If space permitted, it would be interesting to give the biography[6] of this extraordinary man, but here it will be necessary to content ourselves with an examination of his portraits and a brief account of his work. Portraits.--Several portraits of von Baer showing him at different periods of his life have been published. A very attractive one, taken in his early manhood, appeared in _Harper's Magazine_ for 1898. The expression of the face is poetical, and the picture is interesting to compare with the more matured, sage-like countenance forming the frontispiece of Stieda's _Life of Von Baer_ (see Fig. 65). This, perhaps the best of all his portraits, shows him in the full development of his powers. An examination of it impresses one with confidence in his balanced judgment and the thoroughness and profundity of his mental operations. [Illustration: Fig. 65.--Karl Ernst von Baer, 1792-1876.] The portrait of Von Baer at about seventy years of age, reproduced in Fig. 66, is, however, destined to be the one by which he is commonly known to embryologists, since it forms the frontispiece of the great cooperative _Handbook of Embryology_ just published under the editorship of Oskar Hertwig. [Illustration: Fig. 66.--Von Baer at about Seventy Years of Age.] Von Baer's Especial Service.--Apart from special discoveries, Von Baer greatly enriched embryology in three directions: In the first place, he set a higher standard for all work in embryology, and thereby lifted the entire science to a higher level. Activity in a great field of this kind is, with the rank and file of workers, so largely imitative that this feature of his influence should not be overlooked. In the second place, he established the germ-layer theory, and, in the third, he made embryology comparative. In reference to the germ-layer theory, it should be recalled that Wolff had distinctly foreshadowed the idea by showing that the material out of which the embryo is constructed is, in an early stage of development, arranged in the form of leaf-like layers. He showed specifically that the alimentary canal is produced by one of these sheet-like expansions folding and rolling together. Pander, by observations on the chick (1817), had extended the knowledge of these layers and elaborated the conception of Wolff. He recognized the presence of three primary layers--an outer, a middle, and an inner--out of which the tissues of the body are formed. The Germ-Layers.--But it remained for Von Baer,[7] by extending his observations into all the principal groups of animals, to raise this conception to the rank of a general law of development. He was able to show that in all animals except the very lowest there arise in the course of development leaf-like layers, which become converted into the "fundamental organs" of the body.