ABSTRACT
INTRODUCTION AND BACKGROUND In the fourth century BC, Aristotle wrote the first treatise on embryology, including observations, arguments, and speculation. His belief that slime and decaying matter gave rise to living animals was not disproved until 1668 by Redi, although the final repudiation of “spontaneous generation” occurred in 1864 by Louis Pasteur. Until approximately 1800, it was believed that a fully formed organism in miniature was either in the egg (and only needed the sperm to activate it) or it was in the sperm (drawings of a human sperm cell with a miniature “homunculus” squatting in the sperm head, especially by Hartsoeker, were popular in the late 1600s) and only needed the egg for nutrients. This doctrine of “preformation” was seriously questioned initially by Harvey in the 17th century, followed by Wolff in the 18th century, and finally disproved by Driesch in 1900. Physical identification of the human sperm (by Leeuwenhoek in 1677) and the human egg (by von Baer in 1827) and the recognition that the cell was the structural and functional unit of the organism (by Schwann in 1839) occurred in the 19th century.
