SO-CALLED FORMATIVE SUBSTANCES. 171 



of unknown nature again a vitalistic conception --or finally 

 we must admit that the relations not the substances give rise to 

 the final arrangement of parts. 



THE ROLE OF CHEMICAL CONSTITUTION IN MORPHOGENESIS. 



Objection may be made to the conclusions reached above on 

 the ground that the chemical constitution of the various sub- 

 stances plays a part in determining the character of the relations 

 which exist between them, and consequently in determining the 

 character of the result. It is of course quite true that the chem- 

 ical constitution of the substances involved may affect the result, 

 but a single substance can accomplish nothing of itself by virtue 

 of its composition. It must enter into relation with another or 

 others in a more or less typical environment and the composition 

 of the other substances and the factors of the environment are 

 equally important in determining the result. What are the real 

 formative factors in this case ? It is evident that we can properly 

 recognize only a formative complex of substances and relations 

 and in this complex the constitution of the various substances is 

 a condition affecting the character of the result while the relations 

 between the substances are the cause of the result. But again it 

 may be objected that the chemical constitution is the cause of the 

 relations existing. While this may be true in large measure for 

 certain simple chemical reactions it certainly is not true for many 

 of the processes which give rise to organic structure. 



Many other conditions beside those resulting from chemical 

 constitution are usually important. For example, we know that 

 the formation and persistence of various structures is determined 

 primarily by simple mechanical conditions. It may be said that 

 these conditions are not effective unless they act upon a particu- 

 lar substance but unless we make the term particular substance 

 equivalent to protoplasm of a particular species or genus there is 

 no evidence that this is true. 



There is very strong evidence for believing that mechanical 

 conditions resulting from the presence of fluid in a cavity are 

 often fully as important as the character of the cells about it in de- 

 termining the development of hollow, fluid-containing structures. 

 We know that mechanical conditions are potent factors in the de- 



