2l8 BIOLOGY AND ITS MAKERS 



coveries, Von Baer greatly enriched embryology in three di- 

 rections: 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 A\'olft* 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 fold- 

 ing and rolling together. 



Pander, by observations on the chick (1817), had ex- 

 tended 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,* by 

 extending his observations into all the principal groups of 

 animals, to raise this conception to the rank of a general lav/ 

 of development. He was able to show that in all animals 



* It is of more than passing interest to remember that Pander and Von 

 Baer were associated as friends and fellow-students, under Dollingcr at 

 Wiirzburg. It was partly through the influence of Von Baer that Pander 

 came to study with DolUnger, and took up investigations on development. 

 His ample private means made it possible for him to bear the expenses con- 

 nected with the investigation, and to secure the services of a fine artist for 

 making the illustrations. The result was a magnificently illustrated treatise. 

 His unillustrated thesis in Latin (1817) is more commonly known, but the 

 illustrated treatise in German is rarer. Von Baer did not take up his re- 

 searches seriously until Pander's were pul)lished. It is significant of their 

 continued harmonious relations that Von Baer's work is dedicated " An 

 meinen Jugendfreund, Dr. Christian Pander." 



