In the beginning, no one really understood how babies were made. Thinkers puzzled for millennia about how life arose from one generation to the next. But not until the 17th century did scientists start to seriously study the question. At that time, the theory of preformation held that minuscule humans already existed, fully formed, in either the mother’s menstrual blood or the father’s semen, depending on whether you were an “ovist” or a “spermist.”
Little changed until two late-19th century scientists, Oskar Hertwig from Germany and Hermann Fol from France, independently conducted experiments on sea urchins, proving conclusively that creating new offspring takes one egg and one sperm.
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