THE MUTATED GENE 205 



processes of diffusion, separation, changes of viscosity, adsorp- 

 tion, chemical equilibrium, sedimentation, etc. In Liesegang- 

 ring formation, the nature of the solution, the nature of the 

 evocator substance, and the whole system (arrangement and 

 texture of filter paper, etc.) will determine what the pattern of 

 the rings will be. In development also, the nature of the evocator 

 and of the area to be patterned, together with the conditions of 

 the whole system, will decide the pattern within this area. Many 

 different processes of diversification within a given physico- 

 chemical system after the model of Liesegang rings are imaginable. 

 All have in common that an act of evocation (in the model, 

 putting a drop of a certain chemical to a certain point) auto- 

 matically produces a definite equilibrium arrangement of present 

 or newly formed different substances in the form of a pattern, 

 a process designated by the general term stratification. (A 

 point of view practically identical with ours has recently been 

 derived by von Ubisch (1936) from his work on sea urMrins and 

 also by Lindahl (1936), who actually established some chemical 

 patterns.) 



Finally, Harrison's views (1937) may be mentioned, though he 

 has thus far pointed out only the general direction of his delibera- 

 tions. He thinks that change in the spacing of the atomic lattice 

 within the molecules of living matter may turn out to be relevant 

 for cytoplasmic differentiation. 



The important thing that is at the basis of these formulations, 

 and that characterizes the developmental process which ought to 

 be linked directly with the action of the genes, is the automatic 

 physicochemical process of pattern ( = equilibrium) formation, 

 given evocation at definite time and place and the conditions of 

 the whole system and its parts. 



B. Genes Controlling Developmental Processes 



A discussion of the problems of development with the ultimate 

 purpose of understanding the action of the genes presupposes 

 that the decisive steps in development are controlled by gene 

 activity. Most biologists who have discussed the problem of 

 whether the promorphological features of the egg before fertiliza- 

 tion — early formation of pattern (organ-forming areas) — or 

 in early development are controlled by genes have come to the 



