48 The Unity of the Organism 



most embryologists : "As our knowledge of the development 

 from the germ-layers has grown, we have learned with ever- 

 increasing certainty that each germ-layer has its specific 

 role to play." ^ 



Are Germ-layers Developmental Organs and Subservient to 

 the Developmental Requirements of the Organism? 



But after all this has been fully and gladly granted, there 

 still remains much to be said concerning the deeper biologi- 

 cal meaning of the germ-layers, and the different attitudes 

 of mind which different biologists may assume, indeed do 

 assume, toward these layers. What we have to offer on this 

 subject will be from the standpoint of the difference bet- 

 tween the elemental and the organismal ways of looking at 

 biological phenomena generally. Tliis difference may be 

 brought out b}- asking, does the obviously very general rule 

 of origin of the tissues and organs from the different layers 

 hold with genuine universality, that is, in all animals in 

 wliich the three (or four) layers occur, and under all cir- 

 cumstances of development in every animal .'' Or, putting 

 essentially the same question, but modified so as to show 

 more clearly its relevancy to the organismal and elemental 

 standpoints : are the germ-layers, when looked at as "struc- 

 tural units" or elements "of a higher order than cells" so 

 fundamentally independent of one another and of the or- 

 ganism as a whole that they always and under all conditions 

 must give rise to just the tissues and parts typically arising 

 from them, and nothing else? Or, shifting the point of view 

 a little : has the organism, as such, needs and abilities so 

 paramount that it is able to realize these needs by modi- 

 fying to any extent the developmental course usual to the 

 germ-layers? 



