ii 4 POPULAR SCIEN.CE MONTHLY. 



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 ma- 

 terial 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. 



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 except the very lowest, there arises in the course of develop- 

 ment leaf-like layers, which become converted into the ' fundamental 

 organs ' of the body. 



Now, these elementary layers are not definitive tissues of the body, 

 but are embryonic, and therefore, may appropriately be designated 

 ' germ-layers.' The conception that these germ-layers are essentially 

 similar in origin and fate, in all animals, was a fuller and later de- 

 velopment of the germ-layer theory, which dominated embryological 

 studv until a recent date. 



Von Baer recognized four such layers: the outer and inner ones 

 being formed first, and, subsequently budding off a middle layer 

 composed of two sheets. A little later (1845) Remak recognized the 

 double middle layer of Von Baer as a unit, and thus arrived at the 

 fundamental conception of three layers — the ecto-, endo- and meso- 

 derm — which has so long held sway. For a long time after Von Baer, 

 the aim of embryologists was to trace the history of these germ-layers 

 — and so in a wider and much qualified sense it is to-day. 



It will ever stand to his credit, as a great achievement, that Von 

 Baer was able to make a very complicated feature of development clear 



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

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

 burg. It was partly through the influence of Von Baer that Pander came to 

 study with Dollinger, and took up investigations on development. His ample 

 private means made it possible for him to bear the expenses connected with the 

 investigation, and to secure the services of a fine artist for making the illustra- 

 tions. 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 researches seriously until 

 Pander's were published. It is significant of their continued harmonious rela- 

 tions thai Ynn Baer's work is dedicated 'An meinen jugendfreund, Dr. Chris- 

 tian Pander.' 



