Vol. XI. September, 1906. No. 4. 



BIOLOGICAL BULLETIN 



SOME CONSIDERATIONS REGARDING SO-CALLED 

 FORMATIVE SUBSTANCES. 



C. M. CHILD. 



Within the last few years the idea of "formative substances" 

 has received consideration and support from workers in certain 

 fields of zoology and embryology to such an extent that it ap- 

 pears worth while to examine critically some of the assumptions 

 upon which the hypothesis of the existence of such substances 

 rests. 



As the first step in this consideration a definition of formative 

 substances is necessary. But anything like an exact definition is 

 difficult to find even in the literature which has devoted most at- 

 tention to these hypothetical substances. We read of ectodermal 

 substance, myogenic substance, hydranth-forming substance, head- 

 and tail-forming substances, etc., but the general question, what is 

 a formative substance? does not seem to be very clearly answered. 

 It may, however, be inferred from the use of the term that a for- 

 mative substance is a specific substance capable of giving rise 

 under certain conditions to a specific structure. Secondly, we 

 must inquire as to the nature of organic structure to which 

 these formative substances are supposed to give rise ? Organic 

 structure may be defined as a complex of heterogeneous parts 

 of typical constitution and arranged in typical space relations. 

 Strictly speaking this definition is not exact, for organic structure, 

 like other kinds of structure, is fundamentally simply a matter of 

 space-relations or arrangement of parts entirely independent of 

 their constitution. The constitution of the component parts must 

 of course play a role in determining the dynamic or functional 

 activities of the structural complex, and ordinarily we do not 



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