SO-CALLED FORMATIVE SUBSTANCES. 173 



of so-called morphogenetic enzymes seem to be similar to this 

 one in that the structure appears, not as the result of enzyme 

 action, but in consequence of certain physical conditions which 

 may accompany or follow the enzyme action but are not an 

 essential part of it. There is nothing specific in the relation of 

 this process to the enzyme. Coagulating or coagulated colloids 

 may be made to assume a fibrillar structure in many cases 

 " under the influence of pressure and traction " in total absence 

 of enzymes. The specific effect of the thrombin and of other 

 enzymes as well has not in any case been shown to be morpho- 

 genetic in character. 



In general, physical conditions appear to be more important 

 factors in morphogenesis than substances of particular chemical 

 constitution. 



FORMATIVE SUBSTANCES IN ONTOGENY. 



The chief reason for the consideration recently accorded to this 

 old idea of formative substances appears to lie in the fact that 

 many eggs exhibit visibly differentiated regions which normally 

 give rise to particular structural complexes. Moreover, experi- 

 ment has demonstrated that in some cases certain of these regions 

 are capable of continuing the process of visible differentiation in 

 a manner apparently normal after separation from other parts of 

 the egg or embryo. The conclusion in such cases is that these 

 regions must contain a certain substance or certain substances 

 which are responsible for the differentiation. Hence these hypo- 

 thetical substances are called formative. These regions are mostly 

 extensive and give rise in development to a multitude of struc- 

 tures : for example we read of ectodermal formative substances, 

 neurogenic substances, myogenic substances, entodermal sub- 

 stances, etc. In some cases, however, smaller regions giving 

 rise to definite organs or parts of organs appear to possess in 

 greater or less degree the power of " self-differentiation." We 

 must regard these regions from either the preformation or the 

 epigenetic point of view, i. c., each element of the structure to 

 which they finally give rise must be represented by an element 

 existing before visible differentiation or else new elements must 

 arise in the course of development. In any case the region must 



