It wasn't until 1876 that early embryologists used sea urchin embryos to show that babies definitely come from both sperm and egg. But how that happened and how in the world an embryo ultimately became a baby was a field full of creative, fun and—at times—bizarre experiments.
Gregory Pincus was an American biologist studying reproduction in rabbits at Harvard in the 1930s. Fascinated by how chemicals in the body could play a part in reproduction, Pincus was the first to figure out the hormone cycle that gives rise to a mature egg every month. Once he understood that hormones controlled the potential for reproduction in the body, Pincus and his colleague E.V. Enzmann set out to recreate the whole process outside the body. These experiments are the foundation for all IVF that occurs today.
Pincus and Enzmann pulled out immature eggs from a female rabbit and washed the eggs in a cocktail of hormones to bring them to maturity. Adding in rabbit sperm, the matured eggs became rabbit embryos and were implanted into the uterus of a third, unrelated rabbit. One month later—bing, bang, boom—out came the first IVF babies: a litter of rabbits!
Pincus continued his work at Harvard for several years, pushing the boundaries of just how much power hormones could have over reproduction. In one of his most infamous experiments, later named "Franken-bunny," Pincus used only hormones—no sperm needed!—to "fertilize" a rabbit egg.
The "fertilized" embryo eventually grew into a bunny that graced the cover of Look magazine in 1937. This form of reproduction is called parthenogenesis and happens naturally in a variety of animal species, including some bees, fish, reptiles, amphibians and, very rarely, in birds.
Speaking of amphibians, salamanders provided the key to understanding how the embryonic ball of cells morphs into a baby. In the 1920s, Hans Spemann and Hilde Mangold were trying to answer the embryo-to-baby question through transplantation experiments in salamander embryos.
Using a microscope, a tweezer and a fine needle, Spemann and Mangold systematically sliced small parts off one embryo and grafted them onto a second embryo. In one of these experiments, Spemann and Mangold's grafted embryo grew into not one, but two salamander larvae attached at the belly. This chunk of transplanted cells became known as the "organizer" as this experiment showed it was enough to "organize" the formation of an entire individual. Spemann went on to win the Nobel Prize in 1935 for this weird but wildly important experiment.