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 WÃrzburg, 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 KÃnigsberg, and in 1819 he was appointed associate professor of zoology there. In 1822 he achieved the rank of professor.

At KÃnigsberg 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