Karl Ernst von Baer

The Estonian anatomist and embryologist Karl Ernst von Baer (1792-1876) was the first to describe the mammalian ovum. He also developed the germ-layer theory, which became the basis for modern embryology.

Karl Ernst von Baer was born in Piep on Feb. 29, 1792. He began his medical studies at the University of Dorpat in Estonia in 1810, and after graduating in 1814 he continued his studies at Vienna. After realizing his limitations as a practitioner, he studied comparative anatomy at the University of Würzburg, where he was taught by the influential anatomist Johann Döllinger. On completion of his studies, Baer accepted a position as prosector in anatomy at the University of Königsberg, and in 1819 he was appointed associate professor of zoology there. In 1822 he achieved the rank of professor.

At Königsberg he undertook his famous studies in embryology in collaboration with C. H. Pander. He worked first on the embryology of the chick but later investigated the problem of identifying the structure of the ovum of the dog and found it to be a small yellow spot floating in the follicular fluid. As a result of this work, he published in 1827 the first description of a mammalian egg, Epistola de ovi mammalium et hominis genesi (On the Origin of the Mammalian and Human Ovum). His reputation was further increased by the publication of his most famous work, Entwicklungsgeshichte der Tiere (1828-1837; Developmental History in Animals). In this work he developed the germ-layer theory, in which he held that in vertebrate eggs four "layers" of cells are formed and that each layer always gives rise to certain tissues in the adult organism. (The two middle layers were later regarded as one.) In this same work he outlined his discovery of the notochord in the chick embryo. He described it as a rod of cells which runs the length of the vertebrate embryo and around which the future backbone is laid down.

Laws of Development

Baer's work on the embryological development of animals led him to frame four laws. In these laws he was concerned with the question of how closely the development of an embryo of one species resembles that of other species and how closely its various embryonic stages resemble the adult stages of other species. His laws state that the embryo of a given species never resembles the adult of another species and that the embryos of even the most similar species do not pass through the same states but, rather, become progressively different from each other. These "laws of development," though much misunderstood by other biologists and in some cases used by them to support opposite views, were fruitful in later interpretations of embryology and evolution. Herbert Spencer used Baer's law (later known as the biogenetic law) to support his theory that the world is becoming increasingly differentiated and complicated.

In 1834 Baer left Germany to take up the position of librarian of the Academy of Science at St. Petersburg. In this position he advised the Russian government on a number of scientific matters. In 1837 he led a scientific expedition into Arctic regions, and from 1851 until 1856 he studied Russian fisheries and suggested many improvements. He retired to his native Estonia and died on Nov. 28, 1876, in Dorpat.

Further Reading

There is a very good account of Baer's biological work and influence in Edward Stuart Russell, Form and Function: A Contribution to the History of Animal Morphology (1916). Information is also in Joseph Needham, A History of Embryology (1934; 2d ed. 1959); Arthur William Meyer, The Rise of Embryology (1939); and Jane M. Oppenheimer, Essays in the History of Embryology and Biology (1967).

Additional Sources

Baer, Karl Ernst von, Autobiography of Dr. Karl Ernst von Baer, Canton, MA: Science History Publications U.S.A., 1986. □

Karl Ernst von Baer

1792-1876

Estonian Biologist

Karl Ernst von Baer was an Estonian biologist who discovered the mammalian ovum—the reproductive egg in female mammals. He made significant contributions to the study of the embryonic development of animals.

Born in Piep, Estonia, to parents descended from Prussian nobility, Baer studied medicine at the University of Dorpat in Estonia. He continued his studies in Vienna, Austria, and later in Würzburg, Germany. He then accepted a position to teach anatomy, anthropology, and zoology at the University of Königsberg.

From his investigations at Königsberg arose one of Baer's most important discoveries. Scientists had long been trying to determine the nature and location of the mammalian egg. In 1672 Reinier de Graaf (1641-1673) discovered follicles in the ovaries and believed that the follicles themselves might be eggs. When he found structures in the uterus that were even smaller than the follicles, the role of follicles was thrown into doubt. During his research, Baer, using a microscope, found a structure within the follicle. He concluded that this structure was the mammalian egg. Baer's discovery, which he expanded on in his 1827 treatise, called in English On the Origin of the Mammalian and Human Ovum, confirmed that mammals develop in a manner essentially like that of other animals.

Baer's other great achievement was his theory on embryonic development. In On the Development of Animals (1828-1837), he argued that the embryos of all animals develop from a simple and homogeneous stage to a complex and heterogeneous one. Suggesting that the younger the embryos of different species are the closer the resemblance between them, he showed that during embryonic development general characteristics of animals appear before traits specific to a species. This idea came to be known as the "biogenetic law." In addition, Baer argued that vertebrate embryos form distinct layers that later differentiate into all the organs or structures of the animal. These layers are the ectoderm, mesoderm, and endoderm. This theory is called the germ layer theory of development.

In his studies of the embryo, Baer was the first to show that mammalian reproduction involves the fusion of an ovum with sperm, not the mingling of female and male seminal fluids as had been previously thought. He discovered and described the functions of the extraembryonic membranes—the chorion, amnion, and allantois. Baer also discovered the notochord in early vertebrate embryos as well as the neural folds. (The notochord in vertebrates develops into the vertebrae, or backbone, while the neural folds are concerned with formation of the nervous system.)

Following his tenure at Königsberg, Baer accepted a position as librarian at the St. Petersburg Academy of Sciences. He was also appointed Professor of Comparative Anatomy and Physiology at the Medico-Chirugical Academy in St. Petersburg. While in St. Petersburg he took part in various scientific expeditions, including many to Novaya Zemlya, in the Russian Arctic, where he was the first naturalist to collect plant and animal specimens. Baer's greatest accomplishments, however, would remain the discovery of the mammalian egg and the publication of his theory on embryonic development.

GARRET LEMOI

Baer, Karl Ernst von (1792–1876) Estonian-born German biologist and embryologist. He studied medicine and comparative anatomy before becoming professor of zoology at Königsberg University in 1817. Ten years later he discovered the mammalian ovum, and traced its development from the Graafian follicle to the embryo. He demonstrated that similar organs in different animals developed from the same germ layers. He also noted the similarities between the young embryos of widely different species (the biogenetic law), which provided Charles Darwin with important evidence for his theory of evolution.