From antiquity humans have tried to understand how a new individual is generated after sexual intercourse. A popular notion was that a woman's blood and a man's semen were involved. Aristotle, based on studies of chick embryology, proposed that the mixture produced an egg from which the foetus developed in the uterus. This idea lasted for nearly two thousand years. William Harvey disproved the old notion by finding an empty uterus in a variety of animals soon after mating. Although he did not find any eggs he insightfully proposed “ex ovo omnia”, implying that all creatures arise from eggs. This lead to an intensive search for the source of eggs in mammals and resulted in the discovery of the role of the ovaries and fallopian tubes. With the introduction of the microscope in the 17th century sperm were discovered in the semen. Studies involving impregnation of amphibian eggs revealed that single sperm could produce a foetus. Despite this information the mammalian egg remained undiscovered for another 200 years. Finally, Karl Ernst von Baer discovered the ovum by microscopic examination of the ovarian follicle contents of the dog. Studies on fertilization and embryo development in mammals were initiated. With the advent of the cell theory it was recognized that the ovum was a cell, its nucleus was discovered, meiosis was described and the role of the chromosomes in heredity was revealed. Egg and embryo transfer in animals developed quickly and was facilitated by the introduction of tissue culture. The establishment of egg and embryo culture techniques encouraged John Rock and Miriam Menkin to attempt the first studies on human in vitro fertilization and embryo culture. Subsequent refinement of chemically defined culture media set the stage for successful in vitro fertilization, embryo culture and transfer for treating infertility.