ovists

Ovists were early modern embryologists who argued that the embryo exists preformed within the ovum, meaning that fertilization merely activated development of an already formed organism. The term is used by historians to describe proponents of the ovist side of the preformationist debate in the 17th and 18th centuries. They contrasted with spermists, who maintained that the embryo is preformed in the sperm and that the egg mainly provides nourishment.

In this period, scientists observed eggs from birds and other animals with increasingly capable microscopes, leading to different interpretations of how reproduction and development occur. The ovist position was associated with the claim that the egg contains a preexisting miniature form that grows after fertilization, while the rival spermist view held that the sperm carried the preformed embryo and the egg offered only supporting material.

The debate between the ovists and the spermists was part of the broader preformationist era, a historical phase in which some thinkers believed organisms or their parts existed in miniature before birth. Over time, accumulating evidence for fertilization processes, embryonic development, and epigenesis shifted scientific consensus away from preformation toward a more dynamic view of development.

Today, the term ovist is used primarily in historical discussions of embryology to illustrate early models of reproduction and to show how scientists debated the origins of the embryo before modern developmental biology took shape. See also: spermist, epigenesis, preformationism.