2i8 BIOLOGY AXD ITS MAKERS 



covcries, \^on Bacr 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 WoUi had distinctly foreshadowed the idea bv 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 \\'olff. He recognized the presence of three 

 primiary layers — an outer, a middle, and an inner — out of 

 v/hich 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 law 

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



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

 Bacr were associated as friends and fellow-students, under Dollinger at 

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

 came to studv with Dollinger, 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 published. It is significant of their 

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

 meinen Jugendfreund, Dr. Christian Pander." 



