preformationist

Preformationism is a historical doctrine in biology and embryology that holds that an organism exists in miniature within a reproductive cell or within the egg, and that development is essentially the enlargement of this preformed entity rather than the growth and differentiation of undifferentiated tissues.

The theory emerged in the 17th century with the advent of microscopy and sparked a major debate about how organisms develop. Preformationists were divided into two main camps: spermists, who argued that a homunculus—a tiny, fully formed human or animal—resided in the sperm, and ovists, who claimed that the egg contained a preformed miniature embryo. Early observers, including those who studied spermatozoa and ovarian structures, contributed to these competing views. In 1694 Nicolaas Hartsoeker popularized the homunculus concept in the sperm, helping to shape the popular imagination around preformation.

Over time, preformationism faced increasing criticism as embryology advanced. Beginning in the mid-18th century, advocates of epigenesis argued that the embryo forms and differentiates progressively from otherwise undifferentiated material, rather than simply expanding a preformed entity. Key figures in this shift included Caspar Friedrich Wolff, who proposed epigenesis, and later Carl Ernst von Baer, whose work further established developmental ordering and the growth of stereotypes from general to specific.

Today, preformationism is regarded as a historical episode in the history of science. It is cited as an example of how scientific theories evolve in response to new evidence and methodological advances, ultimately giving way to modern developmental biology and genetics.