218 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 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 cxpansions 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 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 
Baer were associated as friends and fellow-students, under Ddllinger at 
Wiirzburg. It was partly through the influence of Von Baer that Pander 
came to study with Déllinger, 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 Von Baer’s work is dedicated ‘‘ An 
meinen Jugendfreund, Dr. Christian Pander.” 
