It's a bright day in 1677, in the city of Delft, and Antonie van Leeuwenhoek is making love to his wife. But moments after he shudders with orgasm, he hurries out of bed to grab his microscope. After all, he's not just spending time with his wife: he's running an important scientific experiment at the request of the Royal Society in London.
Leeuwenhoek has already gained quite a reputation at the Royal Society for his observations of microscopic things, and has — with the Society's urging — looked at a lot of bodily fluids, including blood, milk, spit, and tears. This time, the plan is to see what's inside semen.
He quickly collects his sample from his wife, places some on a pin on the microscope, and lifts the device toward the sun "before six beats of the pulse [have] passed." When he peers through the lens, he becomes the first person to see living sperm cells.
In his November 1677 letter to the Royal Society, "de Natis e semine genital Animalculis," he describes a scene teeming with tiny "animalcules." Each possesses a blunt head and a long, nearly transparent tail. And they're propelling themselves along, "moving like a snake or like an eel swimming in water." Helpfully, he provides drawings.
Leeuwenhoek knew his discovery was important: he went on to find sperm in many other animals and determine that they were made by the testes. But he was also a little overexcited by their potential. He insisted that a sperm cell was the only thing that made an embryo, and that the egg and uterus existed merely nourished it as it grew. That idea completely flipped the prevailing view among natural philosophers at the time: that the embryo grew from the egg alone, after the semen added a "volatile spirit" to spark its development. The real roles of egg and sperm wouldn't get sorted out until the middle of the 19th century. [Sources: Leeuwenhoek 1673 [Leeuwenhoek 1677 [Rustowe 1983 [Snyder 2015 ]