PROBLEMS AND MATERIAL ii 



mental induction and axiate organization may be initiated by transmis- 

 sion of excitation in some of the invertebrates. In its more primitive ex- 

 pressions dominance is intimately associated with physiological gradients. 

 The high region of a gradient is primarily the chief dominant region; but 

 a given gradient level, when isolated from more anterior levels, may, to 

 some degree, dominate lower gradient levels, at least in the polar gradient 

 of hydroids and planarians. Experimental localization of a new dominant 

 region generally results in induction of a gradient or gradient system, with 

 the dominant region as its high end or as its center or otherwise located, 

 according to the form and pre-existing pattern of the material. Trans- 

 portative dominance in its more highly specific forms is not necessarily 

 associated with a gradient, though there may be an indirect relation if the 

 parts concerned were determined in relation to a gradient. 



In many organisms, particularly in the simpler animals, dominance ap- 

 parently decreases in effectiveness with increase in distance from the dom- 

 inant region or is limited in range of effectiveness, and this range varies 

 with physiological condition and changes during the course of develop- 

 ment. In consequence of this hmited range, more or less physiological 

 isolation of parts of the organism which come to lie beyond this range is 

 possible. "Physiological isolation" may be defined as isolation in greater 

 or less degree of a part of an organism from control by a dominant region, 

 without physical discontinuity. Physiological isolation of a part may be 

 brought about in various ways, to be considered in chapter ix. In the 

 simpler organisms the result of physiological isolation is usually alteration 

 of the part of the axiate pattern present or development of a new inde- 

 pendent axiate pattern in the isolated part.^ 



Agamic reproduction in axiate organisms is intimately associated with 

 physiological isolation. In many cases either the degree of isolation is in- 

 suflicient or the potentialities of the isolated part are too limited to per- 

 mit development of a complete individual, and a part — for example, a 

 segment — may develop. Local dominances and physiological isolations 

 maybe concerned in determining the developmental order of various repet- 

 itive parts, such as tentacles. In fact, development apparently involves 

 progressively increasing complexity of relations of dominance and sub- 

 ordination, of both the transmissive and the transportative type. Ques- 

 tions of the origin and nature of physiological dominance in any particular 

 case; of its changes in character during the course of development; of its 



J For earlier discussions of dominance and physiological isolation see Child, 1911a; 1915^, 

 chaps, iv and v; 19246, chaps, x-xii. 



