The Debate on Embryo Development: Epigenesis vs Preformation

The topic of embryo development, also known as germination, sparked debate for almost two thousand years, from Aristotle's time until the 18th century. Aristotle proposed two conflicting theories: preformation and epigenesis, with each having their own proponents.

This is an image of a human embryo obtained from an ectopic pregnancy at nine weeks, or seven weeks post ovulation. Obstetrical dating of pregnancy begins from the first day of the last menstrual period, which typically occurs about two weeks prior to ovulation.

Preformation was based on a religious interpretation of creation, where at the time of conception, the embryo contained a complete set of organs that were too small to be visible and were located in either the mother's egg or the father's semen. It was believed that each organ grew in size during development. In the 17th century, preformationists proposed that the preformed germs of all plants and animals originated within the original parents of each species, suggesting that no new living beings were being created. This belief prevailed from around 1675 until the end of the 18th century.

On the other hand, Aristotle favored the theory of epigenesis, where each individual began as an undifferentiated mass in the egg and gradually differentiated and grew, with male semen providing the form or soul that guided this developmental process. However, epigenesis gained little support during the 17th century, despite being supported by William Harvey.

It wasn't until the 18th century that the German physiologist and embryologist Casper Friedrich Wolff revitalized the theory of epigenesis and became its leading advocate. By studying the chick embryo under a microscope, he saw no evidence supporting the preformation theory. Instead, he observed continuous growth and gradual development of the chick. In his doctoral dissertation in 1759, Wolff described that organs did not exist at the beginning of the generation process, but formed from undifferentiated matter through a series of incremental steps. He also demonstrated that a plant root could regenerate a new plant even after the stem and roots were removed, bolstering his arguments for epigenesis.

Despite facing controversy and damage to his career, Wolff's findings were later validated and served as the basis for the germ-layer theory in 1828.