Study of the formation and development of an embryo and fetus. Before widespread use of the microscope and the advent of cellular biology in the 19th century, embryology was based on descriptive and comparative studies. From the time of Aristotle it was debated whether the embryo was a preformed, miniature individual or an undifferentiated form that gradually became specialized. The latter theory was proved in 1827 when Karl Ernst Baer discovered the mammalian ovum (egg). The German anatomist Wilhelm Roux (1850–1924), noted for his pioneering studies on frog eggs (from 1885), became the founder of experimental embryology.