Karl Ernst von Baer, a Baltic German biologist and a full professor of zoology and anatomy at Königsberg University, had discovered the mammalian ovum in 1826.

Together with Heinz Christian Pander and based on the work by Caspar Friedrich Wolff, Baer describes the germ layer theory of development (ectoderm, mesoderm, and endoderm) as a principle in a variety of species laying the foundation for comparative embryology in the book Über Entwickelungsgeschichte der Thiere (1828).

This forces the abandonment of the preformation theory, which postulates that the mammalian embryo is preformed at its onset and merely has to enlarge during development.