3o6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



In the more highly differentiated animals range of effective dominance 

 is not limited, except perhaps in earlier developmental stages. With at- 

 tainment of all-or-none conduction of nervous impulses nervous domi- 

 nance is effective over an indefinite distance. The limit of individual size 

 in these forms is determined by factors limiting growth rather than by a 

 limited range of dominance. Dominance effected by mass transport of 

 substance is not necessarily limited in range, though it may be so limited 

 if the substance decreases in concentration by spreading over a greater 

 area or is progressively used up or altered in the course of transport. 



In many of the simpler organisms physiological isolation with resulting 

 reorganization and reconstitution of the isolated part may result from 

 any one or more of four factors: (i) increase in length of the polar axis, 

 so that a part comes to lie beyond the range of effective dominance; (2) 

 decrease in effective range of dominance in consequence of decrease in 

 activity of a dominant region; (3) blocking by local inhibition between 

 dominant and subordinate region; (4) alteration of a subordinate part so 

 that it becomes more or less insensitive to dominance, by activation or 

 stimulation from some other source and probably in many cases by pro- 

 gressing determination and differentiation. 



A suf!icient degree of physiological isolation of a part may result in 

 its development into a new individual, as in cases of budding in coelen- 

 terates and other forms and of fission in planarians and annelids. Re- 

 duplication of parts, segments, etc., from a growing region probably also 

 involves at least partial successive physiological isolations. In general, 

 agamic reproduction in axiate organisms appears to be a reconstitution re- 

 sulting from physiological isolation, as reconstitution of pieces results from 

 physical isolation. 



Physiological isolation of a part by its determination or differentiation 

 leads to a different result, that is, independent or self-differentiation. 

 Moreover, attainment of a certain degree of specificity by a part may re- 

 sult in its activation in spite of dominance. The orderly appearance of 

 localized regions of increased developmental activity, increased rate of 

 dye reduction, and increased susceptibility in connection with develop- 

 ment of particular organs, as in the chick embryo (pp. 159-63), suggests 

 this sort of self-isolation. When the region from which the optic vesicle 

 develops attains a certain physiological stage, it undergoes an activation 

 irrespective of any general dominance. Similarly, the otic region, the 

 appendage region, etc., undergo activation at a certain stage. Such physi- 

 ological isolation may or may not be so complete that the part can con- 



