What did Karl Ernst von Baer discover during his research in the 1800s as noted in the nonfiction book, Your Inner Fish?

Karl Ernst von Baer was a natural philosopher in the 1800s. He was supported in his research efforts by his wealthy friend Christian Pander. Von Baer focused his research on the study of embryos. He and Pander were amazed when they saw evidence that all organs in a chicken could be directly traced to of the three layers of tissue in the embryo. Von Baer compared his findings to a variety of species and the result was always the same. The organs in all species came from one of three layers of the embryo. These layers came to be known as the “germ” layers and the research of von Baer and Pander is still held as valid to this day.