PRECIPITATION STRUCTURES SIMULATING ORGANIC GROWTH. 235 



constant as in the majority of organic growths; but much devia- 

 tion from type is to be expected when accidental variables are 

 not compensated or excluded as they typically are in the deli- 

 cately regulated growth-processes of living organisms. Even 

 in organisms, however, the degree of form-regulation varies, 

 and in many plants especially the "habit" of growth is not 

 fixed but changes with the conditions. Precipitation-structures 

 formed according to the directions of Leduc or Herrera from a 

 prescribed combination of materials 1 exhibit many morphological 

 features which are constant and determined mainly by the 

 chemical nature of the structural material. Similarly in the 

 precipitation-structures formed from metals by electrolytic 

 local action, as described in the present and preceding papers, 

 the tubules and vesicles of iron ferricyanide are characteristically 

 different in appearance and structure from those of (e. g.) zinc 

 ferricyanide formed under the same external conditions; in 

 fact each metal (Fe, Co, Ni, Zn, Cd, Cu) forms a specific and 

 easily recognizable type of precipitation-structure. The chem- 

 ical nature of the structural material insoluble precipitate of 

 inorganic salt in this case determines the special form-char- 

 acters assumed by the single filaments, vesicles and other struc- 

 tures which grow out from the metal; and if these structures are 

 allowed to accumulate until conditions of equilibrium are reached 

 and growth ceases, the whole resulting mass of precipitation- 

 formation is then found to present an appearance of a constant 

 and characteristic kind (see photographs, Plates 1-6). 



In the analogous case of the living organism, e. g., a higher 

 animal, where a definite and final form and structure are attained 

 at the end of a long period of development and growth, it is 

 known that the structural material has a constant and highly 

 specific chemical composition; and it is to be assumed that this 

 composition has a determinative relation to the physical and 

 other properties of the organized structure thus arising. In 

 this respect the two types of growing system under comparison 

 exhibit certain fundamental similarities in the general physico- 

 chemical nature of the formative process ; and it seems desirable 



1 Cf. Leduc, "Mechanism of Life," London, Rebman Ltd., 1911; Herrera, 

 Bolelin de la Direcion de Estudios Biologicos, Mexico, Vols. i and 2, 1916 and 1917. 



