PRECIPITATION STRUCTURES SIMULATING ORGANIC GROWTH. 24! 



reactions. Certain insoluble and physically coherent products 

 of these reactions form structure, which as soon as formed in- 

 fluences in a definite manner the chemical and physical processes 

 by which further structure is formed. In its special character 

 this structure varies with the chemical nature of the structural 

 material; as is general w r ith matter in the solid form, this material 

 is deposited from solution in a crystalline or quasi-crystalline 

 state of aggregation, which is structurally specific in the same 

 sense as the molecular structure is specific. Thus the micro- 

 scopic and ultimately the* macroscopic form-characters of the 

 growing system are determined by the special molecular structure 

 of the formative material. 



Organic growth, as Child has so clearly pointed out, depends 

 upon the progressive accumulation of the solid metabolic products 

 which are stable under the conditions. Since the chief of these 

 structure-forming products, the proteins, have the most minute 

 and detailed specificity known among chemical compounds, it 

 is not surprising that the living systems built up from these 

 compounds should be correspondingly individualized and specific 

 in their structure and activities. The formation of precipitation- 

 structures from metals may be regarded as a simplified model- 

 process which makes it easier to understand why organisms 

 whose proteins are similar in chemical configuration (as shown 

 e. g. by precipitin tests) are also structurally and physiologically 

 similar i. e., are closely related in the natural system. Chemical 

 similarity and structural similarity go hand in hand, and struc- 

 tural similarity involves similarity in physiological activity and 

 external behavior. 



We have seen that the formation and activity of these pre- 

 cipitation-growths are the result of processes of electrolysis in 

 the local electrical circuits arising between different areas of the 

 metallic surface. The currents of these circuits control the 

 structure-forming process; and in conformity with this condition 

 we find experimentally that the formation of filaments can be 

 promoted, modified, or suppressed at will by electrical means. 1 

 In this respect also an analogy to organic processes is to be seen. 

 The electrical sensitivity of living matter is one of its most 



1 R. S. Lillie, loc. cit., pp. 148-157. 



