PRECIPITATION STRUCTURES SIMULATING ORGANIC GROWTH. 237 



of this effect are the cyclopia induced in fishes by adding mag- 

 nesium salts, 1 and the production of exogastrula? in sea-urchin 

 eggs by lithium chloride. 2 The germinal protoplasm contains 

 inorganic salts and numerous other non-specific compounds, 

 differing in their nature, concentration, and distribution in 

 different germs; moreover the catalysers present and the various 

 purely physical peculiarities of the system (e. g., viscosity, 

 capillarity, boundary-potentials, permeability to diffusing sub- 

 stances, etc.) must all enter as factors influencing the rate and 

 character of the chemical and physical processes concerned in 

 development. All such factors must be included under the 

 conception of germinal constitution; but we are still far from under- 

 standing why a given germinal constitution determines in such 

 minute detail the course of the proliferative and transformative 

 process constituting development in any particular species. It 

 seems certain, however, that chemical specificity is the primary 

 and fundamental factor. All organic structure is built up by 

 chemical processes, i. e., by metabolism; hence the divergences 

 first appearing in evolution between different species must have 

 been primarily chemical in nature, since they obviously involved 

 or consisted in divergences in the character of the specific 

 metabolic syntheses on which the growth, continued life and 

 special character of any organism depend. The essential dif- 

 ferences in the germinal constitutions of different species must 

 similarly depend on specific chemical differences, i. e., primarily 

 on the specific constitution of the proteins; these differences will 

 determine specific structural differences, as we have seen. For 

 example, it seems probable that the egg of the starfish and that 

 of the sand-dollar which are closely alike in general appearance 

 and properties are not widely dissimilar in their purely chemical 

 constitution, except as regards the specific nature of their struc- 

 tural proteins; and that it is ultimately because of this difference 

 that the two germs effect such different transformations in the 

 food and other materials which each incorporates from the sur- 

 roundings and builds up into an adult organism of constant type. 



1 Stockard, Archiv f. Entwicklungsmech., 1907, Vol. 23, p. 249; Journ. Exper. 

 Zool., 1909, Vol. 6, p. 285. 



- Herbst, Zeitschr. wiss. Zool., 1892, Vol. 4, p. 446; Mitt. zool. Sta. Neapel, 1893 

 Vol. ii, p. 136. 



