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BAER, KARL ERNST VON (1792-1876) , German biologist, was born at Piep, in Esthonia, on the 29th of February 1792. His father, a small landowner, sent him to school at Reval , which he left in his eighteenth year to study medicine at Dorpat University. The lectures of K. F. Burdach (17761847) suggested research in the wider field of life-history, and as at that time Germany offered more facilities for, and greater encouragement to, scientific work, von Baer went to Wiirzburg, where J. I. J. Dellinger (1770-1841), father of the Catholic theologian, was professor of anatomy. In teaching von Baer, Dellinger gave a direction to his studies which secured his future pre-eminence in the science of organic development. He collaborated with C. H. Pander (17941865) in researches on the evolution of the chick, the results of which were first published in Burdach's treatise on physiology. Continuing his investigations alone, von Baer ex-tended them to the evolution of organisms generally, and after asojourn at Berlin he was invited by his old teacher Burdach, who had become professor of anatomy at Konigsberg, to join him as prosector and chief of the new zoological museum (18r7). Von Baer's great discovery of the human ovum is the subject of his Epistola de Ovo Mammalium et Hominis Genesi (Leipzig , 1827), and in the following year he published the first part of his History of the Evolution of Animals (Ueber die Entwickelungsgeschichte der Thiere), the second part following in 1837. In this work he demonstrated first, that the Graafian follicles in the ovary are not the actual eggs, but that they contain the spherical vesicle, which is the true ovum, a body about the one hundred and twentieth of an inch in diameter , wherein lie the properties transmitting the physical and mental characteristics of the parent or grandparent, or even of more remote ancestors. He next showed that in all vertebrates the primary stage of cleavage of the fertilized egg is followed bymodification into leaf-like germ layersskin, muscular, vascular and mucouswhence arise the several organs of the body by differentiation. He further discovered the gelatinous, cylindrical cord, known as the chorda dorsalis, which passes along the body of the embryo of vertebrates, in the lower types of which it is limited to the entire inner skeleton, while in the higher the backbone and skull are developed round it. His " law of corresponding stages " in the development of vertebrate embryos was exemplified in the fact recorded by him about certain specimens preserved in spirit which he had omitted to label. " I am quite unable to say to what class they belong. They may be lizards, or small birds, or very young mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals. The extremities are still absent, but even if they had existed in the earliest stage of the development we should learn nothing, because all arise from the same fundamental form." Again, in his History of Evolution he suggests, "Are not all animals in the beginning of their development essentially alike, and is there not a primary form common to all ?" (i. p. 223). Notwithstanding this, the " telic " idea, with the archetypal theory which it involved, possessed von Baer to the end of his life, and explains his inability to accept the theory of unbroken descent with modification when it was propounded by Charles Darwin and A. R. Wallace in 1858. The influence of von Baer's discoveries has been far-reaching and abiding. Not only was he the pioneer in that branch of biological science to which Francis Balfour, gathering up the labours of many fellow-workers, gave coherence in his Comparative Embryology (1881), but the impetus to T. H. Huxley's researches on the structure of the medusae came from him (Life, i. 163), and Herbert Spencer found in von Baer's " law of development " the "law of all development " (Essays, i. 30). In 1834 von Baer was appointed librarian of the Academy of Sciences of St Petersburg . In 1835 he published his Development of Fishes, and as the result of collection of all available information concerning the fauna and flora of the Polar regions of the empire, he was appointed leader of an Arctic expedition in 1837, The remainder of his active life was occupied in divers fields of research, geological as well as biological, an outcome of the latter being his fine monograph on the fishes of the Baltic and Caspian Seas. One of the last works from his prolific pen was an interesting autobiography published at the expense of the Esthonian nobles on the celebration of the jubilee of his doctorate in 1864. Three years afterwards he received the Copley medal. He died at Dorpat on the 28th of November 1876. (E. CL.)
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