The Animal Organism and its Germ-Layers 49 



A Negative Answer to the Question in the Last Section 

 Expected of Element alist Biology 



Any one who perceives the essence of elementalism and 

 so has seen that it perforce impHes a denial of causal 

 power of the whole organism over its parts in development 

 will foresee what answer biology as strongly elementalist as 

 the science has been in the recent period will be likely to give 

 to these questions. It will not be satisfied with basing its 

 expectation that an organ or tissue has arisen from a par- 

 ticular germ-layer solely on the fact that in all hitherto 

 obsei*ved cases it has so arisen. The contention may be ex- 

 pected that the independence and autocracy of the layers 

 are in no way subject to modification to meet the require- 

 ments of the organism as a unit : that in case of conflict 

 between the needs of the organism as such and the proper 

 powers of the layers, the organism must accommodate itself 

 to the layers if any accommodating is to be done. As a 

 matter of fact the germ-layer theory has been defended with 

 great vigor in just this hard-and-fast way. Nerve tissue 

 must arise from ectoderm if it comes into existence at all. 

 Under no circumstances is it permissible to believe it to have 

 arisen from either of the other layers. Muscle tissue must 

 arise from mesoderm (or mesenchyme) or not at all; and 

 so on, according to this way of viewing developmental phe- 

 nomena. Numerous biologists say in substance that the 

 entire teaching of embryology, anatomy, histology, and 

 pathology, should be based on the doctrine of the germ- 

 layers. 



Evidence That Germ-Layers Are Thus Subservient to the 



Organism . 



This brings us to the facts previously alluded to as 

 having played so considerable a part in generating the sys- 



