Chapter III 



THE ANIMAL ORGANISM AND ITS GERM-LAYERS 



The Germ-layers, Their Role In Development, and the 



Germ-layer Theory 



^^TRICT fidelity to the natural sequences of biological 

 ^^ knowledge as viewed in this work would not permit us 

 to introduce at this early stage of our discussion such a 

 subject as that of germ-layers, or indeed any other purely 

 developmental aspect of the organism, but would require us 

 to deal more fully than we yet have with the completed or- 

 ganism. However, our general attitude having much of the 

 pragmatic about it will be broadly tolerant in the matter of 

 adapting methods to ends sought. This way of beginning 

 is chosen for the two-fold reason that in this domain my 

 own researches first came upon facts which contributed 

 very largely to the ideas underlying this whole undertaking, 

 and also that these and kindred facts constitute some of the 

 most striking evidence we have of the ability of the organism 

 to gain its developmental ends in unusual ways when the 

 usual ways chance to be obstructed — evidence, in other 

 words, of the domination of the organism as a totality over 

 its parts. 



From its very beginning with Wolff and von Baer, mod- 

 ern embryology has recognized that animal embryos pass 

 through a stage in which the body consists of little more 

 than unifomi layers of cells, first one, then two, then three, 

 and finally, in several classes of animals, four ; these being 

 disposed one inside the other and more or less regularly 



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