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Two Become One

Necessary requirements: one sperm and one egg.

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When reproductive fertilization piqued the interest of German scientist Wilhelm August Oscar Hertwig in the early 1870s, the field was dominated by two conflicting -- and ultimately incorrect -- theories [source: Brind'Amour and Garcia]. One camp posited that the mechanical vibrations of so many wiggling sperm around an egg triggered embryonic development, like a clap light switching on in response to the auditory transmission. The other predominating view maintained that sperm deposit a chemical compound into the egg, providing the crucial ingredient to kick-start the process.

In 1872, Hertwig closely observed fertilization in a transparent species of sea urchin and witnessed the fusion of sperm and egg, effectively disproving his academic cohorts [source: Brind'Amour and Garcia]. Not only that, Hertwig also discovered that fertilization isn't a group effort on the part of the 150 million spermatozoa released in the average ejaculate. He realized that, instead, it only requires a single sperm to find its way inside the egg.

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