236 RALPH S. LILLIE AND EARL N. JOHNSTON. 



to recognize these resemblances and to formulate them clearly 

 (without of course ignoring or underestimating the many and 

 obvious differences). In both the living and the non-living 

 growing systems the structural material is the product of chemical 

 reactions whose rate, character, and position depend upon the 

 chemical nature of the interacting substances and upon the 

 structural conditions under which the reactions take place. 

 In the living system, however, on account of the minutely graded 

 chemical and structural specificity and 'the exact regulation of 

 the conditions under which the metabolic and structural trans- 

 formations occur, the constancy or detenninateness of form, 

 structure, and activity when the final or adult stage is reached 

 is much greater than in the inorganic system; and it is only in 

 its broadest and most general features that the growth of the 

 latter can be regarded as a model for that of a living organism. 

 Indeed the most astonishing feature of organisms has always 

 been that from a minute germ a system of the utmost chemical 

 and physical complexity, yet identical in all respects with another 

 system arising similarly from another germ of the same kind, 

 should be built up by the chemical transformation of materials 

 taken from the surroundings. Nevertheless something of the same 

 kind is seen in the building-up of a precipitation-structure under 

 the influence of a piece of metal placed in a ferricyanide solution. 

 Here also the material of metal and solution is chemically and 

 structurally transformed in a constant manner, and a system of 

 definite physico-chemical constitution and activity is formed 

 whose peculiarities of structure and activity have a specific 

 dependence upon the nature of the introduced metal. 



The precise relation between the specific chemical constitution 

 of the proteins of the living germ i. e., of those structural 

 compounds which are demonstrably peculiar to the species in 

 question and the specific type of structure arising in growth or 

 development is unknown. It must be recognized that other 

 factors than the chemical specificity and arrangement of the 

 structural proteins play a part in determining the precise course 

 of the formative process; e. g., changing the nature and propor- 

 tions of the salts of sea-water produces definite modifications of 

 development in the eggs of marine animals; well-known examples 



