Baer, Karl Ernst von (1792-1876), a German-Estonian biologist, was the first to discover the egg of the mammal in the ovary. He is considered the founder of embryology, the study of the early development of animals and plants.

One of 10 children and three sons of an Estonian landholder of Prussian descent, Baer spent several childhood years in the home of an aunt and uncle. Returning to his parents at age 7, he was privately tutored. From 1807 to 1810, he attended a cathedral school for members of the nobility. In 1814, he earned a medical degree from the recently opened University of Dorpat, a city in southeastern Estonia, where he studied with physiologist Karl Burdach. He also took courses in botany and physics. Not satisfied with his training, he continued his studies in Germany and Austria over the next three years. A turning point came in 1815-1816, when he studied with Ignaz Döllinger, professor of anatomy at the University of Würzberg, who suggested he begin a study of chick development. Although Baer lacked the financial resources to buy eggs and pay someone to monitor the incubators, his wealthier friend Christian Pander undertook the task. In the early stages of the study, Baer was part of the study. In 1817, Pander described the early stages of chick development in terms of what later were called the primary germ layers: ectoderm, mesoderm, and endoderm.

In 1817, Baer, at the invitation of his former teacher Burdach, joined the faculty at the University of Königsberg (now Kaliningrad, in Russia), where he taught zoology, anatomy, and anthropology at various times until 1834. From 1819 on, Baer carried out research to extend to all vertebrates Pander's pioneering work in chick germ-layer formation. He thus inaugurated the field of comparative embryology. He developed the theory that all later organs develop from the three germ layers. He also proposed the “law of corresponding stages,” which asserted that at very early stages, there is such a strong resem- blance among embryos of different species that it is quite difficult to classify them. This assertion led Baer to his idea of epigenesis, the theory that an embryo develops in stages from an originally undifferentiated structure.

Baer made many important discoveries. In 1827, Baer published On the Mammalian Egg and the Origin of Man, which publicized his most significant discovery of all: the mammalian ovum. The English physician William Harvey had previously dissected a deer in his attempt to find the mammalian ovum, but Harvey had searched for it in the uterus. Baer found the egg in a follicle in the ovary of Burdach's female dog, which the professor had presented to Baer for experimental purposes. Baer stated clearly that every animal develops from an ovum, not from “simple formative liquid.” His two-volume book, On the Development of Animals (1828, 1837), which became a classic in the field, not only marked the beginning of embryology as a distinct research subject but also outlined some goals for future comparative studies.

Baer's studies of the embryo also led to his detection and description of the function of other vertebrate structures, including the chorion, which contributes to the formation of the placenta, and the notochord, a rodlike cord of cells that supports the structure of vertebrate embryos. Pointing out the neural folds in the notochord, Baer proposed that they are the beginnings of the nervous system.

In 1834, Baer moved to St. Petersburg, Russia. He became a full member of the Academy of Sciences and held a number of administrative positions there. Although he did no further-embryological research, he continued to contribute to science as a naturalist. He made expeditions to the sparsely inhabited Russian north, from which he collected specimens. An island there was named in his honor. Baer used his measurements of skull specimens as the basis for a work in which he proposed that all human skulls might have originated in a single type. That work was published in 1859, the same year as Charles Robert Darwin' s Origin of Species. In 1861, Baer organized a meeting of craniologists (scientists specializing in human skulls) in Germany. This meeting led to the founding of the German Anthropological Society. Baer was also instrumental in founding the Russian Geographical Society and the Russian Entomological Society. In addition, Baer explained the forces behind the characteristic configuration of Russian riverbanks. His Development of Fishes (1835) encouraged new interest, both scientific and economic, in Russian fisheries.