The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology (2015)
Theories of Germination
Aristotle (384–322 BCE), William Harvey (1578–1657), Casper Friedrich Wolff (1733–1794)
The development of the embryo, or germination, remained the subject of debate for almost two millennia from the time of Aristotle until the eighteenth century. Aristotle proposed two possible alternatives: preformation and epigenesis, with each having formidable advocates.
Preformation was based on a Biblical interpretation of creation. At the time of conception, the embryo had a complete set of organs that were too small to be seen and that were located in either the mother’s egg or the father’s semen. It was believed that during the process of development, each body part increased in size. In the seventeenth century, it was further proposed that the preformed germs of all plants and animals originated within the original parents of each species (i.e., Adam and Eve) and, hence, no new living beings were being created de novo. Preformation was the prevailing view from about 1675 until end of the eighteenth century.
Aristotle favored the theory of epigenesis: each individual began entirely new 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. Although supported by William Harvey, epigenesis gained little traction during the seventeenth century.
The German physiologist and embryologist Casper Friedrich Wolff revitalized the theory of epigenesis and became its leading advocate. Microscopically studying the chick embryo, he saw no evidence supporting the notion that a preformed miniature becomes enlarged. Instead, he saw continuous growth and gradual development of the chick. In his 1759 doctoral dissertation Theoria Generationis, Wolff described that organs of the body did not exist at the beginning of the generation process but formed from undifferentiated matter through a series of incremental steps. To bolster his arguments, he also showed that a plant root, despite its differentiated tissues, is capable of regenerating a new plant even after the stem and roots are removed. Wolff’s vigorous support of epigenesis and rejection of preformation only generated controversy about his theory in the scientific community, and damaged his personal career. His findings were later validated and served as the foundation for the germ-layer theory in 1828.
SEE ALSO: Spermatozoa (1677), Germ-Layer Theory of Development (1828), Embryonic Induction (1924).
A nine-week (seven weeks post ovulation) human embryo obtained from an ectopic pregnancy. In obstetrical practice, pregnancy is dated from the first day of the last menstrual period, which is about two weeks prior to the ovulation.