Karl Ernst von Baer (1792–1876) carefully removed a yellowish structure he had spotted in the ovary of a dog and studied it in 1826. Inside was a precious cell – the oocyte. This was the first time mammalian oocytes were observed under the
microscope. Von Baer called it “ovulum” or “little egg” and, at a time when many contradicting theories abounded, he made an important leap to establish that all animals develop from an egg. He also described the initial stages of development in the dog embryo, carefully placing earlier stages near the ovary and more advanced stages of development closer to the uterus. In view of the fact that he had a very rudimentary microscope and the concept of a cell had not been developed yet, his results were nothing short of amazing.