphological changes that occur in the embryo.