(1792-1876) Estonian embryologist who discovered the mammalian ovum 1827. He made a significant contribution to the systematic study of the development of animals, and showed that an embryo develops from simple to complex, from a homogeneous to a heterogeneous stage.
Baer was born in Piep and studied at Dorpat (Tartu); at Vienna, Austria; and in Germany at Würzburg. He taught at Königsberg 1817-34, then moved to St Petersburg, Russia. In 1837 he led the first of many expeditions into Novaya Zemlya, in Arctic Russia, where he was the first naturalist to collect plant and animal specimens. He later led expeditions to Lapland, the Caucasus, and the Caspian Sea. He was professor at the Medico-Chirurgical Academy in St Petersburg 1846-62.
Baer conceived that the goal of early development is the formation of three layers in the vertebrate embryo—the ectoderm, endoderm, and mesoderm—out of which all later organs are formed. He also suggested that the younger the embryos of various species are, the stronger is the resemblance between them.
In his observations of the embryo, von Baer discovered the extraembryonic membranes—the chorion, amnion, and allantois—and described their functions. He also identified for the first time the notochord and revealed the neural folds.