As recently as the 18th century, the prevailing notion in human embryology was preformation: the idea that semen contains an embryo - a preformed, miniature infant, or "''homunculus''" - that simply becomes larger during development.
The competing explanation of embryonic development was ''epigenesis'', originally proposed 2,000 years earlier by Aristotle.
According to epigenesis, the form of an animal emerges gradually from a relatively formless egg.
As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.
Modern embryological pioneers include Gavin de Beer, Charles Darwin, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham, while much early embryology came from the work of Aristotle and the great Italian anatomists: Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppia, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, Mauro Rusconi, etc. Other important contributors include William Harvey, Kaspar Friedrich Wolff, Heinz Christian Pander, Karl Ernst von Baer, and August Weismann.
After the 1950s, with the DNA helical structure being unravelled and the increasing knowledge in the field of molecular biology, developmental biology emerged as a field of study which attempts to correlate the genes with morphological change, and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.
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