Embryo Project Encyclopedia

Ovism was one of two models of preformationism, a theory of generation prevalent in the late seventeenth through the end of the eighteenth century. Contrary to the competing theory of epigenesis (gradual emergence of form), preformationism held that the unborn offspring existed fully formed in the eggs or sperm of its parents prior to conception. The ovist model held that the maternal egg was the location of this preformed embryo, while the other preformationism model known as spermism preferred the paternal germ cell, as the name implies.

Preformationism was a theory of embryological development used in the late seventeenth through the late eighteenth centuries. This theory held that the generation of offspring occurs as a result of an unfolding and growth of preformed parts. There were two competing models of preformationism: the ovism model, in which the location of these preformed parts prior to gestation was the maternal egg, and the spermism model, in which a preformed individual or homunculus was thought to exist in the head of each sperm. Preformationism was a widely-held theory by Enlightenment-era scientists, but by the early 1800s, most scientists had abandoned it, in part because higher magnification in microscopes enabled them to see the very earliest stages of embryos as small collections of cells. Prior to preformationism, naturalists who studied embryo development favored the theory of spontaneous generation in lower animals, such as flies, which appeared to arise from manure. In higher animals, however, scientists used the theory of epigenesis put forth by Aristotle, who said that maternal and paternal fluids came together in the uterus and solidified during early gestation into a fetus. Preformationism was the first theory of generation and development that applied to all organisms in the plant and animal kingdoms.

Victor Albrecht von Haller was an 18th century scientist who did extensive work in the life sciences, including anatomy and physiology, botany, and developmental biology. His embryological work consisted of experiments in understanding the process of generation, and led him to adopt the model of preformationism called ovism (the idea that the new individual exists within the maternal egg prior to conception). Haller was born in Bern, Switzerland, on 16 October, 1708. His mother was Anna Maria Engel, and his father was Niklaus Emanuel Haller. The Hallers were an old family in Bern, and Haller spent the majority of his life there. During his life, Haller made many important contributions to medicine, botany, anatomy, and physiology. Haller became one of the most outspoken supporters of preformationism, and the long debate between Haller and Caspar Friedrich Wolff, who supported the competing theory of epigenesis in which form emerges gradually, is a hallmark of this time period in developmental biology. Haller also formulated an accurate model of the rate of fetal growth during gestation, demonstrating statistically that development at the beginning is more rapid than growth later on.

Spermism was one of two models of preformationism, a theory of embryo generation prevalent in the late seventeenth through the end of the eighteenth century. Spermist preformationism was the belief that offspring develop from a tiny fully-formed fetus contained within the head of a sperm cell. This model developed slightly later than the opposing ovist model because sperm cells were not seen under the microscope until about 1677. Spermism was never as dominant as ovist preformationism, but it had ardent followers whose work and writings greatly influenced the development of embryology in this time period. Spermism was and is now sometimes referred to as animalculism, a name taken from the term most naturalists at the time used to refer to microscopic organisms, or vermiculism, which comes from a specific term for sperm cells referring to their worm-like appearance. The most notable spermist philosophers and scientists were Nicolaas Hartsoeker, Anton Leeuwenhoek, and Wilhelm Gottfried Liebniz.