THE MUTATED GENE 203 



control of development by heredity, it is first bound to control 

 this sequence of patterning. 



5. The process of pattern formation may be evocated by 

 evocator substances diffusing into the area from an adjacent area 

 or center (organizer type) or by substances diffusing from larger 

 or smaller, already formed areas in definite directions. In the 

 latter case, it cannot be decided whether an evocator is actually 

 present or the observed process of diffusion, e.g., sea-urchin egg, 

 amounts to separation of the pattern-forming substances. Some- 

 thing, however, must initiate this process and might be called 

 an evocator, produced within the area upon which it acts, but 

 this need not necessarily be a substance; it might also be the 

 pH situation or an electric charge, etc. 



6. The type of pattern formed depends upon the nature of the 

 evocator (example: head and tail organizer), the nature of the 

 area (example: newt evocator acting upon frog-mouth area), 

 and probably also upon the conditions of the whole developing 

 system (example: different success in transplant and explant.) 

 The problem of regulation has been barred from this general 

 review. This is how we conceive of the general picture of devel- 

 opment as derived from modern work. (The facts of experi- 

 mental embryology are assumed to be known to the reader.) 

 The action of the hereditary material, then, has to be linked with 

 these primary processes. 



Before we begin to study the facts, some information should be 

 presented regarding the decisive process of patterning. Genes 

 or mutant genes that control development ought to be responsi- 

 ble for the initiation of such a process at definite times and 

 places by some type of evocation. What follows, the subdivision 

 of the field, may be a wholly automatic, i.e., physicochemical, 

 consequence of the situation created. The actual process of 

 pattern formation must therefore be of a rather generalized type; 

 as a matter of fact, it may be any type of process that leads to a 

 redistribution of substances within a given system (or eventually 

 of physical conditions like viscosity, charge, dispersion). 



A considerable number of investigators have tried to formulate 

 definite ideas of this process. One point of view, couched in 

 physiological language, is Child's theory of gradients which, 

 applied to our problem, would mean that a gradient of intensity 

 of metabolic processes produces different conditions in a set 



